Naval Research Laboratory

Washington, DC 20375-5320

Validation of HVOF WC/Co Thermal Spray

Coatings as a Replacement for Hard

Chrome Plating on Aircraft Landing Gear

BRUCE D. SARTWELL

Surface Chemistry Branch, Chemistry Division

KEITH O. LEGG

Rowan Technology Group, Libertyville, IL

JERRY SCHELL

GE Aircraft Engines, Cincinnati, OH

JOHN SAUER

Sauer Engineering, Cincinnati, OH

PAUL NATISHAN

Center for Corrosion Science, Chemistry Division

DENNIS DULL AND JOHN FALKOWSKI

Boeing Corporation, Seattle, WA

PHILIP BRETZ

Metcut Research, Cincinnati, OH

DONALD PARKER

NASA Kennedy Space Center, Cape Canaveral, FL

CRAIG EDWARDS

General Atomics, Ogden, UT

JON DEVEREAUX

Naval Aviation Depot, Jacksonville, FL

March 31, 2004

NRL/MR/6170--04-8762

Approved for public release: distribution is unlimited.


iii

TABLE OF CONTENTS

Table of Contents................................................................................................................iii

List of Figures.....................................................................................................................ix

List of Tables..................................................................................................................... xv

List of Acronyms ............................................................................................................. xix

Acknowledgments............................................................................................................ xxi

1. Background................................................................................................................1 

1.1. References ...........................................................................................................8 

2. Technology Description.............................................................................................9 

2.1. Technology Development and Application.........................................................9 

2.2. Process Description...........................................................................................10 

2.3. Previous Testing of the Technology..................................................................13 

2.4. Advantages and Limitations of the Technology ...............................................13 

2.5. References .........................................................................................................13 

3. Materials Testing .....................................................................................................15 

3.1. Summary of Tests Conducted ...........................................................................15 

3.2. Coating optimization and characterization........................................................16 

3.2.1. Rationale/Background....................................................................................17 

3.2.1.1. HVOF Application ..................................................................................17 

3.2.1.2. HCAT Coating Optimization and Deposition Philosophy – Fatigue......17 

3.2.1.3. Fatigue Cycle Life vs. Fatigue Coating Integrity....................................18 

3.2.2. Test methodology...........................................................................................19 

3.2.2.1. DOE Methodology ..................................................................................19 

3.2.2.2. HVOF Optimization................................................................................19 

3.2.2.2.1. General...............................................................................................19 

3.2.2.3. General Example: CCAD Analysis........................................................20 

3.2.3. Test Results....................................................................................................21 

3.2.3.1. General Sequence....................................................................................21 

3.2.3.2. Northwestern/Generic Protocol at Hill AFB JP 5000 .............................22 

3.2.3.2.1. General Test Results..........................................................................22 

3.2.3.2.2. General Discussion............................................................................24 

3.2.3.3. CCAD DOE 1998...................................................................................25 
iv

3.2.3.3.1. General Test Results..........................................................................25 

3.2.3.3.2. General Discussion............................................................................28 

3.2.3.4. LG DOE at Hitemco: 1998-1999 DJ 2600 System...............................29 

3.2.3.4.1. General Discussion............................................................................33 

3.2.3.5. Almen Strip and Temperature Measurement Procedures .......................34 

3.2.4. Interpretation and Discussion ........................................................................35 

3.2.5. Significance....................................................................................................36 

3.2.6. Conclusions....................................................................................................36 

3.3. Fatigue data .......................................................................................................37 

3.3.1. Data Summary ...............................................................................................37 

3.3.2. Test Rationale ................................................................................................37 

3.3.3. Specimen Fabrication.....................................................................................40 

3.3.3.1. Specimen Geometry and Materials: ........................................................40 

3.3.3.2. Specimen preparation..............................................................................41 

3.3.4. Coating deposition methodology ...................................................................42 

3.3.5. Test methodology...........................................................................................43 

3.3.6. Test results .....................................................................................................47 

3.3.6.1. 4340 Substrate.........................................................................................47 

3.3.6.2. A100 substrate.........................................................................................51 

3.3.6.3. 300M substrate ........................................................................................54 

3.3.7. Coating integrity ............................................................................................56 

3.3.8. Discussion of trends in fatigue results ...........................................................59 

3.3.9. Significance....................................................................................................60 

3.3.10. Conclusions....................................................................................................60 

3.4. Corrosion data ...................................................................................................61 

3.4.1. Data summary ................................................................................................61 

3.4.2. Pre-JTP atmospheric corrosion testing ..........................................................61 

3.4.2.1. Specimen preparation..............................................................................62 

3.4.2.2. Experimental Procedures.........................................................................63 

3.4.2.3. Results and Discussion............................................................................63 

3.4.3. Rationale for JTP testing................................................................................66 

3.4.4. Landing gear original JTP testing..................................................................67 

3.4.4.1. JTP specimen fabrication and deposition methodology..........................67 
 v

3.4.4.2. JTP test methodology..............................................................................68 

3.4.4.2.1. Test Description.................................................................................69 

3.4.4.3. Test results...............................................................................................73 

3.4.4.3.1. 4340 Steel in ASTM B-117...............................................................73 

3.4.4.3.2. 300M Steel in B 117..........................................................................75 

3.4.4.3.3. 300M Steel in GM 9540 P.................................................................76 

3.4.4.3.4. Aermet 100 Steel in B 117 ................................................................76 

3.4.4.3.5. Aermet 100 Steel in GM 9540P ........................................................77 

3.4.4.3.6. Comparison of performance of WC-Co and WC-CoCr coatings 

without sealer........................................................................................................77 

3.4.5. Additional B117 Corrosion Testing...............................................................85 

3.4.5.1. Background .............................................................................................85 

3.4.5.2. Specimen Preparation and Coating Deposition.......................................86 

3.4.5.3. Results and Discussion............................................................................90 

3.4.5.3.1. Protection Ratings..............................................................................90 

3.4.5.3.2. Appearance Ratings...........................................................................92 

3.4.5.4. Conclusions .............................................................................................92 

3.4.6. Discussion of overall corrosion results........................................................102 

3.4.7. Significance..................................................................................................103 

3.4.8. Conclusion ...................................................................................................103 

3.5. Wear data.........................................................................................................104 

3.5.1. Data summary ..............................................................................................104 

3.5.2. Rationale ......................................................................................................104 

3.5.3. Specimen fabrication ...................................................................................105 

3.5.3.1. Fretting specimens.................................................................................105 

3.5.3.2. Sliding wear specimens.........................................................................106 

3.5.4. Coating deposition methodology .................................................................108 

3.5.5. Test methodology.........................................................................................108 

3.5.5.1. Fretting ..................................................................................................109 

3.5.5.2. Sliding wear...........................................................................................109 

3.5.5.3. Test matrices .........................................................................................113 

3.5.6. Test results ...................................................................................................116 

3.5.6.1. Pre-DOE................................................................................................116 
 vi

3.5.6.2. L12 fretting tests....................................................................................116 

3.5.6.3. L12 sliding wear tests............................................................................118 

3.5.6.4. L8 sliding wear DOE tests ....................................................................121 

3.5.7. Discussion of wear results ...........................................................................131 

3.5.7.1. Fretting ..................................................................................................131 

3.5.7.2. Sliding wear...........................................................................................131 

3.5.8. Significance..................................................................................................132 

3.5.9. Conclusion ...................................................................................................133 

3.6. Impact data ......................................................................................................134 

3.6.1. Data summary ..............................................................................................134 

3.6.2. Rationale ......................................................................................................134 

3.6.3. Specimen fabrication ...................................................................................134 

3.6.4. Coating deposition methodology .................................................................135 

3.6.5. Test methodology.........................................................................................135 

3.6.5.1. Gravelometry.........................................................................................135 

3.6.5.2. Dropped ball impact..............................................................................136 

3.6.6. Test results ...................................................................................................138 

3.6.6.1. Gravelometry.........................................................................................138 

3.6.6.2. Dropped ball impact..............................................................................139 

3.6.7. Discussion of impact results ........................................................................142 

3.6.7.1. Gravelometry.........................................................................................142 

3.6.7.2. Dropped ball impact..............................................................................142 

3.6.8. Significance..................................................................................................142 

3.6.9. Conclusion ...................................................................................................143 

3.7. Hydrogen embrittlement data..........................................................................143 

3.7.1. Data summary ..............................................................................................143 

3.7.2. Rationale ......................................................................................................143 

3.7.3. Specimen fabrication ...................................................................................144 

3.7.4. Coating deposition methodology .................................................................144 

3.7.5. Scribing (exposing the notch to the test environment) ................................146 

3.7.6. Test methodology.........................................................................................146 

3.7.7. Test sample mechanical properties ..............................................................148 

3.7.8. Coating structure..........................................................................................149 
vii

3.7.9. Test results ...................................................................................................150 

3.7.9.1. Cracking of coatings..............................................................................150 

3.7.9.2. Substrate failure mechanisms................................................................150 

3.7.9.3. Time to failure data ...............................................................................151 

3.7.9.4. Open circuit potential............................................................................158 

3.7.10. Discussion of hydrogen embrittlement results.............................................159 

3.7.11. Significance..................................................................................................162 

3.7.12. Conclusion ...................................................................................................162 

3.8. Summary and Conclusions..............................................................................164 

3.9. References .......................................................................................................165 

4. Coating Integrity and Spalling...............................................................................167 

4.1. Axial integrity tests .........................................................................................167 

4.2. Landing gear bend integrity tests ....................................................................169 

4.3. References .......................................................................................................171 

5. Component Rig and Flight Testing........................................................................173 

5.1. Rig Testing ......................................................................................................173 

5.1.1. F/A-18 E/F Main Landing Gear Pins...........................................................173 

5.1.2. P-3C Main Landing Gear Piston..................................................................176 

5.2. Flight Testing ..................................................................................................179 

5.2.1. E-6A Uplock Hook Shaft.............................................................................179 

5.2.2. P-3C Main Landing Gear Components........................................................180 

6. Producibility...........................................................................................................183 

6.1. Stripping of HVOF coatings ...........................................................................183 

6.1.1. Stripping of HVOF WC/Co – Sacramento ALC, NADEP Cherry Point ....183 

6.1.1.1. Masking, fixturing, and positioning specimens ....................................185 

6.1.1.2. Process Solutions and Operating Conditions ........................................186 

6.1.1.3. Stripping Process Used and Specimen Measurements Conducted .......186 

6.1.1.4. Stripping test measurements..................................................................187 

6.1.1.5. Post Stripping Inspections and Cross-Sectional Examination ..............189 

6.1.1.6. Conclusions and Recommendations......................................................189 

6.1.2. Stripping of HVOF WC/CoCr – Heroux Devtek, Canada...........................190 

6.2. Grinding ..........................................................................................................193 

6.3. Fluid compatibility ..........................................................................................194 
viii

6.4. Non Destructive Inspection.............................................................................195 

6.4.1. Fluorescent penetrant inspection (FPI) ........................................................195 

6.4.2. Magnetic particle inspection (MPI) .............................................................196 

6.4.3. Ultrasonic inspection ...................................................................................196 

6.4.4. Barkhausen Noise measurement of grind burns ..........................................198 

6.5. References .......................................................................................................199 

7. Cost Benefit Analysis ............................................................................................201 

7.1. Introduction .....................................................................................................201 

7.2. Approach .........................................................................................................201 

7.3. Data and assumptions......................................................................................203 

7.3.1. Data Provided by MRO facility ...................................................................205 

7.3.2. HVOF Materials and Equipment Considered..............................................207 

7.3.3. Assumptions About Current and Future Operations....................................208 

7.4. Annual operating cost avoidance ....................................................................210 

7.5. Comparison of calculated results with vendor quotes.....................................210 

7.6. Financial evaluation ........................................................................................211 

7.7. Summary and recommendations .....................................................................213 

7.8. Bibliography....................................................................................................215 

8. Specifications and Standards .................................................................................217 

8.1. Proposed AMS 2448 ......................................................................................218 

8.2. Proposed AMS 7881 .......................................................................................230 

8.3. Proposed AMS7882 ........................................................................................235 

8.4. Proposed HVOF grinding specification ..........................................................240 

8.4.1. Truing & Dressing of Large Diameter Diamond Grinding Wheels ............248 

8.4.2. Calculation of nozzle opening for proper cutting fluid application.............250 

9. Implementation ......................................................................................................253 

9.1. General implementation issues........................................................................253 

9.2. Performance observations – differences between HVOF and EHC ...............254 

9.3. Implementation................................................................................................255 

9.4. Specifications and standards ...........................................................................256 

9.5. Cost observations ............................................................................................257 

9.6. Regulatory Compliance and Acceptance ........................................................257 
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LIST OF FIGURES

Figure 2-1. Schematic of HVOF Gun and Process (Sulzer Metco DiamondJet)............... 9 

Figure 2-2. HVOF Spray of Landing Gear Inner Cylinder.............................................. 10 

Figure 3-1. Microhardness Response for CCAD DOE.................................................... 27 

Figure 3-2. Substrate Temperature Response for CCAD DOE ....................................... 27 

Figure 3-3. Almen Strip Response for CCAD DOE........................................................ 28 

Figure 3-4. Microhardness Response for Hitemco DOE................................................ 31 

Figure 3-5. Substrate Temperature Response for Hitemco DOE ................................... 31 

Figure 3-6. Almen Strip Response for Hitemco DOE .................................................... 32 

Figure 3-7. Typical Hourglass-shaped Fatigue Bar. ........................................................ 39 

Figure 3-8. Typical Smooth Fatigue Bar. ........................................................................ 39 

Figure 3-9. Hourglass Configuration with .500” Gage Section....................................... 41 

Figure 3-10. Small (1/4” dia) Hourglass Bars for Corrosion Fatigue Testing................. 44 

Figure 3-11. Corrosion Cell Set-up for Fatigue Testing-Large Hourglass ...................... 44 

Figure 3-12. Close-up View of Corrosion Cell Set-up for Fatigue Testing – Small 

Hourglass Specimen.................................................................................................. 45 

Figure 3-13. 4340 Fatigue - Hourglass Configuration, Air Environment, R = -1. .......... 48 

Figure 3-14. 4340 - Hourglass vs. Smooth Bar Configuration, Air Environment, R = -1.

................................................................................................................................... 49 

Figure 3-15. 4340 - Air vs. NaCl Environment Comparison, Hourglass Configuration, 

0.003” Thickness, R = -1. ......................................................................................... 49 

Figure 3-16. 4340 - 0.003” vs. 0.010” Thickness Comparison, Air Environment, 

Hourglass Configuration, R=-1, on 0.250” and 0.500” Specimens Respectively. ... 50 

Figure 3-17. 4340 – R-ratio Comparison, Hourglass Configuration, 0.003” Thickness . 50 

Figure 3-18. A100 - Unpeened vs. Peened Substrate Comparison, Hourglass 

Configuration, R=-1, 0.003” Thickness................................................................... 52 

Figure 3-19. A100 - R=-1 vs. R=0.1 Comparison, Hourglass Configuration, 0.003” 

Thickness. ................................................................................................................. 52 

Figure 3-20. A100 - Air vs. NaCl Environment Comparison, Hourglass Configuration, 

R =-1 ......................................................................................................................... 53 

Figure 3-21. A100 - .003” vs. .010” Thickness Comparison, Hourglass Configuration, 

NaCl Environment, R=-1, 0.250” and 0.500” Diameter Bars Respectively............. 53 

Figure 3-22. 300M - Peened vs. Unpeened Substrate Comparison, Hourglass 

Configuration, 0.003” Thickness, R=-1.................................................................... 54 
 x

Figure 3-23. 300M - Smooth Bar vs. Hourglass Bar Comparison, 0.003” Thickness, 

R=-1. ......................................................................................................................... 55 

Figure 3-24. 300M - 0.010” Thick Coating on 0.500” dia Hourglass vs. 0.003” 

Thickness, 0.250” dia Hourglass, R=-1. ................................................................... 55 

Figure 3-25. 300M - Air vs. NaCl Environment, Hourglass, 0.003” Thickness, R=-1. .. 56 

Figure 3-26. Cracking of HVOF Coating in Central Region of Hourglass Specimen. ... 57 

Figure 3-27. Failed 300M Specimens Showing Spalling in Small Zone Near Fracture 

Surface. ..................................................................................................................... 57

Figure 3-28. Failed 300M Specimens Showing Total Spallation on Fracture................. 58 

Figure 3-29. HVOF WC-Co, Tribaloy 400 (T400) and EHC (Cr) on 4340 Steel 

Following Three-year Atmospheric Exposure. Samples Shown Prior to Cleaning. 64 

Figure 3-30. HVOF WC-Co, Tribaloy 400 (T400) and EHC (Cr) on 4340 Steel 

Following Three-Year Atmospheric Exposure. Samples Shown Subsequent to 

Cleaning. ................................................................................................................... 65 

Figure 3-31. Protection Ratings for EHC, HVOF Tribaloy 400, and HVOF WC-17Co on 

4340 Steel After Three-year Atmospheric Exposure................................................ 65 

Figure 3-32. Corrosion Coupon, 1” Diameter Bar........................................................... 67 

Figure 3-33. Illustration of Scratched Corrosion Test Specimen. ................................... 68 

Figure 3-34. 0.003”-thick EHC Coating + Sealer on 4340 Steel After B117 Test: 

a) Before and b) After Cleaning................................................................................ 78 

Figure 3-35. Protection Ratings on 4340 Steel After B117 Testing................................ 79 

Figure 3-36. Radar Plot of the Protection Ratings on 4340 Steel After ASTM B117 

Testing....................................................................................................................... 79 

Figure 3-37. HVOF Coatings on 4340 Steel After the B117 Test: a) 0.003” WC-Co, b) 

0.010” WC-Co, c) 0.010” WC-CoCr, and d) 0.003” WC-CoCr............................... 80 

Figure 3-38. 0.010”-thick EHC Coatings on 4340 Steel With: a) Ni Sublayer, b) Sealer, 

and c) EHC Coating Only. After B117 Testing....................................................... 80 

Figure 3-39. HVOF Coatings on 4340 Steel After the B117 Test: a) EHC Coating Only 

b) EHC With Sealer, c) EHC With Ni Sublayer, and d) 0.010”-thick WC-CoCr. ... 81 

Figure 3-40. 0.003”-thick HVOF WC-Co Coating on 4340 Steel with Sealer After 

Blister has been Removed......................................................................................... 81 

Figure 3-41. Protection Ratings on 300M After ASTM B117 Testing. .......................... 82 

Figure 3-42. Radar Plot of the Protection Ratings on 300M After ASTM B117 Testing.

................................................................................................................................... 82 

Figure 3-43. 0.003”-thick Coatings on 300M After GM 9540 Testing: a) EHC Only, 

b)WC-Co, c) WC-Co With Sealer, and d) EHC With Sealer. .................................. 83 

Figure 3-44. Protection Ratings on Aermet 100 After ASTM B117 Testing.................. 83 
 xi

Figure 3-45. Radar Plot of the Protection Ratings on Aermet 100 After ASTM B117 

Testing....................................................................................................................... 84 

Figure 3-46. WC-Co HVOF Coating 0.003” in Thickness on 4340 Steel, 300M, and 

Aermet 100 After the B117 Test............................................................................... 84 

Figure 3-47. 0.010”-thick HVOF Coatings: a) WC-Co on 4340, b)WC-CoCr on 4340, c) 

WC-Co on 300M, and d) WC-Co on Aermet 100 After the B117 Test ................... 85 

Figure 3-48. HVOF WC-Co Coatings, 0.003”-thick, Ground, on 300M Steel from the 

LGJTP Tests, Left, and the Supplemental Tests, Right............................................ 93 

Figure 3-49. HVOF WC-Co Coatings, 0.003”-thick, Ground, on 4340 Steel from the 

LGJTP Tests.............................................................................................................. 94 

Figure 3-50. EHC Coatings, 0.003”-thick, Ground, on 4340 Steel from the LGJTP Tests, 

Left, and the Supplemental Tests, Right................................................................... 94 

Figure 3-51. HVOF WC-Co Coatings, 0.003”-thick, on 4340 Steel, As-deposited on Left 

and Ground on Right................................................................................................. 95 

Figure 3-52. Left: WC-Co, 0.010”-thick, Ground. Center: EHC, 0.010”-thick, Ground. 

Right: WC-CoCr, 0.010”-thick, Ground. All on 4340 Steel Substrates. ................ 95 

Figure 3-53. Left: WC-Co, 0.010”-thick, Ground. Center: EHC, 0.010”-thick, Ground. 

Right; WC-CoCr, 0.010”-thick, Ground. All on 300M Steel Substrates................ 96 

Figure 3-54. Comparison of Appearance and Protection Ratings for 4340 Plate, NRL.. 96 

Figure 3-55. Comparison of Appearance Ratings for HVOF and EHC Coatings on 4340 

Rod Tested at NRL and KSC.................................................................................... 97 

Figure 3-56. Comparison of Appearance Ratings for HVOF and EHC Coatings on 300M 

Rod Tested at NRL and KSC.................................................................................... 97 

Figure 3-57. Comparison of Protection Ratings for HVOF and EHC Coatings on 4340 

Rod Tested at NRL and KSC.................................................................................... 98 

Figure 3-58. Comparison of Protection Ratings for HVOF and EHC Coatings on 300M 

Rod Tested at NRL and KSC.................................................................................... 99 

Figure 3-59. WC-Co Coatings, 0.003”-thick, on 300M Steel. Left: As-deposited. Right: 

Ground. ..................................................................................................................... 99 

Figure 3-60. WC-Co Coatings on 4340 Steel. Left: 0.003”-thick, Ground. Right: 

0.010”-thick, Ground. ............................................................................................. 100 

Figure 3-61. WC-Co Coatings on 300M Steel. Left: 0.003”-thick, Ground. Right: 

0.010”-thick, Ground. ............................................................................................. 101 

Figure 3-62. Left: EHC Coating, 0.010”-thick, Ground. Right: WC-Co Coating, 

0.010”-thick, Ground. All on 4340 Steel. .............................................................. 101 

Figure 3-63. Left: EHC, 0.010”-thick, Ground. Right: HVOF WC-Co, 0.010”-thick, 

Ground. All on 300M Steel.................................................................................... 102 

Figure 3-64. a) Fretting Block; b) Fretting Shoe. .......................................................... 105 
xii

Figure 3-65. Wear Test Rods and Bushings - a) Rod, b) Straight Bushing, c) Seal Groove 

Bushing. .................................................................................................................. 107 

Figure 3-66. Cross-sectional Schematic of Fretting Wear Test..................................... 109 

Figure 3-67. Cross-sectional Schematic of Piston and Bushing Oscillating Wear Test.110 

Figure 3-68. Sliding Wear Test Equipment. .................................................................. 111 

Figure 3-69. Visual Ranking Standards for Rods, VR 01 to VR 10.............................. 112 

Figure 3-70. Fretting Wear Ranking - Shoe. L12 DOE Matrix. (Red against metal 

bushings; blue against nitrile seals.) ....................................................................... 117 

Figure 3-71. Fretting Wear - L12 DOE Matrix. Yellow: Wear of Coated Shoe; Red: 

Wear of Uncoated 4340 Block; Blue: Wear of Nitrile Seal.................................... 118 

Figure 3-72. Sliding Wear – L12 data. Visual Wear Ranking – Rod. (Red against 

bushings; blue against seals.).................................................................................. 119 

Figure 3-73. Sliding Wear – L12 data. Weight Change – Bushing/seal. Yellow: Wear of 

Coated Rod; Red: Wear of Uncoated Bushing; Blue: Wear of Nitrile Seal. .......... 120 

Figure 3-74. Sliding Wear, L8 DOE – Visual Rankings for Baseline Metal Bushings. 125 

Figure 3-75. Sliding Wear, L8 DOE – Weight Loss for Baseline Metal Bushings....... 125 

Figure 3-76. Pareto Chart and Visualization of the Major Effects in the L8 DOE Based 

on Visual Wear Rating. Only Effects D and A (normal load and number of cyles) 

are Statistically Significant. .................................................................................... 126 

Figure 3-77. Pareto Chart and Visualization of the Major Effects in the L8 DOE Based 

on Rod Wear Volume. Only Effect C (coating material) is Statistically Significant.

................................................................................................................................. 127 

Figure 3-78. Pareto Chart and Visualization of the Major Effects in the L8 DOE Based 

on Bushing Wear Volume. Only Effects A and C (number of cycles and coating 

material) are Statistically Significant...................................................................... 128 

Figure 3-79. L8 DOE Data for 3rd Half Replicate – Visual Ranking for Rod Wear. EHC 

and WC-Co on Metal Bushings and Nitrile Seals. ................................................. 129 

Figure 3-80. L8 DOE Data for 3rd Half Replicate – Weight Loss. EHC and WC-Co on 

Metal Bushings and Nitrile Seals............................................................................ 129 

Figure 3-81. L8 DOE Data for 5th Half Replicate – Visual Ranking for Rod Wear. EHC 

and WC-Co on AlNiBronze Bushings and Karon B Liners. .................................. 130 

Figure 3-82. L8 DOE Data for 5th Half Replicate – Weight Loss. EHC and WC-Co on 

AlNiBronze Bushings and Karon B Liners. ........................................................... 130 

Figure 3-83. Average Rod Wear for L8 matrix - Visual Rating.................................... 131 

Figure 3-84. Average Rod and Bushing/Seal Wear for L8 Matrix - Weight Loss. ....... 131 

Figure 3-85. Gravelometry Chamber (Boeing).............................................................. 136 

Figure 3-86. Close-up of Ball Impact Equipment, Showing Ball Delivery Tube, Sample 
xiii

in Vise, and Ball...................................................................................................... 137 

Figure 3-87. Surfaces of EHC Coatings After Gravelometry Testing. a) and b) 0.003”- 

thick on Rods; c) and d) 0.010”-thick on rods; e) and f) 0.010”-thick on flats. ..... 138 

Figure 3-88. Surfaces of WC-Co Coatings After Gravelometry Testing. a) and b) 

0.003”-thick on Rods; c) and d) 0.003”-thick on Flats........................................... 139 

Figure 3-89. Surfaces of WC-CoCr Coatings After Gravelometry Testing. a) and b) 

0.003”-thick on Rods; c) and d) 0.010”-thick on Flats........................................... 139 

Figure 3-90. Impact Area on 0.010” WC-CoCr , 72” Drop Height............................... 140 

Figure 3-91. Circumferential Cracking Around Impact Point for 0.003” and 0.010” 

Coatings at Different Ball Drop Heights. ............................................................... 141 

Figure 3-92. Radial Cracking Along Rod Axis for 0.003” and 0.010” Coatings at 

Different Ball Drop Heights.................................................................................... 141 

Figure 3-93. F-519, Type 1a.2 Specimen Coated with 0.010” WC-Co......................... 144 

Figure 3-94. Standard F-519 Specimen Coating............................................................ 145 

Figure 3-95. Method for Scribing the Notch (exposing the substrate) on a CNC Milling 

Machine. Left – Schematic of Cutting Blade in Notch; Right – Actual Set-up on 

CNC machine.......................................................................................................... 146 

Figure 3-96. EHC and HVOF WC-Co in Notch of F-519 Specimens........................... 149 

Figure 3-97. Metallographic Cross Sections of Chrome Plate (left) and HVOF WC-Co 

After Stressing to 45% NTS in Air. (Note also the original layer formation in the 

HVOF coating.)....................................................................................................... 150 

Figure 3-98. Sequence 1: Average Hours to Failure (NF = No failure in 200hrs), With 

and Without Hydrogen Bake. All Failures in Notch.............................................. 152 

Figure 3-99. Sequence 2: Average Hours to Failure (NF = No failure in 200hrs), With 

and Without Hydrogen Bake. Failures Occurred on Shank at Button................... 153 

Figure 3-100. Sequence 3 Average Hours to Failure with No Notch Scribing. (Numbers 

are verage time-to-failure data values – note log scale. Error bars indicate high and 

low values.)............................................................................................................. 154 

Figure 3-101. Sequence 3: Average Hours to Failure With Notch Scribed to Expose the 

Substrate. (Numbers are average time-to-failure data values – note log scale. Error 

bars indicate high and low values.) (Note that these tests were run for 0.003” 

coatings only).......................................................................................................... 155 

Figure 3-102. Sequence 3 Time-to-failure Data Summary............................................ 156 

Figure 3-103. Sequence 3 – Comparison of Time to Failure for Scribed and Un-scribed 

Specimens - DI Water............................................................................................. 157 

Figure 3-104. Sequence 3 – Comparison of Time to Failure for Scribed and Un-scribed 

Specimens - 5% NaCl Solution............................................................................... 157 
 xiv

Figure 3-105. Open Circuit Potential for Coated Sheet Specimens in 5% NaCl Solution.

................................................................................................................................. 158 

Figure 3-106. Plot of Log Time-to-failure vs. Average Potential Difference Between 

Coating and 4340 Substrate in 5% NaCl Solution.................................................. 159 

Figure 4-1. HCAT Axial Integrity Big Bar Design (Metcut). ....................................... 167 

Figure 4-2. NAVAIR "Big Bar" Axial Integrity Test Specimen (Eui Lee, NAVAIR). 168 

Figure 4-3. Axial Big Bar Integrity Tests (HCAT and NAVAIR). ............................... 168 

Figure 4-4. Hill AFB Bend Test Fixture........................................................................ 169 

Figure 4-5. Bend Test Spalling for A-10 Landing Gear Cylinder Under Strain-Control. 

0.010” WC-Co, R=-0.33 (OO-ALC) ...................................................................... 170 

Figure 4-6. Spallation of a 0.015” WC/CoCr Coating From a Test Cylinder (OO-ALC).

................................................................................................................................. 171 

Figure 4-7. Same as Figure 4-6 , but Under FPI............................................................ 171 

Figure 5-1. Location of HVOF-coated Pins in Boeing F/A-18E/F Main Landing Gear 

Fatigue Test............................................................................................................. 174 

Figure 5-2. HVOF WC/Co-coated Pins Used in Boeing F/A-18E/F Main Landing Gear 

Rig Test. Left, End of 14,000 hr Spectrum Test; Right, End of CAC Test (Boeing St 

Louis). ..................................................................................................................... 175

Figure 5-3. Application of HVOF WC/17Co Coating to P-3C MLG Piston................. 177 

Figure 5-4. Left-hand MLG Containing HVOF WC/17Co-coated Piston Mounted on P3C Test Article at Lockheed-Martin in Marietta, Georgia ..................................... 178 

Figure 5-5. Close-up of HVOF WC/17Co-coated Piston in MLG ................................ 178 

Figure 5-6. Navy E-6A Reconnaissance Aircraft .......................................................... 179 

Figure 5-7. E-6A Uplock Hook Shaft............................................................................ 180 

Figure 5-8. P-3C Main Landing Gear with HVOF WC/17Co-coated Piston Cylinder 

(indicated) and Axle Journals ................................................................................. 181 

Figure 6-1. Weight Loss as a Function of Stripping Time. .......................................... 189 

Figure 6-2. Polarization Curves for WC/CoCr and 4340 Substrate with the Stripping 

Solutions (Heroux Devtek). .................................................................................... 191 

Figure 6-3. Stripping Rate of WC/CoCr by Type S2-2, Ni Stripper (Heroux Devtek). 192 

Figure 6-4. Stripping Rate of WC/CoCr by Type S2-1, Rochelle Salt Stripper (Heroux 

Devtek )................................................................................................................... 192 

Figure 6-5. Effect of Ultrasonic Agitation. Type S1-3 Stripper. .................................. 193 

Figure 6-6. Damage Pit in HVOF WC/CoCr Coating. Left: Standard; Right: FPI 

(Heroux Devtek). .................................................................................................... 195 

Figure 6-7. FPI of HVOF-coated Landing Gear After Fatigue to Stress Sufficient to 
 xv

Cause Coating Delamination, Showing Circumferential Coating Cracks (C. 

Edwards, OO-ALC). ............................................................................................... 195 

Figure 6-8. Ultrasonic Detection of Crack Beneath HVOF WC/CoCr Coatings (Boeing)

................................................................................................................................. 197 

Figure 6-9. Ultrasonic Measurement of Delamination of HVOF Coatings During Fatigue 

(R. Ware, AFRL). ................................................................................................... 197 

Figure 6-10. Barkhausen Noise Signal vs Coating Thickness (Heroux Devtek)........... 198 

Figure 6-11. Effect of Shot Peening and Grind Burning on Barkhausen Noise of HVOF 

WC/CoCr (Heroux Devtek). ................................................................................... 199 

Figure 7-1. EHC Process Flow at Commercial MRO Shop. ......................................... 202 

Figure 7-2. Projected Process Flow of HVOF for Applying WC/Co or WC/CoCr. ..... 203 

Figure 7-3. Areas Expected to be Transitioned from Hard Chrome Electroplating to 

HVOF Thermal Spraying........................................................................................ 204 

LIST OF TABLES

Table 1-1. Landing Gear Aircraft Applications.................................................................. 4 

Table 1-2. Defense System Programs Potentially Affected............................................... 4 

Table 1-3. Applicable Materials Processing, Coating Deposition and Test Standards. .... 7 

Table 1-4. Commercial Programs Affected and Potentially Affected by HVOF Chrome 

Replacement................................................................................................................ 8 

Table 2-1. Optimized Deposition Conditions for WC-17Co - DJ 2600 and JP 5000 

HVOF Guns. ............................................................................................................. 11 

Table 2-2. Advantages and Limitations of HVOF as a Chrome Replacement................ 14 

Table 3-1. Summary of Tests Conducted in JTP ............................................................. 15 

Table 3-2. Optimized Deposition Conditions for WC-17Co - DJ 2600 and JP 5000 

HVOF Guns. ............................................................................................................. 16 

Table 3-3. Inputs and Outputs for Design of Experiment HVOF Optimization.............. 19 

Table 3-4. Optimization Parameters for the CCAD DJ 2600 Analysis ........................... 21 

Table 3-5. Actual Runs for CCAD Experimentation....................................................... 22 

Table 3-6. DOE Sites and Locations................................................................................ 22 

Table 3-7. DOE Matrix for Hill AFB Work 1996 ........................................................... 23 

Table 3-8. Summary of Results for the Hill JP 5000 DOE............................................. 23 

Table 3-9. Run Order for Hill AFB Work ...................................................................... 24 

Table 3-10. Final Parameters for Hill AFB JP 5000........................................................ 25 
xvi

Table 3-11. DOE Matrix for CCAD 1998 ...................................................................... 26 

Table 3-12. Random Run Listing for CCAD DOE.......................................................... 26 

Table 3-13. Final Summary of Factors in CCAD DOE................................................... 29 

Table 3-14. DOE Matrix for Hitemco Analysis............................................................... 30 

Table 3-15. Random Runs for Hitemco DOE.................................................................. 30 

Table 3-16. Final Deposition Parameters Hitemco LG JTP ............................................ 33 

Table 3-17. Primary and Secondary Determinants of Coating Properties....................... 35 

Table 3-18. Quick Reference to Primary Data. (Click blue links to jump to data). ....... 37 

Table 3-19. Fatigue Testing Variables............................................................................. 40 

Table 3-20. Substrate Materials in LG JTP ..................................................................... 41 

Table 3-21. Machining Procedures for Fatigue Specimens............................................. 42 

Table 3-22. DiamondJet 2600 Process Parameters for Deposition of WC-17Co............ 43 

Table 3-23. Fatigue Test Matrix. ..................................................................................... 45 

Table 3-24. Coating Integrity - Summary of Analysis. ................................................... 59 

Table 3-25. Quick Data Locator. (Click on item number to view.)................................ 61 

Table 3-26. Protection Rating versus Area of Defect From ASTM B 537...................... 63 

Table 3-28. Average Value of the Protection Rating for Samples that had Undergone 

1,000 Hours of ASTM B117 Testing........................................................................ 66 

Table 3-27. Average Value of the Protection Rating for Samples that had Undergone a 

Three-year Atmospheric Exposure. .......................................................................... 66 

Table 3-29. GM 9540 Test Protocol. ............................................................................... 70 

Table 3-30. Table of Protection Rankings/Ratings.......................................................... 70 

Table 3-31. Corrosion Test Matrix. ................................................................................. 71 

Table 3-32. GM Accelerated Corrosion Test Results for 300M Steel. (Coating thickness 

0.003”.) ..................................................................................................................... 76

Table 3-33. GM Accelerated Corrosion Test Results for Aermet 100 Steel. (Coating 

thickness 0.003”.)...................................................................................................... 77 

Table 3-34. Percentage of Specimens with Intact Coatings and the Overall Protection 

Ratings for HVOF Coatings Without Sealer. ........................................................... 78 

Table 3-35. Sample Matrix for Supplemental Corrosion Testing – 4340 Rods. ............. 87 

Table 3-36. Sample Matrix for Supplemental Corrosion Testing – 300M Rods............. 88 

Table 3-37. Sample Matrix for Supplemental Corrosion Testing – 4340 Plates............. 89 

Table 3-38. Failed Coatings on 4340 Substrates. ............................................................ 91 

Table 3-39. Failed Coatings on 300M Substrates............................................................ 91 
xvii

Table 3-40. Quick Data Locator. (Click on item number to view.).............................. 104 

Table 3-41. Wear Test Materials and Coatings - Fretting. (Fretting blocks uncoated, 

shoe 4340 coated) ................................................................................................... 106 

Table 3-42. Wear test Materials and Coatings - Sliding. (Piston coated 4340, bushing 

uncoated.)................................................................................................................ 108 

Table 3-43. Description of Visual Wear Ranking. ........................................................ 112 

Table 3-44. Matrix for Pre-DOE – Fretting Tests Run.................................................. 113 

Table 3-45. Matrix for Pre-DOE (Design of Experiment) - Sliding.............................. 113 

Table 3-46. L-12 Design of Experiment Matrix for Fretting......................................... 114 

Table 3-47. L-12 Design of Experiment Matrix for Bushings/liners............................. 114 

Table 3-48. L-8 Wear Test Matrix. (Note: each test condition was run twice (A and B)).

................................................................................................................................. 115 

Table 3-49. L12 Matrix Data – Fretting......................................................................... 117 

Table 3-50. L12 Test Matrix Data - Sliding Wear......................................................... 119 

Table 3-51. L8 DOE Data - Baseline Metal Bushings................................................... 121 

Table 3-52. L8 DOE Data - 1st and 2nd Half Replicates. ............................................. 122 

Table 3-53. L8 DOE Data - 3rd and 4th Half Replicates............................................... 123 

Table 3-54. L8 DOE Data - 5th and 6th Half Replicates............................................... 124 

Table 3-55. Quick Data Locator. (Click on item number to view.).............................. 134 

Table 3-56. Hydrogen Embrittlement Coating Thicknesses (ground)........................... 135 

Table 3-57. Gravelometry and Ball Impact Coating Thicknesses (ground). ................. 137 

Table 3-58. Summary of Gravelometry Test Observations. Rankings from 1 (best) to 3 

(worst) for Different Evaluation Methods. ............................................................. 138 

Table 3-59. Quick Data Locator. (Click on item number to view.).............................. 143 

Table 3-60. Hydrogen Embrittlement Coating Thicknesses (ground)........................... 144 

Table 3-61. Hydrogen Embrittlement Test Matrix. ....................................................... 148 

Table 3-62. F-519 Test Specimen Hardness.................................................................. 148 

Table 3-63. F-519 Test Specimen Notch Tensile Strength............................................ 148 

Table 3-64. Summary of Crack Initiation Points........................................................... 151 

Table 3-65. F-519 Hydrogen Embrittlement Testing – Average Time to Failure (NF = 

No Failure in 200 hr). (Full data summary in JTR Appendix 6.) .......................... 152 

Table 5-1. F/A-18 E/F Main Landing Gear Fatigue Simulated Flight Hour Success 

Requirements .......................................................................................................... 173 

Table 6-1. Vendor WC-Co and WC-Co-Cr Coatings QC Test Results......................... 184 
xviii

Table 6-2. Coating Thickness of Panels. ....................................................................... 185 

Table 6-3. Percent Coating Thickness Removal............................................................ 187 

Table 6-4. Percent Coating Weight Removal. ............................................................... 188 

Table 6-5. Stripping Solutions Tested by Heroux Devtek for HVOF WC/CoCr. ......... 191 

Table 6-6. Fluid Compatibility of HVOF WC/CoCr (Heroux Devtek)......................... 194 

Table 6-7. NDI Testing of Substrate Cracks (Boeing St Louis).................................... 196 

Table 7-1. Annual Number of C-130 Landing Gear Components Chrome Electroplated 

at MRO facility. ...................................................................................................... 203 

Table 7-2. Estimated Annual Operating Cost Avoidance.............................................. 210 

Table 7-3. Summary of Vendor Quotes......................................................................... 211 

Table 7-4. Summary of Investment Criteria. ................................................................. 212 

Table 7-5. Results of Financial Evaluation for Direct Implementation......................... 212 

Table 7-6. Net Present Value by Scenario...................................................................... 213 

Table 8-1. SAE Aerospace Material Specifications for HVOF..................................... 217 

Table 9-1. Components Approved for HVOF at OO-ALC. .......................................... 255 
xix

LIST OF ACRONYMS

AMS Aerospace Materials Specification 

ANSI American National Standards Institute 

ASTM American Society for Testing and Materials 

CAC constant amplitude test 

CBA cost/benefit analysis 

CCAD Corpus Christi Army Depot 

CFR Code of Federal Regulations 

C-HCAT Canadian Hard Chrome Alternatives Team 

DARPA Defense Advanced Research Projects Agency 

DI de-ionized 

DND Department of National Defence 

DOD Department of Defense 

DOE design of experiments 

ECAM Environmental Cost Accounting Methodology 

EHC electrolytic hard chrome 

EPA U.S. Environmental Protection Agency 

ESTCP Environmental Security Technology Certification Program 

FPI fluorescent penetrant inspection 

FTSH fatigue test spectrum hours 

GEAE GE Aircraft Engines 

gph gallons per hour 

GTE gas turbine engine 

HCAT Hard Chrome Alternatives Team 

HE hydrogen embrittlement 

HVOF high-velocity oxygen-fuel 

IARC International Agency for Research on Cancer 

IC Industry Canada 

IRR internal rate-of-return 

JG-PP Joint Group on Pollution Prevention 

JTP joint test protocol 

JTR joint test report 

ksi thousands of pounds per square inch 

LGJTP Landing Gear Joint Test Protocol 

MLG main landing gear 

MJTR Materials Joint Test Report 

MRO maintenance, repair and overhaul 

NADEP-CP Naval Aviation Depot Cherry Point 

NADEP-JAX Naval Aviation Depot Jacksonville 

NAVAIR Naval Air Systems Command 

NLG nose landing gear 

NPV net present value 

NRL Naval Research Laboratory 

NTS notch tensile strength 

OEM original equipment manufacturer 
xx

OMB U.S. Office of Management and Budget 

OO-ALC Ogden Air Logistics Center 

OSHA Occupational Safety and Health Administration 

PEL permissible exposure limit 

PPE personal protective equipment 

psi pounds per square inch 

PVD physical vapor deposition 

rpm rotations per minute 

SAE Society of Automotive and Aerospace Engineers 

scfh standard cubic feet per hour 

SFH simulated flight hours 

TAT turn-around time 

TWA time-weighted average 
xxi

ACKNOWLEDGMENTS

The financial and programmatic support of the Department of Defense Environmental 

Security Technology Certification Program, under the direction of Dr. Jeffrey Marqusee, 

Director, and Mr. Charles Pellerin, Program Manager for Pollution Prevention, is 

gratefully acknowledged. In addition, the financial and programmatic support of the 

Joint Group on Pollution Prevention and the Air Force Materiel Command, coordinated 

by Ms. Debbie Meredith, is also gratefully acknowledged. 

The authors would also like to express thanks to the following individuals who made 

substantial contributions to the execution of the project: 

Mr. Warren Assink and Mr. Gene Jeunelot, Air Force Materiel Command 

Mr. Doug Wiser, Mr. Craig Edwards, Mr. Grant Cheever, Mr. Paul Trester, and 

Mr. Clint Forrest, Ogden Air Logistics Center 

Mr. Robert Kestler, Naval Aviation Depot Cherry Point 

Mr. James Candela, Naval Air Systems Command 

Mr. Ken McRae, Department of National Defence Canada 

Mr. Lawrence Otupiri, Industry Canada 

Mr. Roque Panza-Giosa and Mr. Ben Evans, Goodrich Landing Gear 

Mr. Roger Eybel, Messier-Dowty, Inc. 

Ms. Mary Gilman, Boeing Long Beach 

Mr. Stephen Gaydos, Boeing St. Louis 

Dr. Jean-Gabriel Legoux, NRC Canada 

Ms. Nihad Ben Salah, Heroux 

Mr. Robert Ware, AFRL 
xxii

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1 Background 1

1.Background 

The replacement of hard chrome plating in aircraft manufacturing activities and 

maintenance depots is a high priority for the U.S. Department of Defense and the 

Canadian Department of National Defence. Hard chrome plating is a technique that has 

been in commercial production for over 50 years and is a critical process that is used both 

for applying hard coatings to a variety of aircraft components in manufacturing 

operations and for general rebuild of worn or corroded components that have been 

removed from aircraft during overhaul. In particular, chrome plating is used extensively 

on landing gear components such as axles, hydraulic cylinders, pins and races. 

Chromium plating baths contain chromic acid, in which the chromium is in the 

hexavalent state, with hexavalent chromium (hex-Cr) being a known carcinogen having a 

level of toxicity greater than arsenic or cadmium. During operation chrome plating tanks 

emit a hex-Cr mist into the air, which must be ducted away and removed by scrubbers. 

Wastes generated from plating operations must be disposed of as hazardous waste and 

plating operations must abide by EPA emissions standards and OSHA permissible 

exposure limits (PEL). 

A significant lowering of the hex-Cr PEL would most likely have the greatest cost impact 

on military and commercial repair facilities. Such a change has been expected for several 

years but has not yet been issued by OSHA. In anticipation of the change, in 1995 a 

Navy/Industry task group under the coordination of the Naval Sea Systems Command 

studied the technical and economic impact of a reduction in the hex-Cr PEL. At the time, 

a reduction in the 8-hour time-weighted average (TWA) from the existing 100 μg/cm3

 to 

between 0.5 and 5.0 μg/cm3

 was being considered. The Navy/Industry task group 

performed the following tasks: 

• Identified the manufacturing and repair operations, materials and processes that 

are used in Navy ships, aircraft, other weapons systems and facilities where 

worker exposure to hex-Cr would be expected 

• Developed data on current worker exposure levels to hex-Cr using OSHA Method 

215 

• Estimated the technical and economic impact of the anticipated reductions in hexCr exposure on Navy ships, aircraft, other weapons systems and facilities 

• Identified future actions required to comply with the anticipated PEL reductions 

The following operations were identified as having the potential for exposing workers to 

hex-Cr: 

• Metal cleaning (including abrasive blasting and grinding) of chromate-coated 

materials 

• Electroplating of chromium 

• Painting and application of chromate paints and coatings 

• Welding, thermal spraying and thermal cutting 

The following conclusions were reached by the task group: 

__________________ 

Manuscript approved February 27, 2004 
2

Regulated areas for hex-Cr would have to be created in much greater numbers 

than have been required for cadmium or lead exposure 

Local exhaust ventilation, which is the presently available engineering control, is 

not completely effective in reducing exposure to below 0.5 ∝g/cm3

 for many 

operations or even below 5 ∝g/cm3

 in some cases 

The inability of engineering controls to consistently reduce worker exposure 

below the anticipated PEL levels will significantly increase the use of respirators 

The costs of reducing the hex-Cr PEL will include costs for training, exposure 

monitoring, medical surveillance, engineering controls, personal protective 

equipment, regulated areas, hygiene facilities, housekeeping and maintenance of 

equipment. There will also be costs due to reduced efficiency of not only the 

operations involving hex-Cr but adjacent operations and personnel as well. 

The estimated costs for compliance with a PEL of 0.5 ∝g/cm3

 at Navy facilities 

include an initial, one-time cost of about $22,000,000 and annual costs of about 

$46,000,000 per year. 

The estimated costs for compliance with a PEL of 5.0 ∝g/cm3

 at Navy facilities 

include an initial, one-time cost of about $3,000,000 and annual costs of about 

$5,000,000 per year 

In addition to the greatly increased cost that would be associated with chrome 

plating, turnaround times for processing of components would be significantly 

increased as well, impacting mission readiness. 

Although OSHA has delayed issuance of new hex-Cr permissible exposure limits, recent 

studies have clearly shown that there are a significant number of excess deaths at the 

current PEL of 0.1 mg/m3

 for hex-Cr emissions in plating facilities. For example, the 

August 2000 issue of the American Journal of Industrial Medicine contained a report on a 

study of 2,357 workers over a 30-year period which correlated the incidence of cancer 

with hex-Cr exposure. An analysis of the study was conducted by the Navy 

Environmental Health Center and it was their conclusion that the study appeared to 

support a lowering of the PEL to less than 0.001 mg/m3

. Although OSHA has not issued 

a schedule for issuance of a proposed new hex-Cr PEL, it appears clear that ultimately the 

PEL will have to be lowered. 

Previous research and development efforts [1.1, 1.2] had established that high-velocity 

oxygen-fuel (HVOF) thermal spray coatings are the leading candidates for replacement 

of hard chrome. Using commercially available thermal spray systems HVOF thermal 

spraying can be used to deposit both metal alloy and ceramic/metal (e.g., WC-Co) 

coatings that are dense and highly adherent to the base material. They also can be 

applied in thicknesses in the same range as that currently being used for chrome plating. 

Although there are a wide number of applications for these coatings, their qualification as 

an acceptable replacement for hard chrome plating has not been adequately demonstrated, 

particularly for fatigue-sensitive aircraft components. 

The Environmental Security Technology Certification Program (ESTCP) was established 

as a program of the U. S. Department of Defense (DOD) in December 1993. The 

ESTCP, which is managed by the Deputy Under Secretary of Defense for Installations 

and Environment, demonstrates and validates lab-proven technologies that target the 

most urgent DOD environmental needs. These technologies provide a return on 
3

investment through reduced environmental, safety, and occupational health (ESOH) 

risks; cost savings; and improved efficiency. The new technologies typically have broad 

application both to the DOD sustainment community and industry. 

In order to conduct the advanced development work required for qualification of the 

HVOF coatings, a project entitled, "Tri-Service Dem/Val of Chromium Electroplating 

Replacements,” principally sponsored by ESTCP, was established in March 1996. A 

project team, designated the Hard Chrome Alternatives Team (HCAT) was established to 

execute the project. From 1996 to early 1998, the HCAT acquired and installed HVOF 

thermal spray systems at the Naval Aviation Depot in Cherry Point, North Carolina 

(NADEP-CP) and the Corpus Christi Army Depot (CCAD). It also performed some 

generic fatigue and corrosion testing on HVOF WC-17Co (83 wt% WC particles in a 17 

wt% Co matrix) and Tribaloy 400 (60% Co, 28% Mo, 9% Cr, 3% Si) coatings compared 

to electrolytic hard chrome (EHC) coatings. Substrate materials included 4340 steel, 

7075 aluminum alloy, and PH13-8 stainless steel. From a fatigue standpoint the HVOF 

coatings generally performed better than the EHC coatings (i.e., there was a reduced 

fatigue debit with respect to the non-coated material for the HVOF coatings compared to 

the EHC coatings). In B117 salt fog corrosion studies, the performance of the WC/Co 

was comparable to the EHC, with the Tribaloy 400 slightly worse. In atmospheric 

corrosion studies, the WC/Co performed substantially better than the EHC, with the 

Tribaloy 400 comparable to the EHC. 

While these studies were valuable, it was realized in early 1998 that because hard chrome 

plating was being used on such a wide variety of aircraft components, it would be 

impossible to develop one test plan or conduct one series of tests that would address all 

materials and component qualification requirements. It was therefore decided to develop 

separate projects related to categories of aircraft components onto which hard chrome 

was being used. At the same time, the DOD Joint Group on Pollution Prevention (JGPP) decided to partner with the HCAT on development and execution of the various 

projects. JG-PP is chartered by the Joint Logistics Commanders (JLC) to coordinate joint 

service pollution prevention activities during the acquisition and sustainment of weapons 

systems. It was jointly determined by the HCAT and JG-PP that the first projects to be 

executed would be on landing gear and propeller hubs, with projects on hydraulic 

actuators and helicopter dynamic components to come later. (Note that there is a fifth 

project being executed between the HCAT and DOD Propulsion Environmental Working 

Group on hard chrome replacement on gas turbine engine components.) 

Since the technology to be demonstrated and validated as a hard chrome replacement had 

already been selected (namely HVOF thermal spray), then the first activity for the 

landing gear project was the development of the Joint Test Protocol (JTP) which would 

delineate all of the materials and component testing requirements necessary to qualify the 

HVOF coatings on landing gear components for all types of DoD aircraft. Table 1-1 and 

Table 1-2 summarize the target hazardous material, current process, application, current 

specifications, and affected defense systems programs (delineated according to the U.S. 

DOD aviation depot at which the overhaul of the landing gear from that aircraft takes 

place). 

A stakeholder meeting was held at the Naval Research Laboratory in July 1998 to discuss 

the types of materials testing that would be required and also explore what avenues were 
4

available for component testing. It was subsequently decided that the JTP would be 

developed and issued in two parts, with Part I for materials (i.e., coupon-type) testing and 

Part II for component testing. 

The Canadian Government, through the Department of National Defence (DND) and 

Industry Canada (IC), facing environmental restrictions on chrome plating similar to the 

U.S., also became interested in qualifying HVOF thermal spray coatings on aircraft 

landing gear both for manufacturing and maintenance operations. The Canadian 

Government formed their own project team, designated the Canadian Hard Chrome 

Alternatives Team (C-HCAT), and a partnership was formed between both projects. A 

formal Project Arrangement, conducted under the auspices of the U.S.-Canadian 

Research and Technology Projects Memorandum of Understanding (MOU), was 

Table 1-1. Landing Gear Aircraft Applications

Target 

HazMat 

Current 

Process Application 

Current 

Specifications 

Candidate Parts/ 

Substrates 

Hexavalent 

Chromium

Hard 

Chromium 

Electroplating 

Rebuilding Worn 

Components 

Wear-resistant 

Coating 

Corrosion-resistant 

Coating

DOD-STD-2182 

MIL-C-14538C 

MIL-C-20218F 

MIL-H-83282 

MIL-STD-1501C 

QQ-C-320B

Landing Gear 

Table 1-2. Defense System Programs Potentially Affected

CCAD NADEP 

Cherry Point

NADEP 

JAX:

Ogden ALC

SH-60 (Navy) AV-8B E-6 A-7 F-16 

UH-60 C-130 EA-6B A-10 F-22 

 E-2/C-2 F-14 B-1B F-104 

 H-1 F-18 B-2 F-106 

 H-46 P-3 B-52 F-111 

 H-53 S-3 C-5 H-3 

 H-60 T-45 C-17 H-53 (USAF/Navy) 

 P-3 C-130 KC-135 

 C-141 T-37 

 F-5 T-38 

 F-15 
5

negotiated and executed in March 1999. In order to achieve maximum impact, the joint 

project concentrated on landing gear systems common to both U.S. and Canadian aircraft 

and on landing gear components that are supplied by Canadian companies for use in U.S. 

aircraft. 

In terms of qualifying specific types of HVOF coatings by the two teams, it was decided 

that the U.S. HCAT would evaluate WC/Co (83%/17%) HVOF coatings whereas the CHCAT would evaluate WC/CoCr (86%/10%4%) HVOF coatings. It was further decided 

that all materials testing would be incorporated into one document constituting Part I of 

the JTP, but that there would be a clear delineation between the testing being conducted 

by the U.S. HCAT and that conducted by the C-HCAT. 

The following organizations were involved with the development of the U.S. portion of 

the Landing Gear JTP: 

 Naval Research Laboratory (NRL) 

 Air Force Materiel Command 

 Naval Air Systems Command 

 Air Force Landing Gear Single Item Manager OO-ALC/LIL 

 Naval Aviation Depot- Jacksonville (NADEP-JAX) 

 Naval Aviation Depot- Cherry Point (NADEP-CP) 

 Ogden Air Logistics Center (OO-ALC) 

 Naval Air Warfare Center, Aircraft Division, Patuxent River 

 Air Force Research Laboratory 

 Rowan Technology Group 

 GE Aircraft Engines (GEAE) 

 National Technical Systems (NTS) 

 Boeing Defense (St. Louis) 

The following are the principal organizations that constitute the C-HCAT and which 

participated in the development of the Canadian portion of the Landing Gear JTP: 

 Department of National Defence 

 Industry Canada 

 Technology Partnerships Canada 

 Messier-Dowty 

 Goodrich 

 Heroux, Inc. 

 Orenda Aerospace Corporation 

 National Research Council of Canada 

Part I of the Landing Gear JTP [1.3] was organized in sections, with each section devoted 
6

to the type of test being conducted. Most sections were divided into two subsections, the 

first for the U.S. portion of the testing and the second for the Canadian portion of the 

testing. The testing by the two teams on the two different HVOF coatings was meant to 

be very similar, but there were some differences. The following were the different 

sections in the Materials JTP: 

1. Introduction 

2. Fatigue 

3. Corrosion 

4. Wear 

5. Impact Testing 

6. Hydrogen Embrittlement

Section 3 of this report, essentially reproducing the Materials Joint Test Report (MJTR) 

(without appendices), provides the results of all of the testing conducted in accordance 

with Part I of the JTP. It only includes the U.S. portion of the testing and thus in general 

only includes results for WC-Co (83%/17%) coatings compared to EHC coatings. A 

separate C-HCAT MJTR has been issued by the Canadian Department of National 

Defence on the WC/CoCr coatings. Section 4 of this report describes the results of 

extended testing carried out to address the issue of coating integrity that was found during 

the JTP fatigue measurements. 

Although there was no formal Component Joint Test Protocol, there were a number of 

component rig and flight tests conducted on landing gear components and these are 

described in Section 5 of this report. 

In addition to materials and component testing, in order to transition HVOF thermal spray 

technology to military repair facilities, it was necessary to address several producibility 

issues including stripping and grinding of the WC/Co coatings, determining their 

compatibility with different fluids used for cleaning and other operations, and performing 

non-destructive inspection of the WC/Co coatings. The results of investigations in these 

areas are described in Section 1. 

Another issue related to successful transitioning of the HVOF technology was relative 

costs compared to hard chrome plating and determining the return-on-investment by 

implementing the WC/Co coatings. The results of a detailed cost/benefit analysis are 

presented in Section 1. 

To ensure uniformity of coating characteristics and performance between military 

facilities using HVOF thermal spray of WC/Co, standards and specifications were 

developed for the WC/Co and WC/CoCr powder and for the deposition conditions. 

These are presented in Section 8. 

Finally, a review of issues associated with implementation of HVOF WC/Co coatings in 

repair facilities is presented in Section 9. 

In this report, there are a number of references to specific standards related to materials 

processing, coating deposition, and testing. These are summarized in Table 1-3. 
7

Table 1-3. Applicable Materials Processing, Coating Deposition and Test Standards

ASTM Standards: 

ASTM E466: Standard Practice for Fatigue testing 

ASTM B117: Standard Practice for Salt Spray (fog) Apparatus, Operating 

ASTM G85: Standard Practice for Modified Salt Spray (FOG) 

ASTM B537: Standard Practice for Ranking Electroplated Panels Subject to Atmospheric 

Exposure 

ASTM D3170: Standard Practice for Coatings, Chipping Resistance 

ASTM F519: Mechanical Hydrogen Embrittlement Testing of Plating Processes and Aircraft 

Maintenance Chemicals, Method for 

Boeing Aircraft Corporation (BAC) Standards: 

P.S. 15169: Heat treatment specifications for Aermet 100 steel 

P.S. 11501: Application of polystyrene resin impregnation sealer on EHC coatings 

BAC 5851: Deposition of HVOF thermal spray coatings 

BAC 5855: Low-stress grinding of coatings 

Military Specifications: 

DOD-STD-2182: Engineering Chromium Plating (Electrodeposit for Repair of Shafting) 

MIL-C-14538: Chromium Plating, Black (Electrodeposited) 

MIL-C-20218: Chromium Plating, Electrodeposited, Porous 

MIL-H-83282: Hydraulic Fluid, Fire Resistant, Synthetic Hydrocarbon Base 

MIL-STD-1501: Chromium Plating Low Embrittlement, Electrodeposition 

MIL-STD-866: Grinding of Chrome Plated Steel and Steel Parts Heat Treated to 180,000 psi 

or over 

MIL-STD-867: Temper Etch Inspection 

MIL-STD-1504: Abrasive Blasting 

QQ-C-320: Chromium Plating (Electrodeposited) 

QQ-N-290: Nickel Plating (Electrodeposited) 

MIL-A-8625: Anodic Coatings for Aluminum and Aluminum Alloys 

SAE Standards: 

AMS-81934: Bearings, Sleeves, Plain and Flanged, Self-lubricating, General Spec for 

AMS-4640: Aluminum Bronze, Bars, Rods, Shapes, Tubes, and Forgings 81.5Cu-10.0 Al4.8 Ni-3.0 Fe, Drawn and Stress Relieve (HR50) or Temper Anneals 

AMS-2432: Shot Peening, Computer Controlled 

AMS-6875: Heat treating of high strength Steels 

SAE J400: Gravelometer Testing 

Other specifications: 

GM9540P/B: GM corrosion test 
8

One final item to note in this section is that the commercial aircraft industry has 

independently moved toward qualifying HVOF thermal spray coatings (especially 

WC/Co and WC/CoCr) on landing gear components. For OEM applications, Boeing has 

specified WC/CoCr on the landing gear for the 767-400, and the current design for the 

new Airbus A380 has established WC/CoCr as the preferred coating on its landing gear. 

For commercial maintenance and repair operations (MRO) at airlines, Boeing has 

approved HVOF WC/Co and WC/CoCr coatings up to 0.010” thick subject to the coating 

facility obtaining a Boeing qualification certification. Delta Airlines is one of several 

U.S. airlines that have aggressively moved toward inserting HVOF into its production 

MRO operations. Table 1-4 is a summary of the various applications of HVOF coatings 

in the commercial aircraft industry. 

1.1. References 

1.1 “High Velocity Oxy Fuel Final Results Report,” Final Report issued by Science 

Applications International Corporation under Government Contract F09603-90-

D2215, Oklahoma City Air Logistics Center, Tinker Air Force Base, May 25, 1994 

1.2 “Hard Chrome Coatings: Advanced Technology for Waste Elimination,” Final Report 

issued by Northwestern University, Evanston, IL, under DARPA Contract MDA972-

93-1-0006, 1996 

1.3 “Joint Test Protocol, Validation of WC/Co and WC/CoCr HVOF Thermal Spray 

Coatings as a Replacement for Hard Chrome Plating on Aircraft Landing Gear, Part I: 

Materials Testing.” Prepared by Hard Chrome Alternatives Team for Environmental 

Security Technology Certification Program, Revision A, September 1999 

Table 1-4. Commercial Programs Affected and Potentially Affected by HVOF 

Chrome Replacement 

Approved Potentially affected 

Boeing - Steering collars, flap and slat tracks, 

pins, sleeves, fittings (>100 items) 

Bombardier Dash 8 (OEM and MRO) 

Boeing 676-400 landing gear (HVOF WCCoCr) Airbus 380 

Commercial airliner landing gear MRO – 

HVOF approved by Boeing up to 0.010” thick. 

HVOF WC-CoCr approved by Delta for MRO 

on landing gear 

Bombardier Q400 Global Express flap tracks 
9

2.Technology Description 

2.1. Technology Development and Application 

Technology background and theory of operation: HVOF is a standard commercial 

thermal spray process in which a powder of the material to be sprayed is injected into a 

supersonic flame of a fuel (usually hydrogen, propylene or kerosene) as shown in Figure 

2-1. The powder particles are accelerated to high speed and soften in the flame, forming 

a dense, well-adhered coating on the substrate. The coating material is usually a metal or 

alloy (such as Tribaloy or stainless steel), or a cermet (such as cobalt-cemented tungsten 

carbide, WC/Co). The technology is used to deposit coatings about 0.003” thick on 

original equipment manufacturer (OEM) parts, and to rebuild worn components by 

depositing layers up to 0.015” thick. 

Applicability: HVOF was originally developed primarily for gas turbine engine (GTE) 

applications. The primary thermal spray processes are Flame Spray, Plasma Spray, Arc 

Spray, HVOF and the recently-developed cold spray. The original high velocity spray 

technology was the pulsed deposition detonation gun (D-gun) developed by Union 

Carbide (later Praxair). The quality of the wear and erosion resistant spray coatings 

produced by this method was much better than the lower speed methods, and continuous 

flame HVOF was developed as a competitive response. 

The original applications for HVOF were wear components in GTEs, such as shafts and 

bearing journals. As the availability and use of the technology grew, it began to be 

applied to a wide range of other types of coatings and applications, including a variety of 

aircraft components such as flap and slat tracks, landing gear and hydraulics for 

commercial aircraft. It is now being used in many applications outside the aircraft 

industry, such as industrial rolls and vehicle hydraulics. The original aircraft wear 

applications, primarily used by Boeing, were for otherwise-intractable spot problems that 

neither the original alloy nor chrome plate could solve. 

The technology can be used to spray a wide variety of alloys and cermets. It is limited 

for high temperature materials such as oxides, most of which cannot be melted in the 

flame. The areas to be coated must be accessible to the gun – i.e., they must be line-ofsight. 

Material to be replaced: HVOF coatings are used to replace hard chrome plate 

(especially using carbide cermets and high temperature oxidation-resistant Tribaloys). 

Figure 2-1. Schematic of HVOF Gun and Process (Sulzer Metco DiamondJet) 
10

The combination of HVOF NiAl with an overlayer carbide is also used to replace the 

combination sulfamate Ni/hard chrome. HVOF coatings can also be used to replace 

some hard Ni and electroless Ni coatings on such components as flap tracks and propeller 

hubs. In the HCAT program, the primary application is hard chrome replacement. 

2.2. Process Description 

Installation and operation: The HVOF 

gun can be hand-held and used in an 

open-fronted booth. However, the 

supersonic gas stream is extremely loud 

and requires that the operator use very 

good ear protection. For this reason the 

unit is usually installed on a six-axis robot 

arm in a sound-proof booth, programmed 

and operated remotely. Most depots 

already use this type of booth for their 

existing plasma spray operations. Since 

the method is frequently used for 

cylindrical items, the most common 

arrangement is to rotate the component on 

a horizontal rotating table and move the 

gun up and down the axis. 

Facility design: The installation requires: 

. A soundproof booth. Booths are 

typically 15 feet square, with a 

separate operator control room, an 

observation window, and a high 

volume air handling system drawing air and dust out of the booth through a 

louvered opening (shown in Figure 2-2). 

. Gun and control panel. The gun burns the fuel and oxygen inside its combustion 

chamber and injects the powder axially into the flame. The gas exits the gun at 

supersonic speed, while the particles are accelerated to high velocity but usually 

remain subsonic. The control panel controls the gas flows, cooling water, etc. 

. Powder feeder. Powder is typically about 60∝m in diameter and is held in a 

powder feeder, which meters the powder to the gun at a steady rate, carried on a 

gas stream. Two powder feeders are commonly used to permit changeover from 

one coating to another without interrupting the spraying. 

. 6-axis industrial robot and controller. Most installations use an industrial robot to 

manipulate the gun and ensure even spraying. The robot is often suspended from 

above to leave the maximum possible floor space for large items. 

. Supply of oxygen. This is frequently a bulk storage container outside the 

building. Alternatively bottled gas can be used, but because of the high usage rate 

of up to 2,000 scfh (see Table 2-1), even a standard 12-bottle setup lasts only a 

Figure 2-2. HVOF Spray of Landing 

Gear Inner Cylinder 
11

few hours in production. 

. Supply of fuel gas or kerosene (bottled or bulk). Hydrogen is the most common 

fuel, supplied in bulk or in bottles. Praxair TAFA guns use kerosene, which is 

significantly cheaper and less dangerous. 

. Dust extractor and bag-house filter system. The air extracted from the booth is 

laden with overspray – particles that have failed to stick to the surface (often 20-

50% of the total sprayed). The air is blown into a standard bag house, often 

located outside the building, where the dust is removed. 

. Dry, oil-free compressed air for cooling the component and gun. Air cooling 

prevents the components being overheated (temperatures must be kept below 

about 400°F for most high strength steels). 

. Water cooling for gun. Not all guns are water cooled, but most are. 

The facility must be capable of supplying the material pressures and flows of Table 2-1. 

Standard commercial equipment currently in service already meets these requirements. 

Equipment vendors are able to supply turnkey systems. 

Performance: From Table 2-1, HVOF guns deliver about 8-10 pounds per hour (4-5 kg 

per hour) of powder, of which 65% typically enters the coating, for a coating rate of 

Table 2-1. Optimized Deposition Conditions for WC/17Co (DJ 2600 and JP 5000 HVOF guns)

Equipment Gun Model 2600 hybrid gun Model 5220 gun with 8” nozzle 

Console Model DJC Model 5120 

Powder feeder Model DJP powder feeder Model 5500 powder feeder 

Powder feed Powder Diamalloy 2005 Stark Amperit 526.062 

Powder Feed Rate: 8.5 lb/hr 80 gm/min (325 rpm, 6 pitch feeder screw) 

Powder Carrier Gas Nitrogen Argon 

Carrier gas pressure 148 psi 50 psi 

Flow rate 28 scfh 15 scfh 

Combustion Gases Fuel Hydrogen Kerosene, Type 1-K 

Console supply pressure 162-168 psi 

Gun supply pressure 135 psi 121-123 psi 

Flow rate 1229 scfh 5.0 gph 

Oxidizer Oxygen Oxygen 

Pressure 148 psi 138-140 psi 

Mass flow 412 scfh 2000 scfh 

Gun Compressed Air Pressure 105 psi 

Mass flow 920 scfh 

Gun Cooling Water Flow rate 5.3-5.7 gph (factory set) 8.3-8.7 gph 

Flow Water Temperature to Gun: 65-80o

F typical (ground water, temp varies) 64-72oF 

Specimen Rotation 2,336 rpm for round bars (0.25 inch dia.) – 

1835 in/min surface speed 

600 rpm for round bars (0.25 inch diam.); 144 

rpm for rectangular bars (at 6.63 inch diam.) 

Gun Traverse Speed 400 linear in/min for round bars 70 in/min for round bars 

Spray Distance 11.5 inches 18 inches 

Cooling Air Pressure 90-110 psi 90-110 psi 

Location 2 stationary nozzle tips at 6 inches pointed 

at coating area 

2 gun-mounted air jets at 14 inches; 1 stationary 

air jet at 4-6 inches pointed at coating area 
12

about 6 pounds per hour (3 kg per hour). For a common 0.010” WC/Co rebuild coating 

(which will be sprayed to a thickness of 0.013-0.015”), an HVOF gun can deposit about 

900 square inches per hour. This permits coating the 23” long, 4” diameter bearing 

surface of an F-18 nose landing gear (NLG) in about 30 minutes, compared with about 15 

hours for chrome plating. 

Specifications: The following specifications and standards apply to HVOF coatings: 

. Prior to the HCAT program, the only aerospace specifications were those issued 

by prime contractors such as Boeing, whose BAC 5851 thermal spray 

specification, supported by BMS 10-67G powder specification, is still one of the 

most quoted standards 

. AMS 2447 was developed with the assistance of the HCAT team and issued by 

SAE in 1998. It is now a widely used standard in the aerospace industry. 

. In order to provide specifications for spraying high strength aircraft steels at 

depots and vendors, HCAT has worked through SAE to promulgate several 

standards: 

o Aerospace Materials Specification (AMS) 2448, issued in 2003, is a 

specification for HVOF spraying of high strength steel. 

o AMS 7881 and AMS 7882 are powder specifications that support AMS 

2448. 

o An AMS standard for grinding of HVOF coatings will be issued in a few 

months. 

Training: Just as plating shops typically have several personnel who handle masking, 

racking, demasking, etc., it is common for HVOF shops to have 3 or 4 technicians 

dedicated to masking and spraying. HVOF training is essential and is usually provided 

by equipment vendors such as Praxair and Sulzer Metco. Training is also available 

through the Thermal Spray Society. Depot personnel taking part in the HCAT program 

have been trained by Jerry Schell, thermal spray coatings expert at GE Aircraft Engines. 

Since thermal spray is a more complex technology than electroplating, plating line 

personnel cannot be transferred successfully to an HVOF shop without extensive 

retraining. 

Health and safety: The process does not produce air emissions or toxic wastes. Co 

powder is an International Agency for Research on Cancer (IARC) Group 2B material, 

which means that “The agent (mixture) is possibly carcinogenic to humans”, whereas 

Cr6+ is an IARC Group 1 material, “Known to be carcinogenic to humans”. However, 

the OSHA PEL for Co (8hr TWA) of 0.1 mg(Co)/m3

, is lower than the 1 mg(Cr)/m3 for 

metallic chrome, and is the same as the 0.1 mg(Cr)/m3

 for Cr6+. Unlike chrome plating, 

the Co is not emitted into the air. Excess Co-containing powder is drawn from the spray 

booth and captured in the bag house. Nevertheless personnel should wear a dust 

respirator when handling the powder, working in the booth, or grinding the coating. 

While the powders are usually about 60∝m in diameter, they can break apart on impact, 

producing 10∝m or smaller particles. The American Welding Society recommends the 

use of a respirator complying with American National Standards Institute (ANSI) Z88.2 
13

Ease of operation: Since in commercial systems the entire system is programmable, 

including the gun control and robot, it is generally easy to operate. The operator must 

create masking (usually shim stock shadow masks) and must develop the correct spray 

parameters and gun motions. While vendors supply standard operating conditions for 

different materials, these may have to be optimized experimentally for new materials and 

powders, and must be adjusted for different components to ensure proper coating speed 

and gun traverse rate. Small diameter components, for example, must be rotated faster 

than large ones to maintain the same deposition rate and coating structure. In this respect 

operating an HVOF system is considerably more complex than electroplating. 

2.3. Previous Testing of the Technology 

Prior to the HCAT program, HVOF technology had been successfully used by Boeing for 

a number of years for their commercial aircraft and by GEAE for GTEs. In the period 

1993-1996 Keith Legg, Bruce Sartwell, GEAE, Cummins Diesel and Corpus Christi 

Army Depot carried out an evaluation of chrome alternatives [2.1], funded by the 

Defense Advanced Research Projects Agency (DARPA). The program evaluated HVOF, 

physical vapor deposition (PVD) and laser cladding, and concluded that HVOF was the 

best overall alternative for use in depots and most OEM aircraft applications. At the 

beginning of the HCAT program, Lufthansa successfully completed flight tests of HVOF 

coatings on commercial landing gear and Delta began to carry out similar flight tests. 

2.4. Advantages and Limitations of the Technology 

Replacing hard chrome plating is a great deal more complex than simply putting down a 

hard coating. The alternative must not only work technically, but it must fit with the 

entire life cycle of use and maintenance, and it must be a reasonable, mature technology 

for depot use. The advantages and limitations of HVOF are summarized in Table 2-2. 

2.5. References 

2.1 “Hard Chrome Coatings: Advanced Technology for Waste Elimination,” Final Report 

issued by Northwestern University, Evanston, IL, under DARPA Contract MDA972-

93-1-0006, 1996 
14

Table 2-2. Advantages and Limitations of HVOF as a Chrome Replacement 

Advantages/strengths Disadvantages/limitations 

Technical: 

Higher hardness, better wear resistance, longer 

overhaul cycle, less frequent replacement 

Brittle, low strain-to-failure – can spall at high 

load. Issue primarily for carrier-based aircraft 

Better fatigue, corrosion, embrittlement Line-of-sight. Cannot coat IDs 

Material can be adjusted to match service 

requirements 

More complex than electroplating. Requires 

careful QC 

Depot and OEM fit: 

Most depots already have thermal spray 

expertise and equipment 

WC-Co requires diamond grinding wheel. 

Only HVOF alloys can be plunge ground 

Can coat large areas quickly 

Can be chemically stripped 

Many commercial vendors 

Environmental: 

No air emissions, no high volume rinse water Co toxicity 
15

3.Materials Testing 

3.1. Summary of Tests Conducted 

As indicated in Section 1, this section reproduces the Materials Joint Test Report, but 

without the appendices. The entire report [3.1] is available from the HCAT web site at 

www.hcat.org. Table 3-1 summarizes the tests conducted under the JTP for the three 

coatings tested on the three substrates of major interest for landing gear. 4340 and 

4340M are primarily used on older landing gear and hydraulic actuators, 300M is 

commonly used on most modern landing gear, while Aermet 100 is used on some modern 

Navy gear. 

Table 3-1. Summary of Tests Conducted in JTP 

Substrates/coatings 

4340 300M A100 

Tests 

WCCo WCCoCr T400 WC-Co WC-CoCr WC-Co WC-CoCr 

Deposition X X O X X X X 

Fatigue X X O X X X X 

Corrosion X X O X X X X 

Wear X X O 

Impact X X 

Embrittlement X X 

X LG JTP 

O Generic 
16

3.2. Coating optimization and characterization 

The optimized deposition conditions for the hydrogen-fueled Sulzer Metco Diamond Jet 

DJ 2600 HVOF gun used at most of the depots and the Praxair kerosene-fueled JP 5000 

gun used at Ogden ALC are shown in Table 3-2. These guns were optimized for WC17Co by design of experiment methods as described below. 

Table 3-2. Optimized Deposition Conditions for WC-17Co - DJ 2600 and JP 5000 HVOF guns 

Equipment Gun Model 2600 hybrid gun Model 5220 gun with 8” nozzle 

Console Model DJC Model 5120 

Powder feeder Model DJP powder feeder Model 5500 powder feeder 

Injector #8 

Shell #8 

Insert #8 

Siphon plug #8 

Aircap DJ2603 

Powder feed Powder Diamalloy 2005 Stark Amperit 526.062 

Powder Feed Rate: 8.5 lb/hr 80 gm/min (325 rpm, 6 pitch feeder screw) 

Vibrator setting 30 

Tube DJP 116 

Pick-up shaft “E” 

Air Vibrator setting 20 psi 

Powder Carrier Gas Nitrogen Argon 

Carrier gas pressure 148 psi 50 psi 

FMR 55 

Flow rate 28 scfh 15 scfh 

Combustion Gases Fuel Hydrogen Kerosene, Type 1-K 

Console supply pressure 162-168 psi 

Gun supply pressure 135 psi 121-123 psi 

Combustion Chamber Pressure 100-102 psi 

FMR 53.4 

Flow rate 1229 scfh 5.0 gph 

Oxidizer Oxygen Oxygen 

Pressure 148 psi 190-235 psi console supply reading 

138-140 psi gun supply pressure 

FMR 28.7 

Mass flow 412 scfh 2000 scfh (corrected flow at 210 psi: 1940 to 2075 actual flow tube 

reading, depending on console supply pressure reading) 

Gun Compressed Air Pressure 105 psi 

FMR FMR – 50.5 

Mass flow 920 scfh 

Gun Cooling Water Flow Flow rate 5.3-5.7 gph (factory set) 8.3-8.7 gph 

Water Temperature to Gun: 

In 65-80oF typical (ground water, 

temp varies) 

64-72oF 

Out 77-100 oF typical 117-125 oF 

Delta 12-20 oF (~ 14 oF typical) 51-54 oF 

Specimen Rotation 2,336 rpm for round bars (0.25 inch 

dia.) – 1835 in/min surface speed 

600 rpm for round bars (0.25 inch diam.); 144 rpm for rectangular 

bars (at 6.63 inch fixture diam.) 

Gun Traverse Speed 400 linear in/min for round bars 70 in/min for round bars; 17 in/min for rectangular bars 

Spray Distance 11.5 inches 18 inches 

Cooling Air Pressure 90-110 psi 90-110 psi 

Location 2 stationary nozzle tips at 6 inches 

pointed at coating area 

2 gun-mounted air jets at 14 inches; 1 stationary air jet at 4-6 inches 

pointed at coating area 
17

3.2.1. Rationale/Background 

3.2.1.1. HVOF Application 

As in all coating methods, the properties and performance of the coating depends on both 

the coating material and the deposition conditions. Optimal coating properties can 

therefore only be obtained when the critical deposition parameters are in the proper 

range. In chrome plating the coating properties are primarily governed by solution 

chemistry, temperature, and current density. HVOF spraying is more complex to 

optimize since there are many more variables in the deposition process. For this reason, 

HVOF coatings were optimized in the HCAT program by a Design of Experiment (DOE) 

approach, which permits optimum conditions to be identified from a limited set of test 

runs, obviating the need for a full test matrix that would entail many hundreds of 

deposition tests.

3.2.1.2. HCAT Coating Optimization and Deposition Philosophy – 

Fatigue 

In order to optimize a coating, it is important to decide at the outset what property, or set 

of properties, is to be optimized. This is especially true for thermal spray coatings, where 

it has been found, for example, that a coating optimized for minimum wear can behave 

poorly in fatigue. Within the HCAT, the fatigue critical nature of applications such as 

landing gear, actuators, propeller hub components and gas turbine engine parts was 

quickly identified as the major life limiting characteristic. This does not eliminate the 

need to evaluate other characteristics such as corrosion, wear, hydrogen embrittlement, 

etc. but coating optimization initially concentrated on fatigue performance, with 

modifications for other properties as necessary. This approach was adopted as the 

philosophy of the HCAT from the onset of the program, as evidenced by the heavy 

concentration on fatigue in the test protocols. 

Coating deposition for the various team members was optimized by Jerry Schell of GE 

Aircraft Engines, an expert in thermal spray optimization. The coating optimization 

process began with the initial “Northwestern or generic” protocol in 1996 and has been 

evolving to the present time. A design of experiment (DOE) test methodology (which 

will be discussed in a later section) was chosen as the mechanism to provide an optimized 

coating deposit. The variables in the process were identified and experiments conducted 

to determine the best parameter set for optimum results. Work was required for the 

varied commercial systems to demonstrate capability across the available equipment sets. 

Thus, data summarized in this section covers both the JP-5000 (Praxair TAFA) and the 

DJ 2600 (Sulzer Metco) systems, and represents work conducted at varied locations (Hill 

AFB, CCAD, Cherry Point, and Hitemco) in the 1996-1999 time frame. This resulted in 

the final coating parameters for the majority of the Landing Gear Joint Test Protocol (LG 

JTP). 

Optimization of the process is carried out for three important reasons: 

1. To define a thermal spray process that will achieve the desired performance and 

property goals. 
18

2. To establish manufacturing robustness and the process window for a reliable 

process. 

3. To understand the process and trends that give an indication of (and can later be 

used as) a trouble shooting guide. When parameters are identified as significant, 

these variables will be the first areas of investigation in problem solving. 

In optimizing the HVOF process, it is important to understand the difference between the 

general output of the process and the characteristics/properties of the final coating 

deposit. As stated earlier, the final goal of the coating optimization was maximized 

fatigue performance with close emphasis on other properties such as corrosion, wear, etc. 

However, when the coating is initially sprayed, only a set of simple measurements can be 

used for quality control of the process, as follows: 

 Microstructure (primarily measurement of porosity, unmelted particles, and 

oxides) 

 Hardness (both macro and micro) 

 Almen Strip (residual stress) 

 Substrate Temperature (during coating) 

 Deposition Rate 

Between them, these measurements have proved to be adequate to define the coating for 

the purpose of quality control. It makes technical sense that characteristics such as 

microstructure and hardness will ultimately determine coating performance in areas such 

as wear or corrosion resistance, while residual stress and substrate temperature are known 

to strongly influence fatigue. Thus, even though the ultimate goal is enhanced fatigue 

performance, that performance can be ensured indirectly by measuring other coating 

properties for quality control. 

Since the deposition process is known to be uniform and stable, these measurements can 

be made on test samples set up to see the same deposition conditions as the components 

to be coated. These test samples may be fabricated prior to deposition (for daily spray 

booth qualification), or sprayed during coating deposition on components (for quality 

control). 

3.2.1.3. Fatigue Cycle Life vs. Fatigue Coating Integrity 

As with any optimization procedure, there is a learning phase to the process and issues 

are sometimes identified that are unexpected from previous experience and planning. 

This was the situation in developing precise testing documentation for measurement of 

the Almen Strip and Temperature during spraying (procedures detailed later in this 

section). 

Another major property issue identified during optimization was differentiation between 

fatigue coating integrity and fatigue cycle life (further detailed in the fatigue section). 

The initial Northwestern-generic protocol addressed fatigue performance with primary 

emphasis on fatigue cycle life, the typical S-N curves which are generated to illustrate 

load vs. cycle performance. This followed the initial work performed by Boeing where 

3-4 coating suppliers were qualified based upon the Q.C. tests listed earlier and 
19

satisfactory fatigue life cycle life performance. Both sets of data were focused upon a 

non-aggressive R ratio of .1 and stress levels to maximum of 180 ksi. 

The LG JTP also addressed fatigue performance with emphasis on cycle life. However, 

the R ratio was increased to a more aggressive level of –1, while cyclic stress was 

increased, in some cases to the yield point. This resulted in concerns with coating 

spalling or delamination and a category or classification for fatigue coating integrity. It 

must be emphasized that the LG JTP coatings were optimized for fatigue cycle life, NOT 

fatigue coating integrity. Details of the optimization process and some subsequent plans 

to address the fatigue coating integrity issue will be discussed and summarized in this 

section. 

3.2.2. Test methodology 

3.2.2.1. DOE Methodology 

For this optimization study, the design of experiment (DOE) methodology was chosen as 

the vehicle to deliver the best spray parameter set. This method is used in many 

manufacturing environments when numerous variables exist and there is insufficient time 

and financial resources to analyze each individual process input (i.e. to carry out a full 

matrix test). Pre-DOE experiments are usually run on a iterative basis to determine the 

limits of the various parameters and which have the most significant effect on the output 

of the process. A DOE matrix is then designed using standard experimental design 

protocols in which the variables selected are usually assigned high and low values for the 

numerous DOE test runs. Statistical analysis is applied and each variable assigned a rank 

as to the effect on the final process output. In subsequent experimentation, insignificant 

variables are eliminated from the analysis and the final outcome is a full parameter set for 

the process in question. For the HCAT program the experimental design was done using 

commercial software made by Minitab, Inc. 

3.2.2.2. HVOF Optimization 

3.2.2.2.1. General 

The DOE method revolves around process inputs and outputs as illustrated below with a 

limited list for the HVOF spray technology: 

Table 3-3. Inputs and Outputs for Design of 

Experiment HVOF Optimization 

Input Output 

Powder size Hardness 

Gas flow Microstructure 

Gas ratio-fuel to oxygen Almen strip 

Spray distance Tensile 

Carrier gas flow Coating deposition rate 

Air flow 

Traverse speed 
20

As stated earlier, even with the DOE methodology, there must be a general starting or 

reference point. This was provided by the earlier HVOF work conducted by Boeing and 

some general experience from Jerry Schell of GEAE. With this knowledge and the 

ultimate goal of fatigue performance, three QC outputs were identified as the major 

drivers to achieve the end goal: 

Hardness – tends to be a general gage of wear resistance, but more 

importantly an indicator of carbide solutioning and phase 

change 

Almen Strip – indication of coating residual stress 

Substrate Temperature – should be below 350°F to avoid degrading substrate fatigue in 

high strength steels 

This information shaped the methodology involved for this optimization, which included: 

Pre-DOE – A series of general experimental runs to achieve a common-sense 

understanding of the process. For example, it would not make sense 

to pick a parameter range for the HVOF system setting that would not 

allow the gun to spray in an efficient manner or provides no Almen 

Strip response. This initial set identifies some reasonable responses 

for the actual DOE experimentation. 

Actual DOE – When a reasonable set of process inputs and ranges have been 

identified, a number of runs/experiments are conducted according to a 

test matrix defined by the DOE software. Outputs are analyzed and 

trends determined. Dependent upon time and funding, further more 

refined studies can be run or the process fine-tuned at this point. 

Validation Runs – Using the optimum determined from the DOE, a small set of runs is 

made to verify the parameter set, and repeat spray cycles are 

conducted to establish consistency.

3.2.2.3. General Example: CCAD Analysis 

A typical example of this process is represented by the work conducted at CCAD in the 

initial optimization of the DJ 2600 system. 
21

Table 3-4 represents the variables that were analyzed in the CCAD study. As illustrated 

by the data, a high/low range was selected for parameters such as gas flow, powder feed 

rate, part stand-off distance (distance from end of gun to part), etc. The inputs are then 

placed into a matrix as typified in Table 3-5 and the runs made in the random order 

specified to eliminate any bias which might exist. 

Trends and relationships are determined by statistical analysis as will be summarized in 

the subsequent test results section. 

3.2.3. Test Results 

3.2.3.1. General Sequence 

The evolving optimization process for LG coatings involved the systems, locations, and 

time frames as listed in Table 3-6. In discussing each location, the general matrix design 

and effect on the three major outputs of Almen, Temperature and Microhardness will be 

presented. No pre DOE or final optimization fine tuning will be summarized but the final 

spray parameter set is listed. 

Table 3-4 Optimization Parameters for the CCAD DJ 2600 Analysis 

Levels

FACTORS: -1 +1 C Pt FIXED:

A Traverse speeds 20 ipm 40 ipm 30 ipm Metco VF grit blast surface prep procedure*

B Spray distance 8 inch 12 inch 10 inch Substrate is 4340 steel, 260-280 ksi

C Stoic Ratio 0.38 0.6 0.44 Powder type is WC-17Co, agglomerated and sintered

D Combustion Gas 1550 scfh 2000 scfh 1775 scfh Spray angle is 90 degrees

E Air flow 735 scfh 965 scfh 850 scfh 100 psi air, 5 AJs @ 6 inch spaced over 8-12 inch coupon area

F Carrier gas flow 42.6 scfh 71 scfh 56.8 scfh Carrier gas equiv N2 mass flow for N2 or Argon

G Turntable RPM 90 RPM 250 RPM 170 scfh N2 x 1.42 = Ar

H Powder Feed Rate** 5.4 lbs/hr 10.7 lbs/hr 15.8 lbs/hr RESPONSES: RELATED CTG FUNCTION:

I Powder size Amp526.062005NS 2005NS 1) Part temperature Fatigue

Lot 10362 Lot 53792 Lot 53558 2) Almen strip Fatigue, ctg residual stress

J Part diameter 2 inch 6.63 inch 4.31 inch 3) Hardness, HV300 Wear

K Spray pattern length 11 inch 19 inch 15 inch 4) Coating dep/pass Cost

5) Porosity Ctg quality, corrosion

*Grit blast 24 grit, 35 psi 6) Oxides Ctg quality

** Actual PFR needs TBD in light of TWIN 10 uncertainities; 7) Carbides Ctg quality, wear

(theorhetical values are given above) 8) Tensile bond Adhesion/cohesion
22

Jerry Schell, an HCAT team member from GEAE, was selected to manage the 

optimization procedure based upon his previous experience with HVOF and his extensive 

knowledge of the DOE methodology with special emphasis on thermal spray processes 

3.2.3.2. Northwestern/Generic Protocol at Hill AFB JP 5000 

3.2.3.2.1. General Test Results 

The initial Northwestern Protocol was run to optimize fatigue for HVOF process 

optimization and not for any specific aircraft application. The R ratio of 0.1 was 

therefore selected as the test condition. This work was conducted at Hill AFB in the 

1996 time frame. 

Table 3-7 to Table 3-9 summarize the DOE for the Hill work. 

Table 3-5 Actual Runs for CCAD Experimentation 

CDEFAGJBKH I

Actual 3 4 5 6 1 7 10 2 11 8 9

Run Order Std Order Random STOIC COMB G Air scfh CG scfh TrvSp ipm TT Sp rpm Part diam SD inches SpPattL,in PFR lb/hr Pwdr size

1 16 1 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

2 14 2 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

3 12 11 0.38 1550 735 30 20 90 2.00 8 11 5.5 Diam 53792

4 8 13 0.60 2000 965 30 20 250 6.63 8 11 15.8 Diam 53792

5 10 5 0.38 1550 965 50 40 250 6.63 8 19 5.5 Stark 10362

6 7 10 0.60 2000 735 50 20 250 2.00 12 11 5.5 Stark 10362

7 6 15 0.60 1550 965 50 40 90 2.00 12 11 15.8 Diam 53792

8 15 12 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

9 13 14 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

10 1 6 0.60 1550 735 50 40 250 2.00 8 19 15.8 Stark 10362

11 9 16 0.38 2000 965 50 20 90 2.00 8 19 15.8 Stark 10362

12 2 3 0.38 2000 785 30 40 90 6.63 12 11 15.8 Stark 10362

13 3 8 0.60 1550 965 30 20 90 6.63 12 19 5.5 Stark 10362

14 11 9 0.38 1550 735 50 20 250 6.63 12 19 15.8 Diam 53792

15 4 7 0.60 2000 735 50 40 90 6.63 8 19 5.5 Diam 53792

16 5 4 0.38 2000 965 30 40 250 2.00 12 19 5.5 Diam 53792

17 17 17 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

18 18 18 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

19 10 repeat 0.38 1550 965 50 40 250 6.63 8 19 5.5 Stark 10362

Table 3-6 DOE Sites and Locations

Location System Time Frame 

Hill AFB JP 5000 1996 

CCAD DJ 2600 1997-8 

Hitemco DJ 2600 1998-9 

NADEP-CP DJ 2600 1998-9 
23

Table 3-7. DOE Matrix for Hill AFB Work in 1996 

Design 1: Use L8 design 8 runs total FIXED:

Levels 54 grit alumina grit blast at 40 psi, 6 inches

FACTORS: -1 +1 Substrate is 4340 steel, 260-280 ksi

A Fuel (gph) 4 5.5

B PFR (gm/min) 5 8

C Nozzle (inches) 4 8 Spray angle is 90 degrees

D Oxygen (scfh) 1900 2100 100 psi cooling air, 4 AJs @ 6 inch spaced over coupon area

E Overlap (RPM) 90 144

F

Spray 

Distance 

(inches) 15 18 Spray pattern length Approximately 13 inch

G

Powder 

Size (μm) 31 39 Fixture diameter 6 inch

RESPONSES: RELATED CTG FUNCTION:

1) Part temperature Fatigue

2) Almen strip Fatigue, ctg residual stress

3) Hardness, HV300 Wear

4) Coating dep/pass Cost

5) Porosity Ctg quality, corrosion

6) Oxides Ctg quality

7) Carbides Ctg quality, wear

8) Tensile bond Adhesion/cohesion

Table 3-8. Summary of Results for the Hill JP 5000 DOE 

Hill AFB JP 5000

% RESPONSES

Parameters Thk/ Pass CY6 Temp Almen HV300 AVG Rank

A Fuel 6.8 43.4 25.5 42.8 29.6 1st

B PFR 47.8 2.5 9.4 12.8 18.1 2nd

C Nozzle 13.4 27.0 22.7 0.2 15.8 4th

D Oxygen 4.5 0.9 6.1 20.6 8.0 5th

E Overlap 22.8 0.3 1.8 1.9 6.7 5th

F Spary Distance 2.1 23.2 27.6 14.0 16.7 3rd

G Powder Size 2.6 2.7 7.0 7.7 5.0 6th

Effects Range Thk/ Pass CY6 Temp Almen HV300

Average 0.644 409 12.6 1054

Lower 0.357 301 6.5 855

Upper 0.917 530 18 1220

Mean +/ - X % 42.4 29.6 42.9 15.7
24

Table 3-8 summarizes the data form the Hill DOE. Table 3-9 Listing of the random run 

order for the varied parameter sets. Table 3-10 lists the final coating parameters for the 

JP 5000 system. 

3.2.3.2.2. General Discussion 

As expected, combustion gas and stand-off distance are the major factors in the spray 

process. The data for microhardness, almen strip values, and substrate temperature 

identifies these variables as the critical parameters for control and the obvious areas to 

investigate in future problem troubleshooting. There is also a degree of process 

robustness with the analysis indicating that a fairly wide range of values can be used to 

achieve a consistent end result. 

Three other areas should also be noted: 

 Deposition rate of the coating is obviously controlled not only by powder feed 

rate but traverse speed of the part being sprayed. This traverse rate dependency is 

evident in data from all systems and guns. This will have a substantial effect on 

almen and substrate temperature because of the heat being transferred to the part 

as the gun sweeps across it. It is therefore critical to keep deposition rate constant 

in spraying test bars, almen strips, or parts to best approximate a consistent and 

repeatable process 

 For the JP 5000 analysis, the nozzle length was a significant factor for all three 

critical outputs. Only the 4 and 8 inch lengths were tested, with the 8 inch 

dimension providing the optimum coating deposit. 

Table 3-9. Run Order for Hill AFB Work 

 FACTOR 

 Fuel PFR Nozle Oxygen Overlap Sp Dist Powder 

Size 

Run No. Std Order gph gm/min inches scfh RPM inches μm 

1 3 4 5 4 1900 90 15 31 

2 5 4 5 4 2100 144 18 39 

3 8 4 8 8 1900 90 18 39 

4 2 4 8 8 2100 144 15 31 

5 7 5.5 5 8 1900 144 15 39 

6 1 5.5 5 8 2100 90 18 31 

7 4 5.5 8 4 1900 144 18 31 

8 6 5.5 8 4 2100 90 15 39 

 
25

 Stoichiometry (i.e. hydrogen/oxygen ratio in the flame) was also identified as a 

major factor in microhardness results. This affects the melting of the cobalt 

binder and dissolution of the carbides. Non-optimal ratios can drive the carbides 

into solution, resulting in poor hardness or toughness. 

3.2.3.3.CCAD DOE 1998 

3.2.3.3.1. General Test Results 

In preparation for the LG JTP and to assist in assimilating HVOF technology into the 

depots, an interim optimization was run at the CCAD facility to optimize spray 

parameters on the DJ 2600 system. Although the JP 5000 and DJ2600 are both HVOF 

Table 3-10. Final Parameters for Hill AFB JP 5000 

JP5000 Process Parameters for WC-17%Co Fatigue Specimens

Equipment Model 5220 gun with 8” nozzle 

Model 5120 

Model 5500 powder feeder 

Powder feed Stark Amperit 526.062 

80 gm/min (325 rpm, 6 pitch feeder screw) 

30 

Argon 

50 psi 

15 scfh 

Combustion Gases Kerosene, Type 1-K 

162-168 psi 

121-123 psi 

100-102 psi 

5.0 gph 

Oxygen 

190-235 psi console supply reading 

138-140 psi gun supply pressure 

2000 scfh (corrected flow at 210 psi: 1940 to 2075 actual flow tube 

reading, depending on console supply pressure reading) 

Gun Compressed Air 

Gun Cooling Water Flow 8.3-8.7 gph 

64-72oF 

117-125 oF 

51-54 oF 

Specimen Rotation 600 rpm for round bars (0.25 inch diam.); 144 rpm for rectangular 

bars (at 6.63 inch fixture diam.) 

Gun Traverse Speed 70 in/min for round bars; 17 in/min for rectangular bars 

Spray Distance 18 inches 

Cooling Air 90-110 psi 

2 gun-mounted air jets at 14 inches; 1 stationary air jet at 4-6 inches 

pointed at coating area 
26

systems, there is enough difference in the design to require optimization of both systems. 

Table 3-11 summarizes the DOE matrix for the CCAD work. 

Table 3-12 summarizes random run order for the DOE matrix of the CCAD work. 

The responses for the DOE are summarized in Figure 3-1., Figure 3-2., and Figure 3-3.. 

Table 3-11. DOE Matrix for CCAD 1998 

Levels

FACTORS: -1 +1 C Pt FIXED:

A Traverse speeds 20 ipm 40 ipm 30 ipm Metco VF grit blast surface prep procedure*

B Spray distance 8 inch 12 inch 10 inch Substrate is 4340 steel, 260-280 ksi

C Stoic Ratio 0.38 0.6 0.44 Powder type is WC-17Co, agglomerated and sintered

D Combustion Gas 1550 scfh 2000 scfh 1775 scfh Spray angle is 90 degrees

E Air flow 735 scfh 965 scfh 850 scfh 100 psi air, 5 AJs @ 6 inch spaced over 8-12 inch coupon area

F Carrier gas flow 42.6 scfh 71 scfh 56.8 scfh Carrier gas equiv N2 mass flow for N2 or Argon

G Turntable RPM 90 RPM 250 RPM 170 scfh N2 x 1.42 = Ar

H Powder Feed Rate** 5.4 lbs/hr 10.7 lbs/hr 15.8 lbs/hr RESPONSES: RELATED CTG FUNCTION:

I Powder size Amp526.062005NS 2005NS 1) Part temperature Fatigue

Lot 10362 Lot 53792 Lot 53558 2) Almen strip Fatigue, ctg residual stress

J Part diameter 2 inch 6.63 inch 4.31 inch 3) Hardness, HV300 Wear

K Spray pattern length 11 inch 19 inch 15 inch 4) Coating dep/pass Cost

5) Porosity Ctg quality, corrosion

*Grit blast 24 grit, 35 psi 6) Oxides Ctg quality

** Actual PFR needs TBD in light of TWIN 10 uncertainities; 7) Carbides Ctg quality, wear

(theorhetical values are given above) 8) Tensile bond Adhesion/cohesion

Table 3-12. Random Run Listing for CCAD DOE 

CDEFAGJBKH I

Actual 3 4 5 6 1 7 10 2 11 8 9

Run Order Std Order Random STOIC COMB G Air scfh CG scfh TrvSp ipm TT Sp rpm Part diam SD inches SpPattL,in PFR lb/hr Pwdr size

1 16 1 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

2 14 2 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

3 12 11 0.38 1550 735 30 20 90 2.00 8 11 5.5 Diam 53792

4 8 13 0.60 2000 965 30 20 250 6.63 8 11 15.8 Diam 53792

5 10 5 0.38 1550 965 50 40 250 6.63 8 19 5.5 Stark 10362

6 7 10 0.60 2000 735 50 20 250 2.00 12 11 5.5 Stark 10362

7 6 15 0.60 1550 965 50 40 90 2.00 12 11 15.8 Diam 53792

8 15 12 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

9 13 14 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

10 1 6 0.60 1550 735 50 40 250 2.00 8 19 15.8 Stark 10362

11 9 16 0.38 2000 965 50 20 90 2.00 8 19 15.8 Stark 10362

12 2 3 0.38 2000 785 30 40 90 6.63 12 11 15.8 Stark 10362

13 3 8 0.60 1550 965 30 20 90 6.63 12 19 5.5 Stark 10362

14 11 9 0.38 1550 735 50 20 250 6.63 12 19 15.8 Diam 53792

15 4 7 0.60 2000 735 50 40 90 6.63 8 19 5.5 Diam 53792

16 5 4 0.38 2000 965 30 40 250 2.00 12 19 5.5 Diam 53792

17 17 17 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

18 18 18 0.49 1775 850 40 30 170 4.31 10 15 10.7 Diam 53558

19 10 repeat 0.38 1550 965 50 40 250 6.63 8 19 5.5 Stark 10362
27

0 20 40 60 80 100 120 140 160 180

TT Sp

SpPattL

Air

PFR

SD

Part Dia

TrSp

CG

Powd Sz

Stoic

Comb G

Pareto Chart of the Effects

(response is HV300, Alpha = .10)

Figure 3-1. Microhardness Response for CCAD 

DOE

0 50 100 150

SpPattL

Powd Sz

CG

PFR

Air

TT Sp

Comb G

Stoic

TrSp

Part Dia

SD

Pareto Chart of the Effects

(response is Cy6 Temp, Alpha = .10)

Figure 3-2. Substrate Temperature Response for 

CCAD DOE 
28

3.2.3.3.2. General Discussion 

As expected, combustion gas and stand-off distance are the major factors in the spray 

process. The data for microhardness, almen strip values, and substrate temperature 

identifies these variables as the critical parameters for control and the obvious areas to 

investigate in future problem troubleshooting. The analysis shows a robust process with 

that a fairly wide range of values that can be used to achieve a consistent end result. 

Three other areas should also be noted: 

 Deposition rate of the coating is obviously controlled not only by powder feed 

rate but traverse speed of the part being sprayed. This traverse rate/deposition 

rate dependency is evident in data from all systems and guns. This will have a 

substantial effect on Almen and Substrate Temperature because of the heat being 

transferred to part. It is therefore critical to keep deposition rate constant in 

spraying test bars, Almen strips, or parts to best approximate a consistent and 

repeatable process . 

 For the CCAD analysis, the powder size/distribution was a significant factor for 

the Almen strip output indicating a variation in the residual stress state of the 

coating. This variable can be controlled by use of material form one powder 

manufacturer which is common in the thermal spray industry. However, this 

indicates that a change to another manufacturer will require some process 

revalidation and is also an indicator that this parameter should also be added to 

the problem trouble shooting guide. 

 Stoichiometry was also identified as a major factor in microhardness results. 

Stoichiometry is the ratio of fuel gas to oxygen in the gun, and because it controls 

0 2 4 6 8 10 12 14

Part Dia

TrSp

Stoic

SpPattL

CG

Air

PFR

TT Sp

SD

Comb G

Powd Sz

Pareto Chart of the Effects

(response is Norm.Alm, Alpha = .10)

Figure 3-3. Almen Strip Response for CCAD 

DOE
29

flame temperature, it affects melting of the matrix Co and dissolution of the 

carbides. High flame temperatures tend to solution the carbides, resulting in 

hardness changes and alloying of the binder, both of which affect mechanical 

properties of the coating. Stoichiometry must therefore be included in the process 

control. 

A summary of the CCAD work is presented in Table 3-13, and more details can be found 

in the Appendix 1 of the Joint Test Report [3.1]. 

3.2.3.4. LG DOE at Hitemco: 1998-1999 DJ 2600 System 

The DOE work at Hitemco was the final coating analysis prior to coating of the fatigue, 

corrosion and wear specimens. The investigation was performed to verify the outcome of 

the DOE at CCAD and to finalize the process prior to test bar coating deposition. Table 

3-14 presents the DOE matrix. 

Table 3-13. Final Summary of Factors in CCAD DOE 

% RESPO N SES

Pa ra m e te rs Th k / Pass CY6 Tem p Alm en H V3 0 0 A V G Ra nk

A TrvSp ipm 9.3 11.3 2.3 9.3 8.0 4th

B SD inches 6.4 18.1 17.0 5.9 11.9 3rd

C STOIC 3.7 11.2 2.3 14.6 8.0 4th

D COMB G 5.3 9.5 17.5 24.3 14.2 1st

E Air scfh 6.0 6.6 6.2 4.2 5.8 5th

F CG scfh 10.3 4.7 6.2 11.4 8.2 4th

G TT Sp rpm 7.4 9.1 9.9 2.6 7.2 4th

H PFR lb/ hr 14.4 6.5 6.4 4.6 8.0 4th

J Pwdr size 11.3 4.6 25.7 12.4 13.5 2nd

K Part diam 19.0 15.6 2.2 7.9 11.2 3rd

L SpPattL,in 6.8 2.6 4.4 2.8 4.2 5th

100.0 100.0 100.0 100.0 100.0

SUM Sha d e d Effe c ts 5 5 .1 5 6 .3 7 0 .1 6 2 .8 51

Effects Range Thk/ Pass CY6 Tem p Alm en HV300

Lower 0.168 325 7.2 1024

Upper 0.746 496 21.5 1203

Mean +/ - X % 63.2 20.8 49.8 8.0
30

Table 3-15 represents the random run sequence for the Hiemco DOE. 

Figure 3-4., Figure 3-5. and Figure 3-6. show the general trends for substrate temperature, 

almen strip and microhardness responses. 

Table 3-14. DOE Matrix for Hitemco Analysis 

Design 1: Use L8 design plus Center Points, 11 runs total FIXED:

Levels 54 grit alumina grit blast at 40 psi, 6 inches

FACTORS: -1 +1 C Pt Substrate is 4340 steel, 260-280 ksi

A Surf Speed,Feed Rate 1335, 5.1 1835, 3.5 1585 ipm, 4.3 Powder size/type is WC-17Co, Diamalloy 2005, Lot 54480

B Combustion Gas 1525 scfh 1825 scfh 1675 scfh Powder Feed Rate** 8.5 lbs/hr

C Stoic Ratio 0.405 0.485 0.445 Spray angle is 90 degrees

D Spray Distance 10 inch 13 inch 11.5 inch 100 psi cooling air, 4 AJs @ 6 inch spaced over coupon area

Carrier gas N2 at 148 psi, 55 flow, air vib @ 20 psi

Turntable Robot Spd Robot % @ Spray pattern length Approximately 13 inch

A Factor: RPM ipm mm/sec 750 mm/sec Spots/Rev Fixture diameter 2 inch

(-1) 212 25 10.6 1.41% 5.1

C Pt 252 35 14.8 1.98% 4.3

(+1) 292 50 21.2 2.82% 3.5

RESPONSES: RELATED CTG FUNCTION:

(B,C) Factor Combinations: 1) Part temperature Fatigue

Comb Gas Stoic Ratio Hyd SCFH Oxy SCFH Air SCFH Point (CG,SR) 2) Almen strip Fatigue, ctg residual stress

1675 0.445 1159 332 920 ( 0, 0) 3) Hardness, HV300 Wear

1525 0.405 1085 258 920 (-1,-1) 4) Coating dep/pass Cost

1525 0.485 1027 314 920 (-1,+1) 5) Porosity Ctg quality, corrosion

1825 0.405 1299 342 920 (+1,-1) 6) Oxides Ctg quality

1825 0.485 1229 412 920 (+1,+1) 7) Carbides Ctg quality, wear

8) Tensile bond Adhesion/cohesion

Table 3-15. Random Runs for Hitemco DOE 

A factor (B,C) Combined Factors D factor

Std.Ord Turn Table Robot Trav Hydrogen Oxygen Air Sp Dist

Run No. RPM Sp mm/s psi/FMR psi/FMR psi, FMR inches

1 9 252 14.8 135 psi, 50.4 148 psi, 23.1 105 psi, 50.5 11.5

2 1 212 10.6 135 psi, 47.2 148 psi, 17.8 105 psi, 50.5 10

3 2 292 21.2 135 psi, 47.2 148 psi, 17.8 105 psi, 50.5 13

4 3 212 10.6 135 psi, 56.5 148 psi, 23.8 105 psi, 50.5 13

5 4 292 21.2 135 psi, 56.5 148 psi, 23.8 105 psi, 50.5 10

6 10 252 14.8 135 psi, 50.4 148 psi, 23.1 105 psi, 50.5 11.5

7 5 212 10.6 135 psi, 44.6 148 psi, 21.8 105 psi, 50.5 13

8 6 292 21.2 135 psi, 44.6 148 psi, 21.8 105 psi, 50.5 10

9 7 212 10.6 135 psi, 53.4 148 psi, 28.7 105 psi, 50.5 10

10 8 292 21.2 135 psi, 53.4 148 psi, 28.7 105 psi, 50.5 13

11 11 252 14.8 135 psi, 50.4 148 psi, 23.1 105 psi, 50.5 11.5
31

0 2 4 6 8 10 12 14 16 18

AD

AB

A

AC

C

B

D

Pareto Chart of the Standardized Effects

(response is DPH 300, Alpha = .10)

A: RPMxIPM

B: CombGas

C: Stoic

D: SpDist

Figure 3-4. Microhardness Response for Hitemco 

DOE

0123

AB

AC

AD

C

B

A

D

Pareto Chart of the Standardized Effects

(response is T 6cycle, Alpha = .10)

A: RPMxIPM

B: CombGas

C: Stoic

D: SpDist

Figure 3-5. Substrate Temperature Response for Hitemco 

DOE
32

Table 3-16 is the final spray parameter set for the test bar spraying for the LG JTP. 

0 5 10 15

A

AB

AC

AD

C

D

B

Pareto Chart of the Standardized Effects

(response is Norm.Alm, Alpha = .10)

A: RPMxIPM

B: CombGas

C: Stoic

D: SpDist

Figure 3-6. Almen Strip Response for Hitemco 

DOE
33

 

3.2.3.4.1. General Discussion 

As expected, combustion gas and stand-off distance are the major factors in the spray 

process. The data for microhardness, Almen strip values, and substrate temperature 

identifies these variables as the critical parameters for control and the obvious areas to 

investigate in future problem troubleshooting. There is also a degree of process 

Table 3-16. Final Deposition Parameters Hitemco LG JTP 

Equipment Gun Model 2600 hybrid gun

Console Model DJC

Powder feeder Model DJP powder feeder

Injector #8

Shell #8

Insert #8

Siphon plug #8

Aircap DJ2603

Powder feed Powder Diamalloy 2005

Powder Feed Rate: 8.5 lb/hr 

Tube DJP 116

Pick-up shaft “E”

Air Vibrator setting 20 psi

Powder Carrier Gas Nitrogen

Carrier gas pressure 148 psi

FMR 55

Flow rate 28 scfh

Combustion Gases Fuel Hydrogen

Gun supply pressure 135 psi

FMR 53.4

Flow rate 1229 scfh

Oxidizer Oxygen

Pressure 148 psi

FMR 28.7

Mass flow 412 scfh

Gun Compressed Air Pressure 105 psi

FMR FMR – 50.5

Mass flow 920 scfh

Gun Cooling Water Flow Flow rate 5.3-5.7 gph (factory set)

Water Temperature to Gun:

In 65-80oF typical (ground water, temp varies)

Out 77-100 oF typical

Delta 12-20 oF (~ 14 oF typical)

Specimen Rotation 2,336 rpm for round bars (0.25 inch dia.) – 

1835 in/min surface speed

Gun Traverse Speed 400 linear in/min for round bars

Spray Distance 11.5 inches

Cooling Air Pressure 90-110 psi

Location 2 stationary nozzle tips at 6 inches pointed 

at coating area
34

robustness with the analysis indicating that a fairly wide range of values can be used to 

achieve a consistent end result. 

Two other areas should also be noted: 

o The deposition rate of the coating is obviously controlled not only by powder 

feed rate but traverse speed of the part being sprayed. This traverse 

rate/deposition rate dependency is evident in data from all systems and guns. 

This will have a substantial effect on Almen and substrate temperature because 

of the heat being transferred to the part. It is therefore critical to keep the 

deposition rate constant in spraying test bars, Almen strips, or parts to best 

approximate a consistent and repeatable process 

o Stoichiometry was also identified as a major factor in microhardness results. 

A summary of the Hitemco work can be found in Appendix 1 of the JTR [3.1]. 

3.2.3.5. Almen Strip and Temperature Measurement Procedures 

Almen strip and substrate temperature control are two of the more critical areas for 

process control. In the initial HVOF trials, it was assumed that these measurements were 

well defined and would not create any inconsistencies. Subsequent experience has shown 

that this is not the case. 

Almen strip results were found to be strongly influenced by preparation and spraying 

methods, leading to large systematic differences between spray sites. Factors to consider 

are: 

 Grit blasting of one side vs. both sides of the strip This can result in a 3-4 mil 

difference in Almen results when spraying. 

 Orientation of the strip (i.e. torch traverse along or across the strip This can 

result in a 0.001”-0.002” difference in Almen results when spraying. 

 Cleaning of Almen block If not performed properly and frequently, coating will 

build up on the block, preventing proper thermal contact and leading to improper 

readings 

 Reduction in Almen Response with Increasing Thickness As thickness increases, 

the Almen response appears to level off. Another almen strip type may be 

required for more substantial deflections 

 Normalized Almen Values Based upon the issue of thickness, many initial values 

have been reported for the .003-.005” thickness range due to interest in those 

areas. However, values for Almen response can vary even over a .002” range. 

HCAT has therefore defined Almen stress as the value measured on a 0.005” 

thick coating. 

It is common practice in many spray shops to measure substrate temperature with a 

contact probe at the end of the spray run. This approach provides no information on the 

true temperature excursions that occur during spraying. The HCAT team therefore 

adopted the approach of continuous infrared temperature measurement during spraying. 

For substrate temperature measurement, the following issues have been identified: 
35

 Use of Real time measurement vs. Touch Probe Temperatures can be as much as 

100o

 F hotter with instantaneous measurement (IR pyrometer) vs. touch probe 

after all spraying is complete. 

 Spot size of IR System When spraying small test bars (as was required for this 

project), the spot size is normally bigger than the specimen diameter. This 

requires a compensation factor when spraying test bars. If the actual reading is 

used, it will give a false indication which is lower than the actual bar value and 

substrate overheating may result. 

 Co-ordination of Touch Probes and IR System Via Emissivity Corrections As 

stated earlier, temperatures can be as much as 100o

 F hotter with instantaneous 

measurement (IR pyrometer) vs. touch probe after all spraying is complete. The 

IR unit must therefore be calibrated to define an emissivity setting that can be 

used as a default value. Although the IR system may not be an exact value, it can 

be used as a conservative guideline to control the process. 

A complete guideline for Almen Strip/Substrate Temperature measurement can be found 

in Appendix 1 of the JTR [3.1]. 

3.2.4. Interpretation and Discussion 

The optimization process for both the JP 500 and DJ 2600 produced the trends found in 

Table 3-17. The primary results are not unexpected for the substrate temperature and 

Almen as the amount of combustion gas will drive the achievable flame temperature, 

while spray distance (end of nozzle to part) will have a substantial effect on how much of 

that heat input is transferred to the part. It is very important to note that these trends were 

observed for both types of gun at three different locations. 

The secondary effects of nozzle and powder size will be controlled by a standard choice 

for each of these parameters. When selected, these variables will be fixed but powder 

size must still be a part of the troubleshooting guide if size/particle distribution issues are 

identified at the powder vendor. 

Fatigue was used as the most important factor in coating optimization. Initial focus was 

on fatigue cycle life which was achieved during the LG JTP evaluation. Additional work 

performed as part of HCAT by the Air Force Research Lab (AFRL) in the same time 

fame provided an example of problems that can be experienced during spraying. When 

Table 3-17. Primary and Secondary Determinants of 

Coating Properties 

Property Primary Secondary 

Almen Combustion Gas 

Spray Distance 

Nozzle 

Powder size 

Microhardness Combustion Gas 

Spray Distance 

Powder size 

Substrate temperature Combustion Gas 

Spray Distance 

Nozzle 
36

coating parameters were chosen that were optimized for wear performance rather than 

fatigue, HVOF cyclic fatigue lives were lower than chrome. Temperature control was 

also suspect. Respraying of bars at the same location with HCAT-optimized parameters 

and better temperature control resulted in satisfactory fatigue performance. This 

highlights not only the importance of process optimization, but also the importance of 

optimizing for the most critical performance parameters. 

However, the question that still remains is coating integrity (which will be further 

discussed in the fatigue section of this report, and will be the subject of a subsequent 

report), and the need for process optimization for optimal integrity. Current industrial 

research indicates that areas such as phase content in the as-sprayed coating and the 

actual residual stress values in the test bar or part will be critical to coating integrity or 

cracking/delamination/spalling tendencies. These characteristics will be important in 

conjunction with particle velocity/temperature measurements that can be obtained with 

systems like the DPV 2000. It will be critical to investigate and understand these issues 

to produce an optimized coating which meets coating integrity criteria. This work, in 

conjunction with the present information, will provide a defined and robust set of 

parameters that can consistently produce reliable coatings on military hardware at the 

depot. 

3.2.5. Significance 

For implementation at military depots on flight critical hardware, a manufacturing 

process must be well defined and process variables highlighted for control. The coating 

optimization procedure has identified parameters that are critical and evaluated those 

variables with relation to manufacturing robustness and the future need for process 

troubleshooting. Further process understanding is in progress and will be required to 

fully comprehend and optimize the spray technology for fatigue coating integrity 

performance. 

3.2.6. Conclusions 

The HVOF process for WC-Co has been successfully optimized for both the JP 5000 and 

DJ 2600 systems with regard to many of the properties such as fatigue cycle life to 

achieve a repeatable process at varied locations. 

More work is required concerning the issue of coating integrity (spalling and 

delamination) and process understanding concerning such variables as phase content, 

actual residual stressing the coating deposit, and velocity/temperature relationships (as 

measured by instruments such as the DPV 2000). 
37

3.3. Fatigue data 

3.3.1. Data Summary 

3.3.2. Test Rationale 

Fatigue is a very critical property in the aerospace industry, because of the repeated 

cyclic loading for landing gear, actuators, airframe parts, and gas turbine engine 

components. Since fatigue performance is driven by material strength and is especially 

related to near-surface effects, fatigue-critical applications require careful definition and 

control of the thermal spray process to 

1. minimize surface heating so as to prevent loss of mechanical properties due to 

overheating, and 

2. deposit thermal spray coatings with compressive stress to minimize or eliminate 

any fatigue debit. 

Although plasma spray processes have been widespread in the aerospace industry for 

many years, they have tended to be limited to non-fatigue critical applications, largely 

due to the heat input of the process and tensile coating stresses. The commercial 

development of the HVOF process, which relies more on kinetic than thermal energy for 

final coating properties and permits compressive coating stress, has started to move the 

design community towards thermal spray in fatigue-driven components. As a result 

HVOF coatings are now specified in numerous commercial aircraft and some military 

aircraft. The fatigue test matrix was defined to assess the impact of HVOF coatings on 

the fatigue life of the substrate. 

The evaluation of fatigue in a coated part is really the analysis of how the coating affects 

the known values of an uncoated component. Baseline data for the alloys used in most 

applications already have been established. There are established methods and 

Table 3-18. Quick Reference to Primary Data (Click blue links to jump to data) 

Item Item Number 

Materials evaluated for fatigue testing Table 3-20 

Specimen types for fatigue testing Figure 3-7, Figure 3-8, 

Fatigue test matrix Table 3-23 

Representative data – 4340 steel Figure 3-13. to Figure 

3-17. 

Representative data – Aermet 100 steel Figure 3-18. to Figure 

3-21. 

Representative data – 300M steel Figure 3-22. to Figure 

3-25. 

Summary of coating integrity analysis Table 3-24 
38

specifications for determining fatigue properties. However, with coatings now applied, 

some of the guidelines used for bulk materials are somewhat different. 

For most chrome-replacement testing, axial fatigue testing (ASTM E466-96) provides the 

most useful data for evaluation (rather than bend testing). In designing a fatigue testing 

protocol, some areas requiring definition are: 

1. What is the load carrying capability of the coating and should this value be used 

in determining the applied stress? How will thickness of the coating affect this 

situation? The cross sectional area of the coating should be much less than that of 

the sample. 

2. What is the best bar design for fatigue evaluation of coatings? 

 Hourglass (smoothly varying cross section, thinnest at the center) 

 Smooth section (constant cross section from some distance in center) 

3. Will the testing mode be load (stress) or strain control? 

4. Can grinding of the coating be repeatable on the fatigue bars to produce a 

consistent thickness and surface for testing? 

As with all fatigue evaluations, considerations must be given to the following: 

Frequency or speed of testing –will determine time for testing, but too high a 

frequency can cause overheating or a shift in results for strain rate sensitive 

materials 

Type of control (load or strain) – will application of force to the bar be 

controlled purely by load or by the deformation induced in the part? 

R ratio or A ratio – R ratio is defined as the ratio of minimum cyclic load to 

maximum cyclic load. For example, an R ratio of –1.0 means the maximum 

and minimum loads are the same but the loading is fully reversed from 

positive to negative. A ratio is defined as 

(maximum stress - minimum stress)/(maximum stress + minimum stress) 

Typical fatigue bar shapes are shown in Figure 3-7 and Figure 3-8. The most common 

designs are the hourglass (which can only be used for load-control fatigue), and the 

smooth bar. The hourglass bar was used for the bulk of the testing for consistency with 

existing data in the landing gear community. The hourglass shape is more sensitive to 

coatings and tends to give a wider separation of curves between coating conditions. 

Some tests with smooth bars were included to permit correlation with earlier HCAT 

generic testing. While for 0.003” coatings a small 0.25” diameter bar was used, for 

0.010” coatings a 0.5” diameter bar was used to ensure that the bar cross section was 

much greater than that of the coating. Most testing was done at R=-1 (fully reversed), 

since that best corresponds with field conditions for military landing gear components. 

Some testing was done at R=0.1 (tension-tension) for comparison with earlier generic 

HCAT data and with Boeing test data. 
39

Figure 3-7. Typical Hourglass-shaped Fatigue Bar 

Figure 3-8. Typical Smooth Fatigue Bar 
40

Fatigue testing for coating comparison must be defined by the important parameters of 

the spray process and the critical controls for consistent property evaluation as shown in 

Table 3-19. 

Corrosion fatigue must also be a consideration, given the corrosive environments for 

many of the hard chrome applications. This usually involves the same considerations as 

testing in air but the test area in question is exposed to the corrosive media for the entire 

test or for specific periods of time. There may also be pre-exposures for the purpose of 

initiating corrosion followed by constant exposure to the environment in question. When 

defining the protocol for testing, the frequency of exposure, and the degree of 

replenishment must be stipulated to best approximate the actual service conditions. 

3.3.3. Specimen Fabrication 

3.3.3.1.Specimen Geometry and Materials: 

In the case of the Landing Gear Joint Test Protocol (LG JTP), both configurations were 

selected for the fatigue evaluation. 

 Hourglass: 0.25” gage diameter for specimens receiving 3-mil-thick coatings 

and 0.50” gage diameter for specimens receiving 0.010”-thick coatings (See 

Figure 3-7 and Figure 3-9). 

 Smooth bar: 0.25” gage diameter over a gage length of 0.75” (See Figure 3-8) 

Table 3-19. Fatigue Testing Variables

Coating/Substrate Information Testing Information 

Peening of substrate Frequency of testing 

machine 

Thickness of coating Bar geometry 

Surface finish (ground, unground, 

or superfinished) 

Load vs. strain control 

Location of coating (patch/full 

length) 

R ratio, or A ratio 

Almen (intrinsic stress) intensity 
41

The two smaller specimens (Figure 3-7 and Figure 3-8) were not threaded on the ends; 

rather, the smooth shank end was clamped in hydraulic grips. A threaded end was used 

for the larger hourglass specimen (Figure 3-9). The materials selected for the protocol 

are listed in Table 3-20. 

3.3.3.2. Specimen preparation 

Specimen preparation involved four major steps: rough grinding, vacuum heat treatment, 

finish grinding/polishing, and shot peening. 

The heat treat for the 4340 and 300M was in accordance with MIL-H-6875 and the heat 

treat for the Aermet 100 in accordance with P.S. 15169. When possible, all specimens 

were heat treated at same time. Grinding was performed to IAW MIL-STD-866 and 

Figure 3-9. Hourglass Configuration with .500” Gage Section 

Table 3-20. Substrate Materials in LG JTP 

Material Heat Treat (tens. Strength) Shot Peen 

4340 260-280 ksi 8-10A, S230, wrought steel 

300M 280-300 ksi 8-10A, S230, wrought steel 

Aermet 100 280-290 ksi 8-10A, S230, wrought steel 
42

Metcut internal specifications (Table 3-21) as listed below. The fatigue specimen surface 

preparation involved low stress grinding followed by 600 grit alumina polishing which 

removed 0.001” minimum on all gage section surfaces in the polishing step of the 

specimens. Nital etching IAW MIL-STD-867 was conducted on all specimens to 

examine for grinding burns. All specimens were baked subsequent to Nital etching to 

remove residual hydrogen. All specimens to be tested (both coated and uncoated) were 

prepared in the same fashion to ensure a common starting condition prior to any surface 

preparation techniques used for the coatings. This approach was used to reduce fatigue 

test data scatter due to variations in the condition of the machined surfaces. 

Where specified in the test matrix, shot peening was conducted to IAW AMS-2432 under 

computer control using the conditions specified in Table 3-20. The entire surface was 

shot peened to 100% surface coverage. 

3.3.4. Coating deposition methodology 

Fatigue specimens were coated with electroplated hard chrome and HVOF WC-17Co. 

The HVOF coatings were applied at Hitemco in Bethpage, NY. The hard chrome was 

applied at Ogden ALC (Hill AFB). All of the fatigue specimens were shot peened and 

solvent wiped with reagent grade acetone and or isopropyl alcohol prior to coating. All 

coatings were applied in a patch 0.5” long (0.75” long for 0.5” diameter bars) centered on 

the middle of the bar and feathered at the patch ends to limit stress concentrations. The 

coatings were deposited around the entire circumference to a thickness of 0.002 – 0.003” 

over the final diameter to allow for grinding after coating. 

The fatigue specimens for hard chrome were prepared for plating by lightly hand 

abrading the areas to be coated with a Scotchbrite pad. Chrome plating was done at 

about 130 0

F following MIL-STD-1501C, Class 1, Type 1. The resultant hard chrome 

was about 0.002” thicker than the final grind dimension and nominally 0.5” long, 

centered in the gage section of the fatigue specimen, and had feathered edges. 

The fatigue specimens with HVOF WC-17Co were coated at Hitemco with a DJ 2600 

system and Diamalloy 2205 agglomerated/sintered powder (nominal size, -270 mesh/+10 

micron). They were prepared for coating by grit blasting with 24 grit aluminum oxide at 

18-20 psi and then coated using the parameters given in Table 3-22. The smooth round 

bar and hourglass bar specimens were coated individually while rotating on axis and 

being traversed parallel to the length. A pair of shadow masks restricted coating 

deposition to the desired 0.5” region centered in the gage section and ensured proper 

feathering of the patch ends. Tape masking restricted grit blasting and the coating 

overspray area to a slightly wider area (0.6” max). Complete deposition conditions are 

Table 3-21. Machining Procedures for Fatigue Specimens 

Machining Procedures

200.1.1 Specimen Preparation Procedure Low Stress Grind 

Axial Polishing Procedures for Test Specimens 

200.4 Metcut Round Specimen Polish Procedures for All Materials 
43

summarized in Table 3-22. 

3.3.5. Test methodology 

Stress Levels : All fatigue tests were conducted in constant amplitude axial loadcontrol in accordance with ASTM E466-96 to generate standard S-N 

curves (a plot of number of cycles to failure for given stress levels). 

Maximum stress: approximately 180 ksi, in accord with Boeing fatigue 

test procedures 

Stress direction: fully reversed (tensile/compressive, R=-1) for most 

Table 3-22. DiamondJet 2600 Process Parameters for Deposition of WC-17Co

Equipment: Model 2700 hybrid gun 

Powder: Diamalloy 2005 

Powder Feed Rate: 8.5 lb/hr (325 rpm, 6 pitch 

feeder screw) 

Vibrator setting, 30 

Powder Carrier Gas: Nitrogen pressure 148 psi 

 Nitrogen flow 28 scfh 

Combustion Gas: Oxygen pressure 148 psi 

Combustion fuel: Hydrogen 

Fuel pressure – 135 psi console supply pressure 

Fuel flow rate – 1229 scfh 

Combustion chamber 

Pressure: 

100-102 psi 

Gun Cooling Water Flow: 8.3-8.7 gph 

Water Temperature to 

Gun: 

In, 64- 72o

F 

Out, 117-125 oF 

Delta, 51- 54 oF 

Specimen Rotation: 2,636 rpm for round bars (0.25” dia.) – 16,560 in/min 

surface speed 

Gun Traverse Speed: 400 linear in/min for round bars 

Spray Distance 11.5” 

Cooling Air: 2 gun mounted AJs at 14”, 90-110 psi 

1 stationary AJ at 4-6” pointed at coating area, 90-110 

psi 
44

tests, some tests under tension-tension at R=0.1 

Number of stress levels: 4 

Number of specimens at each stress level: 5 

Stress calculations: Calculated on as-machined diameters

Environments: The environments were: (1) Laboratory air at ambient temperature 

(2) 3.5% NaCl solution at ambient temperature. The procedure for 

conducting the corrosion fatigue tests was to immerse the entire 

fatigue specimen in a 3.5% NaCl solution for 24 hours. Then the 

specimen was removed, dried, stored in a desiccator for approximately 

24-48 hours and then placed into the fatigue-testing machine in a cell 

(See Figure 3-10., Figure 3-11. and Figure 3-12. below) containing a 

3.5% NaCl solution. The gage section of the fatigue specimen was 

immersed in the NaCl solution during the entire fatigue test. This cell 

was replenished with a constant flow of 3.5% NaCl solution during the 

test. The excess solution was not returned to the system. 

The matrix for fatigue testing is indicated in Table 3-23. 

Acceptance : After testing, the data were plotted in the standard manner with stress 

on the vertical axis and cycles-to-failure on the horizontal axis. A 

least squares regression was used to produce each S-N curve. The 

regression involved a linear fit to the data in ln (N) vs. ln (σ) space, 

then calculating a best fit curve in the traditional S-N space. If the 

curves for the HVOF coatings fell on or above the curves for the EHC, 

then the HVOF coatings were considered to have met the acceptance 

criteria. 

The testing set-up is shown in Figure 3-10., Figure 3-11., and Figure 3-12.. These figures 

illustrate corrosion-fatigue testing, but the same arrangement was used for in-air testing 

(minus the stop-off paint and liquid receptacle). 

Figure 3-10. Small (1/4" dia) Hourglass Bars 

for Corrosion Fatigue Testing 

Figure 3-11. Corrosion Cell Set-up for 

Fatigue Testing: Large Hourglass 
45

Table 3-23. Fatigue Test Matrix

Material Geometry Peen Coating Thickness 

(mil) 

R Environment # of specimens 

4340 Hourglass No WC-Co 3 -1 Air 20 

4340 Hourglass No WC-Co 3 0.1 Air 20 

4340 Hourglass No EHC 3 -1 Air 20 

4340 Hourglass No EHC 3 0.1 Air 20 

4340 Hourglass Yes EHC 3 -1 Air 20 

4340 Hourglass Yes EHC 3 0.1 Air 20 

4340 Hourglass Yes WC-Co 3 -1 Air 20 

4340 Hourglass Yes WC-Co 3 0.1 Air 20 

4340 Hourglass Yes WC-Co 10 -1 Air 20 

4340 Hourglass Yes EHC 10 -1 Air 20 

4340 Hourglass Yes EHC 3 -1 NaCl 20 

4340 Hourglass Yes WC-Co 3 -1 NaCl 20 

4340 Smooth bar Yes EHC 3 -1 Air 20 

Figure 3-12. Close-up View of Corrosion Cell Set-up for 

Fatigue Testing: Small Hourglass Specimen 
46

4340 Smooth bar Yes WC-Co 3 -1 Air 20 

 Total 280 

 

300M Hourglass No Uncoated 3 -1 Air 20 

300M Hourglass No EHC 3 -1 Air 20 

300M Hourglass No WC-Co 3 -1 Air 20 

300M Hourglass Yes EHC 3 -1 Air 20 

300M Hourglass Yes WC-Co 3 -1 Air 20 

300M Hourglass Yes EHC 3 -1 NaCl 20 

300M Hourglass Yes WC-Co 3 -1 NaCl 20 

300M Hourglass Yes EHC 10 -1 NaCl 20 

300M Hourglass Yes WC-Co 10 -1 NaCl 20 

300M Smooth bar Yes EHC 3 -1 Air 20 

300M Smooth bar Yes WC-Co 3 -1 Air 20 

 Total 220 

 

Aermet 100 Hourglass No Uncoated 3 -1 Air 20 

Aermet 100 Hourglass No EHC 3 -1 Air 20 

Aermet 100 Hourglass No WC-Co 3 -1 Air 20 

Aermet 100 Hourglass Yes EHC 3 -1 Air 20 

Aermet 100 Hourglass Yes WC-Co 3 -1 Air 20 

Aermet 100 Hourglass Yes EHC 3 -1 NaCl 20 

Aermet 100 Hourglass Yes WC-Co 3 -1 NaCl 20 

Aermet 100 Hourglass Yes EHC 10 -1 NaCl 20 

Aermet 100 Hourglass Yes WC-Co 10 -1 NaCl 20 

Aermet 100 Smooth bar Yes EHC 3 -1 Air 20 

Aermet 100 Smooth bar Yes WC-Co 3 -1 Air 20 

 Total 220 

 Grand Total 720 
47

3.3.6. Test results 

The US Landing Gear JTP concentrated on fatigue of WC-Co coatings at both a .003” 

and .010” thickness. Substrate materials were 4340, 300M and Aeromet 100. 

The JTP made primary comparisons between the following variables: 

Coating: EHC vs WC-Co 

Peening: Peened vs. unpeened 

Configuration: Hourglass vs. smooth 

Environment: Air vs. NaCl 

Thickness: .003” vs. .010” 

The primary goal of this program was to generate comparative S-N curves to assess 

fatigue performance. The issues of coating integrity (cracking/spalling) were not 

anticipated by the JTP, and the issue did not arise until the US JTP fatigue testing was 

almost complete. Consequently, no provision was made in the test plan to carefully 

monitor coating behavior during each test. For this reason, observations regarding 

coating integrity reported herein were limited to post-test assessments only. 

3.3.6.1. 4340 Substrate 

The comparisons as outlined above were made for the 4340 substrate material. The bar 

configuration comparison was included in the protocol to provide a link to earlier work 

performed on the smooth configuration. An additional comparison of R ratios was also 

made for this same reason. 

General comments on the comparisons are: 

 Figure 3-13. shows the comparison of unpened vs. peened data. While, as 

expected, peening significantly raised the EHC curves, it made little difference to 

the HVOF data. The hourglass configuration is quite sensitive in differentiating 

chrome vs. HVOF in the unpeened condition. 

 Figure 3-14. shows the comparison of bar configurations. As experienced in other 

protocols, the smooth bar configuration moves the curve to the left or shorter 

fatigue lives. However, in contrast to previous observations, the spread between 

chrome and HVOF is not substantially different. 

 Figure 3-15. shows the comparison of environments, namely air vs. NaCl. The 

NaCl curves show roughly equal degradation in fatigue performance between 

chrome and HVOF. 

 Figure 3-16. shows the comparison between the .003” and .010” coatings. Due to 

the .010” coating thickness, this data not only represents a thicker deposit but also 

a larger diameter bar. Since the bar diameters are different, the comparison can 

be somewhat skewed since larger diameter bars sometimes show reduced fatigue 

performance. It is therefore uncertain if the fatigue reduction is due primarily to 

the thicker coating or to the sample diameter. 
48

 Figure 3-17. shows the comparison of R ratio, namely -1 and 0.1 values. As 

expected, the R = -1 curve (solid) is shifted down and to the left (i.e. R=-1 is a 

more severe fatigue condition) of the R=0.1 (dotted). However, surprisingly, the 

R=0.1 data show a much larger spread between chrome and HVOF than the R=-1 

data. 

4340, SMALL HOURGLASS SPECIMEN

(0.003" COATING) R = -1, AIR

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/UNPeened

EHC/UNPeened/FIT

EHC/Peened

EHC/Peened/FIT

WCCo/UNPeened

WCCo/UNPeened/FIT

WCCo/Peened

WCCo/Peened/FIT

Figure 3-13. 4340 Fatigue - Hourglass Configuration, Air Environment, R = -1 
49

4340, SMOOTH VS HOURGLASS PEENED SPECIMENS

(0.003" COATING), R = -1, AIR

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/Smooth

EHC/Smooth/FIT

WCCo/Smooth

WCCo/Smooth/FIT

EHC/HG

EHC/HG/FIT

WCCo/HG

WCCo/HG/FIT

Figure 3-14. 4340 - Hourglass vs. Smooth Bar Configuration in Air Environment , R = -1

4340, R = -1, SMALL HOURGLASS SPECIMEN

AIR VS. NaCl ENVIRONMENT

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/Air

EHC/Air/FIT

EHC/NaCl

EHC/NaCl/FIT

WCCo/Air

WCCo/Air/FIT

WCCo/NaCl

WCCo/NaCl/FIT

Figure 3-15. 4340 - Air vs. NaCl Environment Comparison, Hourglass 

Configuration, 0.003” Thickness, R = -1 
50

4340, R = -1, AIR

LARGE (0.010"CTNG) VS. SMALL (0.003"CTNG) HOURGLASS

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

LgHG/EHC/Peened

LgHG/EHC/FIT

LgHG/WCCo/Peened

LgHG/WCCo/FIT

SmHG/EHC/Peened

SmHG/EHC/FIT

SmHG/WCCo/Peened

SmHG/WCCo/FIT

Figure 3-16. 4340 - 0.003” vs. 0.010” Thickness Comparison, Air 

Environment, Hourglass Configuration, R=-1, on 0.250” and 0.500” 

Specimens, Respectively

4340, SMALL HOURGLASS SPECIMEN

(0.003" COATING), PEENED, AIR

100.0

120.0

140.0

160.0

180.0

200.0

220.0

240.0

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/-1

EHC/-1/FIT

WCCo/-1

WCCo/-1/FIT

EHC/0.1

EHC/0.1/FIT

WCCo/0.1

WCCo/0.1/FIT

R = -1 vs 0.1

Figure 3-17. 4340 – R Ratio Comparison, Hourglass Configuration, 

0.003” Thickness 
51

3.3.6.2. A100 substrate 

Similar comparisons were made for the A100 substrate material. 

General comments on the comparisons are: 

 Figure 3-18. shows the comparison of unpeened vs. peened data. As compared to 

4340, there appears to be a stronger differentiation between chrome and HVOF. 

Again, the hourglass configuration appears to show more sensitivity in 

differentiating chrome vs. HVOF in the unpeened condition, but there is in this 

case a wider spread between the unpeened and peened material. 

 Figure 3-19. shows the comparison of bar configurations. As with the other 

materials, the HVOF hourglass performance is better than the smooth 

configuration. Again, as with previous protocol observations, the spread between 

chrome and HVOF is bigger for the hourglass configuration. 

 Figure 3-20. shows the comparison of environments, namely air vs. NaCl. As 

expected, the NaCl curves show a degradation in fatigue performance, and the 

difference between chrome and HVOF is essentially equal to that in air. 

 Figure 3-21. shows the comparison between the .003” and .010” coatings in an 

NaCl environment. Due to the .010” coating thickness, these data not only 

represent a thicker deposit but also a larger diameter bar. Since the bar diameters 

are different, the comparison can be somewhat skewed since larger diameter bars 

sometimes show reduced fatigue performance, as noted above. We cannot 

therefore separate the effects of bar diameter and coating thickness. However, the 

general trends for either thickness value are similar in comparing chrome with 

HVOF. We can conclude that a thicker coating does not provide significantly 

better corrosion protection than the thinner one under fatigue conditions. 
52

A100, SMALL HOURGLASS SPECIMEN 

(0.003" COATING) R = -1, AIR

90

100

110

120

130

140

150

160

170

180

190

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

Bare/UNPeened

Bare/UNPeened/FIT

EHC/UNPeened

EHC/UNPeened/FIT

EHC/Peened

EHC/Peened/FIT

WCCo/UNPeened

WCCo/UNPeened/FIT

WCCo/Peened

WCCo/Peened/FIT

Figure 3-18. A100 - Unpeened vs. Peened Substrate Comparison, 

Hourglass Configuration, R= -1, 0.003” Thickness 

A100, SMOOTH VS HOURGLASS PEENED SPECIMENS

(0.003" COATING), R = -1, AIR

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/Smooth

EHC/Smooth/FIT

WCCo/Smooth

WCCo/Smooth/FIT

EHC/HG

EHC/HG/FIT

WCCo/HG

WCCo/HG/FIT

Figure 3-19. A100 – R= -1 vs. R=0.1 Comparison, Hourglass Configuration, 

0.003” Thickness 
53

A100, R = -1, SMALL HOURGLASS SPECIMEN

AIR VS. NaCl ENVIRONMENT

120

130

140

150

160

170

180

190

200

1.E+04 1.E+05 1.E+06 1.E+07

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/Air

EHC/Air/FIT

EHC/NaCl

EHC/NaCl/FIT

WCCo/Air

WCCo/Air/FIT

WCCo/NaCl

WCCo/NaCl/FIT

Figure 3-20. A100 – Air vs. NaCl Environment Comparison, Hourglass 

Configuration, R = -1 

A100, R = -1, NaCl

LARGE (0.010") VS. SMALL (0.003") HOURGLASS SPECIMEN

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

SmHG/EHC

SmHG/EHC/FIT

SmHG/WCCo

SmHG/WCCo/FIT

LgHG/EHC

LgHG/EHC/FIT

LgHG/WCCo

LgHG/WCCo/FIT

Figure 3-21. A100 - .003” vs. .010” Thickness Comparison, Hourglass 

Configuration, NaCl Environment, R= -1, 0.250” and 0.500” Diameter 

Bars, Respectively 
54

3.3.6.3.300M substrate 

General comments on the comparisons for 300M are: 

 Figure 3-22. shows the comparison of unpeened vs. peened data. As with 4340, 

there is little differentiation between chrome and HVOF in the peened condition 

and the unpeened HVOF is identical to the two peened conditions. Unpeened 

EHC is substantially lower, as expected, and is little different from the unpeened, 

uncoated baseline. Again, the hourglass configuration appears to show more 

sensitivity in differentiating chrome vs. HVOF in the unpeened condition. 

 Figure 3-23. shows the comparison of bar configurations. In contrast to the 4340, 

the hourglass and smooth performance is almost identical. 

 Figure 3-24. shows the comparison between the .003” and .010” coatings in an 

NaCl environment. As before, we cannot separate the effects of coating thickness 

and bar diameter. 

 Figure 3-25. shows the comparison of environments, namely air vs. NaCl. As 

expected, the NaCl curves show a degradation in fatigue performance but the 

differences between chrome and HVOF are essentially equal to those in air. 

 There is no measurable difference between EHC and WC-Co. 

300M, SMALL HOURGLASS SPECIMEN

(0.003" COATING) R = -1, AIR

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

Bare/UNPeened

EHC/UNPeened

EHC/UNPeened/FIT

EHC/Peened

EHC/Peened/FIT

WCCo/UNPeened

WCCo/UNPeened/FIT

WCCo/Peened

WCCO/Peened/FIT

Figure 3-22. 300M - Peened vs. Unpeened Substrate Comparison, 

Hourglass Configuration, 0.003” Thickness, R= -1 
55

300M, SMOOTH VS HOURGLASS PEENED SPECIMENS

(0.003" COATING), R = -1, AIR

100.0

110.0

120.0

130.0

140.0

150.0

160.0

170.0

180.0

190.0

200.0

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

EHC/Smooth

EHC/Smooth/FIT

WCCo/Smooth

WCCo/Smooth/FIT

EHC/HG

EHC/HG/FIT

WCCo/HG

WCCo/HG/FIT

Figure 3-23. 300M – Smooth Bar vs. Hourglass Bar Comparison, 

0.003” Thickness, R= -1 

300M, R = -1, NaCl

LARGE (0.010") VS. SMALL (0.003") HOURGLASS SPECIMEN

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

CYCLES TO FAILURE, Nf

ENGINEERING STRESS MAX, KSI

SmHG/EHC

SmHG/EHC/FIT

SmHG/WCCo

SmHG/WCCo/FIT

LgHG/EHC

LgHG/EHC/FIT

LgHG/WCCo

LgHG/WCCo/FIT

Figure 3-24. 300M - 0.010” Thick Coating on 0.500” dia 

Hourglass vs. 0.003” Thickness, 0.250” dia Hourglass, R= -1 
56

3.3.7. Coating integrity 

As noted above, the coating integrity analysis (cracking and spalling) performed in this 

testing was primarily post-test since this issue was highlighted as a major concern only as 

the final bars were being finished in the protocol. 

Cracking of the coating as shown in Figure 3-26 was observed on all bars essentially at or 

above the 150 ksi stress level. Spalling behavior is summarized in Table 3-24. For the 

small hourglass configuration and the .003” thickness, the majority of the bars showed 

spalling of the HVOF coating only near the fracture plane, and only after fracture, as 

depicted in Figure 3-27. However, in some cases there was total spalling on fracture, as 

shown in Figure 3-28, which was especially evident for 0.003” WC-Co on the A100 

substrate material. 

For the large hourglass specimens with the .010” coating, spalling was not observed prior 

to failure, but total spalling occurred on all specimens at failure regardless of the 

substrate type or test environment. 

300M, R = -1, SMALL HOURGLASS SPECIMEN

AIR VS. NaCl ENVIRONMENT

100

110

120

130

140

150

160

170

180

190

200

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 CYCLES TO FAILURE, Nf ENGINEERING STRESS MAX, KSI

EHC/Air

EHC/Air/FIT

EHC/NaCl

EHC/NaCl/FIT

WCCo/Air

WCCo/Air/FIT

WCCo/NaCl

WCCo/NaCl/FIT

Figure 3-25. 300M - Air vs. NaCl Environment, Hourglass, 0.003” 

Thickness, R= -1 
57

Figure 3-26. Cracking of HVOF Coating in Central Region of Hourglass 

Specimen

Figure 3-27. Failed 300M Specimens Showing Spalling in 

Small Zone Near Fracture Surface 
58

Figure 3-28. Failed 300M Specimens Showing Total Spallation on 

Fracture 
59

3.3.8. Discussion of trends in fatigue results 

The following are general trends: 

 In all cases HVOF fatigue performance was equal to (and in many cases better 

than) chrome. 

 The fatigue performance of smaller specimens with 0.003” coatings was better 

than that for the larger specimens with the 0.010” coatings in both air and NaCl 

test environments. However, since different bar diameters can also affect fatigue 

life, we cannot confidently ascribe this effect to coating thickness. 

 Fatigue life was significantly lower for R= -1 than for R= 0.1, as would be 

expected for full stress reversal. 

Table 3-24. Coating Integrity - Summary of Analysis

R Enviro

nment 

Coating Appearance at fracture Appearance at runout 

EHC 

(4340 only) 

No spalling. No apparent difference 

between peened and unpeened bars. 

No spalling. No 

apparent difference 

between peened and 

unpeened bars. 

0.1 Air 

WC-Co 

(4340 only). 

Spalling near fracture plane on all 

specimens. No completely spalled 

bars. 

Little/no evidence of 

cracking or spalling. 

EHC No spalling. No apparent difference 

between peened and unpeened bars. 

No spalling. No 

apparent difference 

between peened and 

unpeened bars. 

-1 Air 

WC-Co Spalling near fracture plane. 

Total spalling: 

4340 – 0/20 peened & unpeened 

300M – 4/20 peened, 2/20 unpeened 

A100 – 13/20 peened, 0/20 unpeened 

Cracking/multicracking 

present. Occasional 

spalling evident. 

(4340) 

No run-outs for 300M 

or A100 

-1 NaCl EHC No spalling N/A 

WC-Co 4340: Spalling near fracture plane on 

all bars 

300M: 14/20 bars not spalled 

A100: Spalling on all bars, 1/20 

completely spalled 

N/A 
60

 Surprisingly, there was little difference in hourglass vs. smooth configurations, in 

contrast to earlier protocols where the hourglass showed a more substantial 

degradation with chrome as compared to HVOF. The differences between 

smooth and hourglass configurations appear to be accentuated at R= 0.1, and are 

less evident at R= -1, where most of the JTP testing was done. 

 While many HVOF coatings spalled on fracture, only limited cases of spalling 

were observed on runout specimens (i.e. specimens that had not failed). This was 

true for both hourglass and smooth bars. Macroscopic cracking of the HVOF 

coating was evident in some of the runout bars at R= -1, but only microcracking 

was observed at R= 0.1. 

 If cracking of the HVOF coating did occur during testing in NaCl, it did not 

degrade fatigue performance as compared to chrome. 

Shot peening: Because of the known fatigue debit for hard chrome, high strength steels 

are always required to be peened prior to coating. This causes a very large improvement 

in fatigue performance. Note, however, that the improvement in performance for 

unpeened HVOF over unpeened chrome is generally quite large, whereas for peened 

steels the difference is much less pronounced (see, Figure 3-13. for example). The reason 

for this is believed to be that the hard WC particles in the HVOF spray have the effect of 

shot peening the surface of the steel at the beginning of coating deposition and then shot 

peening the surface of the coating as it becomes thicker. This results in improved fatigue 

performance of the substrate and higher compressive stress in the coatings. It is 

important to remember that this peening effect is expected to be much less important 

when depositing alloys such as T400, where the spray does not include hard, unmelted 

particles. 

3.3.9. Significance 

Military landing gear components obviously are subjected to both static and cyclic 

loading. As the aircraft move from land-based to carrier-based situations, environmental 

factors can influence performance. The fatigue testing is significant in establishing that 

HVOF coatings do not degrade substrate fatigue, with or without corrosion. 

However, the data established in this protocol and subsequent evaluations have identified 

the need for coating integrity (cracking and spalling) measurements to optimize coating 

performance and establish the load-limiting factors. From initial additional testing, 

coating integrity appears to be a concern primarily for thick coatings (i.e. for repair), and 

of much less concern for thin (OEM) coatings. Further evaluations are in progress and 

are planned to accurately establish this performance characteristic, and the results will be 

presented in a subsequent report. 

3.3.10. Conclusions 

The WC-Co coatings meet the JTP acceptance criterion of fatigue performance 

equal to or better than chrome plating in both air and NaCl environments. This 

conclusion is true for both .003” and .010” thicknesses. 
61

The issue of coating integrity (cracking and spalling) has been identified and is being 

addressed in parallel programs to establish application and load ranges for HVOF 

coatings. 

3.4. Corrosion data 

3.4.1. Data summary 

3.4.2. Pre-JTP atmospheric corrosion testing 

As discussed in the Introduction to this report, some generic fatigue and corrosion testing 

were conducted prior to the initiation of the landing gear project [3.2]. ASTM B117 salt 

fog testing and atmospheric corrosion testing were conducted on flat panels of 4340 steel, 

7075 aluminum alloy, and PH13-8 stainless steel coated with HVOF WC-Co (83%/17%), 

HVOF Tribaloy 400 (composition 60%Co, 28%Mo, 9%Cr, and 3%Si), and EHC. 

Coating thickness was nominally 0.004 inches. The results of the B117 testing after 1000 

hours of exposure indicated that the WC-Co and EHC demonstrated equivalent 

performance, with the Tribaloy 400 demonstrating slightly inferior performance [3.2, 

3.3]. Although it was intended that this JTR would report results only for those tests 

delineated in the Landing Gear JTP, because the corrosion results were inconsistent 

between the earlier generic testing and that conducted under the JTP, it was decided that 

the results of the atmospheric corrosion tests just for the same substrate material as used 

in the JTP would also be presented in this report. The deposition parameters used for the 

coatings in the earlier atmospheric corrosion studies were very similar to those used in 

Table 3-25. Quick Data Locator (Click on item number to view) 

Item Item number 

Test matrix Table 3-31 

JTP data – protection ratings on 4340 Figure 3-35, 

Figure 3-36 

JTP data – protection ratings on 300M Figure 3-41, 

Figure 3-42 

JTP data – protection ratings on Aermet 100 Figure 3-44, 

Figure 3-45 

Supplementary data – Comparison of appearance and protection ratings 

on 4340 plate 

Figure 3-54 

Supplementary data – Appearance ratings for 4340 rod Figure 3-55 

Supplementary data – Protection ratings for 4340 rod Figure 3-57 

Supplementary data – Appearance ratings for 300M rod Figure 3-56 

Supplementary data – Protection ratings for 300M rod Figure 3-58 
62

the tests for the JTP. 

3.4.2.1.Specimen preparation 

Samples of 4340 steel (dimensions 4” x 6” x 0.2” thick) were obtained from commercial 

sources. Prior to deposition of the coatings, the samples were degreased and then grit 

blasted with 180 - 220 grit aluminum oxide for those to receive EHC and with 54 grit 

aluminum oxide for those to receive the HVOF coatings. The angle of impingement was 

90 degrees. 

 EHC was applied to samples in accordance with United States military standard 

MIL-STD-1501. 

 The 83WC/17Co coatings were deposited at the Naval Aviation Depot in 

Jacksonville, Florida using a Sulzer Metco Diamondjet hybrid HVOF system with 

hydrogen as the fuel gas. The 83WC/17Co powder material was Sulzer-Metco 

Diamalloy 2005 with a composition of 83% WC and 17% Co. 

 The Tribaloy 400 coatings were deposited at Southwest Aeroservice, Inc. in 

Tulsa, Oklahoma using a Stellite Jet Kote II HVOF system with hydrogen as the 

fuel gas. The Tribaloy 400 powder material was designated as T400 and was 

manufactured by Stellite with a composition of 60% Co, 28% Mo, 9% Cr, 3% Si. 

All of the HVOF coatings were deposited such that they retained residual compressive 

stress as indicated on an Almen N strip, with deflections ranging from 0.005” to 0.010”. 

The porosity was less than 1.5% and the oxide content was less than 2.5%. The average 

Vickers microhardness for the coatings (100 gram load) was 950 for the EHC, 1150 for 

the WC-17Co, and 700 for the T400. The nominal thickness for all of the coatings was 

0.004”. 

All of the coatings were tested in the as-deposited condition, with no surface finishing or 

grinding conducted subsequent to deposition. At least five samples were produced for 

each coating/substrate combination. The T400 coatings covered the entire sample 

including the edges. For the EHC, the coatings were applied to an area of 4” x 4 “, with 

both faces and edges within that area being coated. For the WC-17Co on the 4340 

substrates, the coatings were applied to one face and the edges but not the reverse face. 

For all samples, any uncoated areas were sealed using epoxy. 

The samples were placed in the atmospheric exposure rack at the Naval Research 

Laboratory facility in Key West, FL. The exposure racks were located approximately 50 

yards from the ocean, with the samples facing the ocean at an angle of 45 degrees from 

the horizontal. The test duration was three years. 
63

3.4.2.2.Experimental Procedures 

After exposure, the samples were cleaned in 

accordance with ASTM B537. After the corrosion 

products were removed, blisters and undercutting of 

the coating were noted. The blisters and undercut 

coating were mechanically removed to better assess 

the extent of the corrosion, i.e., determine the area of 

the substrate that was no longer protected by the 

coating. Then each sample was assigned a protection 

rating based on the ASTM B537 standard, which is 

summarized in Table 3-26. This standard only 

considers the face of the samples for rating, but in this 

work the sample edges were also assigned a protection 

rating. The protection rating is an indication of how 

well the coating protected the substrate. 

3.4.2.3.Results and Discussion 

Figure 3-29 and Figure 3-30 show a series of photos 

taken before and after cleaning, respectively, of the 

coatings on 4340 steel after the three-year atmospheric 

exposure. Once the samples were cleaned, it was 

possible to identify surface defects such as blisters or 

pits. Removing the blisters and portions of the coating that were undercut by corrosion 

provided a better representation of the affected areas as shown in Figure 3-30. This 

surface condition was used to determine the protection ratings for the front face and 

edges. 

Table 3-28 and Figure 3-31 present the protection ratings for the coatings on 4340 steel. 

The front face of all samples coated with T400 showed significant undercutting, with the 

percentage of the surface affected ranging from 10 to 50. The protection ratings for the 

face of individual samples ranged from 1 to 3, with an average of 1.9. For the edges, 

greater than 50% were corroded with a significant amount of undercutting of the coating. 

The average protection rating for the edges was 0. The T400 samples were very heavily 

corroded near the edges and it appeared that the undercutting started at the edges and 

moved toward the center of the face. 

The front face of all EHC samples showed that the coating was severely undercut similar 

to the T400. The protection ratings for individual samples ranged from 0 to 2, with an 

average of 0.5. The edges were also heavily corroded with significant undercutting. The 

average protection rating was 0.2 for the edges with the individual values ranging from 0 

to 1. 

The face and edges of the WC-17Co samples showed no sign of corrosion. The 

protection rating for the individual samples, both the front face and edges, was 10. In 

general, the surface of the WC-17Co coatings became darker in color during exposure. 

Upon cleaning the dark layer was removed and it was apparent that the coating itself had 

undergone some general corrosion. This was probably due to dissolution of the cobalt 

Table 3-26. Protection 

Rating Versus Area of Defect 

from ASTM B 537 

Area of defect (%) Rating 

0 10 

0 to 0.1 9 

0.1 to 0.25 8 

0.25 to 0.5 7 

0.5 to 1.0 6 

1.0 to 2.5 5 

2.5 to 5.0 4 

5 to 10 3 

10 to 25 2 

25 to 50 1 

greater than 50 0 
64

binder. The HVOF WC-17Co coating clearly outperformed the EHC and HVOF T400 

coatings in terms of providing protection to both the face and edges. 

As mentioned above, the results of ASTM B117 testing of samples identical to those that 

were subjected to the atmospheric exposure were previously reported [3.2]. Table 3-27 

provides a summary of the B117 results which were significantly different from the 

atmospheric corrosion results. In the B117 tests, both the WC-Co and EHC performed 

equivalently, with fairly extensive corrosion evident. Both had an average protection 

rating for the faces of about 3 after 1,000 hours exposure. The T400 had an average 

protection rating of 1.6. However, as discussed above, in the atmospheric corrosion tests, 

the HVOF WC-Co coating protected the steel significantly better than the other two 

coatings. 

Figure 3-29. HVOF WC-Co, Tribaloy 400 (T400) and EHC (Cr) on 4340 Steel 

Following Three-year Atmospheric Exposure (Samples shown prior to cleaning) 
65

Figure 3-30. HVOF WC-Co, Tribaloy 400 (T400) and EHC (Cr) on 4340 Steel 

Following Three-year Atmospheric Exposure (Samples shown subsequent to 

cleaning) 

0 2 4 6 8 10 12

WC/Co *

Triballoy 400 **

Hard Chrome

Protection Ratings (10 = Best)

Edge

Face

Figure 3-31. Protection Ratings for EHC, HVOF Tribaloy 400, and HVOF 

WC-17Co on 4340 Steel after Three-year Atmospheric Exposure 
66

3.4.3. Rationale for JTP testing 

With concurrence of the stakeholders the corrosion, a non-standard specimen shape was 

used in an attempt to better represent the typical rod-shaped components on which HVOF 

would most likely be employed. The specimen shape was a 1” diameter round rod 6” 

long, scribed when necessary. Both DoD standard ASTM B117 and GM 9540 cyclic 

tests were done to simulate service conditions. Because Boeing usually specifies a Ni 

sublayer beneath the EHC, and a sealant applied to the coating surface, while most depots 

deposit EHC directly on the substrate and do not apply a sealer, the test matrix 

incorporated combinations of these various conditions. 

Table 3-27. Average Value of the Protection Rating for Samples 

that had Undergone 1,000 Hours of ASTM B117 Testing 

Coating Substrate Protection - Face Protection - Edge 

T400 4340 steel 1.6 1.0 

83WC/17Co 4340 steel 3.4 3.2 

EHC 4340 steel 3.2 2.0 

Table 3-28. Average Value of the Protection Rating for Samples 

that had Undergone a Three-year Atmospheric Exposure 

Coating Substrate Protection - Face Protection - Edge 

T400 4340 steel 1.9 0 

83WC/17Co 4340 steel 10.0 10.0 

EHC 4340 steel 0.5 0.2 
67

3.4.4. Landing gear original JTP testing 

3.4.4.1. JTP specimen fabrication and deposition methodology 

A. Electrolytic hard chrome 

(EHC) was deposited in 

accordance with MIL-STD-1501, 

supported by QQ-C-320. The 

thicknesses was 0.003” and 

0.010” +/- 0.0005” subsequent to 

grinding (i.e., coatings were 

deposited approximately 0.002” 

to 0.003” thicker than specified 

and then ground to final 

dimension). The coatings were 

applied to the curved surfaces of 

the rods over a length of 4.75” 

(see Figure 3-32). It was not 

necessary to apply the coatings to 

the ends. A sulfamate Ni 

underlayer was applied to a 

minimum thickness of 0.0015” in 

accordance with specification 

QQ-N-290 on some of the rods that received the EHC coating (see test matrix, 

Table 3-31). Subsequent to deposition, each EHC coating was ground to a 

surface finish of 16 microinches (+0/-4 microinches) using low-stress grinding 

techniques in accordance with specification MIL-STD-866. Subsequent to 

grinding, on some of the EHC coatings (see test matrix, Table 3-31), a 

polystyrene resin impregnation sealer was applied in accordance with P.S. 11501. 

B. WC-17Co HVOF coatings were deposited in accordance with Boeing 

specification BAC 5851, Class 2, Type I, while WC-10Co4Cr HVOF coatings 

were deposited in accordance with Boeing specification BAC 5851, Class 2, Type 

XVII with the following additions or clarifications. The HVOF coatings were 

deposited using a Sulzer Metco Diamondjet hybrid gun with hydrogen as the fuel 

gas. The WC-Co powder material was Sulzer-Metco Diamalloy 2005 and the 

WC-CoCr powder material was Sulzer-Metco 5847. Uniform deposition 

conditions were utilized for all specimens. Air cooling was utilized to ensure the 

specimen surface temperature did not exceed 350o

 F. The thicknesses was 0.003” 

and 0.010” (+/- 0.0005”) subsequent to grinding (i.e., coatings were deposited 

approximately 0.002” to 0.003” thicker than specified and then ground to final 

dimension). All HVOF coatings were deposited to an Almen number of 3-12N. 

Subsequent to deposition, each coating was ground to a surface finish of 8 

microinches (+0/-2 microinches) in accordance with specification BAC 5855 with 

the following modifications: 

 Spec. paragraph 8.3.b.(1): If the excess coating thickness was less than 

0.004”, then rough grinding was not required. A minimum of 0.002” 

Not shot peened

Mask approx 3/4”

1.25” +

top”

Coat

5”

length+

bottom

Grit blast

4.75” 5” 1/4” + bottom

Figure 3-32. Corrosion Coupon, 1” 

Diameter Bar 
68

stock removal (per side, or 0.004” on diameter) was required for finish 

grinding. The finishing in-feeds did not exceed a maximum of 0.0005” for 

100 or 120 grit, 0.0004” for 150 grit, 0.0003“for 180 grit, 0.0002” for 220 

grit, or 0.0001” for 320 or 400 grit. 

 Spec. paragraph 8.3.b.(3): A finishing cross feed or traverse rate of 1/8 to 

1/12 wheel width per workpiece revolution was used. 

 Spec. paragraph 8.3.c.(4): Hardness – L, M, N, P or R 

 Spec. paragraph 8.3.d: When grinding ID or OD surfaces, the work had a 

speed of 50 to 100 surface feet per minute. 

 Subsequent to grinding, on some of the HVOF coatings a Metco URS 

sealer was applied (see test matrix, Table 3-31). 

Subsequent to coating and grinding, 

an inert epoxy (Mil-P-24441 formula 

150 and 152) was placed on the ends 

of the specimens to ensure no 

galvanic couple existed between the 

coated and uncoated sections. An 

arrow was scribed on the upper end 

of each specimen indicating the “up” 

direction during the corrosion tests. 

For those specimens with the 

scratches, the “up” position 

corresponded to the linear scratch 

being uppermost during the corrosion 

test. 

One specimen in each group of five 

had multiple scratches made in the 

coating. These scratches were made 

using a diamond indenter (such as a Rockwell C indenter) and the scratches were such 

that they penetrated through the coating into the substrate. Two scratches were made 

around the circumference of the specimen 3-inches apart and a linear scratch was made 

3.5” inches in length, crossing both circumferential scratches as shown in Figure 3-33. 

3.4.4.2. JTP test methodology 

Specification: ASTM B117, “Standard Practice for Salt Spray (fog) Apparatus, 

Operating”; GM 9540 P/B, “GM Corrosion Test” 

Modifications: Round bar specimens in place of standard flat plate. 

Significance: Landing gear are subject to corrosion environments. (Note, 

however, that because of its microcracked surface, EHC is not 

generally used by itself for corrosion resistance.) 

Sample types: 1” dia rods, 6” long, Circumference ground to a surface finish of 

32-64 microinches prior to coating. 

Figure 3-33. Illustration of Scratched 

Corrosion Test Specimen 
69

Sample materials: 4340, 300M, Aermet 100. 1”-diameter round bar. All bar stock 

for each material came from the same lot. 

Sample heat treat: Vacuum heat treated specimens to the same tensile strength as for 

fatigue specimens (260-280 ksi for 4340; 280-300 ksi for 300M; 

280-290 ksi for A100). 

Pre-coating prep: Shot Peen: The circumference was shot peened in accordance with 

AMS-2432 under computer control using wrought steel, S230, 

Almen 8-10A. All curved surfaces were shot peened to 100% 

coverage, with the exception of a 0.75”-inch length at the end of 

each rod. 

Grit blast prior to coating: For the EHC-coated specimens, a 5” 

length was grit blasted with 180-220 grit aluminum oxide or glass 

beads in accordance with standard procedure for hard chrome 

plating (see Figure 3-32). For HVOF coated specimens a 5” length 

was grit blasted with 54 grit aluminum oxide at 60 psi at 90° angle 

of impingement in accordance with MIL-STD-1504. 

Coatings: EHC, WC-Co, WC-CoCr, ground 

Coating thicknesses: 0.003” and 0.010” 

Test equipment: Q-Fog Model CCT600 or equivalent salt spray chamber at ambient 

temperature. Specimen holders (made from an inert material such 

as teflon) were made to place the rods with at least 4” of the 

specimen extending out from the holder, at 45 degrees to the 

vertical. The scribed arrows on the ends always pointed vertically 

to keep the scribe uppermost. 

3.4.4.2.1. Test Description 

Three types of corrosion tests were conducted. 

1. Salt Fog Corrosion Test. This test was conducted in accordance with 

ASTM B117-94. In this test, the samples were exposed to a salt fog made 

from a 5% NaCl solution maintained within a pH range of 6.5 and 7.2. 

The temperature in the salt fog chamber was held at 35o

C. The specimens 

were visually inspected for surface corrosion after 125 hours, and every 

125 hours thereafter. The samples were removed at 500 and 1,000 hours 

for photographing. 

2. GM Cyclic Corrosion Test. This test was conducted in accordance with 

GM9540P/B protocol, which is indicated in Table 3-29. At each 500 hour 

interval, the specimens were removed from the test chamber, inspected, 

and photographed. 

3. SO2 Salt Fog Test. This test was conducted in accordance with ASTM 

G85-85. The samples were visually inspected at 48 hours, 96 hours, and 

every 100 hours between 200 and 1,000 hours. After 500 and 1,000 hours 

exposure the specimens were removed and photographed. 
70

Based on visual inspection, a ranking was applied to each specimen at each interval of 

inspection and these rankings were tabulated and displayed graphically. The rankings 

were assigned in accordance with ASTM B537-70 as in Table 3-30. In general, when the 

ranking of a specimen reached a value of 1, it was considered to have failed the corrosion 

test and was removed from the test cabinet. 

The corrosion test matrix is given in Table 3-31. 

Table 3-29. GM 9540 Test Protocol 

Solution: 0.9% NaCl, 0.1% CaCl2 and 

0.25% NaHCO3

pH: 6.0 – 8.0 

Test protocol: Atmosphere Temp. Time 

Step 1 Sub-cycle step 2-3 repeat 4 times 

Step 2 Salt mist 25C 15 min 

Step 3 Dry-off 25C 75 min 

Step 4 Dry-off 25C 120 min 

Step 5 RH 95-100% 49C 8 hours 

Step 6 Dry-off 60C 7 hours 

Step 7 Dry-off 25C 1 hour 

Step 8 Final step, go to step 1 

Note: RH = relative humidity 

Test duration: 2000 hrs 

Examined every 125 hrs 

Table 3-30. Table of 

Protection Rankings/Ratings 

Defect area (%) Rank # 

0 10 

>0 – 0.1 9 

>0.1 – 0.25 8 

>0.25 - 0.5 7 

>0.5 – 1 6 

>1 - 2.5 5 

>2.5 – 5 4 

>5 – 10 3 

>10 – 25 2 

>25 – 50 1 

>50 0 
71

Table 3-31. Corrosion Test Matrix

Test Material Coating Thickness Ni sublayer Sealed # specimens 

B117 4340 Uncoated N/A N/A N/A 5 

B117 4340 EHC 3 No No 5 

B117 4340 EHC 3 Yes No 5 

B117 4340 EHC 3 No Yes 5 

B117 4340 EHC 10 No No 5 

B117 4340 EHC 10 No Yes 5 

B117 4340 EHC 10 Yes No 5 

B117 4340 WC/Co 3 no No 5 

B117 4340 WC/Co 3 no Yes 5 

B117 4340 WC/Co 10 no No 5 

B117 4340 WC/Co 10 no Yes 5 

B117 4340 WC/Co-Cr 3 no No 5 

B117 4340 WC/Co-Cr 10 no No 5 

 

B117 300M EHC 3 no No 5 

B117 300M EHC 3 no Yes 5 

B117 300M EHC 3 yes No 5 

B117 300M EHC 10 no no 5 

B117 300M WC/Co 3 no no 5 

B117 300M WC/Co 3 no yes 5 

B117 300M WC/Co 10 no no 5 

 

B117 Aermet 100 EHC 3 no no 5 

B117 Aermet 100 EHC 3 no yes 5 

B117 Aermet 100 EHC 3 yes no 5 

B117 Aermet 100 EHC 10 no no 5 

B117 Aermet 100 WC/Co 3 no no 5 

B117 Aermet 100 WC/Co 3 no yes 5 

B117 Aermet 100 WC/Co 10 no no 5 
72

Table 3-31. Corrosion Test Matrix (continued) 

Test Material Coating Thickness Ni Sublayer Sealed # specimens 

GM 300M EHC 3 no no 5 

GM 300M EHC 3 no yes 5 

GM 300M WC/Co 3 no no 5 

GM 300M WC/Co 3 no yes 5 

GM Aermet 100 EHC 3 no no 5 

GM Aermet 100 EHC 3 no yes 5 

GM Aermet 100 WC/Co 3 no no 5 

GM Aermet 100 WC/Co 3 no yes 5 

 

SO2 4340 Uncoated N/A N/A N/A 5 

SO2 4340 EHC 3 no no 5 

SO2 4340 EHC 3 no yes 5 

SO2 4340 EHC 3 yes no 5 

SO2 4340 EHC 10 no no 5 

SO2 4340 WC/Co 3 no no 5 

SO2 4340 WC/Co 3 no yes 5 

SO2 4340 WC/Co 10 no no 5 

SO2 4340 WC/Co-Cr 3 no no 5 

SO2 4340 WC/Co-Cr 10 no no 5 

 

SO2 300M EHC 3 no no 5 

SO2 300M EHC 3 no yes 5 

SO2 300M EHC 3 yes no 5 

SO2 300M WC/Co 3 no no 5 

SO2 300M WC/Co 3 no yes 5 

 

SO2 Aermet 100 EHC 3 no no 5 

SO2 Aermet 100 EHC 3 no yes 5 

SO2 Aermet 100 EHC 3 yes no 5 

SO2 Aermet 100 WC/Co 3 no no 5 

SO2 Aermet 100 WC/Co 3 no yes 5 

 Total 275 
73

3.4.4.3.Test results 

As of the date of writing this report, the SO2 corrosion testing had been completed but 

analysis of the samples was still in progress. Therefore, the results presented herein will 

be only for the B117 salt fog and GM9540 cyclic corrosion testing. 

Appearance ratings for the samples were determined at 125 hour intervals during the 

B117 test. After exposure to the accelerated tests, all specimens were removed from the 

salt fog cabinet and were cleaned with a Scotch 3M abrasive pad to remove loosely 

adherent corrosion products then dried with cotton cloth. The corrosion product was 

removed using the abrasive pad because in many instances it was so voluminous that it 

masked the surface. After cleaning, it was possible to identify surface defects such as 

blisters or pits (see Figure 3-34 as an example). Removing the blisters and portions of 

the coating that were undercut by corrosion provided a better representation of the area 

that was affected by corrosion. A protection rating for the coating, i.e. how well it 

protected the substrate, was then determined. The appearance ratings are not reported 

here as they are not as good a metric for the degree of corrosion as the protection rating. 

The ability of the coating to protect the substrate and the corrosion of the coating were 

considered when assigning a protection rating for the HVOF coatings. Protection ratings 

for the EHC coatings only considered defects in the coating as the coating did not show 

any visual signs of corrosion. The four replicate samples were examined and given a 

protection rating (0-10) in accordance with ASTM B 537 as indicated above. The 

arithmetic average of these four samples was computed and used for all data analysis. 

The scribed samples were handled differently; the maximum creep in millimeters from 

the center of the scribe line was recorded for the vertical and separately for the horizontal 

scribes. This information was tabulated and used in the subsequent data analysis. The 

scribe creepage between the horizontal and vertical scribes were seen to be comparable in 

all but two sample groups and only the vertical scribe creep was used for the comparison 

of the various coatings’ performance (with the exceptions noted). In several specimens, 

corrosion had severely damaged the sample and it was uncertain whether any of the 

applied coating was intact near the scribe. To confirm the nature of the corrosion 

damage, both optical and scanning electron microscopy were performed on these 

samples. 

The examination of the samples following the 1,000-hour exposure clearly showed that 

the base alloy influenced the corrosion behavior. As a result, the discussion of the results 

is divided into sections according to the base alloy. Appendix 3 of the JTR [3.1] contains 

the individual protection ratings and scribe creep values for all samples. 

3.4.4.3.1. 4340 Steel in ASTM B-117 

The experimental results for these samples are presented on the following pages in both 

bar chart (Figure 3-35) and radar plot form (Figure 3-36). In general, the EHC coating 

outperformed the WC-Co coating in every direct comparison. Figure 3-38 shows the 

4340 steel with the various 0.010” hard chrome coatings. The overall best performance 

for a coating to protect this base metal was the EHC with a nickel sublayer. This 

combination achieved the highest protection ratings of any coating. The use of an 
74

organic sealer generally enhanced the performance of a given coating system but the 

overall performance levels of any coating combination with sealer did not match the 

performance of EHC with the nickel sublayer. Some modest enhancement in corrosion 

performance was seen by increasing the coating thickness; 0.010”-thick coatings 

generally performed better than the same coating composition at 0.003” in thickness. 

The 0.003”-thick WC-Co coatings without sealer performed poorly, receiving a rating of 

1. The coating was cracked and severely undercut. The 0.010”-thick coating without 

sealer received a rating of 2. In this case, there were no cracks in the coating or any signs 

that the coating had been breached. However, corrosion of the coating had occurred. 

The samples had a mottled appearance with portions of the coating corroding but the rest 

of the coating not affected. Under a light microscope, the attack in the corroded regions 

appeared to be a generalized dissolution. This type of attack on the coatings was also seen 

with the other unsealed HVOF coating/substrate combinations. A possible opportunity 

for improvement of the WC-Co was seen by the addition of chromium to the composition 

(see Figure 3-35, Figure 3-36, and Figure 3-37). The 0.003”-thick WC-CoCr had 

portions of the coating that were removed and some undercutting. Corrosion of the 

coating also occurred and the average protection rating was 3.75. The 0.010”-thick WCCoCr without sealer had a protection rating of 7.75. In this series, three of the samples 

showed cracking of the coating in isolated areas and some undercutting of the coating in 

these areas. In two of the latter cases, the epoxy mask at the top of the rod appeared to be 

compromised and this was the site that appeared to be the origin of the undercutting and 

cracking. The corrosion of the 0.010”-thick WC-CoCr coatings was light. The B117 

results clearly demonstrate the performance enhancement of the chromium-containing 

alloy without relying on an organic sealer or on a Ni underlay. Protection ratings of the 

WC-CoCr coating equaled or exceeded those of EHC with no sealer or Ni sublayer. 

Figure 3-39 shows the 0.010”-thick EHC coatings and the 0.010”-thick WC-CoCr 

coating. While the 0.010”-thick EHC with the nickel sublayer was the best overall 

performer with a rating of 9.7, the 0.010”-thick WC-CoCr test specimens certainly 

showed this modification to be promising, receiving an average rating of 7.75 – as good 

as EHC without the Ni underlayer. 

The 0.003”-thick WC-Co with the organic sealer coating provided a protection rating 

which equaled the general performance of the EHC without nickel. This result needs 

confirmation however because the 0.010”-thick WC-Co coating with organic sealer 

showed poor performance. Certainly this is counterintuitive; a 0.010”-thick coating of 

the equivalent composition ought to outperform a 0.003”-thick coating in the same 

corrosion test. The explanation most likely lies in the mechanical integrity of the sealer. 

Both organic sealers evidenced some blistering, which was much more severe in the 

sample with the 0.010”-thick coating. The damage to the organic coating appeared much 

worse for the 0.010”-thick coating, which allowed the salt solution to penetrate and 

undercut this sealer. Once the adhesion between the sealer and the metal clad is 

compromised, corrosion can take place leading to similar results for this tungsten carbide 

coating as with the unsealed versions. Figure 3-40 shows the corrosion that has occurred 

beneath a removed blister. The coating adjacent to the blister is easily peeled away from 

the substrate indicating poor adhesion. 

The samples with the 0.003”- and 0.010”-thick WC-Co coatings and sealer had a scribe 
75

creepage of 12 mm. Without sealer, scribe creepage was 19 mm for the 0.003”-thick 

coating and 15 mm for the 0.010”-thick coating. 

The scribe creepage results for the WC-CoCr coating were distinct from other results. 

Both scribed samples showed large scribe creepage from the horizontal scribes, running 

practically the entire length of the sample. The vertical scribe creepage values were 11 

mm and 18 mm while the maximum creepage from the horizontal scribe was 58 mm and 

47 mm respectively. Severe undercutting of the metal coating was observed and 

disbonding of the coating was evident in both samples. Corrosion products had separated 

the metal coating from the base metal. This occurred at only one location on each sample 

but only at a horizontal scribe line. At present, there is no explanation for this result but it 

seems anomalous in light of the performance of the other WC-Co samples. 

Five uncoated 4340 rods were placed in the B117 test chamber as controls. These rods 

experienced severe corrosion and were removed at 250 hours of exposure. The rating for 

each of these samples was 0. 

3.4.4.3.2. 300M Steel in B 117 

Figure 3-41 and Figure 3-42 present the experimental results in bar chart and radar plot 

formats for the base metal 300M with the EHC and WC-Co coatings after the salt fog 

test. The same general trends observed with the 4340 steel results in salt fog testing were 

seen for this substrate as well. Overall, there were fewer coating variations tested on this 

substrate but the results for those specimens are clear. The general performance of the 

EHC coating exceeds that of the WC-Co. All EHC coatings achieved protection ratings 

of 8 or above, with the EHC with a nickel sublayer achieving an almost perfect protection 

rating. By contrast, the 0.003”-thick WC-Co coating without sealer showed a 0 

protection rating. These samples had severe undercutting and the substrate was heavily 

corroded. Simply increasing the coating thickness to 0.010” did improve the performance 

but the rating, 3.25, was still low. Two of the samples in the latter group had cracks in 

the coating and undercutting occurred. In these latter areas the coating was easily 

removed. The other two of the samples in this group had coatings that were intact. All of 

the unsealed WC-Co coatings showed a degree of corrosion and the corrosion product 

was removed from the samples during the cleaning process. 

The use of an organic sealer with the WC-Co coating did show significant improvement 

in the protection rating. The 0.003”-thick coatings which had such poor performance 

without the sealer, improved to an average protection rating of 8 with sealer application. 

These results should be viewed with some caution however. The organic sealer layer was 

generally intact when the specimens were cleaned but blistering was noted on all these 

samples. The blisters were easily removed. In order to check the adhesion of the organic 

sealer to the underlying tungsten carbide, a water soak tape adhesion test was performed. 

Using 3M brand tape over a 2 cm x 2 cm area, 50% or more of the sealer was removed by 

tape pull normal to the surface, evidencing adhesion problems between the sealer and 

underlying WC-Co. If additional accelerated corrosion testing were performed, it is 

suspected that this organic layer would have failed leading to results similar to those 

above. There is no doubt the presence of an intact sealer layer acts as a barrier but 

adhesion issues should be addressed to ensure this barrier remains effective if WC-Co 

coatings are to be used with this substrate. 
76

The scribe creep for the 0.003”-thick EHC coatings was between 5 and 9 mm whereas it 

was 10mm for the 0.003”-thick unsealed WC-Co coating. The scribe creep was 4 mm for 

WC-Co coating with sealer and 3 mm for the 0.010”-thick WC-Co coating. 

3.4.4.3.3. 300M Steel in GM 9540 P 

The experimental results are tabulated in Table 3-32 for these samples. All coating 

thicknesses were 0.003”. 

The accelerated cyclic corrosion test results indicate the EHC coatings have enhanced 

corrosion protection performance over the WC-Co coatings, although the WC-Co 

coatings did perform well in this test. On average the WC-Co coatings merited an 8 

protection rating but the EHC coatings showed a rating of 10. Figure 3-43 is a photo of 

representative samples from each group. All of the unsealed WC-Co coatings were intact, 

i.e. no cracks or defects exposing the substrate were noted. However, all of the unsealed 

WC-Co samples showed a degree of corrosion and a light corrosion product was removed 

from the samples during the cleaning process. Vertical scribe creep results did not 

differentiate one coating from another, with all samples having a maximum creep of 2 

mm or less. 

3.4.4.3.4. Aermet 100 Steel in B 117 

Figure 3-44 and Figure 3-45 present bar and radar charts that summarize the results for 

this base metal test series. The corrosion testing showed this base metal to be the best of 

the three for corrosion performance. Within the results from salt fog testing for just this 

base metal, EHC was again seen as superior. Consistent results with the previous base 

metal results were seen in that thicker coatings protect more than thinner coatings of the 

same chemical composition. A nickel metal underlayer with EHC offers superb 

resistance to corrosion with all other test samples performing worse. The 0.003”-thick 

WC-Co coatings performed poorly compared to EHC. All of the unsealed WC-Co 

coatings showed a degree of general corrosion and the corrosion product was removed 

from the samples during the cleaning process. The application of an organic sealer to the 

WC-Co did enhance its protection rating, as seen in the previous corrosion results. 

Two of the samples with 0.003”-thick WC-Co HVOF coatings without sealer on Aermet 

100 had one region where the coating had a crack. The cracked region was easily 

Table 3-32. GM Accelerated Corrosion Test Results 

for 300M Steel (Coating thickness 0.003”) 

Coating Sealer 

applied

Protection 

rating 

Vertical scribe 

creep, max (mm) 

Cr No 10 1.5 

Cr Yes 10 2.0 

WC-Co No 8.5 2.0 

WC-Co Yes 8 1.0 
77

removed and showed that some undercutting had occurred. The coating was not 

compromised in the other two samples in this group. All of the unsealed WC-Co coatings 

showed a degree of corrosion and the corrosion product was removed from the samples 

during the cleaning process. The 0.010”-thick WC-Co coatings showed no signs of 

cracks or defects that exposed the substrate. However, all of coatings showed a degree of 

corrosion and the corrosion product was removed from the samples during the cleaning 

process. The corrosion performance of the WC-Co coatings on A100 without sealer was 

better than on the other substrates with the same coating (see, for example, Figure 3-46 

which is a photo of representative samples of the 0.003”-thick WC-Co HVOF coatings 

without sealer on 4340 steel, 300M, and Aermet 100 after the B117 test).

The superior performance of the coatings on Aermet 100 over the other substrate/coating 

combinations was evidenced by the scribe creepage results. Overall, there was almost no 

scribe creepage for the Aermet 100/coating combinations. The only significant deviation 

was the WC-Co coating with the sealer. Even though the overall appearance of the 

sealer-coated WC-Co was better, the scribe creepage increased to 3 mm maximum versus 

less than 1 mm for the other samples in this series. This supports the theory that intact 

organic sealers act as a barrier, but once damaged, they may enhance the corrosion at 

localized points. As with the 300M metal samples, once the organic sealer is damaged, 

adhesion between sealer and metal coating is compromised. 

3.4.4.3.5. Aermet 100 Steel in GM 9540P 

Results from this test set are reported in Table 3-33. All coatings were 0.003” thick. 

Both sets of EHC specimens were rated at 10 while the WC-Co sets were rated at 8 and 9 

for unsealed and sealed, respectively. All of the unsealed WC-Co coatings were intact, 

i.e. no cracks or defects exposing the substrate were noted. However, all of the unsealed 

WC-Co samples showed a degree of corrosion and a light corrosion product was removed 

from the samples during the cleaning process. Scribe creepage results were exceptional 

for this base metal. All test samples showed zero creep on both the horizontal and 

vertical scribes, the best results of the entire set of samples. 

3.4.4.3.6. Comparison of performance of WC-Co and WC-CoCr 

coatings without sealer 

Table 3-34 shows that the unsealed 0.010”-thick HVOF coatings were more effective in 

protecting the underlying substrate from corrosion compared to the 0.003”-thick HVOF 

Table 3-33. GM Accelerated Corrosion Test Results 

for Aermet 100 Steel (Coating thickness 0.003”) 

Coating Sealer 

applied

Protection 

rating 

Vertical scribe 

creep, max (mm) 

Cr No 10 0 

Cr Yes 10 0 

WC-Co No 8 0 

WC-Co Yes 9 0 
78

coatings. The unsealed coatings on the 4340 steel and Aermet 100 were completely 

intact, i.e, there were no cracks and/or defects exposing the substrate. However, the 

overall rating of these samples was lower because the coating had undergone corrosion. 

The WC-CoCr had the least amount of coating corrosion of the unsealed samples 

exposed to the B117 test protocol. Figure 3-47 shows samples with the various 0.010”-

thick HVOF coatings on 4340, 300M and Aermet 100 that are intact. 

Table 3-34. Percentage of Specimens with Intact Coatings and the Overall 

Protection Ratings for HVOF Coatings Without Sealer 

Coating/substrate 0.003” coating 0.010” coating 

% of 

specimens 

with intact 

coating 

Overall 

rating 

% of specimens 

with intact 

coating 

Overall 

rating 

WC-Co on 4340 0 1 100 2 

WC-CoCr on 4340 0 3 25 7 

WC-Co on 300M 0 0 50 2 

WC-CoCr on A100 50 2 100 3 

Figure 3-34. 0.003”-thick EHC Coating + Sealer on 4340 Steel After B117 

Test: a) Before and b) After Cleaning 
79

PROTECTION RATINGS ON 4340 STEEL IN ASTM B 117

0 2 4 6 8 10 12

3mil Cr, No Ni, No Sealer

3mil Cr, Nickel,No Sealer

3mil Cr, No Ni, Sealer

10mil Cr, No Ni, No Sealer

10mil Cr,Nickel, No Sealer

10mil Cr, No Ni, Sealer

3mil WC/Co, No Sealer

3mil WC/Co, Sealer

10mil WC/Co, No Sealer

10mil WC/Co, Sealer

3mil WC/Co/Cr, No Sealer

10mil WC/Co/Cr, No Sealer

Protection Rating

Figure 3-35. Protection Ratings on 4340 Steel After B117 Testing

PROTECTION RATINGS ON 4340 STEEL IN ASTM B 117

0

1

2

3

4

5

6

7

8

9

10

3mil Cr, No Ni, No Sealer

3mil Cr, Nickel,No Sealer

3mil Cr, No Ni, Sealer

10mil Cr, No Ni, No Sealer

10mil Cr,Nickel, No Sealer

10mil Cr, No Ni, Sealer

3mil WC/Co, No Sealer

3mil WC/Co, Sealer

10mil WC/Co, No Sealer

10mil WC/Co, Sealer

3mil WC/Co/Cr, No Sealer

10mil WC/Co/Cr, No Sealer

Figure 3-36. Radar Plot of the Protection Ratings on 4340 Steel After ASTM 

B117 Testing
80

Figure 3-37. HVOF Coatings on 4340 Steel After the B117 Test: a) 0.003” WC-Co, 

b) 0.010” WC-Co, c) 0.010” WC-CoCr, and d) 0.003” WC-CoCr 

Figure 3-38. 0.010”-thick EHC coatings 

on 4340 Steel with: a) Ni Sublayer, b) 

Sealer, and c) EHC Coating Only, After 

B117 Testing 
81

Figure 3-39. Coatings on 4340 Steel After the 

B117 Test: a) EHC Coating Only b) EHC 

with Sealer, c) EHC with Ni Sublayer, and d) 

0.010”-thick WC-CoCr 

Figure 3-40. 0.003”-thick HVOF WC-Co Coating on 4340 Steel with Sealer After Blister 

has been Removed
82

PROTECTION RATINGS ON 300M STEEL IN ASTM B117

0 2 4 6 8 10 12

3mil Cr, No Ni, No Sealer

3m il Cr, Nickel, No Sealer

3m il Cr, No Ni, Sealer

10mil Cr, No Ni, No Sealer

3mil W C/Co, No Sealer

3m il WC/Co, Sealer

10mil W C/Co, No Sealer

Protection Rating

Figure 3-41. Protection Ratings on 300M After ASTM B117 Testing

PROTECTION RATINGS FOR 300M STEEL IN ASTM B 117

0

1

2

3

4

5

6

7

8

9

10

3mil Cr, No Ni, No Sealer

3mil Cr, Nickel, No Sealer

3mil Cr, No Ni, Sealer

3mil WC/Co, No Sealer 10mil Cr, No Ni, No Sealer

3mil WC/Co, Sealer

10mil WC/Co, No Sealer

Figure 3-42. Radar Plot of the Protection Ratings on 300M After ASTM B117 Testing
83

Figure 3-43. 0.003”-thick Coatings on 300M After GM 9540P 

Testing: a) EHC Only, b)WC-Co, c) WC-Co With Sealer, and d) 

EHC With Sealer 

PROTECTION RATING FOR AERMET 100 STEEL IN ASTM B 117

0 2 4 6 8 10 12

3mil Cr, No Ni, No Sealer

3mil Cr,Nickel, No Sealer

3mil Cr, No Ni, Sealer

10mil Cr, No Ni, No Sealer

3mil WC/Co, No Sealer

3mil WC/CO, Sealer

10mil WC/Co, no Sealer

PROTECTION RATING

Figure 3-44. Protection Ratings on Aermet 100 After ASTM B117 Testing
84

PROTECTION RATING FOR AERMET 100 STEEL IN ASTM B 117

0

1

2

3

4

5

6

7

8

9

10

3m il Cr, No Ni, No Sealer

3m il Cr,Nickel, N o Sealer

3mil Cr, No Ni, Sealer

3m il WC/Co, No Sealer 10m il Cr, No Ni, No Sealer

3mil WC /CO, Sealer

10mil WC/Co, no Sealer

Figure 3-45. Radar Plot of the Protection Ratings on Aermet 100 After ASTM B177 

Testing

Figure 3-46. 0.003”-thick WC-Co HVOF coating on 4340 Steel, 300M, and Aermet 100 

After the B117 Test 
85

3.4.5. Additional B117 Corrosion Testing 

3.4.5.1.Background 

It was apparent that there were significant differences between the results that had 

previously been obtained for B117 testing in which the HVOF WC-Co coatings 

performed comparably to the EHC and the results obtained for B117 testing conducted 

under the Landing Gear JTP where the performance of the WC-Co coatings was inferior 

to the EHC. In addition, there were the results for the atmospheric corrosion testing 

discussed in Section 3.4.2 in which the WC-Co coatings performed substantially better 

than the EHC coatings after exposure for three years. 

In order to attempt to determine the reason for the relatively poor performance of the 

WC-Co coatings in the Landing Gear JTP tests, additional corrosion testing was 

conducted. It should first be pointed out that the B117 protocol calls for only flat 

samples to be tested; our use of one-inch-diameter rods was a deviation from the 

protocol. Another difference between the earlier B117 and atmospheric tests and those 

conducted for the Landing Gear JTP was that the former coatings were tested as-sprayed 

and the latter were ground to a surface finish Ra of 8 microinches. Finally, it was 

observed on many of the rod samples tested under the Landing Gear JTP that the epoxy 

sealer that was used to seal the uncoated ends had been breached, which allowed the 

corrosive media to attack the base material, leading to corrosion proceeding underneath 

the WC-Co and subsequent fracturing and spallation of the coatings. 

Figure 3-47. 0.010”-thick HVOF Coatings: a) WC-Co on 4340, b)WC-CoCr on 

4340, c) WC-Co on 300M, and d) WC-Co on Aermet 100 After the B117 Test 
86

3.4.5.2.Specimen Preparation and Coating Deposition 

To attempt to address these issues, additional corrosion testing was performed. This 

involved specimens that consisted of one-inch-diameter 4340 and 300M rods, and 3” by 

4” 4340 plates. To ensure there were no differences between WC-Co coatings deposited 

at different facilities using supposedly the same parameters, two different vendors were 

used, Hitemco and Southwest Aeroservice. Southwest Aeroservice also deposited the 

EHC coatings. 

The sample preparation (e.g., shot peening, grit blasting) and coating deposition 

parameters were identical to those used for the Landing Gear JTP samples. For the sixinch-long rods, the area on each sample that was coated was slightly different than for the 

JTP samples. As before, the flat ends were not coated. On the curved surface, at one end 

of the rod, the coating terminated one-inch from the end. At the other end, the coating 

terminated one-quarter-inch from the end. Thus, the actual length of the coated surface 

was 4-3/4”. On the plates, only one face was coated. On that face, the three-inch-wide 

dimension was coated edge-to-edge and on the four-inch-wide dimension the coating 

terminated one-half-inch from each edge. Thus, the actual area that was coated was three 

by three inches. Application of the coatings followed Boeing specification sheet 

160T1000 which required that the HVOF coatings not terminate with a square edge, but 

that they were tapered from full to zero thickness over a minimum length of 0.006”. 

Following deposition, some of the coatings were ground the same as for the previous 

samples and some were tested as-deposited. 

One major difference was the material used to seal the non-coated areas on the samples. 

Cadmium plating followed by a post chromate treatment per Mil-Spec QQ-P-416F was 

applied to the noncoated areas and the coating/substrate termination areas, with the 

cadmium/chromate overlapping approximately 0.1” onto the HVOF and EHC coatings. 
87

Table 3-35 to Table 3-37 summarize all of the coatings that were deposited onto the 4340 

and 300M rods and the 4340 plates. The testing required a total of 84 samples. 

Table 3-35. Sample Matrix for Supplemental Corrosion Testing 

– 4340 Rods 

Coating Thickness Ground Spray Co. # of 

samples 

WC-Co 0.003” Y H 2 

WC-Co 0.003” Y S 2 

WC-Co 0.003” N H 2 

WC-Co 0.003” N S 2 

WC-Co 0.010” Y H 2 

WC-Co 0.010” Y S 2 

WC-CoCr 0.003” Y H 2 

WC-CoCr 0.003” Y S 2 

WC-CoCr 0.003” N H 2 

WC-CoCr 0.003” N S 2 

WC-CoCr 0.010” Y H 2 

WC-CoCr 0.010” Y S 2 

EHC 0.003” Y S 2 

EHC 0.010” Y S 2 

Notes: Y = Yes H = Hitemco 

N = No S = Southwest 

Aeroservice 
88

Table 3-36. Sample Matrix for Supplemental Corrosion 

Testing – 300M Rods 

Coating Thickness Ground Spray Co. # of 

samples 

WC-Co 0.003" Y H 2 

WC-Co 0.003" Y S 2 

WC-Co 0.003" N H 2 

WC-Co 0.003" N S 2 

WC-Co 0.010" Y H 2 

WC-Co 0.010" Y S 2 

WC-CoCr 0.003" Y H 2 

WC-CoCr 0.003" Y S 2 

WC-CoCr 0.003" N H 2 

WC-CoCr 0.003" N S 2 

WC-CoCr 0.010" Y H 2 

WC-CoCr 0.010" Y S 2 

EHC 0.003" Y S 2 

EHC 0.010" Y S 2 
89

 

As can be seen from Table 3-35 to Table 3-37, there were two samples for each 

condition. One complete set of samples was inserted into the Q-Fog model corrosion test 

cabinet at the Naval Research Laboratory Key West (NRLKW) and the other complete 

set was inserted into the Q-Fog model corrosion test cabinet at the Kennedy Space Center 

(KSC). The samples were exposed for 1000 hours under the exact same conditions as for 

the previous Landing Gear JTP samples. 

After the salt fog tests were complete, the samples were cleaned at NRL using water and 

Scotch Brite abrasive pads to remove loosely adherent corrosion products, then dried 

with laboratory cotton wipes. The corrosion products were removed using the abrasive 

pads to allow a better view of the coating surface, which was often obscured by the large 

volume of corrosion product and salt residue on the samples. After cleaning, it was 

possible to identify surface defects such as blisters or pits that could be more closely 

examined to determine if the coating was undercut and the base metal exposed. All 

examinations were conducted under a 10X stereomicroscope and protection and 

appearance ratings assigned per ASTM Standard B537. 

Table 3-37. Sample Matrix for Supplemental Corrosion 

Testing – 4340 Plates 

Coating Thickness Ground Spray Co. # of 

samples 

WC-Co 0.003" Y H 2 

WC-Co 0.003" Y S 2 

WC-Co 0.003" N H 2 

WC-Co 0.003" N S 2 

WC-Co 0.010" Y H 2 

WC-Co 0.010" Y S 2 

WC-CoCr 0.003" Y H 2 

WC-CoCr 0.003" Y S 2 

WC-CoCr 0.003" N H 2 

WC-CoCr 0.003" N S 2 

WC-CoCr 0.010" Y H 2 

WC-CoCr 0.010" Y S 2 

EHC 0.003" Y S 2 

EHC 0.010" Y S 2 
90

3.4.5.3.Results and Discussion 

In the following discussion, those samples previously tested under the Landing Gear JTP 

will be referred to as the “LGJTP” samples and those samples tested under this 

supplemental corrosion study will be referred to as the “Supplemental” samples. A 

general observation regarding the previous and current results was that the extent of 

corrosion was less for the Supplemental than for the LGJTP samples. Overall, fewer 

surface coating breaches were observed. 

Figure 3-48 compares the 0.003”-thick WC-Co coatings on 300M steel from each data 

set. 

Figure 3-49 is the same coating comparison on 4340 steel. The degree of cracking of the 

tungsten carbide coating in the LGJTP sample set is obvious relative to the Supplemental 

set. Even the EHC coating samples from the LGJTP set had some pitting damage but all 

EHC samples from the Supplemental set were judged to demonstrate excellent 

performance after the 1,000 hours salt fog exposure. Figure 3-50 shows the results from 

the LGJTP 0.003”-thick EHC coating versus the Supplemental equivalent sample. The 

pitting and coating breach in the upper right of the LGJTP sample set is evident. 

The analysis of the LGJTP samples consisted of only recording a protection rating for 

each sample following cleaning to remove loose corrosion products. For these 

Supplemental samples, the recording of both an appearance and a protection rating 

provided a better understanding of the test results. The protection rating details how well 

the coating protected the base metal from corrosive attack by the salt fog. Protection 

ratings of 10 show the underlying metal was completely protected from corrosion while 

ratings of 4 and below show significant corrosion of the base metal. However, the 

appearance rating gives a relative scale to the level of corrosive attack on the coating 

itself. A specimen might have a protection rating of 10 but an appearance rating of 2, 

which indicates a substantial surface attack of the coating material. This may be of 

importance in that a coating’s ability to resist corrosion does impact its service life. 

Coatings which undergo significant surface corrosion in salt fog are more likely to allow 

substrate corrosion in time compared to those coatings more impervious to attack. 

Appendix 3 in the JTR [3.1] provides the protection and appearance ratings for all of the 

tested samples. 

3.4.5.3.1. Protection Ratings 

As mentioned previously, the corrosion levels observed on the Supplemental samples 

were not as severe as those on the LGJTP samples. One likely explanation is that Cd was 

used to protect the top and bottom of each sample. The LGJTP samples used an epoxy 

resin formulation to protect the top and bottom of the samples although, as noted in the 

previous section, there were instances where the epoxy was breached allowing for 

undercutting of the coatings. Cadmium is the least noble (most active) of any of the 

metals used in the coatings or substrate materials. Further, the Cd was in direct contact 

with both the coating and the underlying substrate metal while the samples were in a 

condensing, electrolytic salt atmosphere. It is believed that the Cd metal served to protect 

the coating and substrate by acting as a sacrificial anode, i.e. providing cathodic 

protection, which led to the improved corrosion performance in the ASTM B117 test. 
91

During the rating of the samples, the comment section in JTR Appendix 3 notes the 

condition of the Cd coating on the top (T) and bottom (B or BOT) of each specimen. 

All flat panel specimens demonstrated good protection ratings and similar surface 

corrosion properties to the identical coatings in the rod geometry specimens (see Figure 

3-54). As such, the flat panels added little clarity to this study and the remaining analysis 

will concentrate on the rod geometry samples. 

For the 4340 metal rods, two of the NRL exposed samples had breached coatings as well 

as three of the KSC exposed samples. Those samples with failed coatings are indicated 

in Table 3-38, while the full data are shown in Figure 3-57. 

For the 300M series substrates, three samples from each of the NRL and KSC exposed 

subsets failed, as listed in Table 3-39, while the full data are shown in Figure 3-58. 

The tables presented above show roughly equivalent performance in terms of protection 

results from the corrosion cabinets at NRL Key West and Kennedy Space Center. 

Further, the 0.003”-thick HVOF coated samples accounted for almost all of the failures 

for each metal substrate, indicating a likely problem with the coating as opposed to a 

problem with the underlying metal. Also, 3 of the 5 coatings on the 4340 steel and 4 of 

the 6 coatings that failed on the 300M were EHC. 

It is difficult to draw other conclusions that may be statistically significant regarding 

coating protection from this data set. Inferentially, 0.010”-thick coatings generally 

perform better than 0.003”-thick coatings, though one 0.010”-thick WC-CoCr coating did 

fail. There is insufficient data to really tell if grinding the coating affects corrosion 

performance. The preponderance of the samples that failed to protect the substrate where 

ground but, as will be discussed below, general corrosion of the coating was worse with 

unpolished surface samples. Figure 3-51 shows the comparison of 0.003”-thick WC-Co 

coatings on 4340 steel with those on the left as-deposited and those on the right having 

been ground. The overall surface corrosion attack on the as-deposited samples is evident 

Table 3-38. Failed Coatings on 4340 Substrates

NRLKW Exposed Samples KSC Exposed Samples 

WC-Co 0.003” Ground Hitemco WC-Co 0.003” Ground Hitemco 

WC-CoCr 0.003” Ground SWAero WC-Co-Cr 0.003” Ground SWAero 

 WC-CoCr 0.003” Unground SWAero 

Table 3-39. Failed Coatings on 300M Substrates

NRLKW Exposed Samples KSC Exposed Samples 

WC-Co 0.003” Ground Hitemco WC-Co 0.003” Ground Hitemco 

WC-Co-Cr 0.003” Ground SWAero WC-CoCr 0.003” Ground SWAero 

WC-CoCr 0.003” Unground SWAero WC-CoCr 0.010” Ground SWAero 
92

compared to the ground samples. Note that one of these ground samples has had a 

coating failure. 

While a few more Southwest-Aeroservice-applied coatings failed relative to those applied 

by Hitemco, there was no definitive difference between the coatings deposited by the two 

vendors. 

A final note on substrate protection is the repeat finding that the EHC samples 

outperformed all others. No EHC coating was breached and there was complete 

protection of the substrate. Figure 3-52 and Figure 3-53 compare the 0.010”-thick 

coating types on 4340 and 300M base metals, respectively. The EHC coating integrity 

and surface finish are significantly better than either of the HVOF coatings. 

3.4.5.3.2. Appearance Ratings 

The appearance rating is an important attribute in gauging the extent of corrosion on the 

coating. In most of the samples exposed in this series, the underlying metal substrate was 

protected but the coating itself was degraded to varying degrees. For instance, the EHC 

coatings outperformed all other alloys and received the highest appearance rating (10) for 

seven of the eight EHC samples. The eighth sample received a 9 rating owing to small 

staining of the surface probably derived from Cd corrosion products running onto the 

surface. The HVOF-coated samples received varying appearance ratings, generally lower 

than the EHC samples. Refer to Figure 3-52 and Figure 3-53 for a comparison of surface 

appearance for the 0.010”-thick coating compositions. Figure 3-55 and Figure 3-56 show 

the relative appearance rankings for the coatings on 4340 and 300M steels, respectively. 

Those samples from the KSC cabinet appear to be generally worse with respect to 

appearance, which would indicate a more aggressive corrosion environment. Overall, the 

relative rankings are similar for either sample set with the WC-Co indicating somewhat 

more corrosion than the WC-CoCr. 

The as-deposited samples fared the poorest in surface corrosion as noted by both series of 

data for either the WC-Co or WC-CoCr, and received the lowest appearance ratings. 

Figure 3-59 depicts the 0.003”-thick WC-Co coating on 300M steel and indicates the 

overall surface appearance of the as-deposited coatings to be worse. However, as before 

with the 4340 samples, one of the two ground samples has already failed. There is more 

general surface attack on samples with an as-deposited coating (at least for samples with 

a 0.003” coating thickness) but more of the 0.003”-thick ground samples indicated a 

breach of the coating. The thicker coatings fared better in substrate protection, as 

expected, but since no 0.010”-thick as-deposited coatings were tested, no definitive 

conclusion is able to be drawn from this data set when accounting for general coating 

surface attack AND substrate protection. 

3.4.5.4.Conclusions 

The following conclusions can be drawn from this supplemental study. 

1. 0.010”-thick coatings perform better than 0.003”-thick coatings of any 

composition in terms of protecting the substrate. See Figure 3-60 and Figure 3-61 

for a comparison of the WC-Co coatings. 

2. There was no definitive difference between the coatings applied by the two 
93

vendors. 

3. EHC coatings outperformed either the HVOF WC-Co or WC-CoCr, regardless of 

thickness in this corrosion test. See Figure 3-62 and Figure 3-63 for comparisons 

of 0.010”-thick coatings. 

4. For 0.010”-thick coatings, the data implies WC-CoCr coatings show improved 

performance over WC-Co coatings. Additional studies would be necessary to 

confirm this conclusion since there was one 0.010”-thick WC-CoCr coating that 

was breached. 

5. The 0.003”-thick coatings are suspect in their ability to protect the substrate steel. 

While 0.003”-thick EHC in this data set did protect the substrate (one sample only 

was tested in each location), the LGJTP results with multiple samples being tested 

showed some 0.003”-thick EHC coatings being breached. 

Figure 3-48. HVOF WC-Co Coatings, 0.003” Thick, Ground, on 300M Steel 

From the LGJTP Tests, Left, and the Supplemental Tests, Right 
94

Figure 3-49. HVOF WC-Co Coatings, 0.003”-thick, Ground, on 4340 Steel 

From the LGJTP Tests 

Figure 3-50. EHC Coatings, 0.003”-thick, Ground, on 4340 Steel from the LGJTP 

Tests, Left, and the Supplemental Tests, Right 
95

Figure 3-51. HVOF WC-Co Coatings, 0.003”-thick, on 4340 Steel, As-deposited 

on Left and Ground on Right 

Figure 3-52. Left: WC-Co, 0.010”-thick, Ground. Center: EHC, 0.010”-thick, 

Ground. Right: WC-CoCr, 0.010”-thick, Ground. All on 4340 Steel Substrates 
96

Figure 3-53. Left: WC-Co, 0.010”-thick, Ground. Center: EHC, 0.010”-thick, 

Ground. Right; WC-CoCr, 0.010”-thick, Ground. All on 300M Steel Substrates 

Appearance and protection rating - 4340 plate

0 2 4 6 8 10

WC/Co 3 mil Ground Hitemco

WC/Co 3 mil Ground SW Aero

WC/Co 3 mil Unground Hitemco

WC/Co 3 mil Unground SW Aero

WC/Co 10 mil Ground Hitemco

WC/Co 10 mil Ground SW Aero

WC/CoCr 3 mil Ground Hitemco

WC/CoCr 3 mil Ground SW Aero

WC/CoCr 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/CoCr 10 mil Ground Hitemco

WC/CoCr 10 mil Ground SW Aero

EHC 3 mil Ground SW Aero

EHC 10 mil Ground SW Aero

Protection

Appearance

Figure 3-54. Comparison of Appearance and Protection Ratings for 4340 Plate, NRL
97

Appearance rating - 4340 rod

0 1 2 3 4 5 6 7 8 9 10

WC/Co 3 mil Ground Hitemco

WC/Co 3 mil Ground SW Aero

WC/Co 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/Co 10 mil Ground Hitemco

WC/Co 10 mil Ground SW Aero

WC/CoCr 3 mil Ground Hitemco

WC/CoCr 3 mil Ground SW Aero

WC/CoCr 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/CoCr 10 mil Ground Hitemco

EHC 3 mil Ground SW Aero

EHC 10 mil Ground SW Aero

KSC

NRL

Figure 3-55. Comparison of Appearance Ratings for HVOF and EHC Coatings on 

4340 Rod Tested at NRL and KSC 

Appearance rating - 300M rod

0 1 2 3 4 5 6 7 8 9 10

WC/Co 3mil Ground Hitemco

WC/Co 3 mil Ground SW Aero

WC/Co 3 mil Unground Hitemco

WC/Co 3 mil Unground SW Aero

WC/Co 10 mil Ground Hitemco

WC/Co 10 mil Ground SW Aero

WC/CoCr 3 mil Ground Hitemco

WC/CoCr 3 mil Ground SW Aero

WC/CoCr 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/CoCr 10 mil Ground Hitemco

WC/CoCr 10 mil Ground SW Aero

EHC 3 mil Ground SW Aero

EHC 10 mil Ground SW Aero

KSC

NRL

Figure 3-56. Comparison of Appearance Ratings for HVOF and EHC Coatings on 

300M Rod Tested at NRL and KSC 
98

Protection rating - 4340 rod

0 1 2 3 4 5 6 7 8 9 10

WC/Co 3 mil Ground Hitemco

WC/Co 3 mil Ground SW Aero

WC/Co 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/Co 10 mil Ground Hitemco

WC/Co 10 mil Ground SW Aero

WC/CoCr 3 mil Ground Hitemco

WC/CoCr 3 mil Ground SW Aero

WC/CoCr 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/CoCr 10 mil Ground Hitemco

EHC 3 mil Ground SW Aero

EHC 10 mil Ground SW Aero

KSC

NRL

Figure 3-57. Comparison of Protection Ratings for HVOF and EHC Coatings on 

4340 Rod Tested at NRL and KSC 
99

Protection rating - 300M rod

0 1 2 3 4 5 6 7 8 9 10

WC/Co 3mil Ground Hitemco

WC/Co 3 mil Ground SW Aero

WC/Co 3 mil Unground Hitemco

WC/Co 3 mil Unground SW Aero

WC/Co 10 mil Ground Hitemco

WC/Co 10 mil Ground SW Aero

WC/CoCr 3 mil Ground Hitemco

WC/CoCr 3 mil Ground SW Aero

WC/CoCr 3 mil Unground Hitemco

WC/CoCr 3 mil Unground SW Aero

WC/CoCr 10 mil Ground Hitemco

WC/CoCr 10 mil Ground SW Aero

EHC 3 mil Ground SW Aero

EHC 10 mil Ground SW Aero

KSC

NRL

Figure 3-58. Comparison of Protection Ratings for HVOF and EHC Coatings on 

300M Rod Tested at NRL and KSC 

Figure 3-59. WC-Co Coatings, 0.003”-thick, on 300M Steel. Left: Asdeposited. Right: Ground 
100

Figure 3-60. WC-Co Coatings on 4340 steel. Left: 0.003”-thick, Ground. Right: 

0.010”-thick, Ground 
101

Figure 3-61. WC-Co Coatings on 300M Steel. Left: 0.003”-thick, Ground. Right: 

0.010”-thick, Ground 

Figure 3-62. Left: EHC Coating, 0.010”-thick, Ground. Right: 

WC-Co Coating, 0.010”-thick, Ground. All on 4340 Steel 
102

3.4.6. Discussion of overall corrosion results 

The data show that the overall corrosion performance of EHC was better than WC-CoCr, 

which in turn was better than WC-Co. Based on both the Landing Gear JTP and 

Supplemental testing, it can be concluded that the acceptance criteria from the JTP, 

namely that the corrosion performance of the HVOF WC-Co and WC-CoCr coatings 

must equal or exceed that of EHC, was not met. Putting this work into perspective 

regarding the previous corrosion studies of the HCAT and the studies conducted by 

others, there are several observations that can be made and issues that should still be 

addressed. 

1. The performance of the EHC coatings in the LGJTP and Supplemental testing 

was somewhat unexpected in that B117 testing conducted by others on EHC of 

equivalent thicknesses (see, for example, testing conducted by the Canadian 

HCAT for their portion of the Landing Gear JTP and testing conducted by 

Hamilton Sundstrand for the Propeller Hub JTP) showed substantial corrosion 

after a few hundred hours of exposure. Also note the extremely poor performance 

of EHC in the three-year atmospheric corrosion tests described in this document. 

The HCAT plans on conducting analyses of the EHC coatings from these studies 

and will compare them to coatings from other studies to determine if there are any 

clear differences that would explain the different performance. 

2. The HVOF WC-Co and WC-CoCr coatings do still provide substantial corrosion 

Figure 3-63. Left: EHC, 0.010”-thick, Ground. Right: HVOF WC-Co, 0.010”-

thick, Ground. All on 300M Steel 
103

protection to the steel substrates evaluated in this study as evidenced by the fact 

that uncoated steel subjected to B117 is almost completely corroded after several 

tens of hours exposure. The fact that there was severe undercutting and breaching 

of many of the HVOF coatings indicates that when these coatings are to be used 

in service, extreme care should be taken to ensure that the corrosive medium 

cannot have access to the area where the coating terminates on the base material, 

such as at an edge. This was proven out by the significantly better performance of 

the HVOF coatings when the cadmium/chromate coating was used as a sealer for 

the sample edges instead of the epoxy. 

3. Further evidence of the protective nature of the HVOF coatings was illustrated in 

the results of the cyclic GM9540B testing in which both the EHC and the WC-Co 

coatings showed virtually no corrosion after 1,000 hours. Although no uncoated 

steel samples were evaluated in this series of tests, corrosion engineers at NRL 

indicated that steel such as 4340 would be substantially corroded after only 100 

hours in the GM test. 

4. In terms of corrosion of the coatings themselves, the WC-CoCr coatings do 

perform somewhat better than the WC-Co coatings. However, in terms of 

undercutting and breaching, there was essentially no difference between the 

coatings, again indicating the importance of ensuring the corrosive medium 

cannot have access to the coating termination areas on the base material. 

3.4.7. Significance 

It is difficult to determine the true significance of these data since cabinet corrosion 

testing is notoriously variable, as is the hard chrome baseline itself. Based on the 

experience of most aerospace industry team members (e.g. Boeing, Goodrich, Hill AFB) 

the performance of the Southwest Aeroservice hard chrome appeared to be exceptionally 

good. However, some significant findings should be noted: 

 Clearly, HVOF coatings provide corrosion resistance compared with uncoated 

steel. However, given the wide disparity in results between the various tests 

reported here it is not possible to be sure how they compare with hard chrome 

coatings typical of those deposited in depots. 

 Since WC-CoCr appears to provide better corrosion resistance that WC-Co, 

corrosion considerations would suggest that the former be chosen over the latter if 

both are otherwise acceptable. 

 The data provide evidence that the HVOF coating itself corrodes (especially in the 

case of WC-Co), whereas for EHC coatings the coating itself is untouched while 

the substrate corrodes through coating cracks and breaches. This points to a 

different corrosion mechanism, which is likely to lead to surface roughening or 

release of micron-sized WC particles over sufficient time. This should be borne 

in mind when choosing a coating for use in corrosive environments. 

3.4.8. Conclusion 

HVOF WC-Co and WC-CoCr both fail the JTP acceptance criteria. Note that 

corrosion comparisons were made against what appeared to be an exceptionally high 
104

quality hard chrome. HVOF coatings may or may not fail tests against hard chrome 

coatings commonly used in depot or airline repair. 

3.5. Wear data 

3.5.1. Data summary 

The full data set is provided in Appendix 4 of the JTR [3.1]. 

3.5.2. Rationale 

Prior experience in the aircraft industry (and in other industries) has been that the wear 

performance of components coated with WC-Co or WC-CoCr exceeds that of hard 

chrome. It is primarily for this reason that companies such as Boeing have substituted 

these HVOF coatings in place of hard chrome for specific spot applications on 

components where hard chrome performance in the field was found to be inadequate. 

(These problem-area applications now total more than a hundred.) 

Wear is a very complex phenomenon and depends on a great many factors that are 

specific to each engineering system. Recognizing this, true measurements of 

performance were designed to be made in Part 2 of the Landing Gear Joint Test Protocol, 

using full-scale rig and flight testing. The purpose of the measurements made in Part 1 

was to determine which factors (coating material, surface finish, test conditions, etc.) 

have the most impact on wear performance and therefore must be most carefully 

controlled. 

Table 3-40. Quick Data Locator (Click on item number to view) 

Item Item number 

L12 Matrix Data – Fretting Table 3-49 

L12 Test Matrix Data - Sliding Wear Table 3-50 

L8 DOE Data Table 3-51 - 

Table 3-54 

Fretting wear data Figure 3-70, 

Figure 3-71 

Sliding wear data – L12 matrix Figure 3-72, 

Figure 3-73 

Sliding wear data – final L8 matrix, including average data and data for 

the metal bushing baseline and all half-replicates 

Figure 3-74, 

Figure 3-75, 

Figure 3-79 - 

Figure 3-84 

Statistical analysis of L8 data Figure 3-76 - 

Figure 3-78 
105

Landing gear applications of hard chrome include both long-stroke sliding wear 

experienced by landing gear utility actuators and inner cylinders, and short stroke, 

oscillating wear (dithering and fretting) experienced by pins and hydraulic cylinders 

during ground operations. Testing was designed to evaluate both of these conditions. 

3.5.3. Specimen fabrication 

3.5.3.1.Fretting specimens 

Fretting blocks and shoes were made of 4340 plate stock according to the design shown 

in Figure 3-64. In this design, the fretting shoe (Figure 3-64a) is the same 1” width as the 

fretting block (Figure 3-64b). Only the shoe was coated. 

The block and shoe materials are summarized in Table 3-41. 

a. 

b.

Figure 3-64. a) Fretting Block; b) Fretting Shoe
106

3.5.3.2. Sliding wear specimens 

The design of the sliding wear specimens is shown in Figure 3-65. Bushings were made 

of various materials. All wear rod specimens were fabricated from 4340 round bar, as 

follows: 

 1” diameter, 9” long 

 Tensile strength and heat treat – 260-280 ksi per MIL-H-6875 

 Shot peen – 8-10A, S230, wrought steel shot, per AMS 2432, computer 

controlled, 100% coverage, except a 0.75” section at one end of the rod 

 Grit blast prior to coating – Specimens were gripped on the non-peened area and a 

5” section grit blasted: 

o For EHC with 180-220 grit alumina or glass beads 

o For HVOF coating with 54 grit alumina at 60 psi at 90° angle of 

impingement, per MIL-STD-1504. 

 Coating – per methodology in Section 3.6.4 below 

o Thickness – final thickness 0.003” and 0.010” (see Table 3-42). Coatings 

were deposited 0.002-0.003” thicker to allow for final grinding. 

 Embrittlement relief heat treat - 375°F for EHC plated samples only. 

 Grind after coating 

o For EHC low stress grind to 16∝” finish per MIL-STD-866 

o For HVOF low stress grind to 8∝” finish per BAC 5855 

Table 3-41. Wear Test Materials and Coatings – 

Fretting (Fretting blocks uncoated, 4340 shoe coated) 

Fretting Shoe Coating Thickness 

(inch) 

Fretting 

Block 

Hard chrome plate 0.003, 

0.010 

4340 steel 

HVOF WC-17Co 

(Diamalloy 2005 powder) 

0.003, 

0.010 

Nitrile seals 

in 4340 

HVOF WC-10Co-4Cr 

(SM5847 powder) 

0.003, 

0.010 
107

a) 

b) 

c) 

Figure 3-65. Wear Test Rods and Bushings - a) Rod, b) Straight Bushing, c) Seal 

Groove Bushing 
108

3.5.4. Coating deposition methodology 

Coating materials and thicknesses tested are summarized in Table 3-42. 

Samples were coated as follows: 

 Hard chrome – standard Boeing specifications 

 WC-Co – DiamondJet gun, per HCAT specifications 

o Powder – Diamalloy 2005 

o Fuel – Hydrogen 

 WC-CoCr – DiamondJet gun, per HCAT specifications 

o Powder – Sulzer Metco 5847 

o Fuel – Hydrogen 

 Aluminum anodize – by Southwest Aeroservice per MIL-P-25732 B. 

3.5.5. Test methodology 

In consonance with the aim of determining performance factors rather than measuring 

performance itself, the wear evaluation tests were made in the form of Design of 

Experiment (DOE) protocols, with most of the testing being done for sliding wear, which 

has the broadest application. Test conditions were designed to obtain measurable results 

in a short time rather than to duplicate service conditions. This experimental design 

incorporated the following: 

 A pre-DOE matrix to establish the test conditions. The test conditions in this 

matrix were to be modified by results in order to determine the range of test 

conditions necessary to provide reasonable, measurable ranges of wear rates for 

Table 3-42. Wear Test Materials and Coatings - Sliding. (Piston coated, bushing 

uncoated) 

Piston/coating material Thickness 

(inch) 

Coating vendor and 

specification 

Bushing 

material 

Hard chrome plate 0.003, 

0.010 

Southwest Aeroservice, Tulsa, 

OK; QQ-C-320B, Boeingapproved specification 

4340 steel 

HVOF WC-17Co 

(Diamalloy 2005) 

0.003, 

0.010 

NADEP Cherry Point; HCAT 

specs. 

Al-Ni bronze 

HVOF WC-10Co-4Cr 

(SM5847) 

0.003, 

0.010 

NADEP Cherry Point; HCAT 

specs. 

Anodized 2024-

T3 aluminum 

 Nitrile seal 

 Karon B seal 
109

the material combinations in tests typically averaging no more than 48 hours. 

This information was used to confirm or reassign specific values to the conditions 

for the design factors in the first L12 DOE test matrix. 

 An L12 matrix to evaluate the major potential wear factors for both fretting and 

sliding wear, and establish which factors were the most significant and should be 

evaluated in more detail in the subsequent L8 DOE matrix. 

 An L8 matrix to quantify the relative contributions to sliding wear performance of 

the major wear factors identified in the L12 matrix. Fretting wear was not 

incorporated in the L8 matrix. 

3.5.5.1. Fretting 

Specification: No existing public standard. These tests followed the GE Aircraft 

Engines fretting test method used for engine components. 

Modifications: For some tests the fretting block was modified to incorporate a 

section of seal material. 

Significance: Short-stroke, unlubricated wear is typical of the conditions 

experienced by pins and hydraulics during ground operations. 

Sample types: Blocks and shoes. 

Sample materials: 4340 

Sample heat treat: 260-280 ksi UTS 

Coatings: EHC, WC-Co, WCCoCr, 0.003” and 0.010” 

thick. 

Lubrication: MIL-H-83262 hydraulic 

fluid (Royco 782)

Test equipment: Test equipment is 

illustrated schematically 

in Figure 3-66. Testing 

was done in a standard 

vertical fatigue machine adapted to provide a side load via the 

shoe, as illustrated in the figure. 

Test description: 

Tests were run with the block set in a specially-designed holder on a standard closed-loop 

MTS machine. The shoe was pressed against the block with a normal load, PN, applied 

by a dead weight system. Lubrication, where needed, was supplied by dripping from 

above the fretting region. Test conditions (loads, speeds, lubrication, etc.) are given in 

the test matrices. Wear of the coated shoe and uncoated block was measured by weight 

change. 

3.5.5.2. Sliding wear 

Specification: No existing standard. 

Block 

 Oscillating 

 Motion 

 Shoe 

 LOAD 

Figure 3-66 Cross-sectional 

Schematic of Fretting Wear Test
110

Modifications: None 

Significance: Long-stroke actuation typical of hydraulic cylinder rods in landing, 

takeoff, and ground operations. 

Sample types: Rods: 1” round rod, 9” long; 1.5” OD, 1” ID bushings. 

Sample materials: Rods: 4340 steel heat treated to 260 – 280 ksi UTS. 

Bushings: 4340 steel heat treated to 160-180 ksi; AMS 4640 AlNi-bronze rod in the TQ50 or equivalent HR50 condition (hardness 

HB 201-248); anodized 2024 Al (MIL-A-8625, Type 3, Class I). 

Seals: Nitrile T seals 

type 7214-FT-160-T 

from Green Tweed 

(MIL-HDBK-695C). 

Liners: Karon B from 

Kamatics Corp. (filled 

phenolic 0.015” thick). 

Coatings: EHC, WC-Co, WCCoCr, 0.003” and 

0.010”, ground to 16∝” 

for EHC and to 8∝” Ra 

for HVOF. 

Lubrication: MIL-H-83262 hydraulic 

fluid (Royco 782) 

Test equipment: Test equipment is illustrated schematically in Figure 3-67 and 

shown photographically in Figure 3-68. The test rod was oscillated on four bearings, 

driven by a hydraulic actuator via a load cell to measure the drive load, Pdrive. Normal 

load was applied to the bushing via a spring assembly and load cell so as to create a wear 

couple between the rod and the bushing. The lubricant was supplied at the bushing from 

a suspended bag through a small tube and a constricting valve that controlled the drip 

rate.

Test description: 

For both sliding wear and fretting the coefficient of friction was measured as 

∝ = [Pdrive(max) - Pdrive(min)]/2 PN

Test conditions were as defined in the test matrices. Hydraulic fluid was added to permit 

fully-lubricated, starved-lubricated, or unlubricated testing. Under “Full” lubrication 

fluid was added throughout the test, while under “partial” lubrication lubricant was added 

only for the first hour of the test, then turned off for the remainder of the test. 

Guide

Oscillating piston

Load

Test

bushing Guide

Figure 3-67. Cross-sectional Schematic of 

Piston and Bushing Oscillating Wear Test 
111

For the nitrile seal rod/bushing tests, a pair of standard T-seals was assembled into a 

grooved bushing. These seals were pre-soaked in hydraulic fluid for 24 hours prior to 

assembly to permit any absorption to fluid to occur. After assembly with the rod, the 

seals were energized by filling the inter-seal region with hydraulic fluid from a reservoir 

and pressurizing to 3000 psi with a nitrogen bottle through a small orifice in the bushing. 

For the partial lubrication tests in the L12 and L8 phases, the test was stopped and 

depressurized after 1 hour and disassembled to drain the lubricant. The rod and bushing 

cavity were wiped with a clean, dry rag to remove any excess lubricant, reassembled, and 

repressurized with dry nitrogen to re-energize the seals, and the test was then restarted. 

Wear was judged in two ways: 

 Weight loss – this was found to be an unreliable method since small amounts of 

lubricant trapped in the center hole or thread of the rod caused large errors – up to 

0.010 gm. 

o Measurement accuracy was tested by making a series of measurements: 

 0.00006 gm for WC-Co 

 0.00011 gm for metals and hard chrome 

 0.00063 for seals and anodized Al 

 Visual appearance against standards – this method was used to overcome the 

problems caused by weight measurement errors due to trapped fluids. The 

standards are shown in Figure 3-69 and described in Table 3-43. A visual ranking 

was assigned by the closest match to the standards. 

Side load

Bushing Load cell

Test rod

Figure 3-68. Sliding Wear Test Equipment
112

Table 3-43. Description of Visual Wear Ranking

Rank Description 

1 No visible wear scar 

2 Slight burnishing of circumferential grinding marks on rod 

3 Small, isolated longitudinal wear patches; circumferential marks still partially visible 

4 Larger, more extensive longitudinal marks 

5 General longitudinal marking; circumferential grind lines obliterated 

6 Extensive wear regions, some metal transfer may have occurred 

7 Some deep longitudinal grooving within large general wear scars 

8 Wear scars cover entire stroke length from approximately 10:00 to 2:00 on the rod 

9 Rating 8 plus some longitudinal grooving 

10 Rating 8 plus extensive, deep longitudinal grooving, metal transfer. 

Figure 3-69. Visual Ranking Standards for Rods, VR 01 to VR 10
113

3.5.5.3. Test matrices 

Table 3-44. Matrix for Pre-DOE – Fretting Tests Run

Test DoE Wear Block or Coating 

No. Expt. No. Test Bushing Coating Load, lbs Finish Lube

1 pre-trial Fretting 4340 WC-10Co-4Cr 72 16 no 

2 pre-trial Fretting 4340 Chrome 72 16 no 

3 pre-trial Fretting 4340 WC-17Co 72 16 no 

4 pre-trial Fretting 4340 Chrome 288 16 no 

5 pre-trial Fretting 4340 WC-17Co 288 16 no 

6 pre-trial Fretting 4340 WC-17Co 288 16 yes 

7 pre-trial Fretting Nitrile WC-17Co 40 16 yes 

Table 3-45. Matrix for Pre-DOE (Design of Experiment) - Sliding

Test DoE Wear Block or Coating 

No. Expt. No. Test Bushing Coating Load, lbs Finish Lube

13 pre-trial Bushing 4340 WC-10Co-4Cr 72 16 no 

14 pre-trial Bushing 4340 Chrome 72 16 no 

15 pre-trial Bushing 4340 WC-17Co 72 16 no 

16 pre-trial Bushing 4340 Chrome 144 16 no 

17 pre-trial Bushing 4340 Chrome 36 or 288 16 no 

18 pre-trial Bushing 4340 Chrome 72 4 no 

19 pre-trial Bushing 4340 Chrome 288 or144 16 yes 

20 pre-trial Bushing 4340 WC-17Co 288 or144 16 yes 

21 pre-trial Bushing Nitrile Chrome 40 16 no 

22 pre-trial Bushing Nitrile Chrome 40 16 yes 

23 pre-trial Bushing Nitrile WC-17Co 40 16 yes 

24 pre-trial Bushing Nitrile WC-17Co 40 16 no 
114

*Originally assigned at 288 lb PN. 

Table 3-46. L-12 Design of Experiment Matrix for Fretting

Design 

Factors 2 3 4 5 6 7 8 9 10 

DoE Std. 

Order 

Block or 

Bushing 

Rod/Shoe 

Coating 

Load 

(lbs) 

Coating 

Finish 

Hydraulic 

Fluid 

Lubric’n 

Stroke 

(inches)

Frequency 

(cpm) 

Cycles 

(n) 

Temp 

(Deg F)

1 4340 Chrome 72 4 partial 0.01 600 720,000 70 

2 4340 Chrome 72 4 full 0.05 240 360,000 200 

3 4340 WC-17Co 288 8 partial 0.01 600 360,000 200 

4 Nitrile Chrome 72* 8 partial 0.05 240 720,000 70 

5 Nitrile WC-17Co 72 8 full 0.05 240 720,000 200 

6 Nitrile WC-17Co 72* 4 full 0.01 600 360,000 70 

Table 3-47. L-12 Design of Experiment Matrix for Bushings/Liners

Design 

Factors 2 3 4 5 6 7 8 9 10 

DoE Std. 

Order 

Block or 

Bushing 

Rod/Shoe 

Coating Load, lbs 

Coating 

Finish 

Hydraulic 

Fluid 

Lubric’n 

Stroke 

(inches)

Frequency 

(cpm) Cycles (n)

Temp 

(Deg F)

7 4340 WC-17Co 288 4 partial 1.0 30 108,000 200 

8 4340 WC-17Co 72 8 full 1.0 90 108,000 70 

9 4340 Chrome 288 8 full 0.1 30 54,000 70 

10 Nitrile WC-17Co 72 4 partial 0.1 30 54,000 70 

11 Nitrile Chrome 288 4 full 0.1 90 108,000 200 

12 Nitrile Chrome 72 8 partial 1.0 90 54,000 200 
115

Table 3-48. L-8 Wear Test Matrix. Note: each test condition was run twice (A and B)

Design factors 

C B A D 

Test No. DOE Expt. No. Coating Bushing Cycles Normal Load, lb.

49 A&B L8-01 Chrome 4340 80K 72 

50 A&B L8-02 Chrome 4340 240K 288 

51 A&B L8-03 Chrome AMS 4640* 80K 288 

52 A&B L8-04 Chrome AMS 4640 240K 72 

53 A&B L8-05 WC-17Co 4340 80K 288 

54 A&B L8-06 WC-17Co 4340 240K 72 

55 A&B L8-07 WC-17Co AMS 4640 80K 72 

56 A&B L8-08 WC-17Co AMS 4640 240K 288 

1st L8 Half Replicate (a) 

57 A&B L8-5a WC-10Co-4Cr 4340 80K 288 

58 A&B L8-6a WC-10Co-4Cr 4340 240K 72 

59 A&B L8-7a WC-10Co-4Cr AMS 4640 80K 72 

60 A&B L8-8a WC-10Co-4Cr AMS 4640 240K 288 

2nd L8 Half Replicate (b) 

61 A&B L8-3b Chrome Anodized Al 80K 288 

62 A&B L8-4b Chrome Anodized Al 240K 72 

63 A&B L8-7b WC-17Co Anodized Al 80K 72 

64 A&B L8-8b WC-17Co Anodized Al 240K 288 

3rd L8 Half Replicate (c) 

65 A&B L8-3b Chrome Nitrile seal 80K 288 

66 A&B L8-4b Chrome Nitrile seal 240K 72 

67 A&B L8-7b WC-17Co Nitrile seal 80K 72 

68 A&B L8-8b WC-17Co Nitrile seal 240K 288 

4th L8 Half Replicate (d) 

69 A&B L8-3d Chrome Karon B liner 80K 288 

70 A&B L8-4d Chrome Karon B liner 240K 72 

71 A&B L8-5d WC-17Co Karon B liner 80K 72 

72 A&B L8-6d WC-17Co Karon B liner 240K 288 

5th L8 Half Replicate (e) 

73 A&B L8-1e Chrome Karon B liner 80K 72 

74 A&B L8-2e Chrome Karon B liner 240K 288 

75 A&B L8-3e WC-10Co-4Cr Karon B liner 80K 288 

76 A&B L8-4e WC-10Co-4Cr Karon B liner 240K 72 

77 A&B L8-5e Chrome AMS 4640 80K 288 

78 A&B L8-6e Chrome AMS 4640 240K 72 

6th L8 Half Replicate (f) 

79 A&B L8-1f Chrome Nitrile seal 80K 72 

80 A&B L8-2f Chrome Nitrile seal 240K 288 

81 A&B L8-3f WC-10Co-4Cr Nitrile seal 80K 288 

82 A&B L8-4f WC-10Co-4Cr Nitrile seal 240K 72 

X2 for each 

TOTAL = 68 * Al-Ni Bronze 
116

3.5.6. Test results 

3.5.6.1. Pre-DOE 

The pre-DOE matrix was not a design-of-experiment matrix , but was a screening test 

designed simply to determine general behavior and to decide on the test conditions for 

subsequent testing. It led to the following conclusions: 

Fretting tests:

No meaningful conclusions on relative coating behavior could be drawn from the preDOE data. However, test conditions were determined: 

 For seal tests it was found that a load of 40 lb compressed the seal, leaving only 

about 0.001” between the rod (or in fretting tests the shoe) surface and the 

bushing (or block in fretting tests). Above this load the rod (or shoe) touched the 

bushing (or block). Therefore 40 lb was the maximum test load used in all 

pre-DOE seal tests (see Table 3-44 and Table 3-45).

 The 288 lb. load resulted in greater wear than did the 72 lb. load. 

 Unlike the rod/bushing tests, it was possible to run fretting tests in the dry, 

unlubricated condition at either load without experiencing unstable tests. 

Sliding wear tests: 

 The dry, unlubricated test was much more severe than the lubricated test, even at 

low load. This test did not appear to distinguish between the three coatings, and 

was unsuitable for the L12 DOE. 

 On the other hand, a continuous flooded lubricant test did not adequately 

distinguish among the coatings; nor did a test protocol in which the rod was 

flooded for 1 hr. (5,400 cycles) and then the lubricant shut off. 

 A qualitative discrimination between the EHC and WC-17Co coatings was 

obtained using a slow drip/shutoff test protocol. 

 A 288 lb. normal load provided more significant discrimination of coating 

performance than did a 72 lb. normal load. 

 In general, initial friction values for HVOF coatings were higher than for EHC 

plating. 

 Weight loss had to be measured to 4 decimal places to detect any post-test 

differences. This was a problem, since it was very easy for lubricant to become 

trapped in centering holes and threads. 

Note that the test conditions chosen for subsequent testing do not represent true service 

conditions, but rather conditions for obtaining a measurable difference in wear behavior 

between hard chrome and HVOF coatings. 

3.5.6.2. L12 fretting tests 

Fretting was measured only in the L-12 DOE matrix. 
117

The results are shown in Table 3-49 and in Figure 3-70 and Figure 3-71. Note that, since 

this was a DOE matrix, each point corresponds to a different set of conditions, as shown 

in the matrix of Table 3-46. 

Table 3-49. L12 Matrix Data – Fretting

TEST 

NUMBER 

TEST 

SERIES 

BLOCK 

WEIGHT 

CHANGE

SEAL 

WEIGHT 

CHANGE

SHOE 

WEIGHT 

CHANGE

WEAR 

RANKING 

(SHOE) 

Wear couple 

block/shoe 

1-04 1A 0.0001 0.0003 2 4340/EHC 

3-04 1B 0.0012 0.0008 2 4340/EHC 

12-04 2A 0.0006 0.0016 2 4340/EHC 

13-04 2B -0.0052 -0.0050 2 4340/EHC 

15-04 3A 0.0034 0.0049 2 4340/WC-Co 

20-04 3B -0.0022 0.0011 2 4340/WC-Co 

4-04 4A 0.0102 0.0078 2 Nitrile/EHC 

8-04 4B -0.0759 0.0038 2 Nitrile/EHC 

22-04 5A -0.0171 0.0114 1 Nitrile/WC-Co 

30-04 5B 0.0462 0.0133 1 Nitrile/WC-Co 

6-04 6A -0.0665 0.0057 1 Nitrile/WC-Co 

10-04 6B -0.0991 0.0128 1 Nitrile/WC-Co 

0 

2 

4 

6 

8 

10 

1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B

RA

NK

IN

G

Figure 3-70. Fretting Wear Ranking – Shoe; L12 DOE Matrix (Red against metal 

bushings; blue against nitrile seals) 
118

The L-12 fretting data showed very little differentiation between materials. This is 

especially clear from the visual ranking data (Figure 3-70), which showed almost no wear 

under any conditions. 

3.5.6.3. L12 sliding wear tests 

The data for sliding wear are shown in . Note that, since this was a DOE matrix, each 

point corresponds to a different set of conditions, as shown in the matrix of Table 3-47. 

-0.1200

-0.1000

-0.0800

-0.0600

-0.0400

-0.0200

0.0000

0.0200

0.0400

0.0600

1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B

WEIGHT CHANGE (GRAMS)

Avg. block wt. = 60 gm Avg. seal wt. = 0.84 gm

Figure 3-71. Fretting Wear - L12 DOE Matrix; Yellow Data Wear of Coated Shoe; 

Red Data Wear of Uncoated 4340 Block; Blue Data Wear of Nitrile Seal 
119

Table 3-50. L12 Test Matrix Data - Sliding Wear

TEST 

NUMBER 

TEST 

SERIES 

BUSHING 

WEIGHT 

CHANGE

SEAL 

WEIGHT 

CHANGE

ROD 

WEIGHT 

CHANGE

WEAR 

RANKING 

(ROD) 

14-04 7A -0.1271 -0.0200 10 

23-04 7B -0.0633 -0.0003 6 

7-04 8A -0.0218 0.0018 1 

17-04 8B 0.0014 0.0049 1 

9-04 9A 0.0043 -0.0003 3 

19-04 9B 0.0021 0.0069 3 

24-04 10A-L -0.0223 

24-04 10A-R -0.0383 0.0035 2 

29-04 10B-L -0.0046 

29-04 10B-R -0.0057 0.0000 2 

25-04 11A-L -0.0049 

25-04 11A-R -0.0100 0.0107 3 

27-04 11B-L 0.0509 

27-04 11B-R 0.0560 0.0102 4 

26-04 12A-L -0.4432 

26-04 12A-R -0.1761 -0.0040 7 

28-04 12B-L -0.2973 

28-04 12B-R -0.1348 -0.0184 8 

0

2

4

6

8

10

7A7B8A8B9A9B

10A-L

10AR

10B-L

10BR

11A-L

11AR

11B-L

11BR

12A-L

12AR

12B-L

12BR

RANKING

Figure 3-72. Sliding Wear – L12 Data. Visual Wear Ranking – Rod. (Red against 

bushings; blue against seals) 
120

From the L12 matrix allowed us to draw the following conclusions for design of the final 

L8 DOE matrix: 

1. Rod coating and bushing type were statistically significant design factors, as 

expected. 

2. Normal load, PN, was a significant variable using the 72 and 288 lb. values. 

3. Test duration (cycles) was a significant variable. For better differentiation, it was 

decided to use 80,000 and 240,000 cycles as the design values for the L8 matrix. 

4. The remaining design factors evaluated in this phase did not provide statistically 

differentiable performance over the ranges examined. Therefore, each of these 

became a constant in the L8 matrix, using the following values: 

 Coating finish – 8 microinches Ra 

-0.5000

-0.4000

-0.3000

-0.2000

-0.1000

0.0000

0.1000

7A8A9A

10A-L10B-L11A-L11B-L12A-L12B-L

GRAMS

BUSHING WEIGHT CHANGE

SEAL WEIGHT CHANGE

ROD WEIGHT CHANGE

Avg. bushing wt. = 219 gm

Avg. seal wt. = 2.9 gm

Figure 3-73. Sliding Wear – L12 Data. Weight Change – Bushing/Seal (Yellow data 

wear of coated rod; red data wear of uncoated bushing; blue data wear of nitrile seal) 
121

 Lubrication – partial (1st hour only, 1 drip/4 min.) 

 Stroke – 1” (total range; i.e., +/-0.5”) 

 Frequency – 1.5 Hz (90 cpm) 

 Temperature – 75°F. 

3.5.6.4. L8 sliding wear DOE tests 

The L8 test matrix is given in Table 3-48. The wear results are given in Table 3-51 to 

Table 3-54. The table includes the visual ranking of rod wear as well as weight change 

for rod, bushing, seal, and liner. Negative weight changes, i.e. weight losses, are in 

brackets – numbers not in brackets are weight gains. 

Figure 3-74 and Figure 3-75 show the wear data for the metal (4340 and AlNiBronze) 

bushings. Figure 3-76 to Figure 3-78 shows the statistical analysis for this metal bushing 

wear data. Figure 3-79 to Figure 3-82 show the wear data for bushings, Karon B liners, 

and nitrile seals from the 3rd and 5th half replicates. Average values are given in Figure 

3-83 and Figure 3-84. 

Table 3-51. L8 DOE Data - Baseline Metal Bushings

TEST WEIGHT CHANGE VISUAL

NO. ROD/BUSHING/CYCLES/LOAD ROD BUSHING SEAL A SEAL B RATING

BASELINE 

49A 49A-EHC/4340/80K/72 0.0041 0.0087 3 

49B 49B-EHC/4340/80K/72 (0.0108) (0.0021) 4 

 

50A 50A-EHC/4340/240K/288 0.0012 (0.0061) 9 

50B 50B-EHC/4340/240K/288 (0.0053) (0.0114) 9 

 

51A 51A-EHC/AlNiBr/80K/288 (0.0032) (0.0334) 3 

51B 51B-EHC/AlNiBr/80K/288 (0.0094) (0.0107) 3 

 

52A 52A-EHC/AlNiBr/240K/72 0.0035 (0.0181) 3 

52B 52B-EHC/AlNiBr/240K/72 (0.0149) (0.0104) 4 

 

53A 53A-WCCo/4340/80K/288 (0.0115) (0.0004) 2 

53B 53B-WCCo/4340/80K/288 (0.0036) (0.0168) 5 

 

54A 54A-WCCo/4340/240K/72 (0.0086) (0.2610) 3 

54B 54B-WCCo/4340/240K/72 (0.0045) (0.0471) 2 

 

55A 55A-WCCo/AlNiBr/80K/72 0.0029 (0.0018) 1 

55B 55B-WCCo/AlNiBr/80K/72 (0.0052) (0.0074) 1 

 

56A 56A-WCCo/AlNiBr/240K/288 0.0019 (0.0675) 8 

56B 56B-WCCo/AlNiBr/240K/288 (0.0090) (0.1493) 9 

 
122

Table 3-52. L8 DOE Data - 1st and 2nd Half Replicates

NO. ROD/BUSHING/CYCLES/LOAD ROD BUSHIN

G 

SEAL A SEAL B RATING

1ST L8 HALF-REPLICATE 

57A 57A-WCCoCr/4340/80K/288 (0.0294) (0.3301) 6 

57B 57B-WCCoCr/4340/80K/288 (0.0159) (0.0068) 2 

 

58A 58A-WCCoCr/4340/240K/72 (0.0196) 0.0018 1 

58B 58B-WCCoCr/4340/240K/72 (0.0001) 0.0018 1 

 

59A 59A-WCCoCr/AlNiBr/80K/72 (0.0470) (0.0059) 2 

59B 59B-WCCoCr/AlNiBr/80K/72 (0.0021) (0.0409) 3 

 

60A 60A-WCCoCr/AlNiBr/240K/288 (0.0292) (0.1150) 8 

60B 60B-WCCoCr/AlNiBr/240K/288 0.0033 (0.0370) 4 

 

2ND L8 HALF-REPLICATE 

61A 61A-EHC/Al/80K/288 (0.0033) 0.0055 9 

61B 61B-EHC/Al/80K/288 (0.0095) (0.0040) 7 

 

62A 62A-EHC/Al/240K/72 0.0003 0.0101 7 

62B 62B-EHC/Al/240K/72 (0.0113) (0.0048) 4 

 

63A 63A-WCCo/Al/80K/72 (0.0077) 0.0078 1 

63B 63B-WCCo/Al/80K/72 (0.0017) (0.0016) 1 

 

64A 64A-WCCo/Al/240K/288 (0.0021) 0.0016 2 

64B 64B-WCCo/Al/240K/288 (0.0001) 0.0003 1 

 
123

Table 3-53. L8 DOE Data - 3rd and 4th Half Replicates

TEST WEIGHT CHANGE VISUAL

NO. ROD/BUSHING/CYCLES/LOAD ROD BUSHIN

G 

SEAL A SEAL B RATING

3RD L8 HALF-REPLICATE 

65A 65A-EHC/NITRILE/80K/288 0.2933 (0.0131) (0.0751) (0.0423) 5 

65B 65B-EHC/NITRILE/80K/288 (0.0013) (0.0063) (0.0366) (0.0520) 8 

 

66A 66A-EHC/NITRILE/240K/72 (0.0073) (0.0073) (0.4355) (0.2570) 6 

66B 66B-EHC/NITRILE/240K/72 (0.0071) (0.0044) (0.2583) (0.2772) 9 

67A 67A-WCCo/NITRILE/80K/72 (0.0009) (0.0124) (0.0779) (0.1863) 1 

67B 67B-WCCo/NITRILE/80K/72 (0.0035) (0.0042) (0.0058) (0.0078) 2 

68A 68A-WCCo/NITRILE/240K/288 0.0026 (0.0101) (0.0288) (0.0373) 1 

68B 68B-WCCo/NITRILE/240K/288 0.0013 (0.0016) (0.0226) (0.0335) 2 

4TH L8 HALF-REPLICATE 

69A 69A-EHC/KARON/80K/288 0.0005 (0.0008) 1 

69B 69B-EHC/KARON/80K/288 0.0003 0.0622 1 

 

70A 70A-EHC/KARON/240K/72 (0.0006) (0.0010) 3 

70B 70B-EHC/KARON/240K/72 (0.0016) 0.0776 2 

 

71A 71A-WCCo/KARON/80K/72 0.0002 0.0003 1 

71B 71B-WCCo/KARON/80K/72 (0.0008) 0.0055 1 

 

72A 72A-WCCo/KARON/240K/288 (0.0071) (0.0098) 1 

72B 72B-WCCo/KARON/240K/288 0.0021 0.0541 1 

 
124

Table 3-54. L8 DOE Data - 5th and 6th Half Replicates

TEST WEIGHT CHANGE VISUAL

NO. ROD/BUSHING/CYCLES/LOAD ROD BUSHIN

G 

SEAL A SEAL B RATING

5TH L8 HALF-REPLICATE 

73A 73A-EHC/KARON/80K/72 0.0103 0.0035 2 

73B 73B-EHC/KARON/80K/72 0.0004 0.0091 2 

 

74A 74A-EHC/KARON/240K/288 (0.0017) 0.0034 1 

74B 74B-EHC/KARON/240K/288 (0.0008) 0.0102 1 

 

75A 75A-WCCoCr/KARON/80K/288 (0.0335) 0.0022 1 

75B 75B-WCCoCr/KARON/80K/288 (0.0011) 0.0077 1 

 

76A 76A-WCCoCr/KARON/240K/72 (0.0320) 0.0056 1 

76B 76B-WCCoCr/KARON/240K/72 (0.0320) 0.0101 1 

 

77A 77A-EHC/AlNiBr/80K/288 (0.0016) (0.0328) 9 

77B 77B-EHC/AlNiBr/80K/288 0.0022 (0.0235) 6 

 

78A 78A-EHC/AlNiBr/240K/72 (0.0065) (0.0463) 8 

78B 78B-EHC/AlNiBr/240K/72 0.0005 (0.0590) 7 

 

6TH L8 HALF-REPLICATE 

79A 79A-EHC/NITRILE/80K/72 (0.0016) (0.0018) (0.0604) (0.0783) 8 

79B 79B-EHC/NITRILE/80K/72 (0.0007) 0.0030 (0.0072) (0.0012) 7 

 

80A 80A-EHC/NITRILE/240K/288 (0.0021) (0.0040) (0.1539) (0.1256) 8 

80B 80B-EHC/NITRILE/240K/288 (0.0166) (0.0229) (0.0373) (0.0394) 8 

81A 81A-WCCoCr/NITRILE/80K/288 (0.0049) (0.0391) (0.1289) (0.2392) 2 

81B 81B-WCCoCr/NITRILE/80K/288 (0.0212) (0.0407) (0.0316) (0.0308) 2 

82A 82A-WCCoCr/NITRILE/240K/72 0.0023 0.0006 (0.0926) (0.2962) 2 

82B 82B-WCCoCr/NITRILE/240K/72 (0.0113) (0.0047) (0.0139) (0.0479) 1 
125

0

1

2

3

4

5

6

7

8

9

10

49A-EHC/4340/80K/72

49B-EHC/4340/80K/72

50A-EHC/4340/240K/288

50B-EHC/4340/240K/288

51A-EHC/AlNiBr/80K/288

51B-EHC/AlNiBr/80K/288

52A-EHC/AlNiBr/240K/72

52B-EHC/AlNiBr/240K/72

53A-WCCo/4340/80K/288

53B-WCCo/4340/80K/288

54A-WCCo/4340/240K/72

54B-WCCo/4340/240K/72

55A-WCCo/AlNiBr/80K/72

55B-WCCo/AlNiBr/80K/72

56A-WCCo/AlNiBr/240K/288

56B-WCCo/AlNiBr/240K/288

VISUAL

EHC WC-Co

Figure 3-74. Sliding Wear, L8 DOE – Visual Rankings for Baseline Metal Bushings

(0.0750)

(0.0600)

(0.0450)

(0.0300)

(0.0150)

0.0000

0.0150

49A-EHC/4340/80K/72

49B-EHC/4340/80K/72

50A-EHC/4340/240K/288

50B-EHC/4340/240K/288

51A-EHC/AlNiBr/80K/288

51B-EHC/AlNiBr/80K/288

52A-EHC/AlNiBr/240K/72

52B-EHC/AlNiBr/240K/72

53A-WCCo/4340/80K/288

53B-WCCo/4340/80K/288

54A-WCCo/4340/240K/72

54B-WCCo/4340/240K/72

55A-WCCo/AlNiBr/80K/72

55B-WCCo/AlNiBr/80K/72

56A-WCCo/AlNiBr/240K/288

56B-WCCo/AlNiBr/240K/288

ROD

BUSHING

EHC WC-Co

Figure 3-75. Sliding Wear, L8 DOE – Weight Loss for Baseline Metal Bushings
126

0 1 2 3 4 5 6 7

AC

B

AB

C

AD

A

D

Pareto Chart of the Standardized Effects 

(response is Visual R, Alpha = .10)

A: A

B: B

C: C

D: D

A B C D

-1 1 -1 1 -1 1 -1 1

2.5

3.3

4.1

4.9

5.7

Vis

ual 

Ra

tin

Baseline Visual 

Figure 3-76. Pareto Chart and Visualization of the Major Effects in the 

L8 DOE Based on Visual Wear Rating; Only Effects D and A (normal 

load and number of cyles) are Statistically Significant 
127

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

B

A

D

AB

AC

C

AD

Pareto Chart of the Standardized Effects

(response is Rod Wear Volume,

Alpha = .10)

A: A

B: B

C: C

D: D

A B C D

-1 1 -1 1 -1 1 -1 1

-0.00060

-0.00055

-0.00050

-0.00045

-0.00040

Ro

d 

We

ar 

Vol

um

e, 

cc

LG L8 Baseline Wear Volume Main Effects 

Rods

EHC

WC-CoCr

Coatings

Figure 3-77. Pareto Chart and Visualization of the Major Effects in the L8 

DOE Based on Rod Wear Volume; Only Effect C (coating material) is 

Statistically Significant 
128

A B C D

-1 1 -1 1 -1 1 -1 1

-0.009

-0.007

-0.005

-0.003

-0.001

Bu

shi

ng 

We

ar 

Vol

um

e, 

cc

LG L8 Baseline Wear Volume Main Effects Plots

Bushings

EHC

WC-Co

Cycles Coatings

0 1 2

B

D

AB

AD

C

AC

A

Pareto Chart of the Standardized Effects

(response is Bushing Wear Volume,

Alpha = .10)

A: A

B: B

C: C

D: D

Figure 3-78. Pareto Chart and Visualization of the Major Effects in the L8 DOE 

Based on Bushing Wear Volume; Only Effects A and C (number of cycles and 

coating material) are Statistically Significant 
129

0

1

2

3

4

5

6

7

8

9

10

66A-EHC/NITRILE/240K/72

66B-EHC/NITRILE/240K/72

65A-EHC/NITRILE/80K/288

65B-EHC/NITRILE/80K/288

68A-WCCo/NITRILE/240K/288

68B-WCCo/NITRILE/240K/288

67A-WCCo/NITRILE/80K/72

67B-WCCo/NITRILE/80K/72

50A-EHC/4340/240K/288

50B-EHC/4340/240K/288

49A-EHC/4340/80K/72

49B-EHC/4340/80K/72

53A-WCCo/4340/80K/288

53B-WCCo/4340/80K/288

54A-WCCo/4340/240K/72

54B-WCCo/4340/240K/72

Visual

Nitrile Seals Metal Bushing

EHC

EHC

WC-Co

WC-Co

Figure 3-79. L8 DOE Data for 3rd Half Replicate – Visual Ranking for Rod Wear 

(shows EHC and WC-Co on metal bushings and nitrile seals) 

-0.00600

-0.00500

-0.00400

-0.00300

-0.00200

-0.00100

0.00000

0.00100

0.00200

66A-EHC/NITRILE/240K/72

66B-EHC/NITRILE/240K/72

65A-EHC/NITRILE/80K/288

65B-EHC/NITRILE/80K/288

68A-WCCo/NITRILE/240K/288

68B-WCCo/NITRILE/240K/288

67A-WCCo/NITRILE/80K/72

67B-WCCo/NITRILE/80K/72

50A-EHC/4340/240K/288

50B-EHC/4340/240K/288

49A-EHC/4340/80K/72

49B-EHC/4340/80K/72

53A-WCCo/4340/80K/288

53B-WCCo/4340/80K/288

54A-WCCo/4340/240K/72

54B-WCCo/4340/240K/72

ROD

BUSHING

Nitrile Seals Metal Bushing

EHC WC-Co EHC WC-Co

Figure 3-80. L8 DOE Data for 3rd Half Replicate – Weight Loss (shows EHC and 

WC-Co on metal bushings and nitrile seals) 
130

0.0000

2.0000

4.0000

6.0000

8.0000

10.0000

73A-EHC/KARON/80K/72

73B-EHC/KARON/80K/72

74A-EHC/KARON/240K/288

74B-EHC/KARON/240K/288

75A-WCCoCr/KARON/80K/288

75B-WCCoCr/KARON/80K/28876A-WCCoCr/KARON/240K/72

76B-WCCoCr/KARON/240K/72

77A-EHC/AlNiBr/80K/288

77B-EHC/AlNiBr/80K/288

78A-EHC/AlNiBr/240K/72

78B-EHC/AlNiBr/240K/72

59A-WCCoCr/AlNiBr/80K/7259B-WCCoCr/AlNiBr/80K/72

60A-WCCoCr/AlNiBr/240K/288

60B-WCCoCr/AlNiBr/240K/288

Karon B Seals AlNiBronze Bushing

EHC WC-Co,Cr EHC WC-Co,Cr

Figure 3-81. L8 DOE Data for 5th Half Replicate – Visual Ranking for Rod Wear 

(shows EHC and WC-Co on AlNiBronze bushings and Karon B liners) 

-0.0060

-0.0050

-0.0040

-0.0030

-0.0020

-0.0010

0.0000

0.0010

0.0020

73A-EHC/KARON/80K/72

73B-EHC/KARON/80K/72

74A-EHC/KARON/240K/288

74B-EHC/KARON/240K/288

75A-WCCoCr/KARON/80K/288

75B-WCCoCr/KARON/80K/288

76A-WCCoCr/KARON/240K/72

76B-WCCoCr/KARON/240K/72

77A-EHC/AlNiBr/80K/288

77B-EHC/AlNiBr/80K/288

78A-EHC/AlNiBr/240K/72

78B-EHC/AlNiBr/240K/72

59A-WCCoCr/AlNiBr/80K/72

59B-WCCoCr/AlNiBr/80K/72

60A-WCCoCr/AlNiBr/240K/288

60B-WCCoCr/AlNiBr/240K/288

Weight loss

ROD

BUSHING

Karon B Seals AlNiBronze Bushing

EHC

EHC

WC-Co,Cr

WC-Co,Cr

Figure 3-82. L8 DOE Data for 5th Half Replicate – Weight Loss (shows EHC and WC-Co 

on AlNiBronze bushings and Karon B liners) 
131

3.5.7. Discussion of wear results 

3.5.7.1. Fretting 

Fretting tests showed very little wear. The HVOF coatings showed almost no wear 

against the nitrile seals, but seal wear was very unstable. We conclude that fretting is not 

a good test of wear, making fretting tests poor discriminators between the performance of 

EHC and HVOF coatings. 

3.5.7.2. Sliding wear 

At this point only the baseline wear against metal bushings has been statistically 

0.000000

2.000000

4.000000

6.000000

8.000000

10.000000

Baseline 1st half replicate 2nd half replicate 3rd half replicate 4th half replicate 5th half replicate 6th half replicate

Visual rating

Chrome rod

HVOF rod

Figure 3-83. Average Rod Wear for L8 Matrix - Visual Rating

-0.100000

-0.090000

-0.080000

-0.070000

-0.060000

-0.050000

-0.040000

-0.030000

-0.020000

-0.010000

0.000000

Baseline 1st half replicate 2nd half replicate 3rd half replicate 4th half replicate 5th half replicate 6th half replicate

W

eig

ht loss

Chrome rod

Chrome bushing/seal

HVOF rod

HVOF bushing/seal

Figure 3-84. Average Rod and Bushing/Seal Wear for L8 Matrix - Weight Loss
132

analyzed. The analysis leads to the following conclusions: 

 As we would expect, wear is strongly influenced by load and number of cycles 

(Figure 3-76). 

 As we have found in most rig and flight tests, WC-CoCr shows significantly less 

wear than EHC (Figure 3-77) 

 Also as we have found in rig and flight tests, WC-Co tends to wear the bushings 

more that EHC does (Figure 3-78). 

 Most factors were not statistically significant in discriminating between EHC and 

HVOF coatings in wear testing. Statistically insignificant factors were surface 

finish, lubrication, stroke, frequency, and temperature. This does not mean that 

these factors were not significant in wear, but that they did not discriminate 

between the performance of the different coatings. The only real surprise here is 

that surface finish did not discriminate, whereas in flight testing and rig testing, 

surface finish has been found to strongly affect seal wear and seal life, with 

smooth HVOF coatings giving much better performance than rough coatings1,2. 

Once the detailed statistical analysis is complete it will be possible to analyze the data 

more thoroughly. However, some observations can be made at this time: 

 In general, the two tests at each condition were reasonably consistent, especially 

in visual ranking. Weight change was usually large for both measurements or 

small for both (although small changes were often of opposite sign). This is not 

unexpected since the typical weight change was less than 0.01 grams in 880 

grams, or 0.001%, and small amounts of trapped oil and debris easily exceeded 

the true weight change. 

 Overall, rod wear tended to be less for HVOF coatings than for EHC, but bushing 

or seal wear tended to be higher. This is consistent with the detailed analysis 

above for bushings. The difference between EHC and HVOF coatings was 

especially clear for nitrile seals (Figure 3-83). There was no evident difference 

between WC-Co and WC-CoCr. 

 The visual ratings for rods run against Al-Ni bronze bushings tended to be higher 

(worse) because of metal transfer from the bushing to the rod surface. 

 Visual wear rankings were the lowest (best) for the Karon B liners. In most cases, 

the rod surfaces were nearly pristine after being tested against these liners. 

Weight loss measurements were not nearly as clear. 

3.5.8. Significance 

These wear tests generally bear out the findings of rig and flight tests. HVOF WC-Co 

and WC-CoCr both experience less wear than EHC. However, they tend to cause more 

wear in adjacent seals and bushings. In order to counteract this tendency users have 

successfully used two approaches – superfinishing the HVOF coating, and wear-coating 

the adjacent parts [3.4]. 
133

3.5.9. Conclusion 

Fretting tests are not useful for these types of wear situations, but sliding wear tests can 

discriminate between the different coatings and seal materials. Overall, HVOF coatings 

show less wear than EHC, but tend to cause more wear on adjacent components. Passes 

acceptance criteria
134

3.6. Impact data 

3.6.1. Data summary 

3.6.2. Rationale 

Areas coated with HVOF in place of chrome will often be exposed to potential damage 

from runway debris and larger items such as dropped tools. Impact testing was therefore 

divided into 

 high-velocity small particle impact simulating runway debris (evaluated by 

Gravelometry), and 

 low velocity large item impact simulating dropped tools, etc. (evaluated by 

Dropped Ball Impact testing). 

3.6.3. Specimen fabrication 

All specimens were fabricated from 4340 round bar 

 1” diameter, 6” long 

 Tensile strength and heat treat – 260-280 ksi per MIL-H-6875 

 Shot peen – 8-10A, S230, wrought steel shot, per AMS 2432, computer 

controlled, 100% coverage, except a 0.75” section at one end of the rod 

 Grit blast prior to coating – Specimens were gripped on the non-peened area and a 

5” section grit blasted: 

o For EHC with 180-220 grit alumina or glass beads 

o For HVOF coating with 54 grit alumina at 60 psi at 90° angle of 

impingement, per MIL-STD-1504. 

 Coating – per methodology in Section 3.6.4 below 

o Thickness – final thickness 0.003” and 0.010” (see Table 3-56). Coatings 

Table 3-55. Quick Data Locator (Click on item number to view) 

Item Item number 

Gravelometer test matrix and rankings (visual, touch, Ra) Table 3-58 

Gravelometer surface condition after testing Figure 3-87, 

Figure 3-88, 

Figure 3-89 

Circumferential cracking around the ball impact zone Figure 3-91 

Radial cracking around the ball impact zone Figure 3-92 
135

were deposited 0.002-0.003” thicker to allow for final grinding. 

 Embrittlement relief heat treat - 375°F for EHC plated samples only. 

 Grind after coating 

o For EHC low stress grind to 16∝” finish per MIL-STD-866 

o For HVOF low stress grind to 8∝” finish per BAC 5855 

3.6.4. Coating deposition methodology 

Specimens were coated , and where 

necessary embrittlement heat treated, by 

Southwest Aeroservice. Coatings were 

deposited under the SWA Boeing-approved 

specifications for EHC, WC-Co, and WCCoCr. Coating materials and thicknesses 

tested are summarized in Table 3-56. 

Samples were coated at Southwest 

Aeroservice (Tulsa, OK). Coating 

conditions were Southwest Aeroservice 

proprietary specifications, as follows: 

 Hard chrome – QQ-C-320B 

 WC-Co – DiamondJet gun. 

o SWA Control Specification UHP 520.1, technique SWATS 141 

o Powder – Amperit 526.062/895 

o Fuel – Hydrogen 

 WC-CoCr – DiamondJet gun 

o SWA Control Specification UHP 545, technique SWATS 140 

o Powder – Sulzer Metco 5847 

o Fuel – Hydrogen 

3.6.5. Test methodology 

3.6.5.1.Gravelometry 

Specification: ASTM D 3170, “Standard Test Method for Chipping Resistance of 

Coatings” 

Modifications: The standard test calls for flat panels and gravel sized between 9.5 

and 16 mm. 1” dia round rods were used to more closely 

approximate landing gear parts, and a non-standard gravel size was 

used. 

Significance: For use on landing gear, coatings must be able to withstand debris 

impact on landing and takeoff, when the coated sections may be 

Table 3-56. Hydrogen Embrittlement 

Coating Thicknesses (ground) 

Coating Low 

thickness 

High 

thickness 

EHC 0.003” 0.010” 

WC-Co 0.003” 0.010” 

WC-CoCr 0.003” 0.010” 
136

fully exposed. 

Sample types: 1” round rod (traditional flat samples were coated by Boeing and 

used for comparison.) 

Sample materials: 4340 

Sample heat treat: 260-280 ksi UTS 

Coatings: EHC, WC-Co, WC-CoCr, 0.003” and 0.010”, ground to 16∝” for 

EHC and to 8∝” Ra for HVOF. 

Test equipment: Gravelometer 

(see Figure 3-85). This 

equipment feeds a stream of 

gravel into an air jet, striking the 

sample at a defined angle. 

Impact on a round rod is normal 

incidence to grazing incidence 

since the gravel stream covers 

the entire diameter. 

Test description:

Tests were carried out at Boeing 

Seattle, using a non-standard 

gravel and a standard air pressure 

of 70 psi (approximately 200 

mph air-jet speed). 

1” diameter round bars were used. The round bars were coated by Southwest 

Aeroservice using their Boeing-approved specifications for EHC, WC-Co, and WCCoCr. Flat specimens coated independently for Boeing were included for comparison 

with prior Boeing work. 

Coatings and thicknesses tested are summarized in Table 3-57. 

After testing the surfaces were examined for damage and roughening as follows: 

 microscopically (visual ranking) 

 touch (finger nail) 

 Ra measurement by profilometry 

The JTP required only visual ranking of coated round rods. However, the additional 

ranking methods were added by Boeing for better comparison with prior experience. 

3.6.5.2.Dropped ball impact 

Specification: No existing standard. Followed method similar to that used by 

NAWCAD for similar evaluations. 

Modifications: NAWCAD method modified to evaluate impact within coated area 

Vibrating

gravel hopper

Plexiglass door Door

Air valve

Control

panel

Air

pressure

regulator

Access

door

Sizing

screen

Air vent

Dust

filter

Test

panel

Rubber

lining

13 3/8”

Figure 3-85. Gravelometry Chamber (Boeing) 
137

(rather than on edge). 

Significance: For use on landing gear, coatings must be able to withstand FOD 

by impact from large pieces of debris, dropped tools, etc. 

Sample types: 1” round rod 

Sample materials: 4340 

Sample heat treat: 260-280 ksi UTS 

Coatings: EHC, WC-Co, WC-CoCr, 

0.003” and 0.010”, ground 

to 16∝” for EHC and to 

8∝” Ra for HVOF. 

Test equipment: Impact system comprising 

guidance tube and 52100 steel ball, diameter 1 

7/8”, weight 0.97 lb (see Figure 3-86). 

Test description: 

Tests were carried out at Dayton 

T. Brown using the apparatus of 

Figure 3-86. A 0.97 lb ball 

bearing was dropped down a 

plastic tube (to ensure correct 

aim) to strike the rod radially. 

When the ball rebounded a stop 

was quickly inserted between the 

sample and the tube to limit each 

drop to a single impact. The ball 

was checked after each drop to 

ensure that it had not suffered 

any obvious damage or 

flattening. 

The damage zone was observed 

as a function of impact height. 

Single-impact tests were 

evaluated microscopically to 

determine the extent of cracking. 

Multiple impact tests (up to 50 

impacts at a single point) were 

also made. 

The coatings and thicknesses 

tested are summarized in Table 

3-57. Drop heights were 24”, 

60”, 72”, 93”, and 102”. 

Table 3-57. Gravelometry and Ball 

Impact Coating Thicknesses (ground) 

Coating Low 

thickness 

High 

thickness 

EHC 0.003-

0.004” 

0.010-

0.011” 

WC-Co 0.003-

0.004” 

0.010-

0.011” 

WC-CoCr 0.003-

0.004” 

0.010-

0.011” 

Figure 3-86. Close-up of Ball Impact Equipment, 

Showing Ball Delivery Tube and Sample in Vise
138

3.6.6. Test results 

3.6.6.1. Gravelometry 

Table 3-58 summarizes the evaluations made by Boeing. 

To supplement the observations in Table 3-58, micrographs were taken of selected 

sample surfaces. These are shown in Figure 3-87 to Figure 3-89. 

Table 3-58. Summary of Gravelometry Test Observations (Rankings from 1 (best) 

to 3 (worst) for different evaluation methods) 

Evaluation 0.003-0.004” 0.010-0.011” 

method EHC WC-Co WC-CoCr EHC WC-Co WC-CoCr

Visual 3 1 2 3 1 2 

Touch 1 2 3 3 2 3 

Ra* 1 1 2 1 1 2 

* Note: Boeing regards the profilometer readings as very questionable 

100∝m

0.003” 0.010” 0.010”

a

b

c

d f

e

Figure 3-87. Surfaces of EHC Coatings after Gravelometry Testing; a) and b) 

0.003”-thick EHC on Rods; c) and d) 0.010”-thick on Rods; e) and f) 0.010”-thick 

on Flats 
139

3.6.6.2. Dropped ball impact 

See Appendix 5 of the JTR [3.1] for raw impact data and micrographs. 

100∝m a

b

c

d

Figure 3-88. Surfaces of WC-Co Coatings after 

Gravelometry Testing; a) and b) 0.003”-thick on 

Rods; c) and d) 0.003”-thick on Flats 

100∝m a

b

c

d

Figure 3-89. Surfaces of WC-CoCr Coatings After 

Gravelometry Testing; a) and b) 0.003”-thick on 

Rods; c) and d) 0.010”-thick on flats 
140

For all impacts, including up to 50 drops from 72” onto a single point, there was no 

visible sign of cracking or coating damage on any sample. The only observable 

damage was an approximately 5mm x 2.5mm flattened area. Microscopic observation 

showed this area to be bounded by circumferential cracks of varying visibility, shown 

arrowed in Figure 3-90, and often some radial cracking along the bar axis (3 o’clock and 

9 o’clock locations in the figure). 

In order to make a comparative assessment, 100X micrographs were made of the 24”, 

60”, and 102” drop impact zones. The micrographs were taken at the 12 or 6 o’clock 

position on Figure 3-90 (whichever gave the clearest photograph) and similarly at the 3 or 

9 o’clock position. The number of large and fine circumferential cracks visible in the 

micrographs around the impact were counted separately, as well as the number of radial 

cracks at the 3 and 9 o’clock positions (i.e. parallel to the rod axis). (There were no 

radial cracks at the 12 and 6 o’clock positions on any sample.) 

Figure 3-91 shows the total number of circumferential cracks, and Figure 3-92 the total 

number of radial cracks. 

Figure 3-90. Impact Area on 0.010”-thick WC-CoCr , 72” Drop Height – 

Grazing Incidence Light (Bar axis is horizontal) 
141

Cr 24

WC-Co 24

WC-CoCr 24

Cr 60

WC-Co 60

WC-CoCr 60

Cr 102

WC-Co 102

WC-CoCr 102

0.003"

0.010"

0

1

2

3

4

5

6

7

# 

circumferential 

cracks

Figure 3-91. Circumferential Cracking Around Impact Point for 

0.003”- and 0.010”-thick Coatings at Different Ball Drop Heights Cr 24 WC-Co 24 WC-CoCr 24 Cr 60 WC-Co 60 WC-CoCr 60 Cr 102 WC-Co 102 WC-CoCr 102

0.003"

0.010"

0

1

1

2

2

3

3

# radial 

cracks

Figure 3-92. Radial Cracking Along Rod Axis for 0.003”- and 0.010”-

thick Coatings at Different Ball Drop Heights 
142

3.6.7. Discussion of impact results 

In both types of impact tests, results are determined by visual inspection. Consequently 

they are primarily qualitative and can only show quite significant differences. 

3.6.7.1.Gravelometry 

The results of Table 3-58 are based on the full tested area of the rods and flats, while the 

micrographs of Figure 3-87 to Figure 3-89 represent selected areas, chosen to illustrate 

different types of surface damage. Clearly, both coating materials can be damaged by 

gravel impact, but there is no obvious major difference between the types or extent of 

damage. Boeing’s overall conclusion, as shown in Table 3-58, is that HVOF coatings are 

a little better than chrome using the visual comparison specified in the JTP, and about 

equal using the touch and Ra rankings. 

3.6.7.2.Dropped ball impact 

Note: Because the reflectivity of the chrome was higher than that of the HVOF coatings, 

cracking was easier to see in the chrome, which could slant the data to some degree, since 

it is easier to see thin cracks in chrome than in HVOF coatings. However, the difference 

is not substantive enough to prevent observation of most cracks and invalidate the results. 

From Figure 3-91 and Figure 3-92 we conclude the following: 

 Cracking is clearly more extensive in hard chrome than in the HVOF coatings. 

 The number of cracks increases with drop height (i.e. with impact velocity or 

energy). The increase was more significant in chrome than in HVOF. 

 Chrome shows some radial cracking along the rod axis and the amount of 

cracking increases somewhat with drop height. HVOF coatings show no radial 

cracking,. 

 There is no evident dependence of damage on coating thickness. 

 WC-CoCr may show somewhat less cracking than WC-Co, but if so the 

difference is marginal. 

3.6.8. Significance 

While gravel impact performance is very similar, HVOF coatings are likely to be less 

sensitive than chrome to FOD by dropped tools or debris thrown up from the runway. 

This is likely to lessen the frequency of I-level repair due this type of problem. However, 

the need for repair is largely dictated by post-damage performance (such as corrosion, 

spalling, etc.) rather than by the initial damage itself. These tests provide no information 

on this issue, and because of the variability of field conditions this aspect of performance 

is likely only to be learned in the field. 
143

3.6.9. Conclusion 

Ball impact performance of both WC-Co and WC-CoCr is better than chrome, while 

gravel impact is equal to or better than chrome. Passes acceptance criteria.

3.7. Hydrogen embrittlement data 

3.7.1. Data summary 

3.7.2. Rationale 

In common with most other aqueous plating methods, the chrome plating process 

generates hydrogen at the surface being plated. This hydrogen can be absorbed into high 

strength steels, where it concentrates in high-stress regions and causes hydrogen 

embrittlement (HE). Under static stress cracks can initiate and grow rapidly in this 

embrittled steel, causing catastrophic failure. For this reason most electroplating 

specifications for high strength steels require a hydrogen bakeout (typically 375°F for up 

to 23 hours, depending on the type and strength of the steel) to remove the absorbed 

hydrogen. 

Corrosive liquids can breach chrome plate and HVOF coatings by penetrating through 

cracks and defects, causing substrate corrosion. This can happen during processing steps 

subsequent to chrome plating or HVOF coating, during service, or even during later 

overhaul cycles. The hydrogen produced during the corrosion process can also cause 

hydrogen embrittlement and failure under static load. In high strength steels, this is 

termed “environmental hydrogen embrittlement” and is commonly associated with the 

more general term “stress corrosion cracking”. 

The purpose of embrittlement testing in the JTP was to determine the relative effects of 

Table 3-59. Quick Data Locator (Click on item number to view) 

Item Item number 

Test matrix Table 3-61 

Average time to failure – data summary Table 3-65 

Sequence 1 average time to failure (process embrittlement) Figure 3-98 

Sequence 2 average time to failure (diffusion of H through HVOF) Figure 3-99 

Sequence 3 average time to failure (environmental embrittlement, 

unscribed) 

Figure 3-100 

Sequence 3 average time to failure (environmental embrittlement, 

scribed) 

Figure 3-101 

Sequence 3 – comparison notched and un-notched, DI water Figure 3-103 

Sequence 3 – comparison notched and un-notched, 5% NaCl Figure 3-104 
144

HVOF and EHC coatings on the susceptibility of 4340 steel to HE, in three test 

sequences: 

Sequence 1. To determine if the HVOF process causes hydrogen embrittlement. 

(Mechanical tests run at Metcut.) 

Sequence 2. To dermine if hydrogen can permeate through HVOF coatings, 

permitting areas adjacent to the HVOF coating to be electroplated and baked out by 

hydrogen diffusion through the HVOF. Since this Sequence was designed only to test 

hydrogen diffusion through HVOF coatings, there was no chrome baseline. 

(Mechanical tests run at Metcut.) 

Sequence 3. To determine whether HVOF-coated steel is more or less sensitive to 

environmental hydrogen embrittlement than chrome plated steel. (Tests run at 

Boeing.) 

As with all other JTP testing, the baseline was EHC. 

3.7.3. Specimen fabrication 

Specimens were standard ASTM F519, Type 1a.2 notched bars fabricated from 4340 

steel by Smith-Williston of Seattle. The notch tensile strength of the bars was measured 

at 373 ksi. A typical specimen is shown in Figure 3-93. 

In accordance with ASTM F-519, the uncoated specimen had a diameter of 0.333”, with 

a notch 0.049” deep and a notch tip radius of 0.010”. 

3.7.4. Coating deposition methodology 

Specimens were coated, and ground or 

given a hydrogen bakeout where necessary 

(see Table 3-61), by Southwest Aeroservice. 

Coatings were processed in accordance with 

Boeing-approved specifications for EHC, 

WC-Co, and WC-CoCr. Coating materials 

and thicknesses tested are summarized in 

Table 3-60. 

Southwest Aeroservice coating conditions 

were, as follows: 

 Hard chrome – QQ-C-320B 

 WC-Co – DiamondJet gun. 

o SWA Control Specification UHP 520.1, technique SWATS 141 

Figure 3-93. F-519, Type 1a.2 Specimen Coated with 0.010”-thick WC-Co

Table 3-60. Hydrogen Embrittlement 

Coating Thicknesses (ground) 

Coating Low 

thickness 

High 

thickness 

EHC 0.003” 0.010” 

WC-Co 0.003” 0.010” 

WC-CoCr 0.003” 0.010” 
145

o Powder – Amperit 526.062/895 

o Fuel – Hydrogen 

o SWA Control Specification UHP 545, technique SWATS 140 

o Powder – Sulzer Metco 5847 

o Fuel – Hydrogen 

Testing was done in three test Sequences as shown in Table 3-61, with the coating 

varying for each Sequence. 

Sequence 1 coating: 

Specimens were coated over most of the gauge length, as 

shown in Figure 3-94. Specimens were not ground, so as to 

represent a typical post-plating hydrogen bake prior to 

grinding. The EHC-coated specimens in the second column 

of Table 3-61 Sequence 1 were hydrogen baked (within 4 

hours of plating), while column 1 specimens were not. 

Sequence 2 coating: 

This Sequence required charging the specimen with 

hydrogen, which was accomplished by bright Cd plating and 

stripping without baking. Coating was as follows: 

 The entire specimen was bright Cd plated to a 

thickness of 0.0002-0.0005” in accordance with BAC 

5701. 

 Within 4 hours of plating the Cd was chemically 

stripped in the gauge section to a distance of 

approximately 0.7” from the end cap. 

 The exposed surface was grit blasted for HVOF 

coating. 

 Within 6 hours of Cd plating, HVOF was coated over 

the gauge section, overlapping the Cd on the ends, 

per Figure 3-94. The specimen was completely coated – with HVOF in the gauge 

section, and bright Cd on the ends. 

 For specimens in Sequence 2, column 2 (Table 3-61), a hydrogen bakeout (24 hrs 

at 375°F) was done within 4 hours of coating. 

Specimens were not ground. 

Sequence 3 coating: 

Specimens were coated as in Figure 3-94, and the coating ground to a 16∝” (Ra) finish. 

The coating in the notch was not ground. 

During HVOF spraying, the overspray material, which would normally bounce back off 

the surface, tended to become trapped in the notch, producing a thicker and more porous 

coating. This entrapment was minimized by directing a strong air stream into the notch 

6”

Coating

~0.5”

~0.5”

0.333”

0.50”±0.05” dia

0.6”

0.37”

0.235”

R=0.010”

Figure 3-94. Standard 

F-519 Specimen Coating 
146

and by spraying with the gun at an angle of 30° to the normal. During HVOF coating the 

specimen was rotated while the gun was traversed, angling the gun at +30° from the 

normal when traveling in one direction and -30° when traveling in the other. 

3.7.5. Scribing (exposing the notch to the test environment) 

The standard method used for scribing the notch is to cut through the coating with a 

diamond file. This works well for soft coatings but was not possible with hard HVOF 

coatings or hard chrome plating. Therefore a new method was developed using a 

commercially available shaped circular blade with a 45° angle and a 0.010” radius OD, 

which was coated with diamond. The method is illustrated in Figure 3-95. The blade 

was driven into the notch to cut just into the underlying steel. It was then rotated around 

the sample and withdrawn 45° away from the starting point, exposing a 45° segment of 

the substrate metal in a reliable and reproducible manner. Each scribed substrate was 

visually examined at 10x to ensure complete coating removal in the scribed area before 

the specimen was removed from the machining holder. 

3.7.6. Test methodology 

Specification: ASTM F-519, “Mechanical hydrogen embrittlement evaluation of 

plating processes and service environments” 

Modifications: Various modifications of sample preparation as described in text. 

Significance: Embrittlement is a primary failure mechanism for landing gear. 

Sample types: F-519 Type 1a.2 notched bar: 

Length: 6” 

Gage OD: 0.333” 

F 519 sample

Notch

Blade

Sample holder

Figure 3-95. Method for Scribing the Notch (exposing the substrate) on a CNC Milling 

Machine (Left – schematic of cutting blade in notch; right – actual set-up on CNC 

machine) 
147

Notch angle: 60°

Notch depth: 0.049” 

Notch radius: 0.010” 

Sample materials: 4340 per MIL-S-5000 

Sample heat treat: 52 Rc

Coatings: EHC, WC-Co, WC-CoCr, as-sprayed, and ground 

Coating thicknesses: 0.003” and 0.010” 

Test equipment: Static load creep rupture frame – one specimen per frame. 

 Test description: All tests were run under static load in air or in solution, 

depending on the experimental Sequence, as described in Table 3-61. Tests were 

stopped at 200 hours unless the specimen failed (broke) before this. 

For Sequences 1 and 2, samples were tested as-coated at 75% NTS. For Sequence 3, 

samples were tested at 45% NTS. During Sequence 3 tests, the test solution covered only 

the central portion of the gauge section, excluding any areas outside the coating. 

In all cases, because coating and testing were done in different locations, several weeks 

passed between coating and testing. 
148

* Chrome plated specimens hydrogen baked, HVOF specimens not baked 

** Loads determined by testing (see Table 3-63). 

3.7.7. Test sample mechanical properties 

Prior to 

embrittlement 

testing, the 

specimens were 

tested to verify 

Rockwell 

hardness and to 

determine their 

notch tensile 

strength (by 

loading to 

failure). Table 

3-62 and Table 

3-63 show the 

Table 3-61. Hydrogen Embrittlement Test Matrix 

Sequence 1 Sequence 2 Sequence 3 

Baseline Hydrogen 

pre-test 

loading 

Environmental embrittlement 

(re-embrittlement) in standard 

solutions 

Pretest H load No No Yes Yes No No No No 

Test environment Air Air Air Air DI 

H2O 

DI 

H2O 

5% 

NaCl 

5% 

NaCl 

Hydrogen bake No Yes No Yes No* No* No* No* 

Grind coating No No No No Yes Yes Yes Yes 

Notch partly exposed Yes Yes 

Load (% of NTS**) 75% 75% 75% 75% 45% 45% 45% 45% 

Coatings: 

None 3 3 3 6 6 

Hard chrome, 0.003” 3 3 3 6 3 6 

WC-Co, 0.003” 3 3 3 3 6 3 6 

WC-Co-Cr, 0.003” 3 3 3 3 6 3 6 

Hard chrome, 0.010” 3 3 3 3 

WC-Co, 0.010” 3 3 3 3 3 

WC-Co-Cr, 0.010” 3 3 3 3 3 

Total samples 21 6 15 15 18 24 18 24 

Table 3-62. F-519 Test 

Specimen Hardness 

Sample # Rc hardness 

H 2015 52 

 51.9 

 51.9 

H 2494 52.1 

 51.6 

 52.2 

H 2567 51.8 

 51.7 

 51.9 

Average 51.9 

Table 3-63. F-519 Test Specimen 

Notch Tensile Strength 

Sample # NTS, ksi 

H2494 362 

H2015 372 

H2567 383 

H2281 370 

H2537 380 

NTS 373 

75% NTS 280 

45% NTS 168 
149

hardness and notch tensile strength results. 

3.7.8. Coating structure 

Chrome plating into the notch produced a coating whose 

thickness in the groove bottom was 20%-50% that on the 

outer surface (see Figure 3-96, top picture). The HVOF 

coating in the base of the notch (Figure 3-96, bottom picture) 

was 2.5 - 7 times thicker than on the outer surface and had a 

layered structure with high porosity. This structure appears 

to comprise relatively dense layers sprayed directly into the 

notch, interleaved with porous overspray material. 

The cross-sectional details of HVOF WC-Co and of EHC in 

the notch are shown in Figure 3-97. The HVOF layering was 

only present in the notch, while the coating structure on the 

OD was that of a typical dense HVOF coating. 

Both EHC and HVOF coatings in the notch were left asdeposited because of concern that any machining or grinding 

in the notch would have been likely to lead to uncontrolled 

cracking or other specimen damage. 

Chrome

93.5∝m

0.0037”

93.5∝m

0.0037”

243.1∝m

0.0096”

HVOF WC-Co

31°

0.005”

0.001”

Figure 3-96. EHC and 

HVOF WC-Co in Notch 

of F-519 Specimens 
150

3.7.9. Test results 

3.7.9.1. Cracking of coatings 

On stressing to 45% NTS both the chrome and HVOF coatings cracked (Figure 3-97). 

The chrome plate exhibited many fine cracks around the notch radius, but the WC-Co 

showed only a few cracks penetrating to the substrate. After stressing, liquid penetration 

to the substrate would be expected through cracks in the chrome plate and through both 

cracks and porosity in the HVOF coating. 

3.7.9.2. Substrate failure mechanisms 

In Sequences 1 and 3 all failures were in the notch. All Sequence 2 failures were shank 

failures at the button (cracks emanating from the join between the button end and the 

gauge section), with the exception of a notch failure specimen H1216 (a specimen coated 

with 0.010”-thick WC-CoCr), which failed because of overloading. 

Sequence 3 failure surfaces were examined metallographically. Specimens that failed on 

loading showed no sign of embrittlement (i.e. they failed by overload instead of 

cracks

Figure 3-97. Metallographic Cross Sections of Chrome Plate (left) and HVOF 

WC-Co After Stressing to 45% NTS in Air (Note also the original layer 

formation in the HVOF coating) 
151

embrittlement), while the rest of the Sequence 3 specimens showed the following results: 

Bare specimens: 

 Single site nucleation; thumbnail-like appearance in test environment 

 Crack length (depth) about 1/4 notch diameter deep by 1/2 notch diameter wide 

 Classic intergranular fracture surface 

Chrome plated specimens: 

 Multiple through (surface to substrate) cracks occurred in the Cr plating at the 

notch when loaded to 45% of NTS in air 

 Multiple nucleation sites around notch in test environment 

 Shallow multiple-cracks approximately 1/8 notch diameter deep around most of 

circumference 

 Classic intergranular fracture surface 

HVOF coated surfaces: 

 Some cracking of coatings at notch when loaded to 45% NTS 

 Single site nucleation; thumbnail-like appearance in test environment 

 Crack length (depth) about 1/4 notch diameter deep by 1/2 notch diameter wide 

 Classic intergranular fracture surface 

In contrast to expectations, Sequence 3 crack initiation sites were most frequently not

associated with the exposed substrate produced by scribing in the scribe. This is 

summarized in Table 3-64. 

3.7.9.3. Time to failure data 

The test results are summarized in Table 3-65 and Figure 3-98 to Figure 3-104. The full 

data set summary is provided in Appendix 6 of the JTR [3.1]. The graph bars show the 

Table 3-64. Summary of Crack Initiation Points

Coating Unscribed area of notch Scribed area of notch 

 Crack assoc. 

with scribe 

Crack not

assoc. w/scribe 

Bare 6 specimens – thumbnail shaped crack N/A 

Cr plating 12 specimens – shallow 

circumferential crack 

none 6 specimens 

WC-Co 24 specimens – thumbnail shaped 

crack 

1 specimen 5 specimens 

WC-Co-Cr 3 specimens 3 specimens 
152

average Time-to-failure values. Because there were only three data points per condition, 

where error bars are given in these charts, they indicate the data range (high and low 

values), rather than the standard deviation. 

Table 3-65. F-519 Hydrogen Embrittlement Testing – Average Time to Failure (NF = 

No Failure in 200 hr) 

 Sequence 1 Sequence 2 Sequence 3 

 Distilled water 5% NaCl solution 

Not 

baked 

Baked Not 

baked 

Baked Unscribed Scribed Unscribed Scribed 

None NF NF NF 33.8 15.3 

EHC, 0.003” 0 NF 0.1 2.2 0.1 0.1 

WC-Co, 0.003” NF 6 139 4.7 9.8 3.4 6.8 

WC-Co-Cr, 0.003” NF 6 NF 15.6 36.8 8.3 14.3 

EHC, 0.010” 0 214 0.1 0.1 

WC-Co, 0.010” NF 6 NF 5.0 3.6 

WC-Co-Cr, 0.010” NF 27 NF 19.4 4.8 None EHC, 0.003” WC-Co, 0.003” WC-Co-Cr, 0.003” EHC, 0.010” WC-Co, 0.010”

WC-Co-Cr, 0.010”

Not baked

Baked

NF

NF NF

0

NF NF

0

NF NF

0

50

100

150

200

250

Avg hours to 

failure

Figure 3-98. Sequence 1: Average Hours to Failure (NF = No failure in 

200hrs), With and Without Hydrogen Bake (All failures in notch) 
153

 

None

EHC, 0.003”

WC-Co, 0.003”

WC-Co-Cr, 0.003”

EHC, 0.010”

WC-Co, 0.010”

WC-Co-Cr, 0.010”

Not baked

Baked

NF

139

NF NF NF

NF

6 6 6 27

0

50

100

150

200

250

Avg hours to 

failure

Figure 3-99. Sequence 2: Average Hours to Failure (NF = No failure in 200hrs), 

With and Without Hydrogen Bake (Failures occurred on shank at button) 
154

8.3 4.8

3.6

3.4

0.1 0.1

23.3

19.4 15.6

5.0

4.7

0.1 0.1

33.8

0.0

0.1

1.0

10.0

100.0

None EHC,

0.003”

EHC,

0.010”

WC-Co,

0.003”

WC-Co,

0.010”

WC-CoCr, 0.003”WC-CoCr, 0.010”

Time to failure (hr)

5% NaCl

DI H2O

Figure 3-100. Sequence 3 Average Hours to Failure With No Notch Scribing. (Numbers 

are average time-to-failure data values – note log scale; error bars indicate high and low 

values) 
155

23.3

0.1

6.8

14.3

33.8

2.2

9.8

36.8

0.0

0.1

1.0

10.0

100.0

None EHC, 0.003” WC-Co, 0.003” WC-Co-Cr,

0.003”

Time to failure (hr)

5% NaCl

DI H2O

Figure 3-101. Sequence 3: Average Hours to Failure With Notch Scribed to Expose 

the Substrate (Numbers are average time-to-failure data values – note log scale; 

Error bars indicate high and low values) (Note that these tests were run for 0.003”-

thick coatings only) 
156

None

EHC, 0.003”

WC-Co, 0.003”

WC-Co-Cr, 0.003”

EHC, 0.010”

WC-Co, 0.010”

WC-Co-Cr, 0.010”

NaCl not scribed

NaCl scribed

DI H2O not scribed

DI H2O scribed

0.0

10.0

20.0

30.0

40.0

Avg hours to 

failure

Figure 3-102. Sequence 3 Time-to-failure Data Summary 
157

0.1

33.8

15.6

4.7

2.2

36.8

9.8

0.0

0.1

1.0

10.0

100.0

None EHC, 0.003” WC-Co,

0.003”

WC-Co-Cr,

0.003”

Time to failure (hr)

Unscribed

Scribed

Figure 3-103. Sequence 3 – Comparison of Time to Failure for 

Scribed and Un-scribed Specimens - DI Water 

0.1 0.1

23.3

8.3

3.4

14.3

6.8

0.0

0.1

1.0

10.0

100.0

None EHC, 0.003” WC-Co,

0.003”

WC-Co-Cr,

0.003”

Time to failure (hr)

Unscribed

Scribed

Figure 3-104. Sequence 3 – Comparison of Time to Failure for 

Scribed and Un-scribed Specimens - 5% NaCl Solution 
158

3.7.9.4. Open circuit potential 

Open circuit potentials were also measured as a function of time for rods and unscribed 

coated sheets in 5% NaCl solution. Figure 3-105 shows the results for 100 hr exposure. 

Plotting time-to-failure against this long term average potential difference gave the graph 

of Figure 3-106. 

-0.800

-0.700

-0.600

-0.500

-0.400

-0.300

-0.200

-0.100

0.000

0 20 40 60 80 100

Time (hr)

Open Circuit Potential (V vs SCE)

4130 steel

Cr plating

WC-Co coating

WC-Co-Cr coating

Sheet Specimens

Figure 3-105. Open Circuit Potential for Coated Sheet Specimens in 5% NaCl Solution
159

3.7.10. Discussion of hydrogen embrittlement results 

Sequence 1: Figure 3-98 shows that chrome plated specimens fail very rapidly due to 

hydrogen embrittlement if they are not baked, but pass when baked. HVOF coated 

specimens pass with or without baking. Therefore the HVOF coating process does not 

cause hydrogen embrittlement. 

Sequence 2 is more complex. Figure 3-99 shows that hydrogen-charged specimens 

failed within a few hours without baking, but mostly passed when baked. (Note that this 

Sequence did not include chrome plated specimens since it is already known that 

hydrogen baking after chrome plating is required to remove the hydrogen and prevent 

embrittlement.) 

(Note that specimens that were Cd-plated, stripped in the gauge section, but not recoated 

or baked did not fail when tested several weeks later, whereas the traditional expectation 

is that they would fail very quickly. On the other hand Sequence 1 samples that were 

chrome plated but not baked, and also tested after several weeks failed very rapidly. This 

suggests that the hydrogen diffuses slowly out of an uncoated surface over time, but 

0 .0 1

0 .1 0

1 .0 0

1 0 .0 0

1 0 0 .0 0

0 100 200 300 400 500

A v e ra ge P o t e n t ia l D i ffe re n c e ( m V )

T ime t o Failure (ho urs)

Bare

Cr P latin g

3 mil

1 0 m il

W C-Co

W C -C o - C r

Figure 3-106. Plot of Log Time-to-failure vs. Average Potential Difference Between 

Coating and 4340 Substrate in 5% NaCl Solution 
160

remains trapped if the surface is coated.) 

Without baking the HVOF-coated specimens exhibited shank/end failures (i.e. failure on 

the shank at the button, not at the notch). This means either 

1. that the notch Kt was reduced because the coating blunted the notch and carried 

some of the load, moving the highest stress point to the shank/end, or 

2. most of the hydrogen was driven out from the notch by the local surface heating 

during the HVOF process, but this did not remove the hydrogen beneath the 

bright Cd at the ends of the specimen, because those areas were not heated by the 

HVOF flame. Failure therefore occurred at the shank/end. 

Metallography shows that the HVOF coating in the notch cracks at 45% NTS, preventing 

its supporting the load and reducing the Kt. Furthermore, the material was very porous 

and would have had poor mechanical properties, also reducing its ability to carry load. 

Based on this together with the data for Sequence 3 (discussed below) we believe the lack 

of notch failures to be entirely due to factor 2 above. 

With baking most of the specimens passed the test. Those that did not pass failed at the 

button. We interpret this to mean that the hydrogen at the high stress join between button 

and shank diffused out through the adjacent HVOF coating, since hydrogen cannot 

readily diffuse through bright Cd. Those samples that still failed did not have the 

hydrogen fully removed, probably because of insufficient bakeout time (i.e. the long time 

required to diffuse hydrogen either along the long path between the button and the 

HVOF-coated region or to diffuse it through the bright Cd). Note that the loss of 

hydrogen from the button/shank join on baking suggests that if most of the hydrogen 

diffused through the HVOF coating, it would have done so primarily through the highquality coating on the gauge section adjacent to the Cd-plated end, not through the poor 

quality coating in the notch, which was much further away. Therefore, hydrogen appears 

able to diffuse through high-quality HVOF coatings. 

Sequence 3 clearly shows that hard chrome performs very poorly in this type of test. 

HVOF coatings perform much better than chrome, with WC-CoCr outperforming WCCo, as expected. Figure 3-100 shows time to failure for samples with the notch not 

exposed (i.e. unscribed), while Figure 3-101 shows time to failure with the notch partially 

exposed (i.e. scribed). All of the data are summarized in Figure 3-102 for comparison. 

Figure 3-100 (unscribed coatings) shows the following: 

 While the specimens failed sooner in NaCl there was little difference between 

time to failure in 5% NaCl solution or distilled water. 

 Coating thickness had no significant effect on time to failure. 

 Failure occurred in EHC specimens within minutes – very much sooner than 

HVOF-coated specimens. 

 WC-CoCr had slightly increased time-to-failure than WC-Co. 

The relative time-to-failure performance of the HVOF coatings and chrome plating can 

be understood in terms of the open circuit potential difference between the substrate and 
161

the coating. Figure 3-105 shows that EHC has a much larger potential difference from 

the 4340 substrate than either HVOF coating, and over the long term the potential 

difference of EHC>WC-Co>WC-CoCr. When log time-to-failure is plotted versus this 

potential difference (Figure 3-106) the result is roughly linear, with the shortest time-tofailure being given by the largest galvanic potential difference. 

The data for scribed specimens (Figure 3-101) show results that are similar to the 

unscribed specimens, with HVOF coatings far outperforming EHC (note that a single 

EHC sample lasted 6 hours, while all others lasted less than 10 minutes). Again, WCCoCr slightly outperformed WC-Co. However, the data show that the scribed 

specimens in general outperformed the un-scribed specimens (see Figure 3-102 to 

Figure 3-104), but the time-to-failure of the uncoated specimens was longer than 

both. 

The data from scribed coatings, in which coating was removed around a quarter of the 

notch to expose the substrate, are more difficult to understand. One would have expected 

a priori that clear exposure of the surface would enhance substrate corrosion in the 

exposed area, and hence hydrogen generation (and with it embrittlement). One would 

therefore have expected that specimen failure cracks would be associated with scribes. 

However, in general this is not the case (Table 3-64). 

We believe that cracks in the substrate occurred in unscribed areas because corrodant 

could easily penetrate through cracks and porosity in the chrome and HVOF coatings. 

With corrodant easily reaching the metal substrate, the galvanic coupling effect is greater 

in coated (un-scribed) areas, because. the ratio of the cathodic coating area to the anodic 

substrate area is higher, and the anode and cathode are in very close proximity. This 

creates higher potential gradients with higher corrosion current densities and hence more 

local hydrogen. In contrast the scribed areas have lower current densities and hence a 

lower local hydrogen concentration. 

A question arises as to the effect of the poor quality of the HVOF coatings (thick, 

layered, high porosity) in the notch. It has been suggested that the combination of the 

thick coating in the notch that will lower the Kt, and the high elastic modulus of the 

coating will lead to much longer time-to-failure. On the other hand we might well expect 

that a porous coating in the notch will make corrosive attack easier and reduce time-tofailure. We can draw some inferences from our data: 

 As noted above, the thickness of the HVOF coatings in the notch could have 

reduced the Kt by increasing the notch radius. The fact that failures occurred at 

all (and especially occurred in a few tens of hours) implies that any diminution of 

Kt was fairly small. 

 When the coating was scribed, the true Kt was re-established in the scribed area 

because the notch was recut to the correct (smaller) radius. Rather than being 

reduced, which is what we would expect on sharpening the notch, the time-tofailure of notched specimens actually increased, again showing that the thick 

HVOF coating in the notch did not lower Kt. Furthermore, cutting through the 

coating would have eliminated any effect due to elastic modulus, because the 

coating was no longer continuous and therefore no longer able to carry load. If 

the elastic modulus of the coating had contributed to increased time-to-failure 
162

then failure time should have dropped on scribing, not risen. 

 We would expect a similar electrochemical potential between the substrate and 

either good quality or poor quality HVOF coatings, since the coating chemistry is 

the same. Therefore we would not expect any effect due to electrochemical 

potential, as seen between HVOF and hard chrome in Figure 3-105 and Figure 

3-106. 

 With low quality, porous HVOF coatings, liquid should penetrate to the substrate 

through the pores, as well as through the cracks that form on loading (which can 

be seen in Figure 3-97). With high quality, low porosity HVOF coatings, liquid 

will penetrate to the substrate primarily through the cracks that form on loading. 

We cannot tell a priori which type of coating will have higher time-to-failure, 

although it would be reasonable to expect that the more porous, lower quality 

coating would show shorter time-to-failure. 

 Therefore, although we cannot make a firm conclusion on the overall effect of the 

poor HVOF quality in the notch, we can say that it did not increase time-tofailure by decreasing the Kt or by providing a high-modulus surface layer. 

Therefore, although the quality and thickness of the HVOF coatings in the notch reduced 

the reliability of the results, it does not appear to have invalidated the overall trend. 

3.7.11. Significance 

These results imply the following 

 Since the HVOF process does not cause hydrogen embrittlement, it avoids any 

technical need for stress relief baking prior to coating (which is done to prevent 

HE during plating) as well as for hydrogen baking after (to prevent subsequent 

HE failure). 

 Since hydrogen can diffuse through HVOF coatings during a hydrogen bakeout, 

we would expect it to be possible to strip and electroplate areas adjacent to 

HVOF coatings without trapping the hydrogen and creating an embrittlement 

problem. This makes it a little easier to conduct depot maintenance, although in 

practice only Cr and LE Cd are generally used on high strength steels, both of 

which permit diffusion of hydrogen during baking. 

 The reduction in galvanic coupling on HVOF coated parts should reduce the 

incidence of environmental hydrogen embrittlement and stress corrosion 

cracking. Use of HVOF WC-Co in place of hard chrome should therefore reduce 

both post-maintenance HE failure and subsequent stress corrosion failure in 

service. According to OO-ALC and the Air Force Aging Landing Gear Life 

Extension Program, these are two common failure mechanisms for military 

landing gear. 

3.7.12. Conclusion 

Sequence 1 – The HVOF coating process does not cause hydrogen embrittlement. 

Passes acceptance criteria.
163

Sequence 2 – Hydrogen diffuses through HVOF coatings at 375°F, making it easier to 

strip and plate areas adjacent to them without trapping hydrogen and causing 

embrittlement. However, diffusion through HVOF coatings may occur at a slower rate 

than diffusion through EHC, and may therefore require a longer hydrogen bakout of any 

non-porous coating electroplated contiguous with an existing HVOF coating. Since the 

data on this point are not definitive, additional testing may be needed to verify this. No 

pass/fail criterion.

Sequence 3 – HVOF coatings perform much better than chrome under galvanic coupling 

conditions likely to cause environmental hydrogen embrittlement. Passes acceptance 

criteria.
164

3.8. Summary and Conclusions 

 Fatigue 

o The fatigue lives of landing gear steels coated with HVOF WC-17Co are 

in all cases equal to or better than those coated with EHC. 

o The HVOF coatings develop a circumferential crack pattern, and in some 

cases spall at high loads. 

 This coating integrity issue of cracking, delamination, and spalling 

is presently under detailed investigation, and the results will be 

reported when they are completed. Early data indicates that this 

behavior is a strong function of deposition conditions and coating 

thickness. Under axial stress, 0.003”-thick WC-17Co have 

retained their integrity to yield (220ksi) at R= -1, while 0.012”-

thick coatings have spalled. Under very limited R= -0.33 bending 

load testing, 0.010”-thick WC-Co coatings have survived to 

stresses above yield. Thus the problem appears not to be an OEM 

issue, but to be an issue primarily for thick repair coatings, and will 

require the definition of reliable repair schemes involving 

monolithic or duplex coating structures. 

 Corrosion 

o Corrosion results are highly variable. HVOF coatings clearly provide 

significant protection against corrosion. However, while pre-JTP work 

showed significantly better performance for HVOF, the JTP work and 

supplementary evaluations showed WC-Co and WC-CoCr performance as 

being inferior to EHC. 

 Since EHC is highly variable in corrosion performance, and since 

the chrome baseline material appeared to be exceptionally 

corrosion-resistant, we cannot draw conclusions on how HVOF 

coatings compare with EHC commonly used in the DOD repair 

community. 

 Combining data from our corrosion and embrittlement testing we 

conclude the following: EHC coatings do not corrode, but permit 

substrate corrosion through cracks and other imperfections. The 

Co binder in WC-Co coatings can itself corrode. The corrosion 

mechanism is therefore different between the two materials. In 

WC-CoCr the corrosion of the CoCr binder appears to proceed at a 

rate intermediate between that of Co and that of Cr. 

 Wear 

o Fretting tests were found not to be useful discriminants between coatings, 

but sliding rod/bushing wear tests could discriminate. 

o Although detailed statistical analysis has not been done, the overall 

conclusion is that HVOF WC-Co and WC-CoCr coatings show less wear 
165

than EHC, but that they tend to cause more wear on bushings. This is 

consistent with various rig and flight tests, and the industry has generally 

reduced bushing and seal wear (usually below that caused by chrome) by 

superfinishing the HVOF coatings. 

o There was no significant difference between the performance of WC-Co 

and WC-CoCr. 

 Impact 

o The performance of both WC-Co and WC-CoCr was better than EHC in 

both gravelometry and dropped ball impact. 

 These measurements relate only to damage sustained on impact 

and do no address the subsequent performance of the material. 

 Hydrogen embrittlement 

o HVOF does not cause embrittlement during deposition 

o Hydrogen can diffuse through HVOF coatings to permit embrittlementrelief heat treatment of electroplating or stripping operations subsequent to 

HVOF coating, although the diffusion rate may be slower than through 

EHC. 

o In environmental embrittlement testing WC-Co performed very 

significantly better than EHC, while WC-CoCr was slightly better than 

WC-Co. 

 These data are understandable in terms of the electrochemical 

potentials of the coating materials. 

3.9. References 

3.1 “Joint Test Report, Validation of WC/Co and WC/CoCr HVOF Thermal Spray 

Coatings as a Replacement for Hard Chrome Plating on Aircraft Landing Gear, Part I: 

Materials Testing,” Prepared by Hard Chrome Alternatives Team for Environmental 

Security Technology Certification Program, November 2002 (available from 

www.hcat.org) 

3.2 P. M. Natishan, S. H. Lawrence, R. L. Foster, J. Lewis and B. D. Sartwell, “Salt Fog 

Corrosion Behavior of High-Velocity Oxygen-Fuel Thermal Spray Coatings 

Compared to Electrodeposited Hard Chromium,” Surface and Coatings Technology, 

v. 130, pp. 218-223 (2000). 

3.3 Jay Randolph, Delta Airlines, “Service Evaluation Status and Impact of HVOF 

Coatings on Landin Gear at Delta Airlines,” Gorham Advanced Materials Conference 

on Advanced Coating Systems for Gas Turbine Engines and Aircraft Components, 

San Antonio, March 2000 

3.4 Tony DeGennaro, Green, Tweed & Co., “Evaluation of Chrome Rod Alternative 

Coatings,” Report #GTE0644, September 1999 
166

3.5 Naval Air Warfare Center Aircraft Division Report No. NAWC-ADLKE-DDR481400-0001

 
167

4. Coating Integrity and Spalling 

The coating integrity issue was first reported by C-HCAT when fatigue testing WC/CoCr 

specimens. They found that above a certain stress coatings tended to delaminate and that 

the stress at which this occurred was lower for thicker coatings. The finding led to 

extensive testing to understand and quantify the phenomenon. 

The Canadian team carried out extensive materials analysis and concluded that when the 

strain-to-failure of the coating was exceeded it would crack on the tensile cycle and the 

cracks would propagate into the coating to the interface with the underlying steel, where 

they would bifurcate and spread through the coating parallel to the interface. Eventually 

the cracks would spread far enough for the coating to delaminate, usually during a 

compressive cycle. 

4.1. Axial integrity tests 

Initial findings were made with standard 0.25” diameter fatigue specimens. Because of 

concern over whether coatings made on such small diameters were truly representative of 

those made on larger items, additional tests were run on 2” diameter hollow bars (Figure 

4-1 and Figure 4-2). However, data obtained from the large and small bars showed that 

the coating integrity behavior was very similar. 

The team has carried out testing on three primary materials: 

. WC/Co (HCAT and NAVAIR) 

. WC/CoCr (HCAT, small bars) 

. T400 (HCAT) 

Figure 4-1. HCAT Axial Integrity Big Bar Design (Metcut) 
168

In general it has been found that the spalling threshold for WC/10Co4Cr is lower than for 

WC/17Co, presumably because the WC/Co has more matrix metal, making it more 

ductile. 

However, more metallic materials, such as T400, do not show significantly superior 

integrity. As a result the team is currently engaged on a Process Mapping program 

funded by HCAT, ALGLE, and the thermal spray companies (Deloro Stellite, Sulzer 

Metco, and Praxair) to develop test methods capable of predicting spalling behavior. 

These methods will in turn be used in the development of higher integrity materials. 

Initial data from these tests indicates that the initial WC/CoCr optimization used for the 

Canadian tests accidentally had the effect of significantly reducing the ductility of the 

coating [4.1]. 

Tests have been run using large bars 

at R= -1, which NAVAIR [4.2] 

chose to simulate carrier landings, 

and at R= -0.33, which OO-ALC 

[4.3] chose to simulate land-based 

aircraft landings. Specimens were 

subjected to several cycles at stepped 

loads until spalling occurred. 

The results are shown in Figure 

4-3. It can be seen that the HCAT 

and NAVAIR tests are similar, but 

that thresholds vary considerably. 

Note that spalling occurs at lower 

stress for higher coating thickness 

and in general R= -1 conditions 

cause spalling at lower stress than 

R= -0.33. The areas below the red 

lines are considered by NAVAIR to 

be the operating regions of stress 

and coating thickness in which 

spalling will not occur. 

In axial tests the coatings tended to 

“rumple” somewhat prior to failure, 

and then to delaminate over most 

of the surface simultaneously. 

On this basis NAVAIR have so far 

concluded that for most cases they 

are not comfortable with the use of 

HVOF coatings for landing gear on 

carrier-based aircraft, although 

land-based Naval aircraft and 

helicopters do not appear to be a 

serious issue. 

5.0 in.

2.29 in.

Figure 4-2. NAVAIR “Big Bar” Axial 

Integrity Test Specimen (Eui Lee, NAVAIR) 

120

140

160

180

200

220

0.005 0.01 0.015

Coating thickness (inch)

Failure load (ksi)

NAVAIR, R=-1

HCAT, R=-1

NO VISIBLE 

SPALLING

120

140

160

180

200

220

0.005 0.01 0.015

Coating thickness (inch)

Failure load (ksi)

NAVAIR, R=-0.33

HCAT, R=-0.33

NO VISIBLE 

SPALLING

Figure 4-3. Axial Big Bar Integrity Tests (HCAT and 

NAVAIR). 
169

4.2. Landing gear bend integrity tests 

Ogden ALC has taken 

the approach that the 

test configuration should 

mirror the service 

conditions, in that 

almost all major landing 

gear components are 

tubes that experience 

bending stresses rather 

than tensile stresses 

(with the exception of 

some drag brace struts). 

For this reason OOALC testing has been 

carried out using scrap 

A-10 landing gear inner 

cylinders. The A-10 

landing gear was chosen 

since scrap items are 

readily available and A-10 gear sometimes come in for overhaul with obvious bending 

damage. Bend tests were performed on a rig set up at the University of Pennsylvania. 

The test rig is shown in Figure 4-4, with a specimen under stress. Even though this 

arrangement is designed to mimic service conditions Hill considers it a worst-case 

scenario since the cylinder is held in bushings in rigid blocks attached to a rigid bar. It 

has none of the flexibility provided by an actual landing gear assembly, which tends to 

spread the stresses between components. All tests were also made at full extension, 

whereas in actual landings the extension varies. This has the effect of concentrating the 

maximum bending stress and bushing line load at a single point in the test, rather than 

spreading them over a region of the inner cylinder as in an actual landing gear. 

All coating was done at OO-ALC using a TAFA JP5000 HVOF gun and optimized 

deposition conditions. The spalling thresholds for 0.010”-thick WC-Co coatings are 

shown in Figure 4-5. Most tests were carried out at R= -0.33, which the Air Force 

calculates is their expected R ratio for deflection and spring back. Only the brown point 

was taken at R= -1. Testing was done in a series of steps of increasing strain, using 50 

cycles per step to 190 ksi, and 10 cycles per step thereafter until coating failure. The 

force at the outer end of the bushing at the top of the bar was computed by the standard 

equation for the maximum stress at the outer fiber of a beam: 

I

Mc

σ max = , where M is the moment, c the radius, and I the moment of inertia. 

All data were taken while recording strain with a strain gauge. Even given the 

complexity and size of the system, the failure stress/strain points fell close to the 

expected elastic Mc/I curve. (In fact, as expected, they actually fell somewhat to the left 

of the Mc/I curve due to the small but finite contribution of the high modulus coating to 

Figure 4-4. Hill AFB Bend Test Fixture
170

the stiffness of the tube.) Since a structural engineer would not include the coating 

contribution in stress and strain calculations, in Figure 4-5 the contribution of the coating 

to the modulus is removed and these points are displayed on the Mc/I line. The stresses 

for these points can be read from the vertical axis, while the corresponding strains are 

indicated by the positions of the equivalent color points on the lower horizontal axis. The 

stress/strain curve computed by finite element analysis for this test cylinder, as well as 

the standard reference stress/strain curve (applicable to axial stress) are also shown on the 

graph. Note that all the R= -0.33 data lie above the yield point of the steel indicated by 

the 0.2% offset dashed line. 

It is important to note that under tensile stress 300M steel would broadly follow the black 

reference curve of Figure 4-5. At lower stress the bending and tensile stress/strain curves 

approach each other, but diverge strongly above yield. Clearly, under tensile stress in a 

stress-controlled test rig (such as those used by HCAT/Metcut and NAVAIR) the strains 

equivalent to these same stresses are larger, and increase rapidly as the curves diverge. 

Therefore we would expect to see failures at lower axial stress than bending stress once 

the proportionality limit is exceeded. 

CANTILEVER CYCLIC TEST RESULTS

Applied Stre ss a nd Strain (Ma xim um s) a t onset of Spa llation of HVOF Coa ting (0.010 in., as ground) W C-17Co)

on A-10 Piston (300M Stee l) Te st Article s

140,000

180,000

220,000

260,000

0.004 0.006 0.008 0.010 0.012 0.014

Strain in Me ta l

Stress (psi)

Te st #3 Te st #5 Te st #6 Test #8 Test #9 Test #1

Te st 

Strain

ANSYS FEA Bare Tube 

D/t=17, D=3.227 in. 

Reference Stress-Strain Curve 

300M Steel

0.2% Offset Yield Strength 

Elastic Bending Stress 

M c/I 

R = -1.0 

Test

R = -0.33 

Tests 

Figure 4-5. Bend Test Spalling for A-10 Landing Gear Cylinder Under Strain 

Control (0.010”-thick WC-Co, R= -0.33) (OO-ALC) 
171

The delamination seen in these 

tests takes place over a wide area 

in the region of maximum strain 

and is accompanied by an array of 

circumferential cracks (visible 

with FPI) where the strain in the 

coating exceeds its strain-tofailure. This is shown for a 0.015” 

coating in Figure 4-6, where the 

brown mark in the center of the 

field of view is the location of the 

bushing. Outside the bushing, in 

the area of maximum tensile strain, 

the coating has completely 

delaminated. (This does not 

happen in the maximum 

compressive strain area 180°

around the tube.) The area within the bushing shows some delamination and peeling. A 

view of the same area under FPI clearly shows the crack array (Figure 4-7). Note that the 

cracks are only found on the maximum tensile stress side of the tube and do not extend 

below the centerline. 

On the basis of their testing OOALC have concluded that 

spalling is not likely to be a 

serious issue for Air Force 

aircraft, and have begun to 

qualify components for HVOF 

overhaul, starting with the 

lowest-risk items. Twelve 

different items are qualified as 

of the time of writing. 

4.3. References 

4.1 Jean-Gabriel Legoux, National Research Council Canada, 22nd HCAT Program 

Review, San Diego, CA, April 2003 (available at www.hcat.org) 

4.2 Eui Lee, Naval Air Systems Command, 22nd HCAT Program Review, San Diego, 

CA, April 2003 (available at www.hcat.org) 

4.3 Craig Edwards, Hill Air Force Base, 22nd HCAT Program Review, San Diego, CA, 

April 2003 (available at www.hcat.org) 

Figure 4-6. Spallation of a 0.015”-thick WC/CoCr 

Coating from a Test Cylinder (OO-ALC) 

Figure 4-7. Same as Figure 4-6 , But Under FPI 

173

5. Component Rig and Flight Testing 

5.1. Rig Testing 

5.1.1. F/A-18 E/F Main Landing Gear Pins 

In 1999, Boeing St. Louis conducted a full-scale fatigue test, designated FT66, on the 

left-hand main landing gear (MLG) from the F/A-18 E/F aircraft. The FT66 assembly 

included a trunion, lever, axle, shock absorber, simulated side brace, retract actuator, and 

planing mechanism. The purpose of the fatigue test was to demonstrate, with a full-scale 

test article, that the re-designed MLG met the repeated load requirements of MIL-A8867. The success criterion for the entire test was that selected gear components had to 

complete the simulated flight hours (SFH) presented in Table 5-1 without failure. Failure 

was defined as any deformation or crack that would require repair, replacement or 

inspection to maintain structural integrity. In addition to the SFH testing, some 

components were subjected to constant amplitude testing (CAC) as well. In general, the 

simulated flight hours plus the CAC represented two lifetimes for the MLG. 

Prior to initiation of this test, there was an opportunity to include some attach bolts and 

pins that are normally coated with electrolytic hard chrome but which were instead 

Table 5-1. F/A-18 E/F Main Landing Gear Fatigue Simulated Flight 

Hour Success Requirements 
174

coated with HVOF WC/Co. These were: 

1. Upper oleo pin, part number 74A400645-7001 

2. Side brace to universal attach pin, part number 74A400748-2001 

3. Upper side brace attach pin, part number 74A400527-7001 

These components were not considered to actually be a part of the formal test and their 

failure would not have been considered a FT66 test article failure. Thus, this provided an 

excellent opportunity to assess the performance of the HVOF WC/Co coatings without 

increasing the risk to the entire test program. 

Initially the EHC plate was stripped from one each of the components and then WC/17Co 

coatings were applied in accordance with Boeing specification BAC 5851, Class 2, Type 

I using a Sulzer-Metco Diamondjet hybrid gun with hydrogen as the fuel gas and 

Diamalloy 2005 powder. Air cooling was used to ensure that the surface temperature of 

the components did not exceed 350o

 F during coating deposition. The coatings were 

deposited to a thickness of 0.006” and were then ground to a final thickness of 0.004” and 

surface finish of 8 (+0/-2) microinches Ra using low-stress grinding techniques. 

Figure 5-1. shows the location of the WC/Co-coated components in the MLG. For the 

actual test procedure, the MLG was cycled in 1000 SFH blocks to the total of 14,000 

SFH. During testing, general walk-around visual inspections were conducted at 500 SFH 

block intervals. Major non-destructive inspections were conducted at least every 2000 

SFH, beginning at 8000 SFH. 

Figure 5-1. Location of HVOF-coated Pins in Boeing F/A-18E/F 

Main Landing Gear Fatigue Test 
175

At the end of spectrum 

testing the HVOF coated pins 

showed only some darkening 

and staining, but no coating 

damage and no measurable 

wear. At the end of the CAC 

test the results were the same, 

except that there had been 

some material transfer from 

the Cu-Be bushings onto the 

HVOF surfaces. The three 

pins are shown at the ends of 

these two tests in Figure 5-2.. 

The conclusion of the testing 

was that the performance of 

the HVOF-coated pins was at 

least as good as that of EHCcoated pins, and that they 

“passed” the test. Boeing 

would require more testing to 

qualify the coating for other 

components in the landing 

gear such as the piston and 

axle journals. 

74A400645 

Oleo Attach 

Pin

74A40052

7 Upper 

Sidebrace

Attach Pin

74A400748 

Lower 

Sidebrace

Attach Pin

74A400645 

Oleo Attach 

Pin

74A40052

7 Upper 

Sidebrace

Attach Pin

74A400748 

Lower 

Sidebrace

Attach Pin

Figure 5-2. HVOF WC/Co-coated Pins Used in Boeing F/A18E/F MLG Rig Test (Left, end of 14,000 hr spectrum test; 

right, end of CAC test (Boeing St Louis)) 
176

5.1.2. P-3C Main Landing Gear Piston 

The four-engine turboprop P-3C was first introduced into the Navy fleet in 1968. 

Currently, the Navy operates 237 P-3C aircraft around the globe generally for antisubmarine warfare, search and rescue, mining and anti-surface warfare, and over-land 

intelligence, surveillance and reconnaissance. The Navy and its Foreign Military Sales 

partners initiated the P-3C Service Life Assessment Program (SLAP) to re-assess the P3C fatigue life in 1997. This program also determines structural inspections, 

modifications, replacements and redesigns necessary to extend the P-3C operational 

service life to meet inventory requirements through at least the year 2015. 

Phase II of the SLAP included the design of structural parts/assemblies to be included in 

Service Life Extension Program (SLEP) modification kits, development of a test spectra 

representative of projected P-3C Navy fleet usage, and testing of a P-3C aircraft 

containing the SLEP modification kits to the equivalent of two times the desired service 

life as a minimum. Lockheed-Martin was selected as the prime contractor for 

performance of the testing under the SLAP program. The total fatigue life capability of 

the P-3C aircraft was demonstrated using a production fleet aircraft. The test article 

consisted of the fuselage, wing, empennage, nose landing gear (NLG), main landing gear 

(MLG), engine nacelles and control surfaces. The test duration, which included all aging 

and testing cycles, was based on total accumulated fatigue damage from all loading 

sources rather than just from the Fatigue Test Spectrum Hours (FTSH) alone. The testing 

was intended to simulate the ultimate planned service life of the P-3C as follows: 

Flight hours: 30,000 

Number of flights: 8,802 

Total Landings: 47,154 

Intermediate full-stop landings 8,599 

Final full-stop landings 8,802 

Touch-and-go landings 29,753 
177

It was decided that as part of the test, the piston on the left-hand MLG would be coated 

with HVOF WC/17Co instead of EHC. A piston was shipped to Naval Aviation Depot 

Jacksonville where the hard chrome was removed using standard stripping techniques. 

Then the piston was sent to Southwest Aeroservice where it was grit blasted and 

WC/17Co coatings were applied in accordance with Boeing specification BAC 5851, 

Class 2, Type I using a Sulzer-Metco Diamondjet hybrid gun with Diamalloy 2005 

powder and with hydrogen as the fuel gas. Air cooling was used to ensure that the 

surface temperature of the components did not exceed 350o

 F during coating deposition. 

The coatings were deposited to a thickness of 0.006” and were then ground to a final 

thickness of 0.003” and surface finish of 8 (+0/-2) microinches Ra using low-stress 

grinding techniques. Figure 5-3. shows the MLG piston during application of the 

WC/17Co. 

Following processing the piston was delivered to Lockheed-Martin in Marietta, Georgia 

where it was assembled into the left-hand MLG which then was mounted onto the test 

article as shown in Figure 5-4. and Figure 5-5.. 

Figure 5-3. Application of HVOF WC/17Co Coating to P-3C 

MLG Piston 
178

The test was initiated in August 2001 and 

was suspended in April 2002 to perform 

repairs on some of the equipment that was 

applying the loads to the structure. The 

test was resumed in September 2002 and 

concluded in March 2003. The test was 

extended to a total of 38,000 simulated 

flight hours (SFH), with approximately 

250,000 cycles on the MLG representing 

47,000 landings (equivalent to two fatigue 

lifetimes). 

Following the test, the WC/17Co-coated 

piston was visually inspected with no 

evidence of any coating cracking or 

delamination. Fluorescent penetrant 

inspection of the WC/Co coating also 

showed no evidence of coating cracking or 

other type of deterioration. 

The conclusion was that the WC/Co 

coating successfully completed the fullscale fatigue test and therefore is qualified 

for use on operational aircraft. 

Figure 5-4. Left-hand MLG Containing HVOF WC/17Co-coated Piston 

Mounted on P-3C Test Article at Lockheed-Martin in Marietta, Georgia 

Figure 5-5. Close-up of HVOF 

WC/17Co-coated Piston in MLG 
179

5.2. Flight Testing 

5.2.1. E-6A Uplock Hook Shaft 

The E-6A is a Navy reconnaissance aircraft similar to the Boeing 707 (see Figure 5-6.). 

In February 1999 Naval Aviation Depot Jacksonville arranged through the E-6A program 

office to have two uplock hook shafts from the main landing gear coated with HVOF 

WC/17Co and installed on an aircraft operating out of Tinker Air Force Base. NADEP 

Jacksonville obtained two of the shafts and stripped the EHC coating from the journal 

areas. Then the components were shipped to Hitemco where the areas to be coated were 

grit blasted and then WC/17Co coatings were applied in accordance with Boeing 

specification BAC 5851, Class 2, Type I using a Sulzer-Metco Diamondjet hybrid gun 

with Diamalloy 2005 powder and with hydrogen as the fuel gas. Air cooling was used to 

ensure that the surface temperature of the components did not exceed 350o

 F during 

coating deposition. The coatings were deposited to a thickness of 0.008” and were then 

ground to a final thickness of 0.005” and surface finish of 8 (+0/-2) microinches Ra using 

low-stress grinding techniques. Figure 5-7. shows one of the uplock hook shafts 

following application of the WC/Co coating. 

Two WC/17Co-coated uplock hook shafts were installed on Aircraft 164388 in March 

1999 and immediately commenced operations at Tinker AFB. A third uplock hook shaft 

was installed on Aircraft 162784 in February 2000. Periodic inspections of the shafts 

were conducted. As of 28 February 2003, Aircraft 164388 had 3446 flight hours and 

2731 landings, and Aircraft 162784 had 3871 flight hours and 2756 landings. Inspections 

of the components indicated that there was no evidence of wear, delamination, cracking 

or other type of degradation of the WC/Co coatings. They are continuing in service. 

Figure 5-6. Navy E-6A Reconnaissance Aircraft 
180

5.2.2. P-3C Main Landing Gear Components 

In early 1999, Naval Aviation Depot Jacksonville obtained approval from NAVAIR to 

conduct flight testing of HVOF coatings on a P-3C landing gear. NADEP Jacksonville 

obtained a MLG Piston Assembly (Part Number 901022-1, Serial Number 703) and 

stripped the EHC coating from the piston cylinder and axle journals. Then the assembly 

was shipped to Hitemco where the areas to be coated were grit blasted and then 

WC/17Co coatings were applied in accordance with Boeing specification BAC 5851, 

Class 2, Type I using a Sulzer-Metco Diamondjet hybrid gun with Diamalloy 2005 

powder and with hydrogen as the fuel gas. Air cooling was used to ensure that the 

surface temperature of the components did not exceed 350o

 F during coating deposition. 

The coatings were deposited to a thickness of 0.008” and were then ground to a final 

thickness of 0.004”-0.005” and surface finish of 8 (+0/-2) microinches Ra using lowstress grinding techniques. 

The piston assembly was returned to NADEP Jacksonville where it was installed on 

Aircraft Number 156522. The aircraft completed overhaul in December 1999 and was 

returned to Squadron VP-30 operating out of Jacksonville Naval Air Station. Figure 5-8. 

shows the landing gear and the WC/Co-coated area of the piston. The coated axle 

journals cannot be seen since they are obscured by the wheels. 

Figure 5-7. E-6A Uplock Hook Shaft 

WC/Co WC/Co
181

The Navy P-3 Flight Support Team (FST) issued a Maintenance Engineering Advisory 

message to VP-30 indicating that this was a special test article. The P-3 FST and 

NADEP Jacksonville Material Engineering Laboratory coordinated periodic inspections 

in conjunction with normal squadron inspection intervals (normally 224 days) to monitor 

and record the performance of the WC/Co coating. Inspections were both visual and 

fluorescent penetrant. In August 2000 the component had been subjected to 850 landings 

and it was removed from service due to an internal oil leak not related to the WC/Co 

coating. The component was repaired, returned to VP-30, and installed on Aircraft 

160284 in April 2001. As of 29 March 2003 the component had been subjected to a total 

of 2133 landings combined on both aircraft. Visual and fluorescent penetrant inspection 

indicated no evidence of coating wear, delamination, cracking, corrosion, or other type of 

degradation of the WC/Co coatings. The landing gear is continuing in service. 

Figure 5-8. P-3C Main Landing Gear with HVOF 

WC/17Co-coated Piston Cylinder (indicated) and Axle 

Journals 

WC/Co

183

6. Producibility 

6.1. Stripping of HVOF coatings 

Stripping is a very important process for depot overhaul, since old coatings must be 

removed in order to rebuild worn or damaged components. Although any coating can of 

course be ground off, chemical stripping is generally preferred since it does not remove 

base material and since it minimizes limited personnel resources. Stripping of WC/Co 

and WC/CoCr takes place through dissolution of the matrix metal, which liberates the 

carbides. 

Several sets of tests have demonstrated the efficacy of the environmentally benign 

Rochelle salt stripping solution for both WC/Co and WC/CoCr. 

6.1.1. Stripping of HVOF WC/Co – Sacramento ALC, NADEP 

Cherry Point 

This work was done under AF Contract F04699-98-C-0002 CLIN 1 AY, and was headed 

by Elwin Jang of SM-ALC and Robert Kestler of NADEP Cherry Point, who authored 

the following report. 

Preparation of Test Specimens: Each specimen was engraved with a unique laboratory 

number. 

. Steel panel, AISI 1010, size 0.060” x 3.0” x 5”, 12 each 

. Steel rod, solid, Type 4340, size 5/8” diameter x 5”L, 1 each 

. Stainless steel rod, solid, Type PH13-8Mo, size 0.9 1.0” diameter x 5- 6”L, 1 

each 

Southwest Aeroservices in Tulsa, OK applied both HVOF coatings, using a Jet-Kote II 

spray-coating system equipped with robotic arm and hydrogen fuel. 

The coating materials were H.C. Starck’s Amperit 526 (WC-Co, 83/17) and Praxair’s AI1186 (WC-Co-Cr, 86/10/4). The vendor product descriptions and QC test results are 

summarized in Table 6-1. The coating thickness of each panel is tabulated in Table 6-2. 

The vendor subsequently aged the coated panels by shot peening to 0.008”-0.012” 

thickness, 200% coverage, using MIL Standard 330 high hard shot. 
184

Table 6-1. Vendor WC-Co and WC-Co-Cr Coatings QC Test Results 

Requirements Units Control Limit WC-Co WC-Co-Cr 

Visual inspection N/A No cracks, blisters, 

lumps, splatter, 

chipping, flaking, 

or discoloration 

allowed 

No cracks, 

blisters, lumps, 

splatter, 

chipping, 

flaking, 

discoloration 

No cracks, blisters, 

lumps, splatter, 

chipping, flaking, 

or discoloration 

Part temperature Deg F <350 <350 <350 

Substrates preheat 

temp. 

Deg F As required Remove 

moisture 

Remove moisture 

Powder composition Wt. % Co 15-18, C 4.9-

5.3, Fe <0.2 

Co 10, Cr 4, C 

3.25-3.9, Fe 0.7-

0.8 

Powder 

manufacturing 

method 

N/A Agglomerated & 

sintered 

Sintered & crushed

Powder size range Micron 10-53 10-53 10-53 

Macrohardness 

(superficial 

hardness) 

Rc >55 94.6, avg. of 10 95, avg. of 10 

Microhardness DPH300 >1000 1171, avg. of 10 1225, avg. of 10 

Tensile bond 

strength 

PSI >10,000 >13,254, avg. of 

5 

>12,933, avg. of 

5 

Bend test N/A No separation 

between coating & 

substrate 

No separation 

between coating 

& substrate 

No separation 

between coating & 

substrate 

Oxide content Vol. % <1 <1 <1 

Roughness (assprayed) Microinch 

Ra 

150-250 111 150 

Porosity (void 

volume) 

Vol. % <1 <1 <1 

Microstructural 

uniformity, 200x 

field 

Unmelted 

particle 

<3 0 0 
185

 

6.1.1.1.Masking, fixturing, and positioning specimens 

Masking specimens. All four sides of the panels were masked off by a 1/8-inch band of 

Military Specification MIL-P-23377 epoxy primer to preclude possible solution entry 

from the panel edges into the coating and substrate interfaces. Two coats of primer were 

applied, air dried for over 72 hours, and baked at 325o

 - 330oF for 23 hours. The 

effective dimensions of the average exposed panel area were about 2.25” by 4.25”, or 

about 9.56 square inches. Each specimen was stored in a sealed plastic bag. 

Specimen fixturing. A titanium fixture for anodizing was used for positioning the panels 

because a stainless steel fixture was not available. The panels were fastened to the fixture 

with screws. Each panel was positioned horizontally and alternated at a 90o angle. 

Table 6-2. Coating Thickness of Panels 

Specimen Number Coating Initial Coating Thickness, x 0.001”

1 WC-Co 4.86 

2 WC-Co 5.00 

3 WC-Co 5.16 

Avg. of 1-3 WC-Co 5.00 

5 WC-Co 11.50 

6 WC-Co 8.37 

7 WC-Co 8.60 

Avg. of 5-7 WC-Co 9.49 

9 WC-Co-Cr 3.70 

10 WC-Co-Cr 3.50 

11 WC-Co-Cr 4.10 

Avg. of 9-11 WC-Co-Cr 3.76 

13 WC-Co-Cr 8.13 

14 WC-Co-Cr 8.26 

15 WC-Co-Cr 8.04 

Avg. of 13-15 WC-Co-Cr 8.14 

Note: Panels #4, #8, #12 were used for laboratory tests. 
186

6.1.1.2.Process Solutions and Operating Conditions 

Rochelle salts solution: 

Sodium carbonate: 20 – 30 oz/gal 

Sodium potassium tartrate (Rochelle salts): 8 – 12 oz/gal 

Deionized water: Balance as needed 

pH: 11 – 12 

Operating temperature: 130o – 150oF 

Current: 40 – 80 A/ft2 DC, anodic 

Voltage: As required (~4 – 6 V DC) 

Tank dimensions: 48” x 48” x 48” 

Tank volume: 200 gallons 

Note: This was a new solution. 

Proprietary general purpose alkaline cleaner: 

Make-up Proprietary (Oakite) 

Operating temperature: 130o – 150oF 

Tank dimensions: 48” x 48” x 48” 

Tank volume: 200 gallons 

6.1.1.3.Stripping Process Used and Specimen Measurements Conducted 

The test procedure was as follows: 

1. Weigh each coated panel to 0.1 gram and measure the panel thickness to 0.1 mil 

at three points (1” from top, middle, and 1” from bottom) near the edges with a 

micrometer. (Note: The measuring accuracy was about 0.0002”-0.0005”) 

2. Fasten the panels numerically from the top of the fixture with the coating side up 

and in parallel. 

3. Immerse the fixture in the hot alkaline cleaner for 5 minutes. 

4. Immerse the fixture in a cold water rinse tank for 5 minutes. 

Run 1: 

1. Immerse in the Rochelle salt solution tank maintained at 150o

F at potential of 6 

volts and electrolytically strip for one hour. (Note: Since the titanium fixture was 

a poor electrical conductor, the current density was about 20-25 A/ft2 which was 

below the 40-80 A/ft2

 requirement. Therefore, the stripping rate was relatively 

slow.) 

2. Remove the fixture from the process solution tank. Rinse in two different cold 

water tanks for 5 minutes in each tank. Rinse in a hot water tank (140o

F) for 5 

minutes. Blow dry with filtered compressed air. 

3. Remove panels #2 (WC-Co, 0.005”-thick) and #6 (WC-Co, 0.0084”-thick) from 

the fixture to inspect and weigh. (Note: Inspection revealed that more coating 
187

was removed near the sides than in the center, apparently due to higher current 

density near the edges. There was discoloration or a very thin smudge film 

deposited on the panel surfaces. The epoxy maskant appeared to be intact.) 

Run 2, 3 and 4: 

1. Immerse the fixture in the Rochelle salt tank for 1.5 hours at a potential of 9 volts, 

to generate a current density of about 40 A/ft2. 

2. Repeat 2 above. Remove all panels from the fixture. Inspect panel surfaces, 

measure panel thickness and weigh. 

6.1.1.4.Stripping test measurements 

The coating areas around the edges of the panels were stripped faster than the central 

area, presumably because of higher current density around the edges (edge effects). 

Since the thickness was only measured at three locations at one edge in each panel using 

a micrometer, the stripping rate was characteristic of one edge, rather than the entire 

Table 6-3. Percent Coating Thickness Removal 

Specimen No. Coating/Avg. 

Mils 

% Thickness 

Removed 

After 2.5 Hr. 

% Thickness 

Removed 

After 4.0 Hr. 

% Thickness 

Removed 

After 5.5 Hr. 

1 WC-Co Not available 91.8 ~100 

2 WC-Co 94.6 ~100 ~100 

3 WC-Co 81.4 96.9 ~100 

Avg. 1-3 WC-Co/5.00 88.0 96.2 ~100 

5 WC-Co 78.0 <100 <100 

6 WC-Co 78.5 <100 <100 

7 WC-Co 65.5 87.6 <100 

Avg. 5-7 WC-Co/9.49 74.4 98.4 <100 

9 WC-Co-Cr 91.9 99.2 ~100 

10 WC-Co-Cr 96.3 ~100 ~100 

11 WC-Co-Cr 81.2 95.1 ~100 

Avg. 9-11 WC-Co-Cr/3.76 89.6 98.9 ~100 

13 WC-Co-Cr 50.4 94.7 <100 

14 WC-Co-Cr 62.1 99.6 <100 

15 WC-Co-Cr 87.1 89.2 <100 

Avg. 13-15 WC-Co-Cr/8.14 66.5 94.6 <100 
188

panel. At the completion of the last run, there was almost no coating remaining on the 

thinly coated panels: panels #1-#3 and #9-11. There were some paint residue remaining 

on the thickly coated panels: #5-#7 and #13-#15. It did not appear that the minor stains 

or smudges on the coating surfaces would have impeded the stripping rate. The epoxy 

primer maskant was in good condition with minor amounts flaked off. 

The thickness measurements in Table 6-3 and the weight measurements in Table 6-4 

indicate that the stripping rates of WC-Co and WC-Co-Cr coatings are quite similar. 

Almost all of the coatings were stripped from all panels after four hours. In the last 

column of Table 6-3 and Table 6-4, “~100%” means practically no coating residue left on 

the panel; whereas, “<100%” means small amounts of coating residue remained near the 

center of the panel. 

The data are summarized in Figure 6-1 

Table 6-4. Percent Coating Weight Removal 

Specimen 

Number 

Coating % Wt. 

Removal 

After 1.0 

Hr. 

% Wt. 

Removal 

After 2.5 

Hr. 

% Wt. 

Removal 

After 4.0 

Hr. 

% Wt. 

Removal 

After 5.5 

Hr. 

1 WC-Co 92.85 98.57 ~100 

2 WC-Co 29.28 92.14 97.86 ~100 

3 WC-Co 91.49 97.87 ~100 

Avg. 1-3 WC-Co 92.16 98.10 ~100 

5 WC-Co 55.07 84.06 <100 

6 WC-Co 20.19 53.99 82.63 <100 

7 WC-Co 67.06 99.22 <100 

Avg. 5-7 WC-Co 58.71 88.64 <100 

9 WC-Co-Cr 95.92 98.98 ~100 

10 WC-Co-Cr 94.95 97.98 ~100 

11 WC-Co-Cr 97.06 99.02 ~100 

Avg. 9-11 WC-Co-Cr 95.98 98.66 ~100 

13 WC-Co-Cr 62.32 86.51 <100 

14 WC-Co-Cr 86.42 98.64 <100 

15 WC-Co-Cr 83.48 98.21 <100 

Avg. 13-15 WC-Co-Cr 77.41 94.45 <100 
189

6.1.1.5.Post Stripping Inspections and Cross-Sectional Examination 

Visual Inspection: The coatings on the thinly coated panels, #1-#3 and #9-#11, were 

completely stripped. The thickly coated panels, #5-#7 and #13-#15, had some coating 

residue on central areas. 

The bare sides of the panels showed stains and rust, apparently caused by handling, and 

immersion in alkaline solution and stripping solution. The epoxy primer maskant was 

mostly intact with minor chipping. 

Microscopic Inspection: After stripping the panels showed staining but no etching. 

6.1.1.6.Conclusions and Recommendations 

Proposed chemical or electrochemical stripping acceptance criteria for HVOF coatings is 

as follows: 

. Achieve a stripping rate of at least 0.001” per hour. 

. Does not etch or deplete the substrate after subjecting to 10 hours of immersion or 

electrolytic etching. (The allowable amounts of etching or depletion are to be 

established based on future test results.) 

. After subjecting the specimens to 10-hours of the stripping processing, they are 

Percent Weight Loss

0

20

40

60

80

100

120

0 Hr 1.0 Hr 2.5 Hrs 4.0 Hrs. 5.5 Hrs

Stripping Time

Percent Weight Loss

WC-Co, 5.00 mils

WC-Co, 9.49 mils

WC-Co-Cr, 3.76 mils

WC-Co-Cr, 8.14 mils

Figure 6-1. Weight Loss as a Function of Stripping Time 
190

able to comply with the hydrogen embrittlement requirements of ASTM 519. 

. The process solution does not contain known carcinogens or extremely toxic 

chemicals. 

. A standard process in a conventional wastewater treatment plant can recycle the 

rinse water. 

The Rochelle salt electrochemical stripping process for WC-Co and WC-Co-Cr coatings 

nearly complies with the above criteria. The substrate weight loss due to prolong etching 

was inconclusive. However, visual and microscopic (20X) examinations showed no 

evidence of depletion or pitting. The stripping rates for WC-Co and WC-Co-Cr coatings 

are comparable. The team recommended that a stripping test procedure and a stripping 

specification for production parts be developed for standardization. 

Recommendations for future stripping tests: 

. Use a stainless steel fixture to obtain higher current density in electrolytic 

stripping. 

. Use rods as test specimens as most production parts are cylindrical. Because of 

non-uniform stripping due to edge effects use flat panels for stripping tests only if 

actual production parts to be HVOF-coated and stripped have flat surfaces. 

. Allow the use of glass bead blasting for removing coating residue on parts. 

. Use the same specimen substrates as the production parts (such as 4340 steel, 

PH13-8Mo stainless steel, and 7075 aluminum). 

. Accurately weigh and measure all test specimens at the following times: before 

coating, after coating, at several intervals during stripping, and after stripping 

(when clean and dry). Take thickness measurements that represent the entire 

surface, near its centers, edges, and corners. 

. Conduct tests to identify more effective maskants than Mil-P-23377 epoxy primer 

for use in electrochemical stripping. Determine suitable maskants for each 

stripping chemistry. 

. Develop standards and procedures for aging coated specimens. 

. Apply a thin film of corrosion protective oil to stripped low alloy steel parts to 

prevent rusting. 

6.1.2. Stripping of HVOF WC/CoCr – Heroux Devtek, Canada 

As part of their producibility testing for the Canadian HCAT, Heroux Devtek has tested 

various stripping solutions for WC/10Co4Cr on 4340 high strength steel. They measured 

dissolution rates with cyclic voltammetry Tafel curves, weight loss, and hydrogen 

embrittlement. Heroux checked several chemistries (Table 6-5). The S1 series were 

strippers commonly used for EHC and Ni, while the S2 series were strippers 

recommended for WC-Co. 
191

Figure 6-2 shows that WC/CoCr coatings can be stripped at potentials at which the 

substrates are inactive, making it possible to strip the coating without substrate 

dissolution. 

The solution that worked best, was the cyanide Ni stripper, type S1-3. It had the highest 

stripping rate and a uniform strip. This solution is often used for stripping Ni, but it has 

obvious ESOH issues. 

Rochelle salt was found to work well (Figure 6-4), but the WC particles tended to be 

eliminated before the alloy matrix was dissolved. Note that the higher potential gave 

much more rapid stripping. Metal XB929 Ni stripper also worked well, but was slower 

than Rochelle salt (Figure 6-3). 

Figure 6-2. Polarization Curves for WC/CoCr and 4340 

Substrate with the Stripping Solutions (Heroux Devtek) 

Table 6-5. Stripping Solutions Tested by Heroux Devtek for HVOF WC/CoCr

Designation Chemistry Performance 

S1-1 NaOH + Na2CO3 (Cr strip, MIL-STD-871) Self-limiting 

S1-2 NaOH (Cr stripping, MIL-STD-871) Self-limiting 

S1-3 Meta-nitrobenzane sulfonate + NaOH + Na 

cyanide (Ni strip, MIL-STD-871) 

Works best, uniform, but 

ESOH concerns. Rate = 5.2 

mils/hr 

S2-1 Na2CO3 + sodium tartrate (Rochelle Salt), 

used for WC-Co stripping 

Works well. Rate = 4.1 

mils/hr 

S2-2 Metal XB929 (commercial Ni strip) Works well. Rate = 3.6 

mils/hr 
192

Ultrasonic agitation was found to greatly improve the stripping rate (Figure 6-5), 

presumably by removing stripped material (especially the carbide particles) from the 

surface. Ultrasonic agitators can be immersed in a standard stripping bath. 

As we would expect from Figure 6-2, there was no hydrogen embrittlement since there 

was no dissolution of the substrate metal.

Figure 6-3. Stripping Rate of WC/CoCr by Type S2-2, Ni 

Stripper (Heroux Devtek) 

Figure 6-4. Stripping Rate of WC/CoCr by Type S2-1, 

Rochelle Salt Stripper (Heroux Devtek ) 
193

6.2. Grinding 

Grinding methods have been developed by Jon Deveraux of NADEP Jacksonville for 

HVOF carbide coatings, and these are the subject of an AMS specification currently in 

the approval process (see Section 8.4). 

For grinding it is particularly important that the coating be ground efficiently with as little 

danger as possible of overheating (grind burning) the substrate steel. Grind burning is 

quite common with EHC and many manufacturers check for it as a matter of course with 

Barkhausen Noise (Rollscan) equipment. Prior qualitative history had suggested that it 

was difficult, but not impossible, to grind burn through an HVOF coating. 

As part of their Canadian-HCAT work, Heroux Devtek carried out extensive grind testing 

[6.1]. They came to several important conclusions: 

. Higher wheel speed (5000 to 7000 SFM) improves the finish. 

. High work speed with low crossfeed improves the finish and decreases the total 

time of grinding, but optimum infeed is about 0.0002 in. Higher infeed decreases 

the finish and damages the coating. 

. Lower grit number (larger abrasive particles) decreases productivity. Higher grit 

numbers obtain the same finish in a shorter time than lower ones. 

. It is possible to grind burn the substrate when grinding the HVOF coating, 

although it is not as easy to do as with EHC. 

. Abusive grinding is not necessarily evident to the naked eye. A finish that looks 

good to the eye can mask grind burns or coating damage. 

. The best finish is obtained by grinding to a reasonable (but not very low) Ra and 

finishing with superfinish stone or tape. This typically gives a surface finish of 

<2∝” Ra and 95-99% bearing ratio, Tp. (Note that for hydraulics a Tp in the 60-

Figure 6-5. Effect of Ultrasonic Agitation (Type 

S1-3 Stripper) 
194

70% range may be preferred to better retain hydraulic fluid.) 

6.3. Fluid compatibility 

Heroux Devtek has assessed fluid compatibility through a combination of Tafel plots of 

dissolution rate, immersion test, and F-519 hydrogen embrittlement testing [6.2]. A wide 

range of production, overhaul, and service fluids were tested, with the results outlined in 

Table 6-6. 

The only serious issues were possible hydrogen embrittlement effects from 

. Oakite alkaline cleaning solution – a high dissolution rate was measured in Tafel 

plots for Oakite electrochemical cleaning. Embrittlement failure occurred after 54 

– 148 hours (spec calls for >200 hours). 

. Nital etch (used as a test for grind burns on steel after grinding and prior to 

coating) – Since by definition the solution etches and creates hydrogen, standard 

practice is to bake for 4 hours at 375°F. The 4 hour bake did not completely 

eliminate embrittlement failure, suggesting that hydrogen cannot migrate through 

HVOF WC/CoCr as readily as through EHC. A 12 hour bake did eliminate 

failure (Note: Slower hydrogen evolution through the HVOF coating in accord 

Table 6-6. Fluid Compatibility of HVOF WC/CoCr (Heroux Devtek) 

Fluid Compatibility results 

Organic materials

Corrosion preventive 

products 

No weight or finish change, no dissolution or embrittlement 

Greases No weight or finish change, no dissolution or embrittlement 

Degreasers No weight or finish change, no dissolution or embrittlement 

Hydraulic fluids No weight or finish change, no dissolution or embrittlement 

Grease + hydraulic 

fluids 

No weight or finish change, no dissolution or embrittlement 

Aqueous solutions

Cleaning solutions No weight or finish change, no dissolution or embrittlement. 

Except: Oakite cleaning solution – Tafel plot shows 0.5 mil/hr 

dissolution rate after 5 min exposure; some embrittlement 

failures seen if not baked. 

Inspection related 

products (FPI, MPI, 

nital etch) 

No weight or finish change, no dissolution or embrittlement. 

Except: Nital etch – embrittlement failure if not baked or given 

standard 4 hr, 375°F bake. Heroux recommended 12hr bake. 

Plating and stripping 

solutions 

No weight or finish change, no dissolution or embrittlement 
195

with the results of the HCAT embrittlement testing, see Section 3.7). 

The only serious concern is with Nital etching, which is very commonly done to test for 

grind burns (the part is immersed, so nital etching to check one area exposes the entire 

part to the acid). If a much longer bake is required this will seriously impact the 

efficiency of the production or overhaul process, losing some of the efficiency gained 

through the use of HVOF coating. However, the nital etch sequence is as follows: 

Cleaning in oakite . Immersion in nital . Rinse . Immersion in HCl . Rinse .

Immersion in NaOH . Rinse. Since Oakite itself was found to cause embrittlement it is 

possible that the cleaning contributed to the hydrogen level and therefore to the time 

required for bakeout. 

6.4. Non Destructive Inspection 

6.4.1. Fluorescent penetrant inspection (FPI) 

The simplest method for observing 

cracks, pull outs, delamination, and 

other coating damage has been 

found to be fluorescent penetrant 

inspection (FPI). The major 

advantage of FPI is that all depots 

are set up for it and it permits rapid 

examination of large areas. Figure 

6-6 shows that FPI can clearly bring Figure 6-6. Damage Pit in HVOF WC/CoCr 

Coating (Left: standard; right: FPI) 

Figure 6-7. FPI of HVOF-coated Landing Gear After Fatigue to Stress 

Sufficient to Cause Coating Delamination, Showing Circumferential Coating 

Cracks (C. Edwards, OO-ALC) 
196

out damage in a coated surface. 

FPI is also capable of detecting cracks in the coated surface caused by high strain cycling. 

Figure 6-7 shows the surface of an A-10 landing gear piston coated with 0.015” WC/Co 

and cycled at 125 ksi [6.3]. FPI clearly shows both a major delamination and cracking of 

the coating. 

6.4.2. Magnetic particle inspection (MPI) 

MPI has been found to be not very useful for detection of cracks and damage in HVOF 

coatings (see Table 6-7). 

6.4.3. Ultrasonic inspection 

Ultrasonic inspection is not widely used in the depots as it is a specialized technique that 

is very slow when inspecting large areas. 

Rupel and Pfeiffer of Boeing St Louis have tested several measurement methods for 

detection of cracks beneath HVOF and EHC coatings [6.4]: They made cracks in large 

tensile specimens by machining a starter notch, stressing to create a crack 0.25” long, and 

then machining off the starter notch and overcoating. The results of their evaluation of 

several detection methods are summarized in Table 6-7. 

The best method was ultrasonic flaw detection using shear waves. This method is 

geometry-dependent and worked equally well under hard chrome or HVOF. Surface 

wave ultrasonic detection was also able to detect the crack, but required far more gain for 

HVOF than for EHC. An example of the ultrasonic crack response is illustrated in Figure 

6-8 for thin and thick HVOF coatings. 

Table 6-7. NDI Testing of Substrate Cracks (Boeing St. Louis) 

Test method Results 

Shear wave ultrasonic, 

2.25MHz 

Most consistent results. Able to easily detect through 

both 0.003” and 0.009” coatings 

Surface wave ultrasonic, 

2.25MHz 

Easily detected through 0.003” coatings; marginal with 

0.009” coatings 

MPI Unable to detect cracks even when propagated through 

coating 

Eddy current Marginal crack detection beneath coating; excellent 

detection when crack propagated through coating 
197

Bob Ware of AFRL has used an 

ultrasonic immersion pulse-echo 

technique to detect debonding prior 

to delamination in HVOF-coated 

tensile bars subjected to fatigue 

testing [6.5]. This approach is not 

easy, requiring immersion and 

automatic mapping of the returned 

ultrasonic pulse around the entire 

surface. It depends on using the 

signal strength of the echo from the 

back surface of the test item to 

determine the degree of delamination 

of the coating at the front, based on 

the principle that any debonded area 

will not transmit the ultrasonic pulse 

into the underlying steel. The 

method is capable of clearly 

detecting delamination before it can 

be seen visually. An example is 

shown in Figure 6-9, which shows 

that under the same high load fatigue 

conditions WC/CoCr delaminates at far fewer cycles than an equivalent WC/Co coating. 

This method has not been used for depot evaluation as depots generally do not have test 

equipment of the required sophistication. 

Figure 6-8. Ultrasonic Detection of Crack Beneath 

HVOF WC/CoCr Coatings (Boeing) 

Figure 6-9. Ultrasonic Measurement of Delamination of HVOF Coatings 

During Fatigue (R. Ware, AFRL) 

100% reflection 

“good” (no delamination) 

0% reflection 

(total delamination) 

WC/CoCr 3,500 cycles 

@ 160 ksi; R = -1 

WC/Co 10,000 cycles 

@ 160 ksi; R = -1

Standard 

0.003 

0.009 
198

6.4.4. Barkhausen Noise measurement of grind burns 

Heroux Devtek has evaluated the Barkhausen Noise response for detection of grind 

burning and other damage beneath HVOF coatings [6.6]. 

Figure 6-10 shows that the Barkhausen signal drops off more slowly with HVOF than 

with EHC coatings, permitting somewhat greater sensitivity with HVOF coated 

components. Figure 6-11 shows that the Barkhausen method can readily detect shot 

peening and grind burns beneath an HVOF coating. It can therefore be used for grind 

burn detection of HVOF-coated components just as it is with EHC-coated ones. 

BNI Vs Thickness - GAIN=50 MAGN=70

40

50

60

70

80

90

100

110

120

130

140

0 2 4 6 8 10

Thickness (mils)

Barkhausen Noise (MP)

Chrome

HVOF WC-Co-Cr

Substrate: 4340 HT 260-280 ksi

Figure 6-10. Barkhausen Noise Signal vs Coating Thickness (Heroux 

Devtek) 
199

6.5. References 

6.1 Nihad Ben Salah, Heroux Devtek, 21st HCAT Program Review, Toronto, Canada, 

September 2002 (available at www.hcat.org) 

6.2 Nihad Ben Salah, Heroux Devtek, 22nd HCAT Program Review, San Diego, CA, 

April 2003 (available at www.hcat.org) 

6.3 C. Edwards, Hill Air Force Base, at Meeting on Materials Substitution for Pollution 

Prevention in Advanced Aircraft – Technology Exchange, Baltimore, MD, September 

2001 (available at www.hcat.org) 

6.4 K. Rupel and L. Pfeiffer, Boeing St. Louis, 21st HCAT Program Review, Toronto, 

Canada, September 2002 (available at www.hcat.org) 

6.5 R. Ware, Air Force Research Laboratory, 17th HCAT Program Review, Kennedy 

Space Center, December 2000 (available at www.hcat.org) 

6.6 Nihad Ben Salah, Heroux Devtek, 21st HCAT Program Review, Toronto, Canada, 

September 2002 (available at www.hcat.org) 

0

20

40

60

80

100

120

140

0 20 40 60 80 100

MAGN

MP

Shot peen

No shot peen

HVOF Coating on SP surface

HVOF coating on burned

surface

Figure 6-11. Effect of Shot Peening and Grind Burning on Barkhausen 

Noise of HVOF WC/CoCr (Heroux Devtek) 

201

7. Cost Benefit Analysis 

7.1. Introduction 

To quantify the economic feasibility of implementing HVOF WC/Co and WC/CoCr, a 

CBA was performed focusing on a commercial landing gear overhaul facility that 

manufactures and maintains the majority of U.S. and Canadian C-130 landing gear. The 

company is considering implementation of HVOF equipment, and is a member of the 

Hard Chrome Alternatives Team (HCAT). 

Information about current hard chrome electroplating operations at the facility was used 

to estimate the economic impact that may be expected if some hard chrome electroplating 

is replaced by HVOF thermal spraying. This CBA focuses on the impact on the C-130 

Canadian and United States programs. The results of this CBA were intended to assist 

OEMs and DoD facilities in decisions related to replacing hard chrome electroplating. 

 

7.2. Approach 

Data collection and financial analyses of the data were performed using the JG-PP CBA 

Methodology. In accordance with this methodology, baseline process flow diagrams 

associated with current hard chrome electroplating processes were developed (refer to 

Figure 7-1). This generic flow diagram is based on information provided by the 

company. In general, each of the repaired and overhauled C-130 components addressed 

in this CBA requires two plating steps (shown as “Repeat as needed” in Figure 7-2). 

New production parts require one plating step. Rework steps are shown because some 

components may be improperly coated and require stripping and re-plating. 

Data collection forms were developed to collect information on the baseline hard chrome 

electroplating operations. A site visit was performed collect the data, and information 

was collected in accordance with the JG-PP CBA Methodology. During the site visit, 

interviews were held with plating engineers, plating operators, plating supervisors, 

chemists, and other employees throughout the facility. The information gathered during 

the site visit was supplemented with correspondence after the visit. Where available, 

material usage rates and costs, labor hours, and waste treatment and disposal costs were 

identified. Where data were not available, values were assumed based on data from other 

facilities and using engineering judgement. 

Environmental, safety, and occupational health (ESOH) activity costs were also obtained 

where available, or estimated. Some costs that may be associated with ESOH activities 

are listed below (refer to the data collection forms in the Cost Analysis Report Appendix 

for all ESOH activities considered): 

. Lost productivity from worker exposure to the HazMats associated with hard 

chrome electroplating and from the use of personal protective equipment (PPE) 

. Maintaining an accumulation point for waste 

. Purchasing and maintaining PPE 
202

. Purchasing and storing drums, labels, and shipping materials associated with 

waste 

. Heating and cooling air from losses due to hoods used for the chrome plating 

tanks. 

The collected operating information was used to estimate the potential financial impact of 

the project, in accordance with the JG-PP CBA Methodology. A process flow diagram 

relating to the application of WC/Co or WC/CoCr by HVOF thermal spraying was also 

developed to aid in analysis of the data. A generic process flow diagram for HVOF 

WC/Co and WC/CoCr is shown in Figure 7-2. Note that five process steps (Rinse, Clean, 

Hot Rinse, Dry, and Bake) are expected to be eliminated when transitioning from hard 

chrome electroplating to HVOF thermal spraying. 

The analysis does not include any effects on aircraft operating costs caused by the 

difference between the density of the HVOF coatings and the density of electroplated 

hard chrome. All dollar figures in this report are U.S. dollars. 

Chrome

Plate

Clean

Mask

Hot

Rinse

Grind/ 

Polish

Rinse De-Mask

Strip

(chemical)

Abrasive

Blast

Inspect Repeat as needed

Acceptable

Parts Rework as needed

Unacceptable

Parts

Parts Inspect Detailed

Inspection Mask De-mask

Shot

Peen

Dry Bake

Plate

Req’d

Plate Not

 Req’d

Figure 7-1. EHC Process Flow at Commercial MRO Shop
203

7.3. Data and assumptions 

The approximate number of U.S. and Canadian C-130 landing gear components that are 

hard chrome electroplated annually at the MRO facility is shown in Table 7-1. This 

number does not include rework of components; the actual number of components 

processed may be larger. Workloads can vary from year to year, but the values were 

considered to be representative of a standard annual workload. 

Based on the annual chromic acid usage, it was estimated that the operation uses 

approximately 13,000 pounds (lb) of chromic acid each year for hard chrome 

electroplating the affected areas on the C-130 landing gear components. Affected areas 

of the components, which are areas forecasted for transitioning from hard chrome 

electroplating to HVOF thermal spraying for the purposes of this CBA, are shown in 

Figure 7-3. This figure identifies the components and areas to receive HVOF during 

fabrication and repair/overhaul. These parts were selected as likely components for 

transitioning because they have the largest external chrome plated surfaces, which would 

be suitable for transitioning to HVOF coatings. Additionally, these components 

frequently require stripping and replating during the overhaul process. 

HVOF

Mask Spray

Grind/ 

Polish De-Mask

Strip

(electrochemical)

Abrasive

Blast

Inspect

Repeat as needed

Acceptable

Parts

Rework as needed

Unacceptable

Parts

Parts Inspect Detailed

Inspection Mask De-mask

Shot

Peen

HVOF

Req’d HVOF Not

 Req’d

Figure 7-2. Projected Process Flow of HVOF for Applying WC/Co or WC/CoCr

Table 7-1. Annual Number of C-130 Landing Gear Components Chrome 

Electroplated at MRO Facility 

Component Fabrication (#/yr) Repair/Overhaul (#/yr) 

Main landing gear piston 200 500 

Nose landing gear piston 30 250 

Nose landing gear cylinder 20 250 
204

Due to sensitivity of operating data, remaining detailed data and calculations are not 

shown in this report. However, all data collected are on file. Types of data that were 

collected are listed in Section 7.3.1. HVOF materials and equipment considered in this 

CBA are described in Section 7.3.2. Assumptions made to complete the analysis are 

described in Section 7.3.3. 

NLG Piston NLG Cylinder

MLG Piston

Figure 7-3. Areas Expected to be Transitioned from Hard Chrome Electroplating to HVOF 

Thermal Spraying 
205

7.3.1. Data Provided by MRO facility 

The information listed below was collected. Estimated cost impact on the C-130 was 

based on eliminating chrome plating from their MRO facilities. The quantities of 

materials used were based on data collected for the year 1999 and the forecast for the 

year 2000. 

Current Hard Chrome Electroplating Operations: 

A. The process steps focused on in this study include abrasive blast, mask, 

chrome plate, de-mask, and bake (refer to Figure 7-1). 

B. Annual number of affected C-130 components that are fabricated or 

repaired and overhauled at the facility (refer to Table 7-1). 

C. The frequency that the chrome plated areas on the C-130 landing gear that 

are processed for repair and overhaul require re-plating (after “Detailed 

Inspection” in Figure 7-1). 

D. The average rework rate of C-130 landing gear components due to 

rejections after chrome plating was estimated to be 2%. The annual cost 

of reworking the components was estimated by the MRO facility. 

E. Average size of a lot at the chrome plating operation. 

F. Total number of electroplating personnel. 

G. Labor requirements and turn around time (TAT) for each of the chromium 

electroplating process steps (refer to Figure 7-1). 

H. Labor requirements for maintenance of the chrome plating baths, 

including cleaning, make-ups, and addition of chemicals to the tanks. 

I. Types of inputs (i.e., materials, energy, and labor) and outputs (e.g., air 

emissions, wastewater, and hazardous waste) associated with the hard 

chrome electroplating process steps identified in Figure 7-1. 

J. Chrome plating tank sizes and volume. 

K. The temperature of the chrome plating tanks. 

L. The average chrome plated coating thickness for fabricated components is 

5 mils (prior to grinding). The average chrome plated coating thickness 

for repaired and overhauled components is 10 mils (prior to grinding). 

M. Material usage and costs for chromic acid, sulfuric acid, lead anodes, 

sodium hydroxide, sodium carbonate, vinyl tape, lead tape, wax, and 

deionized water. 

N. Energy usage related to baking the three affected components was 

provided. Bath heating costs were difficult to evaluate since they are 

hidden in a larger central heating system cost. However, since bath 

temperatures were 130-140°F, it was assumed that the energy 

consumption for heating the baths was considerably less than the energy 

costs for baking. 

O. Annual energy usage for plating the affected components. 

P. Chrome plated volume prior to grinding on each of the three affected 

components. 

Q. Types and quantities of PPE used for electroplating activities and cost per 

PPE item. 
206

R. Laboratory tests associated with process control for the chrome plating 

line, frequency of the tests, and labor required per test. 

S. Labor requirements for metallurgical tests for the three affected 

components. 

T. The cost of test panels for tests associated with the three affected 

components. 

U. Heating and cooling costs from losses due to hoods for the chrome plating 

tanks was provided. 

V. A description of the current wastewater treatment process was provided. 

W. The annual costs associated with treatment of rinse water and wastewater 

from the scrubbers, and disposal of chromium solutions and solid waste 

from the scrubbers was provided. 

Transitioning from Hard Chrome Electroplating to HVOF WC/Co and WC/CoCr: 

The company provided information and assumptions related to implementing HVOF 

WC/Co or WC/CoCr to replace hard chrome electroplating on the affected C-130 landing 

gear components. These data and assumptions are described below. 

A. SM5847 is being considered as the WC/CoCr material to be implemented. 

B. Affected C-130 components and the areas on the components expected to 

transition from hard chrome electroplating to HVOF thermal spraying 

(refer to Figure 7-3). The parts were selected because they have large 

chrome plated surfaces, represent a large percentage of the total overhaul 

workload, and are suitable for transitioning to HVOF. 

C. If HVOF is implemented, two chrome plating tanks and one chrome 

stripping tank could be shut down. 

D. Upgrades of hard chrome electroplating equipment that could be avoided 

in the next 15 years if these tanks are shut down due to implementation of 

HVOF thermal spraying. 

E. The surface areas of the affected C-130 component areas. 

F. Transitioning to HVOF will not affect the current abrasive blasting 

process. 

G. The HVOF coating will be applied to the same thickness as the current 

hard chrome electroplated coatings. 

H. No labor or other cost changes are expected for: 

Reporting associated with air emissions 

Sampling and analysis of air emissions (no sampling is currently 

performed), wastewater effluents, and hazardous waste 

Record keeping and reporting related to the use of HazMats 

Other reporting, such as spill/emergency plans 

Maintaining a hazardous waste accumulation point 

Internal ESOH auditing and other auditing 

Maintaining Material Safety Data Sheets (MSDSs) 

Providing safety and other training for employees 
207

Providing medical exams (same requirements regardless of location at 

facility). 

I. The company is currently in compliance with all associated regulatory 

permits, and expects to remain in compliance, so no savings from avoiding 

fines are expected by transitioning to HVOF. 

J. Hard chrome electroplating processes are not expected to be moved to 

other locations because of compliance issues, so the project will not 

eliminate future relocation expenses. 

7.3.2. HVOF Materials and Equipment Considered 

In this analysis, three types of HVOF spray equipment were evaluated: Diamond Jet 

2600, Diamond Jet 2700, and TAFA JP5000. For each type of equipment, estimates of 

effects on the C-130 fleet were made for WC/Co and WC/CoCr materials. The materialand equipment-specific information and assumptions are listed below. 

WC/Co powder

A. WC/Co powder costs approximately $32/lb, based on a quote from a potential 

supplier of this material. 

WC/CoCr powder

A. WC/CoCr powder costs approximately $32/lb, based on quote from a potential 

supplier of this material. 

Diamond Jet 2600 (Sulzer Metco)

A. This type of equipment uses hydrogen as a fuel. 

B. For Diamond Jet 2600 equipment, the average cost of fuels and related materials 

is approximately $33.33 per hour of WC/Co or WC/CoCr powder sprayed. 

Diamond Jet 2700 (Sulzer Metco)

A. This type of equipment uses propylene as a fuel. 

B. For Diamond Jet 2700 equipment, the average cost of fuels and related materials 

is approximately $33.33 per hour of WC/Co or WC/CoCr powder sprayed. 

TAFA JP5000 (TAFA)

A. This type of equipment uses kerosene as a fuel. 

B. For JP-5000 equipment, the average cost of electricity, gases, air, and water is 

approximately $1.20 per pound of WC/Co or WC/CoCr powder sprayed. 

General material and equipment information and assumptions

A. The HVOF spraying rate used for applying HVOF coatings will be 10 lb/hr. 

B. HVOF WC/Co deposit efficiency is approximately 50%. 

C. An average HVOF gun barrel costs approximately $108, and must be replaced 

after 10 hours of spraying powder. 
208

D. On average, one HVOF spray cell can process one landing gear component 

with HVOF in approximately 40 minutes. Actual spraying of WC/Co powder 

is assumed to be approximately 50% of this time, while the rest of the total 

processing time is assumed to be used for setting up the part and spray pattern. 

7.3.3. Assumptions About Current and Future Operations 

The other data collected and assumptions used to complete the economic analysis for this 

CBA are listed below. 

A. The average number of U.S. and Canadian C-130 aircraft was provided by the 

U.S. Air Force and the Canadian Department of Defence. 

B. On average, U.S. C-130 landing gear are overhauled approximately every eight 

years (provided by the U.S Air Force). It was assumed that the Canadian C-130 

landing gear are on the same eight year overhaul cycle. 

C. The number of landing gear components hard chrome electroplated annually will 

remain constant for the entire 15-year study period unless HVOF is implemented. 

D. Current electroplating area labor rates are $65 per hour (fully loaded). 

E. The labor rate for HVOF thermal spraying is the same as the labor rate for 

electroplating ($65/hr fully loaded). 

F. The cost of rework for HVOF coating is the same as the cost of rework for 

electroplating. 

G. The peening, grinding, and inspection steps will remain essentially unchanged 

when transitioning to HVOF coating. The company had also indicated that 

abrasive blasting would not be impacted. Note that application of WC/Co or 

WC/CoCr will lead to a requirement for diamond grinding wheels (the chrome 

plating process uses aluminum oxide wheels). The diamond grinding wheels are 

expected to last longer than conventional grinding wheels, even when grinding the 

HVOF coatings. The additional useful lifetime of the diamond grinding wheels is 

expected to offset the higher purchase cost of the diamond wheels. 

H. It was assumed that the HVOF systems evaluated have a useful lifetime of at least 

15 years. 

I. Labor costs for masking and fixturing C-130 landing gear components for HVOF 

was assumed to be the same as for hard chrome electroplating. 

J. Fixturing costs were estimated to cost $3,000 per landing gear part and would be 

replaced every 5 years. 

K. The operation of the HVOF system to apply WC/Co or WC/CoCr to C-130 

landing gear components will be optimized before full implementation to reach 

maximum efficiency. 

L. The traditional technique of Fluid Penetrant Inspection is not expected to work on 

HVOF coatings. A new technique will be required, which may increase or 

decrease costs. For the purposes of this report, the cost was assumed to remain 

constant. 

M. Stripping material costs were estimated for chrome plating and HVOF thermal 

spraying (based on published literature). 

N. The net cost for disposing of waste WC/Co or WC/CoCr is zero, because the 

material can be sold to a third party for reprocessing, with the proceeds offsetting 

any internal handling costs. 
209

O. Lifetime cartridge-type air filters will be used for filtering particulates from the 

HVOF WC/Co spray booth so material costs will not be affected by filters. This 

type of filter is currently used at other facilities, such as Naval Aviation Depot - 

Jacksonville, Florida. 

P. The cost of installing electrolytic stripping tanks for removing HVOF coatings 

was not included in capital costs. The annual material and utility costs for 

stripping WC/Co or WC/CoCr were included in the analysis. 

Q. The company will eliminate hard chrome electroplating of C-130 landing gear 

components on both fabrication and repair operations. As a result, removal of 

hard chrome electroplated coatings from C-130 landing gear components will 

eventually cease altogether. 

R. The annual quantity of C-130 landing gear fabricated during the 15-year study 

period was assumed to remain constant. 

S. The useful lifetime of a landing gear component coated with WC/Co by HVOF 

will be approximately 50% longer than the useful lifetime of hard chrome 

electroplated landing gear components. This increase in landing gear lifetime is 

based on results of current ESTCP testing, published literature, and engineering 

judgement; some recent testing has shown wear resistance between 2.5 and 4 

times as great as that of electroplated chrome. However, the effects of increasing 

the useful lifetime of C-130 landing gear components was not considered in this 

analysis per the request of the MRO shop. 

T. Reducing the number of C-130 landing gear components HVOF coated will 

proportionally reduce material, labor, waste disposal, and worker health and 

safety costs. 

U. An estimate of approximately 1,200 landing gear components processed per year 

was used for this analysis; this number will remain the same after HVOF 

implementation. The coating thickness of electroplating and HVOF will also 

remain the same. However only 88% of the total component surface area will be 

transitioned to HVOF due to line of site issues, 12% of the total component 

surface area will continue to be electroplated after HVOF implementation. 

Benefits from decreased turn-around time (TAT) of landing gear was also included in this 

CBA. Because the company has reported no difficulty in meeting required schedules for 

processing aircraft C-130 landing gear components, the cost avoidance associated with 

decreased TAT is not expected to accrue directly to the company after implementing 

HVOF. The assumptions relating to TAT are listed below. 

A. The average TAT for C-130 landing gear components coated with WC/Co by 

HVOF will be approximately five days less than the average TAT for hard 

chrome electroplated C-130 landing gear components. This reduction includes 

process time, but not waiting/staging time. 

B. The average value of a C-130 landing gear component is estimated at $60,000. 

C. The annual interest rate used to calculate the “inventory cost” for landing gear is 

3.2%; this is consistent with the 10-year real interest rate listed in Office of 

Management and Budget (OMB) Circular Number A-94 (January 2001 revision). 
210

7.4. Annual operating cost avoidance 

Data and assumptions described in Section 3 were used to calculate the current annual 

operating costs for coating C-130 landing gear components using the baseline hard 

chrome electroplating process. These data and assumptions were also used to estimate 

the annual operating costs for fabricating and overhauling C-130 landing gear 

components with HVOF. The annual operating cost avoidances reported in this section 

were derived from comparing the operating costs of the baseline hard chrome 

electroplating process to those calculated for the three types of HVOF equipment and two 

types of materials described in Section 7.3.2. 

Table 7-2 shows the average annual operating cost avoidances that were estimated for 

implementing HVOF to replace hard chrome electroplating of C-130 landing gear 

components. It is expected that the average number of landing gear that need to be 

repaired and overhauled will decrease beginning in the eighth year after implementation, 

because of superior performance and durability of WC/Co coatings applied by HVOF. 

However, Heroux did not want to include these cost saving because they are not sure 

when the coating will be proven enough to decrease their refabrication schedule. 

Scenario 2 includes a benefit from reduced TAT, which is not expected to accrue directly 

to the MRO company. 

7.5. Comparison of calculated results with vendor 

quotes 

In addition to performing the calculations operating data to determine the cost avoidance 

that may result from transitioning from hard chrome electroplating to HVOF thermal 

Table 7-2. Estimated Annual Operating Cost Avoidance

Annual Operating Cost Avoidancea 

Labor $113,540 

Materials $75,520 

Utilitiesb $11,390 

Waste Disposal $2,900 

Additional Cost Avoidance due to Reduced TAT (Scenario 3) $32,880 

Total $236,230 

 

a Based on 1,250 per year total components processed and 1700 in3

 of the three components 

electroplated; after HVOF implementation assumes 1500 in3

 HVOF coated and 200 in3

electroplated. 

b

 Average of equipment and material types 
211

spraying, quotes from shops were obtained for chrome plating and HVOF spraying the 

three affected components. Quotes for hard chrome plating the components were 

obtained from Southwest Aeroservices in Tulsa, Oklahoma. Quotes for HVOF thermal 

spraying the components were obtained from Engelhard in East Windsor, Connecticut, 

Southwest Aeroservices in Tulsa, Oklahoma, and Hitempco in Old Bethpage, New York. 

For the quotes, the shops assumed that the parts were provided to them with the paint 

removed; one shop quoted for parts ready to coat (plating removed) the other two shops 

quoted for removing the chrome plating and re-applying either the chrome plating or 

applying HVOF thermal sprayed coating. A summary of their quotes and the cost 

difference are provided in Table 7-3. 

7.6. Financial evaluation 

To measure the financial viability of this project, three performance measures for 

investment opportunities were used: net present value (NPV), internal rate of return 

(IRR), and payback period. The NPV is the difference between capital investments and 

the present value of future annual cost benefits associated with the alternatives. The IRR 

is the discount rate at which NPV is equal to zero. NPV and IRR account for the time 

value of money, and discount the future capital investments or annual cost benefits to the 

current year. The payback period is the time period required to recover all of the capital 

investment with future cost avoidance. For NPV and IRR, a 3.2.0% discount rate was 

used for this financial evaluation, which is consistent with the OMB Circular Number A94 and the ECAM. Guidelines for these performance measures are listed in Table 7-4. 

Table 7-3. Summary of Vendor Quotes

Part In-house 

Chromium 

plating 

(including 

stripping) 

Out-source 

Chromium 

plating 

(including 

stripping) 

In-house 

HVOF 

(including 

stripping) 

Out-source 

HVOF 

(including 

stripping) 

In-house 

HVOF 

(without 

stripping) 

Out-source 

HVOF 

(without 

stripping) 

Main Landing 

Gear Piston 

$2,002 $3,400 $1,587 $3,700 $1,322 $934-$942 

Nose Landing 

Gear Cylinder 

$1,335 $1,600 $1,058 $1,250 $881 $166-$262 

Nose Landing 

Gear Piston 

$418 $3,000 $331 $3000 $276 $508-$1,012 
212

A summary of the financial evaluation for implementing HVOF to replace hard chrome 

electroplating of C-130 landing gear components is shown in Table 7-5. This financial 

evaluation includes the range of annual operating cost benefits reported in Section 4 and 

the NPV based on a 15-year study period. The evaluation summarized in Table 7-5 does 

not include the costs of validation testing. The 15-year NPV for the surveyed facility 

ranges from a minimum of $1,799,700 to $1,977,500. The actual savings achieved at the 

facilities will vary depending on the number of actual applications converted, future 

workloads, and other factors specific to each facility. 

NPVs for 5-year, 10-year, and 15-year study periods were calculated to facilitate 

comparing other financial analyses to this economic analysis of implementing HVOF. 

Table 7-6 shows the 5-year, 10-year, and 15-year NPVs calculated for the different 

scenarios. 

Table 7-4. Summary of Investment Criteria

Criteria Recommendations/Conclusions 

NPV > 0 Investment return acceptable 

NPV < 0 Investment return not acceptable 

Highest NPV Maximum value to the facility 

IRR > discount rate Project return acceptable 

IRR < discount rate Project return not acceptable 

Shortest payback period Fastest investment recovery and lowest risk 

Adapted from ECAM Handbook.

Table 7-5. Results of Financial Evaluation for Direct Implementation

Category Calculated Result

Annual Operating Cost Avoidance $195,600 - $210,700 

Initial Capital Investment $700,450 

Net Present Value a $1,156,100 - $1,283,700 

Internal Rate of Return a 30.5 – 32.8% 

Discounted Payback a 3.29 – 3.53 years 

a This value was calculated with Pollution Prevention Financial Analysis and Cost Evaluation System 

(P2/FINANCE) software program. This software program is proprietary and copyrighted by Tellus 

Institute of Boston, Massachusetts. A 15-year analysis and 3.2% discount rate were assumed. 
213

7.7. Summary and recommendations 

HVOF application of WC/Co and WC/CoCr is being investigated as an alternative to 

hard chrome electroplating for fabricating and repairing/overhauling aircraft landing gear. 

A CBA was performed to identify the potential financial impact of implementing these 

HVOF coatings for application to C-130 aircraft landing gear components. Data were 

collected at this facility and the potential economic effects were calculated in accordance 

with the JG-PP CBA Methodology. 

It was estimated that the use of HVOF for C-130 landing gear components will result in a 

net decrease in annual operating costs at Heroux. The annual operating costs savings 

range from $195,600 to $210,700 based on a number of differing assumptions described 

in this CBA. This analysis assumes that HVOF WC/Co or WC/CoCr will be 

implemented only for use on the affected C-130 landing gear components. Although the 

material costs for the HVOF WC/Co or WC/CoCr powders are the same, their densities 

and deposit efficiencies vary. This is the cause of higher operating costs for the 

WC/CoCr material as shown in Table 7-6. The primary difference between the 

equipment types is the fuel type used. The Diamond Jet 2600 uses hydrogen as a fuel, 

the Diamond Jet 2700 uses propylene, and TAFA JP5000 uses kerosene. The difference 

in fuel costs is the reason that the Tafa equipment shows a more positive NPV. A 

reduced labor requirement with HVOF implementation was the primary cost driver in this 

analysis. 

Additional cost savings that would be realized is the estimated 50% extension of service 

life over the service life of electroplated chrome that is expected to be realized with 

HVOF WC/Co or WC/CoCr implementation. This was not done at the request of 

Heroux. Since a WC/Co and WC/CoCr applied by HVOF has reportedly shown wear 

resistance up to four times as great as that of electroplated chrome in some recent 

laboratory investigations. Therefore, greater benefits to the C-130 program may be 

Table 7-6. Net Present Value by Scenario

Scenario 5-year NPV 10-year NPV 15-year NPV 

Tafa 

equipment/WCCo 

coating 

$423,150 $1,283,700 $1,977,500 

Tafa 

equipment/WCCoCr 

coating 

$398,600 $1,238,200 $1,914,000 

Sulzer-Metco 

equipment/ WCCo 

coating 

$383,900 $1,211,000 $1,876,100 

Sulzer-Metco 

equipment/ WCCoCr 

coating 

$354,300 $1,156,100 $1,799,700 

a

 The numbers were rounded to the hundreds place. 
214

realized through implementation of the HVOF coatings due to increased service life of 

landing gear components. This scenario would further reduce labor requirements and 

have a corresponding reduction in labor and material costs.

Outsourcing landing gear coating was analyzed and this analysis indicates that it is more 

cost effective to conduct chromium plating and HVOF in house. One vendor quote 

indicated that outsourcing HVOF after stripping in-house would be a cost effective 

decision. However it should be noted that labor costs for stripping were based on 

estimates and further evaluation of this scenario should be conducted to validate these 

results. 

The MRO facility is considering implementing, independent of the results of this CBA, 

because they believe that the environmental issues associated with hexavalent chromium 

will eventually lead to further regulation of the material and of hard chrome 

electroplating. If this is the case, operating costs for chromium plating would increase 

above that shown in this report, making HVOF implementation even more financially 

beneficial.
215

7.8. Bibliography 

Aldrich. Catalog Handbook of Fine Chemicals. 1998-1999. 

Boy, Jeffrey H.; Ashok Kumar; Patrick March; Paul Willis; and Herbert Herman. 

Construction Productivity Advancement Research (CPAR) Program. Cavitation- and 

Erosion-Resistant Thermal Spray Coatings. U.S. Army Corps of Engineers. 

USACERL Technical Report 97/1997. July 1997. 

http://www.nationalthermospray.com/hydroele.htm 

Cost Benefit Analysis (HV-C-1-1) for High-Velocity Oxygen-Fuel Deposition of 

Tungsten Carbide Cobalt as an Alternative to Hard Chrome Electroplating for 

Aircraft Landing Gear Components. Engineering and Technical Services for Joint 

Group on Acquisition Pollution Prevention (JG-APP) Pilot Projects. Contract No. 

DAAA21-93-C-0046. Task No. N.072. CDRL No. A010. Concurrent Technologies 

Corporation, Johnstown, PA. Draft July 14, 1998. 

Cost Benefit Analysis (PH-C-1-1) for High-Velocity Oxygen-Fuel Deposition of 

Tungsten Carbide Cobalt as an Alternative to Hard Chrome Electroplating for C2/E-2 and P-3 Propeller Hubs. Engineering and Technical Services for Joint Group 

on Pollution Prevention (JG-PP) Projects. Contract No. DAAA21-93-C-0046. Task 

No. N.072. CDRL No. A010. Concurrent Technologies Corporation, Johnstown, 

PA. November 29, 1999. 

Environmental Cost Analysis Methodology (ECAM) Handbook. Environmental 

Security Technology Certification Program (ESTCP) Validation Tasks. Contract No. 

DAAA21-93-C-0046. Task No. 098. CDRL No. A013. Concurrent Technologies 

Corporation, Johnstown, PA. March 29, 1999. 

Gibb, Herman J.; Peter S. J. Lees; Paul F. Pinsky; and Brian C. Rooney. “Clinical 

Findings of Irritation Among Chromium Chemical Production Workers.” American 

Journal of Industrial Medicine. Vol. 38. 2000. pp. 127-131. 

Gibb, Herman J.; Peter S. J. Lees; Paul F. Pinsky; and Brian C. Rooney. “Lung 

Cancer Among Workers in Chromium Chemical Production.” American Journal of 

Industrial Medicine. Vol. 38. 2000. pp. 115-126. 

Hard Chrome Alternatives Team website. http://www.hcat.org. As of June 21, 2000. 

JP-5000 Comparative Operating Costs. Technical Data. Hobart-Tafa Technologies. 

Concord, NH. 1992. 

OMB Circular No. A-94 (Revised), Guidelines and Discount Rates for Benefit-Cost 

Analysis of Federal Programs. Washington, DC. October 29, 1992. Revised 

February 2002. 

Spray Parameters. Technical Data. Hobart-Tafa Technologies. Concord, NH. 

1997. 

Sulzer Metco. Email and phone correspondence between March and October 2001. 

Peter Longobardi. 

Praxair. Email and phone correspondence March 2001. Richard Thorpe. 

217

8. Specifications and Standards 

In order to use the HVOF process in manufacturing or repair it is essential for engineers 

to be able to refer to specifications for HVOF deposition and processing. To facilitate 

this HCAT has developed specifications for both HVOF materials and deposition on high 

strength steel and for grinding. 

Prior to the HCAT program only the Boeing specifications were widely used: 

. BAC 5851 for thermal spray deposition, supported by Boeing Materials Spec 

BMS 10-67, defining thermal spray powders, and 

. BAC 5855, which specifies grinding methods for thermal spray coatings. 

The primary standards and specification body for aerospace specifications is the Society 

of Automotive Engineers (SAE), who issue Aerospace Materials Specifications (AMS 

specs). Obtaining AMS specifications involves drafting the specifications and working 

closely with the SAE AMEC committees over a period of one to three years to create a 

document that is satisfactory for all committee members. A general HVOF specification, 

AMS 2447, was developed by Bruce Bodger of Hitemco, a member of the HCAT team. 

During this program specifications for HVOF powders and deposition methods were 

developed by Donald Parker (NASA KSC), and specifications for grinding of HVOF 

coatings were developed by Jon Deveraux (NADEP Jacksonville), both members of the 

HCAT team. These specifications and their status are shown in Table 8-1. The 

specifications developed in this program are reproduced below. 

Table 8-1. SAE Aerospace Material Specifications for HVOF

Specification Title Notes 

AMS 2447B 

(1998) 

Coating, Thermal Spray High Velocity 

Oxygen/Fuel Process 

Original general HVOF 

spray specification developed 

by Bruce Bodger 

AMS 2448 

(2003) 

Application Of Tungsten Carbide 

Coatings On Ultra High Strength Steels 

High Velocity Oxygen / Fuel Process 

Developed for use on landing 

gear and other high strength 

steel by Don Parker 

AMS 7881 

(2003) 

Tungsten Carbide-Cobalt Powder Powder specification for 

HVOF WC/Co coatings, 

developed by Don Parker 

AMS 7882 

(2003) 

Tungsten Carbide-Cobalt Chrome Powder Powder specification for 

HVOF WC/CoCr coatings, 

developed by Don Parker 

Not yet 

issued 

Grinding Of HVOF Sprayed Tungsten 

Carbide Coatings Applied To High 

Strength Steels 

HVOF grinding specification 

developed by Jon Deveraux 
218

8.1. Proposed AMS 2448 

 APPLICATION OF TUNGSTEN CARBIDE COATINGS ON ULTRA 

HIGH STRENGTH STEELS 

High Velocity Oxygen / Fuel Process 

AMEC00AB DRAFT

Donald S. Parker, Sponsor Page 1 of 12

1.0 SCOPE 

1.1 Purpose 

 This specification covers engineering requirements for applying tungsten carbide thermal 

spray coatings to ultra high strength steels (220 ksi and above) utilizing high velocity 

oxygen fuel (HVOF) combustion driven processes and the properties for such coatings. 

The processes and procedures herein apply only to the properties of the as-deposited 

coating. 

1.2 Application 

 This process has been used typically to provide coatings of lower porosity and higher 

adhesive, and/or cohesive, strength than generally attainable with plasma spray; and for 

applications requiring wear, heat, and corrosion resistance or dimensional restoration that 

were traditionally chrome plated. However, usage is not limited to such applications. 

1.3 Safety - Hazardous Materials 

 While the materials, methods, applications, and processes described or referenced in this 

specification may involve the use of hazardous materials, this specification does not 

address the hazards which may be involved in such use. It is the sole responsibility of the 

user to ensure familiarity with the safe and proper use of any hazardous materials and to 

take necessary precautionary measures to ensure the health and safety of all personnel 

involved. 

2.0 APPLICABLE DOCUMENTS 

2.1 The following publications form a part of this specification to the extent specified herein. 

The latest issue of SAE publications shall apply. The applicable issue of other 

ublications shall be the issue in effect on the date of the purchase order. 

2.2 S.A.E. Publications: Available from SAE, 400 Commonwealth Drive, Warrendale, PA 

15096-0001. 

 AMS 2447 Coating, Thermal Spray High Velocity Oxygen/Fuel Process 

 AMS S13165, Shot Peening of Aircraft Components 

 SAE J442 Test Strip, Holder, and Gage for Shot Peening 

 AMS 6484 Steel, Bars, Forging, and Tubing 

 AMS 6454 Sheet Steel, Strip, and Plate 

 AMS 7881 (AMEC 99B) Tungsten Carbide-Cobalt Powder 
219

 AMS 7882 (AMEC 99C) Tungsten Carbide-Cobalt Chrome Powder 

2.3 ASTM Publications: Available from ASTM, 100 Barr Harbor Drive, West 

Conshocken, PA 19428-2959 

 ASTM C 633 Adhesion or Cohesive Strength of Flame-Sprayed Coatings 

 ASTM E 384 Microhardness of Materials 

ASTM E 18 Rockwell Hardness and Rockwell Superficial Hardness of Metallic 

Materials 

ASTM E3 Standard Practices for Preparation of Metallographic Specimens 

3.0 TECHNICAL REQUIREMENTS 

3.1 Equipment 

3.1.1 Torch: A specially constructed gun that utilizes combustion products to generate a high 

velocity gas stream for heating of the coating material to a molten or plasticized state, and 

transfer of the coating material to the work piece. 

3.1.2 System: The system shall be microprocessor controlled and fitted with an automated 

device for regulating the gas(es) and fuel(s) used to operate the torch. The torch shall be 

mounted on an automated manipulating device during the deposition process to maintain 

a constant working distance and traverse rate, or maintained in a fixed position with the 

component mounted on a manipulating device that will maintain a constant working 

distance. 

3.1.3 Gauges: 

3.1.3.1 Pressure gauges shall have minumum accuracy of ± 1.5% of full scale 

3.1.3.2 Flow meters shall have a minimum accuracy of ± 2% of full scale 

3.1.4 Powder Feeder: The powder feed system shall supply a metered flow of material. 

3.2 Materials 

3.2.1 Gases and Fuels: Specifications utilized by the processor for procurement of these 

materials shall be acceptable to the purchaser. 

3.2.2 Coating Material: Shall conform to AMS xxxx (AMEC 99B) or AMS xxxx (AMEC 99C) 

as required by the drawing or otherwise specified by purchaser. All powders shall be dry, 

free flowing and uniformly blended. 

3.3 Preparation 

3.3.1 Metallurgical Treatments: All metallurgical processes such as heat treating, shot 

peening, or similar should be completed prior to surface conditioning for spraying 

3.3.2 Cleaning: Surfaces to be coated shall be thoroughly cleaned to remove moisture, oil, 

grease, dirt, scale, paint and foreign material. Final cleaning shall take place no more 

than four hours prior to coating. Cleaning procedures shall not cause hydrogen 

embrittlement or other detrimental surface contamination on surfaces that are to be 

coated. 

3.3.3 Masking: Parts shall be masked by an appropriate means to protect all surfaces that are 
220

not being coated. 

3.3.4 Surface Conditioning: After cleaning, surface to be coated shall be grit blasted with 

aluminum oxide blast media at 60 – 80 psi and 6-8 inch standoff distance to clean and 

prepare the surface for HVOF deposition. Grit shall be free from moisture, oil, dirt and 

other contaminants. Grit size shall be of the finest possible size necessary to achieve a 

120 – 150 Ra surface roughness, but shall never be coarser than 54 grit. A surface 

profilometer shall be used to verify the proper surface conditioning for each procedure 

used. 

3.4 Application 

3.4.1 Process: The parameters for gas flows and pressures, powder feed rates, and spray 

distance, as well as deposition rates and traverse speeds shall be determined by a 

statistical method designed to achieve the desired coating properties specified by this 

document and / or the purchaser. 

3.4.2 Preheating: Surfaces to be coated shall be preheated to a sufficient temperature to remove 

moisture; however the maximum temperature during preheating shall not exceed 350 

degrees F. Preheating may be accomplished by use of the torch or by other suitable 

means and shall be monitored as specified in 3.4.4. 

3.4.3 Coating: The coating material shall be deposited on the designated surface in sufficient 

thickness to permit finishing to specified dimensions. The minimum finished coating 

thickness shall be greater than or equal to 0.003 inch. 

3.4.3.1 Areas on which coating is optional, shall be exempt from minimum thickness limitation; 

however, if coated, the areas must be prepared and handled in the same manner as the 

area on which coating is required and adhesion requirements still apply. 

3.4.3.2 A spray angle of 90 + 5 degrees should be maintained. For cylindrical components, angle 

is measured relative to the centerline axis of the cylinder. For any application that 

requires spraying at less than 85 degrees, all test specimens shall be sprayed at the same 

angle as the component and all minimum mechanical property requirements will apply. 

3.4.3.3 Spray deposition shall be continuous, except for interruptions to measure coating 

thickness and/or for cooling cycles to maintain part below maximum allowable 

temperature. 

3.4.4 Substrate Temperature: Unless otherwise specified, maximum temperature of the 

substrate during preheating and coating application shall be controlled to not exceed 350 

degrees F. Temperature measurements shall be made utilizing a laser sighted infrared 

thermometer with adjustable emissivity (0.1 to 0.99) and response time of less than 1 

second. Measurement location shall be taken on the parent metal adjacent to the to the 

edge of the coating as it traverses the area to be coated. If geometric, or part size 

constraints do not allow this procedure to be followed, then the temperature shall be 

measured immediately adjacent to the coated area along the same path of travel as the 

gun plume. Emissivity shall be set to a standard value for high strength steel. 

Resolution of the IR thermometer shall be 1C or 1 F, depending on the scale used and 

spot diameter should be less than 2.5 inch or equivalent to the actual size of the IR beam. 

3.4.5 Test Specimens: Specimens required under 3.7 shall be coated, as far as practicable, 

using the process procedures identified on the Coating Process Control Sheet for the parts 

that they represent. Coupons representing rotating components shall be sprayed at the 

same rotational speed and incremental step rate as the component. If there are multiple 

application angles on the same component, each of the deposition angles shall be 
221

evaluated with coupons for compliance with minimum mechanical property limits. 

Coupons shall be evaluated prior to coating application on production components. 

3.4.6 Specimen Material: Bond strength specimens shall be manufactured from AMS 6484, 

heat-treated to produce a hardness Rockwell C 40 minimum and no higher than the 

hardness of the component substrate. Metallographic and Bend test specimens shall be 

manufactured from AMS 6454 and heat-treated to produce a Rockwell C hardness greater 

than 40. Alternatively, test specimens can be made from the same material, in the same 

condition as the component. 

3.5 Surface Finishing: Procedures for finishing shall be in accordance with purchaser’s 

specifications. 

3.6 Properties 

3.6.1 Adhesion 

3.6.1.1 Bend Test: Specimens prepared and tested in accordance with 3.7.1 shall not show 

separation of the coating from the substrate, when examined visually without 

magnification. Cracking of the coating and minimal separation at the specimen edges 

shall be considered acceptable. 

3.6.1.2 Bond Strength: Specimens, prepared and tested in accordance with 3.7.2, shall be 10 ksi 

minimum. 

3.6.2 Coating Hardness: The coating hardness, tested in accordance with 3.7.3 shall be HV300g

950 minimum. 

3.6.3 Microstructure: A detailed standard procedure for metallographic specimen preparation 

and examination shall be used to ensure consistent results as in 4.4.1. Examination of a 

suitably prepared cross-sectioned specimen shall show the coatings to be free from cracks 

and delaminations. Repolishing can only be performed on specimens that show flaws 

induced by the polishing method. Oxide content cannot be induced and is not grounds 

for re-polishing. Microstructural properties shall be evaluated in accordance with the 

following: 

3.6.3.1 Voids / Oxides: Shall be uniformly distributed and not greater than 1% in any field of 

view (approximately 0.02 inch (0.51mm) length) when examined at 400x magnification 

on the cross sectioned specimen. Any single void greater than 0.002 inch (0.2mm) shall 

be cause for rejection. 

3.6.3.2 Unmelted Particles: None in any field of view (approximately 0.04 inch (1.0mm) length) 

when viewed at 200x magnification on the cross sectioned specimen. 

3.6.3.3 Interface: Contamination of the coating / substrate interface with surface preparation 

media shall not exceed 10% in any field of view (approximately 0.04 inch (1.0mm) 

length) when viewed at 200x magnification on the cross sectioned specimen. Any 

coating separation at the interface will not be acceptable. Separation is defined as gap 

between the coating and substrate greater than 0.002 inch in length following directly 

along the bond line. Any length less than this will be considered acceptable voids or 

porosity. 

3.6.3.4 Carbide Distribution: In any 400x field of view, all carbides shall be uniformly 

distributed with no banding or clustering. 

3.6.4 Residual Stress 
222

3.6.4.1 Almen Strip: Coatings shall be evaluated for residual stress by preparing, and spraying a 

standard Almen Type "N" strip in identical fashion as the component being coated. Both 

the surface to be coated and uncoated surfaces of the Almen strip shall be grit blasted to 

minimize curvature of the Almen strip to less than 0.002 inches arc height. The arc height 

of the Almen strip surface to be coated shall be measured after grit blast surface 

preparation (first reading) and again after coating application (second reading). The first 

reading and subsequent coating application shall be with the convex surface of the Almen 

strip in the up position if the strip is not flat after grit blast and the second reading with 

the same surface, now coated, in the up position. The Almen strip shall be restrained flat, 

in the transverse direction by four screws located as indicated in AMS S13165C (Note: 

also covered in SAE J442) during coating deposition. The measured change in deflection 

of the Almen strip resulting from the application of the coating shall be reported as the 

difference of the two readings, second reading minus first reading, and is indicative of the 

desired compressive coating stresses if the sign of the difference is positive. Acceptable 

values for Almen type “N” strip arc heights are positive 0.003-0.012 inches for a 0.005-

inch thick coating. 

3.7 Test Methods 

3.7.1 Bend Test: Test panels, 0.05 x 1x 3 inches +/- 0.05 inch (1.3 x 25 x 76mm) shall be 

coated on one side to a thickness of 0.001 to .003 inch (0.025 to 0.076 mm). Panels shall 

be tested by being bent around a 0.5 inch (13 mm) diameter bar, with the coated surface 

on the outside of the bend, at a rate of approximately ten degrees per second. Panels shall 

be bent to obtain a 90-degree permanent set. 

3.7.2 Bond Strength: Test specimens, approximately 1 inch (25 mm) in diameter by 2 inches 

(51 mm) long, +/- 0.05 inch (0.5 mm) shall be coated to a thickness of 0.009 to 0.012 

inch (0.2 to 0.3mm). Specimens shall be prepared and tested in accordance with ASTM 

C633. Adhesive only qualification tests shall be run under the same test conditions to 

verify integrity of the adhesive. 

3.7.3 Microhardness: Test specimens, 0.05 x 1 x 3 inches, +/- 0.05 inch (1.3 x 25 x 76mm +/- 

0.5 mm) shall be coated on one side to a minimum thickness of 0.009 inch (0.20 mm). 

The hardness shall be the average of a minimum of ten evenly spaced indentations in 

accordance with ASTM E 384. 

3.8 Quality: The coating, as received by purchaser, shall be adherent to the basis material and 

shall have a uniform, continuous surface free from spalling, chipping, flaking, and other 

imperfections detrimental to usage of the coating. 

3.9 Tolerances: Unless otherwise specified by the purchaser, a tolerance of 

 -0 to +0.125 inch (3.2 mm) is allowed on the boundaries of the coated area. 

3.10 Definition 

3.10.1 Lot: A lot shall be all parts of a similar configuration, coated sequentially on the same 

machine setup using the same batch of coating material and process parameters, within a 

shift or eight hours of torch time, and presented for processor’s inspection at one time. 

3.11 Mechanical Coating Removal 

3.11.1 Coatings may be removed by mechanical methods of grinding, or any method approved 

by purchaser as long as all original substrate conditions are duplicated. 

3.12 Chemical Coating Removal 
223

3.12.1 Equipment 

3.12.1.1 Tank: A container of adequate volume for the stripping solution and the parts being 

stripped. The tank must be manufactured from materials such that the structure will not 

be degraded by the stripping solution. 

3.12.1.2 Heaters: Heating devices approved for use with the required solution(s) and capable of 

reliably maintaining the specified solution temperature. 

3.12.1.3 Rectifier: Approved electrical device for delivery of controlled DC current to the 

electrolytic stripping solution. 

3.12.1.4 Controls: Indicating devices used to determine and maintain process temperature and 

current flow shall have minimum accuracy as follows: 

 Temperature Gages: +/- 2% of full scale 

 Current and Voltage Gages: +/- 5% of full scale 

3.12.1.5 Agitation: Agitation or circulation of the solution shall be accomplished by either 

pumping low pressure, clean, dry air through the solution, or by mechanical agitation 

using impeller or propeller driven flow. 

3.13 Materials 

3.13.1 Approved stripping solutions and their methods of use are listed in Table 1 

TABLE 1 – Stripping Solutions 

Solution # Solution Name Immersion Use Electrolytic Use

1 Rochelle Salt X 

2 Metal-X B929 Nickel 

Plating Strip 

X 

3.14 Preparation 

3.14.1 Composition and Control of Solutions: 

Solution 1 – Rochelle Salt 

Variable Control Limits

Anhydrous Sodium Carbonate 20-30 oz./gal. Water (de-ionized optimum) 

Sodium Tartarate (Rochelle Salt) 8-12 oz./gal. Water (de-ionized optimum) 

pH 11.0 - 12.0 

Temperature 104o

 – 150o F (140o – 150o Optimum) (40o – 650 C) 

Voltage 4-6 volts DC 

Current Density 4-8 amps / square inch 

Solution Agitation Dry, Oil Free Compressed Air 

Solution 2 – Nickel Plate Strip Solution 
224

Variable Control Limits

METAL-X B929 Nickel Strip Solution 

(or equivalent) 

2.5 lb./ gal. Water (de-Ionized optimum) 

pH 9.2 – 9.8 

Temperature 120o

 – 150o F (50o – 650 C)

Solution Agitation Dry, Oil-Free Compressed Air 

3.14.2 Solution Processing Requirements: 

 The processing solutions and equipment shall be kept clean at all times. 

 Operations shall be suspended and corrected when the following has occurred: 

a) A non-removable smut is formed or foreign material is deposited on the component 

surface. 

b) The solution temperature or composition is outside the control limits. 

c) The solution surface has developed a surface film. 

d) The parts have been etched. 

e)The coating removal rate has diminished to less than 1 mil per hour. 

3.14.3 Masking: The specified solutions may have an affect on other plated, painted or surface 

treated areas. Masking of those areas by impermeable means is required. 

3.15 Stripping Procedures 

3.15.1 Electrolytic Method: 

 Step 1 - A stainless steel cathode is required for this method. The coated part to be 

stripped is connected to the positive terminal conductor to act as the anode. Connection 

shall be made such that arcing is prevented. 

 Step 2 – Immerse the part into the bath and apply 4-6 volts DC at a current density of 4-8 

amps per square inch of coated area to be stripped. 

Step 3 – Check progress at least every 45 minutes to remove an accumulated smut. Use 

Scotch Brite  or equivalent to remove smut. Removal rate should be approximately 2-6 

mils of coating per hour. 

Step 4 – When coating is adequately stripped, remove the part from the tank and rinse 

thoroughly with water at room temperature. All remaining coating can be removed by 

grit blasting prior to re-application. 

Step 5 – Dry part with compressed, dry, oil free air and apply any necessary in process 

corrosion preventative compound to prevent corrosion damage while part is in transit. 

3.15.2 Immersion Method 

 Step 1 – Immerse part into prepared solution. 

 Step 2 – Check progress every 45 minutes and remove any accumulated surface smut. 

Use Scotch Brite  or equivalent to remove smut. Removal rate should be 
225

approximately 1 mil of coating per hour. 

 Step 3 – When coating is adequately stripped, remove component from the tank and rinse 

thoroughly with water at room temperature. 

 Step 4 – Dry part with compressed air and apply any necessary in process corrosion 

preventative compound to prevent corrosion damage while part is in transit. 

3.16 Post Strip Processing 

3.16.1 The processes outlined above DO NOT involve evolution of atomic hydrogen at the 

coating surface, or base metal interface. Therefore, there is no susceptibility to hydrogen 

embrittlement of the component and post processing bake is not required. 

4.0 QUALITY ASSURANCE 

4.1 Responsibility for Inspection: Processor shall supply all test specimens for processor’s 

tests and shall be responsible for performance of all required tests. When parts are to be 

tested, the purchaser shall supply such parts. Purchaser reserves the right to sample and 

to perform any conformity testing deemed necessary to ensure that the coating conforms 

to the requirements of this specification. 

4.2 Classification of Tests 

4.2.1 Acceptance Tests: Bend Test (3.6.1.1), Hardness (3.6.2), Microstructure (3.6.3), 

Thickness (3.4.3), and Tolerances (3.9) are acceptance tests, and shall be performed on 

each lot. For fatigue sensitive applications, Residual Stress (3.4.6) should be evaluated 

periodically as deemed appropriate by the processor and/or anytime process variables are 

changed. 

4.2.2 Periodic Tests: The Coating Material Composition Verification (3.2.2) and Bond Strength 

(3.6.1.2) are periodic tests and shall be performed at a frequency selected by the 

processor unless otherwise specified by the purchaser. 

4.2.3 Pre-production Tests: All technical requirements of this specification are pre-production 

tests and shall be performed prior to, or on the initial shipment of coated parts to a 

purchaser. In addition, these tests shall be performed when a significant change in 

material, and/or processing requires approval as in 4.4.3, and when purchaser deems 

conformity testing to be required. 

4.3 Sampling and Testing 

4.3.1 For Acceptance Tests: One or more sets of test specimens to represent each lot of parts. 

4.3.2 For Periodic and Pre-production Tests: Sample quantity shall be selected at the discretion 

of the processor unless otherwise specified by the purchaser. 

4.4 Approval 

4.4.1 The processor shall establish a written process description of preparation, application, 

and inspection for each part number. The description shall include control factors and 

parameters that provide coated parts meeting specified requirements. Control factors 

considered proprietary by the processor shall be assigned codes within the process 

description. The processor shall maintain a complete record of proprietary factors and 

codes. These factors shall include sufficient information to reproduce the process. An 

example is shown in Figure 1. 

4.4.2 The process description and control factors, and first article, whichever is specified, shall 
226

be approved by the cognizant engineering organization before production coated parts are 

supplied. 

4.4.3 The processor of coated parts shall make no significant change to the process description, 

or to the materials, processes, or controls referenced in the process description (see 

Figure 1) unless the change is approved by the cognizant engineering organization. A 

significant change is one, which could affect the properties or performance of the coating 

or substrate. 

4.5 Reports: Processor shall furnish with each lot a report stating that the parts have been 

processed and tested in accordance with specified requirements and that they conform to 

the acceptance test requirements. The report shall include the purchase order number, lot 

number, AMS-xxxx, part number, quantity and numerical test results. 

4.6 Re-sampling and Re-testing: If any acceptance test fails to meet specified requirements, 

disposition of parts may be based upon the results of tests on three additional specimens 

for each nonconforming specimen if coated within the same lot. Except as specified in 

4.6.1, 4.6.2 and 4.6.3, failure of any retest to meet specified requirements shall be cause 

for rejection of parts represented. Results of all tests shall be recorded. 

4.6.1 If any lot acceptance test fails to meet specified requirements, or the original sampling as 

in 4.3 and upon re-sampling as in 4.6, the parts in that lot may be stripped by the method 

specified in this document, or an alternate method acceptable to the purchaser. The parts 

may then be re-coated and re-tested. Whichever method is selected, the process shall not 

roughen, pit, or embrittle the basis material. Alternatively, all parts in that lot may be 

inspected for the nonconformance and nonconforming parts stripped and reprocessed. 

4.6.2 If the results of any periodic test fail to meet the specified requirements, the process is 

nonconforming. No additional parts shall be processed until the process is corrected and 

new specimens are coated and tested. Results of all tests shall be recorded and when 

requested, reported. Purchaser shall be notified of all parts coated since the last 

successful test. 

4.6.3 If a bond strength specimen fails at the adhesive bond joint, at a value less than the 

adhesion control specimen, or at a value less than the required minimum, the test result 

may be declared invalid and the test repeated. Such retest shall not be considered as one 

of the re-tests specified in 4.6. 

5.0 PREPARATION FOR DELIVERY 

5.1 Coated parts shall be handled and packaged to ensure that the required physical 

characteristics and properties of the coating are preserved. 

5.2 Packages of parts shall be prepared for shipment in accordance with commercial practice 

and in compliance with applicable rules and regulations pertaining to the handling, 

packaging, and transportation of the parts to ensure carrier acceptance and safe 

delivery. 

6.0 ACKNOWLEDGMENT 

6.1 Processor shall mention this specification number and its revision letter in all quotations 

and when acknowledging purchase orders. 
227

7.0 REJECTIONS 

7.1 Parts, which have non-compliant coating that does not conform to this specification, or to 

modifications authorized by purchaser, will be subject to rejection. 

8.0 NOTES 

8.1 Dimensions and properties in inch/pound units and the Fahrenheit temperatures are 

primary; dimensions in SI units and Celsius temperatures are shown as the approximate 

equivalent of the primary units and are presented only for information. 

8.2 Procurement documents should specify not less than the following: 

 Type of coating desired and AMS designation 

 Coating thickness required 

 Coating acceptance criteria if not specified herein 

 Quantity of parts to be coated 

8.3 Coatings meeting the requirements of this specification have been classified under 

Federal Standardization Area Symbol "MFFP." 

8.4 Key Words: HVOF, Wear Resistant, Corrosion Resistance, and Dimensional Restoration 

 
228

Figure 1 

Coating Process Control Sheet 

PROCESSOR:__________________________________ COATING SPECIFICATION:_____________ 

PURCHASER:________________________________________________________________________ 

APPLICATION: 

PART NAME:_________________________________________________________________________ 

PART NUMBER: ________________________________ BASIS MATERIAL:____________________ 

PREPARATION: 

PRE-BLAST CLEANING:_______________________________________________________________ 

BLASTING GRIT TYPE:______________GRIT SIZE:____________PRESSURE:_________________ 

BLASTING TIME, INTENSITY, and COVERAGE: __________________________________________ 

SPRAY EQUIPMENT & ACCESSORY EQUIPMENT: 

MANUFACTURER: ______________________________________ TORCH: ____________________ 

POWDER PORT: ________________ HEAD: _________________ NOZZLE: ____________________ 

INJECTOR: OXYGEN: ____________ FUEL: _________________ POWDER: ____________________ 

ADAPTORS:_________________________________________________________________________ 

CONSOLE PARAMETERS: 

OXYGEN: SUPPLY PRESSURE: ____________________ 

 TORCH PRESSURE: ____________________FLOW RATE: _______________________ 

FUEL: TYPE: __________________________________ SUPPLY PRESSURE: __________________ 

TORCH PRESSURE: ______________________________ FLOW RATE: _______________________ 

POWDER FEEDER: 

FEEDER TYPE: ______________________________________________________________________ 

CARRIER GAS: ______________________________SUPPLY PRESSURE:______________________ 

FEEDER PRESSURE:_______________FLOW RATE: ________________DIAL: _________________ 

VIBRATOR USED: [ ]YES [ ]NO AMPLITUDE: _______________________________________ 

FEEDER HOSE: DIAMETER:__________________LENGTH:_________________________________ 

COATING PROCESS DATA: 

PREHEAT TEMPERATURE: ________________ MAXIMUM PART TEMPERATURE: ___________ 

COOLING, METHOD: _____________________ POSITION: _________________________________ 

COOLING, CYCLE TIME: __________________ SPRAY, CYCLE TIME:_______________________ 

SPRAY, NO. OF CYCLES:__________________ SPRAY, COATING THICKNESS:_______________ 

WORK HANDLING: 

PART MOTION: ____________________________________ SPEED: ___________________________ 

GUN MOTION: _____________________________________SPEED: ___________________________ 

GUN-TO-WORK: DISTANCE: _________________________ ANGLE: __________________________ 

SPRAY MASKING/FIXTURES: __________________________________________________________ 

METALLOGRAPHY: 

MICROSTRUCTURE: ________________________________ HARDNESS: ______________________ 
229

BOND STRENGTH: _________________________________ BEND TEST: ______________________ 

COATING MATERIAL: _______________________________ LOT/BATCH NO:_________________ 

OPERATOR: _______________________________________ CERTIFICATION NO: ______________ 

APPROVAL: ____________________________________________________ DATE: ______________

PREPARED UNDER THE JURISDICTION OF AMS COMMITTEE “B” 
230

8.2. Proposed AMS 7881 

AMEC 99B DRAFT 

SPONSOR: BRUCE BODGER 

TUNGSTEN CARBIDE-COBALT POWDER 

Agglomerated and Sintered 

1. SCOPE: 

1.1 Form: 

 This specification covers tungsten carbide-cobalt in the form of powder. 

1.2 Application: 

 This powder has been used typically for producing thermal spray coatings to provide 

wear and fretting resistant surfaces, but usage is not limited to such applications. 

2. APPLICABLE DOCUMENTS: 

 The following publications form a part of this specification to the extent specified herein. 

The applicable issue of referenced publications shall be the issue in effect on the date of 

the purchase order. 

A. S.A.E. Publications: 

 Available from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001. 

 AMS xxxx (AMEC 00AB) 

2.2 ASTM Publications: 

 Available from ASTM, 1916 Race Street, Philadelphia, PA 19103-1187. 

 ASTM B 215 Sampling Finished Lots of Metal Powders 

2.3 U.S. Government Publications: 

 Available from DODSSP, Subscription Services Desk, Building 4D, 700 Robbins 

Avenue, Philadelphia, PA 19111-5094. 

 MIL-STD-2073-1 DOD Materiel, Procedures for Development and Application of 

Packaging Requirements 

3. TECHNICAL REQUIREMENTS: 

3.1 Composition: 

 Shall conform to the percentages by weight shown in Table 1, determined by methods 

acceptable to purchaser. 

TABLE 1 – Composition % by Weight 

Element Minimum % Maximum 

% 

Cobalt 15.0 18.5 
231

Carbon, Total 4.8 5.6 

Tungsten Balance 

Iron -- 1.5 

Others (if 

determined) 

-- 0.5 

3.2 Condition: 

 As manufactured. 

3.3 Properties: 

 Powder shall conform to the following requirements: 

3.3.1 Particle Size Distribution: Shall be as follows: each lot shall show the following 

cumulative volume percentages when measured by laser light scattering method 

(Microtrac). 

TABLE 2 - Particle Size Distribution % by Volume 

Micron Size Minimum % Maximum % 

 

+62 -- 10 

+44 18 35 

+22 60 95 

+11 92 -- 

TABLE 3 – Particle Size distribution by Sieve Size 

Sieve Size Minimum % Maximum % 

 

+270 - 6% 

+325 - 25% 

3.3.2 Thermal Spraying: Powder shall be capable of producing spray coatings to meet the 

requirements of AMEC 00AB. Purchaser and vendor shall agree upon standards for 

acceptance. 

3.4 Quality: 

 Powder, as received by purchaser, shall be thoroughly blended, uniform in color and 

quality, dry, free flowing and free from foreign materials, clumps and imperfections 

detrimental to its spraying qualities. 
232

4. QUALITY ASSURANCE PROVISIONS: 

4.1 Responsibility for Inspection: 

The vendor of powder shall provide all samples for vendor’s tests and shall be responsible for 

performance of all required tests. Purchaser reserves the right to sample and to perform 

any confirmatory testing deemed necessary to ensure that the powder conforms to the 

requirements of this specification. 

4.2 Classification of Tests: 

Tests for all technical requirements are acceptance tests and pre-production tests and shall be 

performed prior to or on the initial shipment of powder to a purchaser, on each lot, when 

a change in ingredients and/or processing requires approval by the cognizant engineering 

organization (see 4.4.2), and when purchaser deems confirmatory testing to be required. 

4.2.1 For direct U.S. Military procurement, substantiating test data and, when requested, 

 pre-production material shall be submitted to the cognizant agency as directed by 

the procuring activity, contracting officer, or request for procurement. 

4.3 Sampling and Testing: 

 Shall be in accordance with ASTM B 215; sufficient powder shall be taken from each lot 

 to perform all required tests. The number of determinations for each requirement 

shall be as specified in the applicable test procedure or, if not specified therein, not 

less than three. 

4.3.1 When purchaser and vendor have agreed upon a statistical sampling plan, sampling shall 

be in accordance with such plan in lieu of sampling as in 4.3 and the report of 4.5 shall 

state such a plan was used. 

4.4 Approval: 

4.4.1 The process and control procedures, a preproduction sample, or both, whichever is 

specified, shall be approved by the cognizant engineering organization before production 

powder is supplied. 

4.4.2 The supplier shall make no significant change in ingredients, processes, or controls 

from those on which the approval was based, unless the change is approved by the cognizant 

engineering organization. A significant change is one, which, in the judgment of the 

cognizant engineering organization, could affect the properties or performance of the 

powder. 

4.5 Reports: 

 The vendor of a powder shall furnish with each shipment a report showing the results of 

 tests for chemical composition and particle size distribution of each lot and 

stating the purchase order number, lot number, AMS XXXX, vendor’s product 

designation, and quantity. 

4.6 Resampling and Retesting: 

 If any sample used in the above tests fails to meet the specified requirements, disposition 

of the powder may be based on the results of testing three additional samples for each 

original nonconforming sample. Failure of any retest sample to meet the specified 

requirements shall be cause for rejection of the powder represented. Results of all tests 

shall be reported. 
233

5. PREPARATION FOR DELIVERY: 

5.1 Packaging and Identification: 

5.1.1 Powder shall be packaged in sealed containers to protect it from contamination during 

shipment and under normal dry storage conditions. Seals used on containers shall be so 

designed that they must be destroyed in order for the container to be opened. 

5.1.2 Each individual container shall be legibly identified with not less than the following 

 information, using characters that will not be obliterated by normal handling: 

 TUNGSTEN CARBIDE-COBALT POWDER 

 AMS XXXX 

 MANUFACTURER’S IDENTIFICATION

_____________________________________ 

PURCHASE ORDER NUMBER 

____________________________________________ 

 QUANTITY 

____________________________________________________________ 

 LOT NUMBER 

__________________________________________________________ 

5.1.3 Containers of powder shall be prepared for shipment in accordance with commercial 

practice and in compliance with applicable rules and regulations pertaining to the 

handling, packaging, and transportation of the powder to ensure carrier handling and safe 

delivery. 

5.1.4 For direct U.S. Military procurement, packaging shall be in accordance with MIL-STD2073-1, Commercial Level, unless Level A is specified in the request for procurement. 

6. ACKNOWLEDGMENT: 

 A vendor shall mention this specification number and its revision letter in all quotations 

and when acknowledging purchase orders. 

7. REJECTIONS: 

 Powder not conforming to this specification, or to modifications authorized by purchaser, 

will be subject to rejection. 

8. NOTES: 

8.1 Revision Indicator: 

 The (R) symbol is for the convenience of the user in locating areas where technical 

revisions, not editorial changes, have been made to the previous issue of this 

specification. If the symbol is next to the specification title, it indicates a complete 

revision of the specification. 

8.2 Definition of terms used in AMS are presented in ARP1917. 

8.3 For direct U.S. Military procurement, purchase documents should specify not less than 

the following: 

 Title, number, and date of this specification 
234

 Quantity of powder desired 

 Thermal spray acceptance standards and methods for testing (see 3.2) 

 Level A packaging, if required (see 5.1.4) 

8.4 Powder meeting the requirements of this specification has been classified under Federal 

Standardization Area Symbol “MFFP” 

8.5 Key Words: 

 Thermal spray coatings, wear resistant coatings 
235

8.3. Proposed AMS7882 

AMEC 99C 

SPONSOR: BRUCE BODGER 

TUNGSTEN CARBIDE-COBALT CHROME POWDER 

Agglomerated and Sintered 

1. SCOPE: 

1.1 Form: 

 This specification covers tungsten carbide-cobalt chrome in the form of powder. 

1.2 Application: 

 This powder has been used typically for producing thermal spray coatings to provide 

wear and fretting resistant surfaces with a high level of corrosion resistance, but usage is 

not limited to such applications. 

2. APPLICABLE DOCUMENTS: 

 The following publications form a part of this specification to the extent specified herein. 

The applicable issue of referenced publications shall be the issue in effect on the date 

 of the purchase order. 

 S.A.E. Publications: 

 AMS xxxx (AMEC 00AB) 

2.2 ASTM Publications: 

 Available from ASTM, 1916 Race Street, Philadelphia, PA 19103-1187. 

 ASTM B 215 Sampling Finished Lots of Metal Powders 

2.2 U.S. Government Publications: 

 Available from DODSSP, Subscription Services Desk, Building 4D, 700 Robbins 

Avenue, Philadelphia, PA 19111-5094. 

 MIL-STD-2073-1 DOD Materiel, Procedures for Development and Application of 

Packaging Requirements 

3. TECHNICAL REQUIREMENTS: 

3.1 Composition: 

 Shall conform to the percentages by weight shown in Table 1, determined by methods 

acceptable to purchaser. 

TABLE 1 – Composition % by Weight 
236

Element Minimum % Maximum 

% 

 

Cobalt 8.5 11.5 

Chromium 3.0 5.0 

Tungsten Balance 

Carbon, Total 3.2 5.5 

Iron -- 1.0 

Others (if 

determined) 

-- 2.5 

3.2 Condition: 

 As manufactured. 

3.3 Properties: 

 Powder shall conform to the following requirements: 

3.3.1 Particle Size Distribution: Shall be as follows: each lot shall show the cumulative 

volume percentages when measured by laser light scattering method (Microtrac). 

TABLE 2 - Particle Size Distribution % by Volume 

Micron Size Minimum % Maximum % 

 

+62 -- 10 

+44 18 35 

+22 60 95 

+11 92 -- 

TABLE 3 – Particle Size distribution by Sieve Size 

Sieve Size Minimum % Maximum % 

 

+270 - 5% 

+325 - 25% 

3.3.2 Thermal Spraying: Powder shall be capable of producing spray coatings to meet the 
237

requirements of AMEC 00AB. Purchaser and vendor shall agree upon standards for 

acceptance. 

3.4 Quality: 

 Powder, as received by purchaser, shall be thoroughly blended, uniform in color and 

quality, dry, free flowing and free from foreign materials, clumps and imperfections 

detrimental to its spraying qualities. 

4. QUALITY ASSURANCE PROVISIONS: 

4.1 Responsibility for Inspection: 

 The vendor of powder shall provide all samples for vendor’s tests and shall be 

responsible for performance of all required tests. Purchaser reserves the right to sample 

and to perform any confirmatory testing deemed necessary to ensure that the powder 

conforms to the requirements of this specification. 

4.2 Classification of Tests: 

 Tests for all technical requirements are acceptance tests and pre-production tests and shall 

be performed prior to or on the initial shipment of powder to a purchaser, on each lot, 

when a change in ingredients and/or processing requires approval by the cognizant 

engineering organization (see 4.4.2), and when purchaser deems confirmatory testing to 

be required. 

4.2.1 For direct U.S. Military procurement, substantiating test data and, when requested, preproduction material shall be submitted to the cognizant agency as directed by the 

procuring activity, contracting officer, or request for procurement. 

4.3 Sampling and Testing: 

 Shall be in accordance with ASTM B 215; sufficient powder shall be taken from each lot 

to perform all required tests. The number of determinations for each requirement shall be 

as specified in the applicable test procedure or, if not specified therein, not less than 

three. 

4.3.1 When purchaser and vendor have agreed upon a statistical sampling plan, sampling shall 

be in accordance with such plan in lieu of sampling as in 4.3 and the report of 4.5 shall 

state such a plan was used. 

4.4 Approval: 

4.4.1 The process and control procedures, a preproduction sample, or both, whichever is 

specified, shall be approved by the cognizant engineering organization before production 

powder is supplied. 

4.4.2 The supplier shall make no significant change in ingredients, processes, or controls from 

those on which the approval was based, unless the change is approved by the cognizant 

engineering organization. A significant change is one, which, in the judgment of the 

cognizant engineering organization, could affect the properties or performance of the 

powder. 

4.5 Reports: 

 The vendor of a powder shall furnish with each shipment a report showing the results of 

tests for chemical composition and particle size distribution of each lot and stating the 

purchase order number, lot number, AMS XXXX, vendor’s product designation, and 

quantity. 
238

4.6 Resampling and Retesting: 

 If any sample used in the above tests fails to meet the specified requirements, disposition 

of the powder may be based on the results of testing three additional samples for each 

original nonconforming sample. Failure of any retest sample to meet the specified 

requirements shall be cause for rejection of the powder represented. Results of all tests 

shall be reported. 

5. PREPARATION FOR DELIVERY: 

5.1 Packaging and Identification: 

5.1.1 Powder shall be packaged in sealed containers to protect it from contamination during 

shipment and under normal dry storage conditions. Seals used on containers shall be so 

designed that they must be destroyed in order for the container to be opened. 

5.1.2 Each individual container shall be legibly identified with not less than the following 

information, using characters that will not be obliterated by normal handling: 

 TUNGSTEN CARBIDE-COBALT CHROME POWDER 

 AMS XXXX 

 MANUFACTURER’S IDENTIFICATION

______________________________________ 

PURCHASE ORDER NUMBER 

____________________________________________ 

QUANTITY

____________________________________________________________ 

LOT NUMBER

__________________________________________________________ 

5.1.3 Containers of powder shall be prepared for shipment in accordance with commercial 

practice and in compliance with applicable rules and regulations pertaining to the 

handling, packaging, and transportation of the powder to ensure carrier handling and safe 

delivery. 

5.1.4 For direct U.S. Military procurement, packaging shall be in accordance with MIL-STD2073-1, Commercial Level, unless Level A is specified in the request for procurement. 

6. ACKNOWLEDGMENT: 

 A vendor shall mention this specification number and its revision letter in all quotations 

and when acknowledging purchase orders. 

7. REJECTIONS: 

 Powder not conforming to this specification, or to modifications authorized by purchaser, 

will be subject to rejection. 

8. NOTES: 

8.1 Revision Indicator: 

 The (R) symbol is for the convenience of the user in locating areas where technical 
239

revisions, not editorial changes, have been made to the previous issue of this 

specification. If the symbol is next to the specification title, it indicates a complete 

revision of the specification. 

8.2 Definition of terms used in AMS are presented in ARP1917. 

8.3 For direct U.S. Military procurement, purchase documents should specify not less than 

the following: 

 Title, number, and date of this specification 

 Quantity of powder desired 

 Thermal spray acceptance standards and methods for testing (see 3.2) 

 Level A packaging, if required (see 5.1.4) 

8.4 Powder meeting the requirements of this specification has been classified under 

 Federal Standardization Area Symbol “MFFP” 

8.5 Key Words: 

 Thermal spray coatings, wear resistant coatings 
240

8.4. Proposed HVOF grinding specification 

Proposed 

 GRINDING OF HVOF SPRAYED TUNGSTEN CARBIDE COATINGS 

APPLIED TO HIGH STRENGTH STEELS 

DRAFT

Jon L. Devereaux, Sponsor Page 1 of 11

1.0 SCOPE 

1.1 Purpose 

 This specification covers engineering requirements for the grinding of tungsten carbide 

High Velocity Oxygen/Fuel (HVOF) thermal spray coatings applied to high strength 

steels (220 ksi and above). 

1.2 Application 

 This process has been used typically to grind tungsten carbide HVOF coatings applied in 

accordance with AMS 2447 to high strength steels for applications requiring wear, heat, 

and corrosion resistance or dimensional restoration, such as aircraft landing gear 

components. However, usage is not limited to such applications. This process 

specification does not cover superfinishing of HVOF applied coatings. 

1.3 Safety - Hazardous Materials 

 While the materials, methods, applications, and processes described or referenced in this 

specification may involve the use of hazardous materials, this specification does not 

address the hazards which may be involved in such use. It is the sole responsibility of the 

user to ensure familiarity with the safe and proper use of any hazardous materials and to 

take necessary precautionary measures to ensure the health and safety of all personnel 

involved. 

2.0 APPLICABLE DOCUMENTS 

2.1 The following publications form a part of this specification to the extent specified herein. 

The latest issue of SAE publications shall apply. The applicable issue of other 

publications shall be the issue in effect on the date of the purchase order. 

2.2 S.A.E. Publications: Available from SAE, 400 Commonwealth Drive, 

Warrendale, PA 15096-0001. 

 AMS 2447 Coating, Thermal Spray High Velocity Oxygen/Fuel Process 

 AMEC 99B Tungsten Carbide-Cobalt Powder 

 AMEC 99C Tungsten Carbide-Cobalt Chrome Powder 

2.3 ASTM Publications: Available from ASTM, 100 Barr Harbor Drive, West 

Conshocken, PA 19428-2959 
241

 ASTM E 1417 Fluorescent Penetrant Inspection Methods 

3.0 TECHNICAL REQUIREMENTS 

3.1 Equipment 

3.1.1 Grinding equipment: Grinding equipment shall be capable of maintaining grinding wheel 

speed, workpiece speed, traverse or cross feed speed, and down feed (infeed) in 

increments necessary to avoid surface degradation of the part. Provisions shall be made 

to supply a constant application of cutting fluid (coolant) to the working surface of the 

wheel at the grinding zone interface. 

3.2 Materials 

3.2.1 Grinding wheels: Grinding wheels shall be labeled with the complete grinding wheel 

specification including abrasive type, grit size, grade, structure, bond type and maximum 

operating speed. Unless otherwise specified, diamond abrasive resin bonded grinding 

wheels shall be used. 

3.2.2 Cutting Fluids (Coolants): A suitable cutting fluid shall be used which does not have an 

adverse effect on the part being ground. Recirculating cutting fluids shall be 

continuously filtered to minimize recycling grinding residue and swarf. A coolant nozzle 

sufficiently wide to flood the entire width of the grinding wheel shall be used. For proper 

application of the cutting fluid, the cutting fluid nozzle should be designed to deliver 

cutting fluid at a speed equal to or slightly faster than the peripheral grinding wheel 

speed. (See Section 8.4.2). 

3.3 Preparation 

3.3.1 Cleaning: Protective coatings and other foreign materials shall be removed from parts 

prior to grinding to preclude contamination of coolant and wheels. Coolants and grinding 

residuals that have a deleterious effect on the part shall be removed after grinding. 

Cleaning materials shall not corrode or otherwise degrade the surfaces of the part. Where 

process delay time is such that corrosion might occur, parts shall be adequately protected 

after cleaning. 

3.4.1 Grinding Process Control: The grinding process shall be performed in accordance with 

Paragraph 3.5 to result in metallurgically sound parts. All speeds, feeds, and stock 

removal parameters are actual and not necessarily machine or indicator readings. Prior to 

grinding, clean all surfaces to be ground as stated in Paragraph 3.3.1. 

3.4.2 Balance the grinding wheel assembly at the time the wheel is mounted. If the grinding 

wheel remains on the machine arbor it will not need to be balanced again. If the grinding 

wheel is removed from the machine/arbor it should be re-balanced at the next mounting. 

True the wheel face so that it is geometrically correct for the application and runs 

concentric with the centerline of the arbor assembly (see Section 8.4.1). Dress the 

grinding wheel frequently during use to keep the wheel open with sharp grit exposed to 

freely cut the work material. 

3.4.3 Flood the entire width of the grinding wheel at the wheel-work interface with a filtered 

continuous flow of approved cutting fluid. 

3.5 Grinding Parameters. 

3.5.1 Use a peripheral grinding wheel speed of 5200 to 6500 surface feet per minute (SFPM). 
242

3.5.2 Use cross feeds/traverse rate and infeeds as follows: 

3.5.2.1 For cylindrical and internal grinding, the roughing infeed (on the diameter) shall not 

 exceed a maximum of 0.002 inch for 100 or 120 grit, 0.0015 inch for 150 grit, 

 0.001 inch for 180 grit, 0.0008 inch for 220 grit, 0.0006 inch for 320 grit, or 

 0.0004 inch for 400 grit for each pass. Incremental infeed shall be done at each end 

 of the traverse or crossfeed to maintain wheel face flatness. 

3.5.2.2 For cylindrical and internal grinding, a minimum of 0.002 inch stock 

 removal per side (0.004 inch on diameter) is required for finish grinding. The 

 finishing infeeds (on the diameter) shall not exceed a maximum of 0.0005 inch for 

 100 or 120 grit, 0.0004 inch for 150 grit, 0.0003 inch for 180 grit, 0.0002 inch for 220 

 grit, or 0.0001 inch for 320 or 400 grit for each pass. Incremental infeed shall be done 

 at each end of the traverse or crossfeed to maintain wheel face flatness. 

3.5.2.3 For cylindrical and internal grinding, use a roughing cross feed or traverse rate 

 of 1/6 to 1/8 wheel width per workpiece revolution. Use a finishing cross feed 

 or traverse rate of 1/8 to 1/12 wheel width per workpiece revolution. 

3.5.2.4 For surface grinding, the cross feeds shall not exceed 0.080 inch per pass. 

3.5.2.5 For surface grinding, the roughing infeed (depth of cut) shall not exceed a maximum of 

0.001 inch for 100 or 120 grit, 0.0008 inch for 150 grit, 0.0005 inch for 180 grit, 0.0004 

inch for 220 grit, 0.0003 inch for 320 grit, or 0.0002 inch for 400 grit for each pass. 

3.5.2.6 For surface grinding, a minimum of 0.002 inch stock removal is required for 

finish grinding. The finishing infeeds (depth of cut) shall not exceed a maximum of 

0.0005 inch for 100 or 120 grit, 0.0004 inch for 150 grit, 0.0003 inch for 180 grit, 

0.0002 inch for 220 grit, or 0.0001 inch for 320 or 400 grit. 

3.5.2.7 When grinding ID or OD surfaces, the work should have a speed of 60 to 100 

 surface feet per minute. 

3.5.2.8 The following chart contains general guide lines for surface finish generation for the grit 

 sizes indicated above. Traverse grinding with spark-out passes may produce finer 

 finishes than shown here. 

Grit Size Roughing Mode Finishing Mode 

100-120 32–36 AA 20-32 AA 

150 22-26 AA 14-20 AA 

180 18-22 AA 12-14 AA 

220 12-16 AA 10-12 AA 

320-400 8-11 AA 6-10 AA 

a. Note: Superfinishing of surfaces is generally indicated by measured surface finishes 

less than 5 AA. Grit sizes used in the ranges above are not capable of producing 

super finished surfaces. 
243

b. AA = Arithmetic Average (Micro-inch) surface finish. 

3.6 Each of the following parameters shall be met when selecting a grinding wheel. 

3.6.1 Abrasive – Diamond. 

3.6.2 Bond – Resin 

3.6.3 Grit or Grain Size – 100 to 400 grit 

3.6.4 Hardness or Grade – H, L, M, N, P, or R 

3.6.5 Concentration – 75 to 125 

3.7 Inspection requirements: All ground surfaces shall be visually inspected without 

magnification for evidence of overheating (discoloration), cracks, flaking or 

peeling. 

 Ground surfaces shall be checked for surface finish. 

3.8 Fluorescent Penetrant Inspection: All ground surfaces shall be inspected by fluorescent 

penetrant inspection in accordance with ASTM E 1417 or approved alternate procedure. 

4.0 QUALITY ASSURANCE 

4.1 Monitoring of the process and examination of the finished items shall be in compliance 

with local quality assurance provisions which ensure that the requirements of this 

specification are met. 

4.2 The coated surface shall be free from heat checks, pull-outs, cracks, and separation from 

the base metal. Examine for heat checks and pull-outs visually using up to 40X 

magnification. Penetrant inspection, in accordance with ASTM E 1417, shall be used to 

check for cracking caused by, or revealed by, grinding; and separation of the 

thermal spray coating from the base metal along the periphery of the coating. Etching 

prior to penetrant inspection is not required. 

4.3 The finished surface shall meet the machine finish required by the drawing. Unless 

otherwise specified, finishes which are finer than the drawing callout shall not be cause 

for rejection. 

4.4 The process description and control factors, a ground part, or both, whichever is 

 specified, shall be approved by the cognizant engineering organization before 

 production finish ground parts are supplied. 

4.5 The processor of parts ground in accordance with this specification shall make no 

significant change to the process description, or to the materials, processes, or controls 

referenced in the process description (see Figure 1) unless the change is approved by the 
244

cognizant engineering organization. A significant change is one which deviates more 

than plus or minus ten percent of the approved baseline. 

4.6 If the results of any inspection fail to meet the specified requirements, the process is 

nonconforming. No additional parts shall be processed until the process is corrected and 

new specimens are coated and ground. Results of all tests shall be recorded and when 

requested, reported. Purchaser shall be notified of all parts ground since the last 

successful inspection. 

5.0 PREPARATION FOR DELIVERY 

5.1 Finish ground parts shall be handled and preserved and packaged to ensure that the 

required physical characteristics and properties of the ground coating are preserved. 

5.2 Packages of parts shall be prepared for shipment in accordance with commercial practice 

and in compliance with applicable rules and regulations pertaining to the handling, 

packaging, and transportation of the parts to ensure carrier acceptance and safe delivery. 

6.0 ACKNOWLEDGMENT 

6.1 Processor shall mention this specification number and its revision letter in all quotations 

and when acknowledging purchase orders. 

7.0 REJECTIONS 

7.1 Parts, which have non-compliant coating that does not conform to this specification, or to 

modifications authorized by purchaser, will be subject to rejection. 

8.0 NOTES 

8.1 Dimensions are in inches. 

8.2 Procurement documents should specify not less than the following: 

 AMS xxxx 

 Type of coating to be ground 

 Coating thickness required 

 Coating acceptance criteria if not specified herein 

 Quantity of parts to be ground 

8.3 Definitions 

8.3.1 Balance (Dynamic): Dynamic balance is the balancing of the complete rotating 
245

 assembly; grinding wheel, mounting arbor, and the machine spindle. Depending on 

 the balancing machine, the balance condition is measured and corrected at wheel 

 speed. 

8.3.2 Balance (Static): Static balance is the balance of the grinding wheel mounted 

 on an arbor, typically balanced on a pair of knife-edges. 

8.3.3 Concentration: The amount of superabrasive material contained in a unit volume of 

 the grinding wheel. The measurement is based on the number of carats of 

 superabrasive material per unit volume. Typical concentration numbers are in the 

 range of 30 to 175. A concentration of 100 does not mean 100 percent and therefore 

 is not the maximum concentration. A concentration of 100 means that there are 4.4 

 carats of abrasive per cm3

 (or 72 carats/cu. in. or 25 volume percent). The abrasive 

 concentration of the selected grinding wheel, for grinding HVOF applied tungsten 

 carbide coatings, is based upon the area of contact between the wheel and workpiece. 

 A large contact area dictates a low concentration and a small area of contact a higher 

 concentration so the wheel can hold form and resist premature wear. Since OD 

 grinding generally has smaller contact areas than ID grinding, the concentration of the 

 abrasive in the grinding wheel should generally be higher for OD grinding and lower for 

 most ID applications. 

8.3.4 Coolant: Coolant is the misnomer for cutting fluid. The cutting fluid not only cools; 

 it also lubricates and washes away chips and debris. The better term is cutting fluid. 

8.3.5 Cutting Fluid: Cutting fluid is the fluid used to cool, lubricate, and wash clean a 

 machining process. Cutting fluids may be oils, water-based synthetic, or soluble oils 

 and gases. 

8.3.6 Dressing: Dressing/sharpening is the process of conditioning the wheel surface so as 

 to achieve a certain grinding behavior. Dressing a resin-bonded grinding wheel entails 

 removing some of the bond to expose the grain. This is accomplished by using an 

 aluminum oxide or silicone carbide stick to "stick" the wheel. "Sticking" is best 

 performed wet. 

8.3.7 Grade: The grade of a grinding wheel usually refers to the hardness of the bond 

 system. This is determined by the size, type, and amount of filler materials 

 incorporated in the plastic binder material. There is a standard code for grinding wheel 

 grades. 

8.3.8 Grain Size: The grain size refers to a number which corresponds to the U.S. Standard 

 wire mesh size screen used for sizing the abrasive grain. This size is also defined by 

 average particle diameter. 

8.3.9 Grit Size: Another term for abrasive grain size. 

8.3.10 Infeed: The infeed or down-feed is the feed motion of the grinding wheel into the 

 workpiece. 

8.3.11 Spark-out: This is the grinding of a workpiece at the end of the stock removal cycle 

 without engaging any further downfeed (infeed). The grinding forces are allowed 

 to subside with time, ensuring a precision surface. 
246

8.3.12 Superabrasive: Diamond or Cubic Boron Nitride (CBN) abrasives. 

8.3.13 Superfinishing: A process used to produce high surface finishes with an average 

 surface roughness of 8 ∝inches Ra, or better, and a higher bearing area ratio, Tp, than 

 ground surfaces. 

8.3.14 Surface Finish: Surface finish is the smoothness of the machining marks on the 

 surface of a workpiece. Surface finish is measured by moving a precision stylus 

 across the surface and measuring the amplitude of the fluctuations of the stylus. 

8.3.15 Swarf: Swarf is the mass of chips and debris remaining after a grinding process. 

8.3.16 Thermal Damage: Thermal damage is metallurgical damage which occurs to a 

 material when it is subjected to temperatures which will affect its metallurgical 

 structure. 

8.3.17 Truing: Truing a grinding wheel is the procedure of making the grinding wheel 

 geometrically correct for the application and to ensure it is rotating concentric to the 

 center line of the spindle. 

8.3.18 Brake Truing Device: A brake controlled truing device is a grinding wheel mounted on 

 an arbor and bearing housing, which has a friction brake to limit its speed. Such a 

 brake-controlled truing device can be used to true resin-bonded superabrasive grinding 

 wheels. Note: Brake controlled truing devices are useful for truing grinding wheels up 

 to 12 inch diameter. 

8.3.19 Wheel Speed: Wheel speed is the peripheral speed of the grinding wheel. It is 

calculated by the following formula: Wheel Speed (Surface Feet per Minute) = Wheel 

Diameter (in inches) x RPM x .262. For metric equivalent, Meters per Second (m/sec. 

x 196.85 = SFPM 

8.4 Key Words: HVOF, Wear Resistant, Corrosion Resistance, Dimensional Restoration, 

Superabrasive, Concentration, Cutting Fluid, Dress, True 

Figure 1 

Grinding Process Control Sheet

APPLICATION: 

PART NAME:__________________________________ MACHINE NAME: __________ 

PART NUMBER (VENDOR CODE):___________________________________(_____________)__ 

WHEEL CLASSIFICATION:______________________________________________________________ 

WHEEL MANUFACTURER:______________________________________________________________ 

WHEEL: DIA____________ RPM______________ SFPM_______________ WIDTH____________ 
247

WORK PIECE DATA: 

DIAMETER____________ RPM___________ SFPM____________ LENGTH OF TRAVERSE_____ 

AMOUNT OF MATERIAL TO BE REMOVED BY GRINDING____________________________________ 

BASE MATERIAL_________________________ UTS OR HARDNESS__________________________ 

CUTTING FLUID_______________________________________ CONCENTRATION______________ 

GRINDING PARAMETERS 

 TRAVERSE GRIND: ROUGHING TRAVERSE GRIND: FINISHING 

__________ Workpiece RPM 

__________ Roughing Traverse Rate 

__________ Traverse Rate (Inches/Minute) 

______Sec. Traverse Time for Length of Traverse 

__________ Rough Infeed per End 

Rough Until __________ From Finish Size 

__________ Finish Infeed per End 

______Sec. Dwell Time Left 

______Sec. Dwell Time Right 

__________ No. of Spark-out Passes 

__________ Total Cycle Time 

__________ Workpiece RPM 

__________ Finishing Traverse Rate 

__________ Traverse Rate (Inches/Minute) 

______Sec. Traverse Time per Length of Traverse 

__________ Finish Infeed per End 

__________ Dwell Time Left 

__________ Dwell Time Right 

__________ No. of Spark-out Passes 

__________ Total Cycle Time 

TOTAL DIMENSIONAL TOLERANCE___________________ SURFACE FINISH REQUIRED___________∝in 

Ra

SPECIAL MACHINING INSTRUCTIONS: Check wheel speed before starting._____________________________ 

_Verify roughing traverse rate (xxx Seconds/xxx inches length of roughing traverse) prior to grinding.____________

 Verify finishing traverse rate (xxx Seconds/xxx inches length of finishing traverse) prior to grinding._____________

____________________________________________________________________________________________ 

____________________________________________________________________________________________ 

____________________________________________________________________________________________ 

____________________________________________________________________________________________ 

OPERATOR: _______________________________________ CERTIFICATION NO: 

______________________ 

APPROVAL: ____________________________________________________ DATE: 

_____________________ 

PREPARED UNDER THE JURISDICTION OF AMS COMMITTEE “B” 
248

8.4.1. Truing & Dressing of Large Diameter Diamond Grinding 

Wheels 

Small brake controlled truing devices, which typically have a 3” diameter truing wheel, 

are designed for use on grinding wheels up to 12” diameter. The use of small brake 

controlled truing devices on large diameter grinding wheels (14” to 36”) results in 

excessive time required to true a wheel with a number of truing wheels being consumed 

in the process. 

There are two options available; one is to send the wheel back to the grinding wheel 

manufacturer for re-truing. The second option is to re-true the wheel locally. This can be 

accomplished on the machine; it does however require the manufacture of tooling to 

accept a conventional grinding wheel for the machine. 

Procedure for truing large diameter diamond grinding wheels (14” to 36”): 

1. Manufacture an arbor and flange assembly to mount a 10” x 1” or 2” wide x hole size 

conventional grinding wheel with provisions for a dog to turn the assembly. Wheel 

diameter can be larger or smaller depending on the size of the O.D. grinding machine. 

The conventional abrasive wheel mounted on this arbor/flange fixture is mounted in the 

work position on the O.D. machine. 

2. The recommended conventional abrasive truing wheel would be 39C60I8VK or 

equivalent. As a general guideline, the truing wheel should be 60 to 100 grit. 

3. At the point of contact of the two wheels, the diamond wheel that needs truing and the 

conventional abrasive truing wheel, the preferred method is for both wheels to be running 

in the same direction. Some machines do not allow for this set-up (i.e. some machines do 

not have capability to reverse the direction of rotation of the workpiece). 

4. The diamond wheel should be running at spindle speed. Usually on O.D. grinders this 

is about 5600 SFPM (28 m/s). 

5. Adjust the conventional abrasive wheels mounted in the work position to 1500 SFPM 

(7 m/s) or about 1/3 the surface footage of the diamond wheel. 

6. Traverse the diamond wheel across the conventional abrasive wheels face at 30 to 60 

inches per minute. Traverse the entire width to ensure that a radius is not formed into the 

wheel face. 

7. Infeed the diamond wheel into the truing wheel 0.0005” to 0.001” per traverse until 

the wheel face is trued. Color the wheel face using a wax china marker, this will aid in 

visually being able to see when truing is complete. DO NOT use liquid ink or Dykum, 

these liquids leach into the diamond wheels pores and will result in truing off more 

usable diamond depth than is necessary. 

8. After truing, stick dress the diamond wheels face to sharpen the wheel for grinding. 

The recommended dressing stick specification is 38A (one or two grit sizes finer than the 
249

diamond in the wheel) HVBE. 

9. Dial indicate the wheel to ensure wheel is round and face is flat. 

This results in a very large brake controlled truing device, significantly more efficient and 

capable of truing large diameter wheels than the small brake controlled truing devices 

designed and intended for use on wheels 12” and less in diameter. 
250

8.4.2. Calculation of nozzle opening for proper cutting fluid 

application 

The velocity of the cutting fluid should be equal to or slightly greater than peripheral 

velocity of the grinding wheel. An example of how to size the cutting fluid nozzle 

opening is provided here. For this example, assume the cutting fluid delivery pump is 

rated at 50 U.S. gallons per minute (gpm). Also, for this example assume a peripheral 

grinding wheel speed of 6,000 surface feet per minute (SFPM). 

For a one inch wide grinding wheel, make the nozzle slightly wider than the wheel width 

or about plus 1/8” on each side. Therefore, for a one inch wide wheel, select nozzle 

width of 1.25”. 

Area (of nozzle opening) = 1.25 x d in2

Grinding wheel speed 6,000 SFPM x 12 in/ft = 72,000 IPM 

Volume flow = Nozzle area x wheel speed 

50 U.S. gal/min x 231 in3

/U.S. gallon = (1.25 x d in2) (72,000 in/min) 

1 1⁄4” wide 

d
251

 11,550 in3

/min = 90,000 x d in3/min 

 d = 0.128” 

To get fluid exit velocity slightly higher than grinding wheel speed, 

select a nozzle opening, d, of 0.125”. 

.

253

9.Implementation 

9.1. General implementation issues 

Implementing HVOF in place of EHC on landing gear has proved to be an important 

learning experience in material and process substitution. The major issues are 

summarized here and some items are discussed in more detail below: 

. Teaming – In attempting to qualify and implement a new technology on flightcritical components, it is essential to involve the entire stakeholder community 

from the outset and identify important areas of concern. Contributions from 

program offices, system support offices, depot engineers, and OEMs were made 

toward development of the JTP and all results, positive and negative, were 

presented to them for evaluation and consideration. To make the program 

manageable, ad hoc subteams were put together to resolve specific issues, such as 

fatigue testing and coating integrity. Various team members and organizations 

volunteered their services and test facilities to acquire data, develop 

specifications, and carry out other tasks critical to the success of the overall 

project. Although commercial competitors were involved in the team, they 

exhibited a remarkable degree of cooperation in developing a common technology 

base. 

. Data acquisition – The whole team has been kept aware of all significant data 

through biannual program review meetings and a team web site where all data is 

posted. When an unexpected issue arose, such as the delamination of the HVOF 

coatings at high stress, it was again important to involve the stakeholder 

community and obtain their criteria for acceptable performance. Team member 

orgainizations contributed their own data to the effort whenever items of 

particular concern or interest arose. There must be flexibility (both programmatic 

and financial) built into any project of this type so that unplanned testing can be 

conducted to address unforeseen issues. 

. Implementation – From the outset the project was designed to implement 

directly in the depots and vendors. Equipment was installed by the program in 

several depots and team experts from industry trained depot personnel and helped 

the depots to develop process specifications. While the original expectation was 

that all depositions on test items would be done in the depots, this was found to be 

impractical in some cases because of the production demands of the depots. 

Therefore in most cases materials analyis specimens were coated at commercial 

vendors. 

. Specifications and standards – It was made clear early in the program that 

implementation would require the existence of standards and specifications to 

which engineers could refer. For this reason the team assigned specific members 

to develop specifications and shepherd them through the relevant AMEC 

committees. 
254

9.2. Performance observations – differences 

between HVOF and EHC 

Based on all of the materials testing, it was concluded that on high-strength steels that are 

used to fabricate landing gear components, the fatigue, wear and impact-resistant 

properties of the HVOF WC/17Co and WC/10Co4Cr coatings are superior to those of 

EHC coatings. Inconsistent results were obtained for corrosion testing of both the HVOF 

and EHC coatings, but examination of all of the B117 salt fog and atmospheric corrosion 

test results, together with the favorable performance of WC/17Co coatings in service on 

the P3 aircraft, lead to the conclusion that the HVOF coatings should perform at least as 

well as EHC coatings. There are no hydrogen embrittlement issues associated with 

HVOF coatings and the testing indicated that re-embrittlement is less of an issue with 

HVOF coatings than with EHC. All of the rig and flight testing performed to date has 

indicated excellent performance of the WC/17Co and WC/10Co4Cr coatings. This is 

true even of components in situations where materials testing had raised coating integrity 

and delamination concerns. 

The one issue that still can impact implementation of the HVOF coatings on landing gear 

components is delamination at high stresses and strains. It has been shown that the 

delamination is dependent on the thickness of the coating and on how the stresses are 

applied. Thus, for example, OO-ALC has shown that on a hollow cylinder representative 

of a landing gear piston, a WC/17Co coating will remain intact in tension/compression 

bending (R = -0.33) to beyond the yield stress of the base material, whereas it will 

delaminate at about 80% of the yield stress (for a 0.010”-thick coating) for fully-reversed 

tension/compression stresses applied axially. The Air Force believes that the bending test 

is more representative of real-life stresses applied to landing gear and thus they are 

continuing to implement HVOF coatings on their landing gear components. On the other 

hand, Navy structural engineers believe that the HVOF coatings should be able to remain 

intact up to the yield stress for fully-reversed axially applied stress and therefore are 

reluctant to approve the coatings on carrier-based aircraft that are subjected to high stress. 

But this concern should not prevent implementation of the HVOF coatings on land-based 

aircraft and helicopters. 

The original specifications for surface finishes and seals that have been developed over 

the years for EHC plating cannot be used for HVOF coatings. If a surface finish of 16∝” 

Ra, typical for EHC, is used on an HVOF-coated landing gear inner cylinder it will 

rapidly degrade the seal and cause excessive leakage. On the other hand, a superfinished 

HVOF surface with <4∝” Ra gives much longer seal life than is obtained with EHC, and 

shows little or no degradation of the coated surface over time. It has been found that it is 

no longer sufficient simply to specify an average roughness (Ra), or a root mean square 

roughness (RMS), surface finish, as is traditionally done with EHC. For many 

applications users are specifying more detailed surface parameters that can be achieved 

with superfinishing equipment, such as bearing area ratio (Tp), peak-to-valley height 

(Rt), peak height (Rp), and valley depth (Rv). 

For applications such as landing gear it is possible that HVOF WC/Co or wC/CoCr will 

approach the ideal of a “lifetime coating” that will not need to be reworked for the life of 

the component (or at least for many maintenance cycles). However, this requires a 
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recognition that the maintenance methods commonly used for chrome plated parts can no 

longer apply. For example, many plated parts are assumed always to deteriorate in 

service and are therefore always stripped and the substrate examined by NDI (magnetic 

particle, MPI, or fluorescent penetrant inspection, FPI) for damage or cracks. If the cost 

savings associated with the extended life of HVOF coatings are to be realized 

maintenance cannot be done in this way since it will result in unnecessary stripping and 

recoating. Some users are adopting the approach that FPI of the coated part is sufficient, 

while other users are endeavoring to develop better through-coating inspection 

procedures. 

The corrosion data is another area in which HVOF coatings perform in a different way 

from EHC coatings. EHC-plated parts tend to corrode by liquid penetration followed by 

substrate corrosion and undercutting of the coating. The chrome plate itself does not 

corrode, but it tends to remain in place until it loses its support and flakes off. HVOF 

coatings, on the other hand, experience corrosion of the matrix alloy. This could lead to 

gradual dissolution and roughening of the surface, leading to seal damage and a gradual 

increase of seal leakage. Neither of these failure mechanisms is necessarily always 

“better” than the other, but the difference needs to be recognized in planning 

implementation and maintenance. 

9.3. Implementation 

The Air Force is proceeding with implementation of HVOF coatings on landing gear 

components at Ogden ALC, the primary overhaul location for landing gear. As of March 

2003, a project implementation team had been established at OO-ALC and they expect a 

phased transition based on capacity. They have approved the application of WC/17Co 

and WC/10Co4Cr coatings up to 0.010” thickness on the components summarized in 

Table 9-1. 

Fixturing is being developed for spraying these parts and the coatings will be applied 

under local engineering authority. 

The Air Force has agreed to invest $3.5 million of non-environmental funds to expand 

Ogden’s HVOF spraying capabilities to meet their workload. OO-ALC is projecting a 

total of 10 spray booths, four considered small, four medium, and two large for 

processing different size components. These spray booths would be used for processing 

more than just landing gear components as most of the actuators on Air Force aircraft are 

also processed at Ogden. 

The HVOF systems currently in operation at OO-ALC and NADEP-JAX are fullproduction systems with fixturing for manipulation of various types of components and 

Table 9-1. Components Approved for HVOF at OO-ALC

C-5 MLG outer pitch actuator C-5 gudgeon pin 

C-5 MLG ball screw F-16 MGL tension strut 

F-16 MLG axle A-10 NLG charging pin 

A-10 NLG inner piston KC-135 MLG aft axle 

KC-135 MLG forward axle KC-135 NLG piston 

F-15 brake drive keys B-2 rub strips 
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robots on which the HVOF spray guns are mounted. NADEP-JAX has not projected the 

total number of spray booths required, but does plan on acquiring more as the number of 

components approved for HVOF processing increases. 

As a result of the HCAT program on landing gear Goodrich has specified HVOF 

WC/CoCr as the baseline coating in place of hard chrome plate on the landing gear for 

the Joint Strike Fighter (JSF). HVOF will therefore be used for seal surfaces and axle 

bearings, lugs, pins, and landing gear hydraulic actuators. 

As a result of the program Messier-Dowty has installed HVOF coating equipment at their 

Ajax, Ontario plant, which manufactures F-18 landing gear, among others. At this point, 

the technology has not been approved for carrier-based F-18 use because of the coating 

integrity issues outlined in Section 9.2. 

Early in 2003 Ogden ALC demonstrated their first “green landing gear” overhaul, using 

HVOF for all wear surfaces (previously chrome), IVD aluminum for external surfaces 

and sputtered aluminum for internal surfaces subject to corrosion (previously cadmium 

plated). 

9.4. Specifications and standards 

One of the key end user/OEM issues is the availability of standards and specifications 

related to the powder used for HVOF coatings, application procedures for the coatings, 

and grinding procedures for the coatings. The HCAT has worked with the SAE 

Aerospace Metals Engineering Committee to develop four separate specifications in these 

areas (see Section 8). Those related to powder and coating deposition were completed 

and forwarded to SAE Aerospace Materials Committee B, who approved them in 

February 2003. The following are the designations: 

. AMS 2448 – “Application of Tungsten Carbide Coatings on Ultra-High-Strength 

Steels, High-Velocity Oxygen/Fuel Process” 

. AMS 7881 – “Tungsten Carbide-Cobalt Powder, Agglomerated and Sintered” 

. AMS 7882 – “Tungsten Carbide-Cobalt Chromium Powder, Agglomerated and 

Sintered” 

A specification for grinding and superfinishing of the coatings has been drafted and is in 

the approval process. All of these specifications can now be utilized by any 

manufacturing or overhaul depot and their use will result in consistency between facilities 

with respect to coating properties. 

In addition, aerospace companies have developed their own specifications: 

. Boeing has updated BAC 5851, the company’s specification for thermal spray. 

. United Technologies Hamilton Sundstrand has developed HS 4412 for HVOF in 

place of EHC. 

. Delta Airlines is now qualified for applying HVOF in place of EHC in landing 

gear repair, and has been particularly forward in developing surface finish 

specifications. 
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. Messier Dowty uses AMS 2447, and will doubtless specify AMS 2448 where 

appropriate. 

9.5. Cost observations 

As with all material and process replacements the life cyle costs vary from one location 

to another. 

As indicated in Section 7, the annual operating cost savings at a major commercial 

landing gear overhaul facility for replacing 88% of their chrome plating operations with 

HVOF thermal spraying range from $195,600 to $210,700. Payback on the capital 

investment of installing HVOF systems would be realized within about 31⁄2 years. 

Although the annual operating costs for both EHC and HVOF would be different at other 

facilities based on labor rates, types of equipment used and other factors, it is still 

anticipated that there would be substantial savings by replacing as much of their chrome 

plating operations as possible with HVOF. Additional cost savings that would be 

realized are associated with the estimated 50-100% extension of service life for the 

WC/Co or WC/CoCr coatings over EHC coatings. In the future, as HVOF-coated 

landing gear components come into the facility for inspection, fewer will require 

overhaul compared to what would have been expected with EHC coatings and thus there 

will be a reduction in labor and material costs associated with overhaul operations since 

the components could be returned to service. In addition, the elimination of process 

hydrogen embrittlement and the reduction of environmental embrittlement will reduce the 

risk (and therefore the annual cost) of landing gear service failures. 

It should also be mentioned that the CBA described in Section 7 did not take into account 

any increases in costs associated with EHC resulting from more stringent environmental 

or worker safety regulations. If, for example, the PEL for hexavalent chromium is 

reduced as is expected, then the cost for chrome plating will increase, thereby making the 

operating cost savings even greater for HVOF. 

Although not discussed in Section 1, a cost analysis was performed to determine if it 

would be more cost-effective to outsource the HVOF coating operations to a job shop. 

The results showed that it would be less costly to establish an in-house HVOF coating 

capability. Therefore, the facility for which the CBA was performed has decided to 

acquire and install HVOF systems in its repair shop to process landing gear components. 

9.6. Regulatory Compliance and Acceptance 

The principal environmental and worker safety issues associated with HVOF thermal 

spraying are air emissions containing overspray particles and the noise of the gun itself. 

At this point there are no serious regulatory issues associated with HVOF deposition of 

WC/Co or WC/CoCr at any of the depots. In each of the depots that have adopted the 

technology, the equipment is properly installed in soundproof booths and excess 

(overspray) powder is trapped in a bag-house filter system. The deposition is carried out 

in a well-controlled manner using robotic systems to prevent any operator exposure to 

noise or dust during the deposition process. 

All of the depots involved in the HCAT project already had other types of thermal spray 
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equipment in operation, such as flame or plasma spray, and therefore they had the 

appropriate air handling equipment (e.g., exhaust hoods, bag houses) available and also 

had the appropriate air permits to cover operation of the HVOF systems.