# 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

B RUCE D. S ARTWELL Surface Chemistry Branch, Chemistry Division

K EITH O. L EGG Rowan Technology Group, Libertyville, IL

J ERRY S CHELL GE Aircraft Engines, Cincinnati, OH

J OHN S AUER Sauer Engineering, Cincinnati, OH

P AUL N ATISHAN Center for Corrosion Science, Chemistry Division

D ENNIS D ULL AND J OHN F ALKOWSKI Boeing Corporation, Seattle, WA

P HILIP B RETZ Metcut Research, Cincinnati, OH

D ONALD P ARKER NASA Kennedy Space Center, Cape Canaveral, FL C RAIG E DWARDS General Atomics, Ogden, UT J ON D EVEREAUX Naval Aviation Depot, Jacksonville, FL

March 31, 2004 NRL/MR/6170--04-8762

Approved for public release: distribution is unlimited.ii i 

# T ABLE OF 

# C ONTENTS

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

.. iii
List of F igures................................................................................................................

.... . i x List of T ables................................................................................................................. .... x v List of Acronyms ............................................................................................................

. xi x Acknowledgments............................................................................................................ xx i 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 Optimizati on 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...................................................................................25i v 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..........................67v 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 performan ce 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................................................................................................116v i 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 notc h to the test environment) ................................146 3.7.6. Test methodology.........................................................................................146 3.7.7. Test sample mechanical properties ..............................................................148 3.7.8. Coating structure..........................................................................................149v ii 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 hydrog en 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/CoC r – Heroux Devtek, Canada...........................190 6.2. Grinding ..........................................................................................................193 6.3. Fluid compatibility ..........................................................................................194v ii i 6.4. Non Destructive Inspection.............................................................................195

6.4.1. Fluorescent penetr ant 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 fo r proper cutting fluid application.............250 9. Implemen tation ......................................................................................................253 9.1. General implem entation issues........................................................................253
9.2. Performance observations – di fferences between HVOF and EHC ...............254 9.3. Implementation................................................................................................255 9.4. Specifications and standards ...........................................................................256
9.5. Cost observations ............................................................................................257 9.6. Regulatory Complia nce and Acceptance ........................................................257i x 

# L IST OF FIGURES

Figure 2-1. Schematic of HVOF Gun a nd Process (Sulzer Metco DiamondJet)............... 9 Figure 2-2. HVOF Spray of La nding Gear Inner Cylinder.............................................. 10 Figure 3-1. Microhardne ss Response for CCAD DOE.................................................... 27 Figure 3-2. Substrate Temper ature Response for CCAD DOE ....................................... 27 Figure 3-3. Almen Strip Response for CCAD DOE........................................................ 28 Figure 3-4. Microhardne ss Response for Hitemco DOE................................................ 31 Figure 3-5. Substrate Temperat ure Response for Hitemco DOE ................................... 31 Figure 3-6. Almen Strip Response for Hitemco DOE .................................................... 32 Figure 3-7. Typical Hourgl ass-shaped Fatigue Bar. ........................................................ 39 Figure 3-8. Typical Smooth Fatigue Bar. ........................................................................ 39 Figure 3-9. Hourglass Configura tion with .500” Gage Section....................................... 41 Figure 3-10. Small (1/4” dia) Hourgla ss 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 Confi guration, 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 Environmen t 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 Comp arison, Hourglass Configuration, 0.003” Thickness. ................................................................................................................. 52 Figure 3-20. A100 - Air vs. NaCl Environmen t Comparison, Hourglass Configuration, R =-1 ......................................................................................................................... 5 3 Figure 3-21. A100 - .003” vs. .010” Thickne ss 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.................................................................... 54x Figure 3-23. 300M - Smooth Bar vs. Hourgl ass Bar Comparison, 0.003” Thickness, R=-1. ......................................................................................................................... 5 5 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 Environmen t, 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 Sh owing 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 Scra tched 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 Protec tion Ratings on 4340 Steel After ASTM B117 Testing....................................................................................................................... 7 9 Figure 3-37. HVOF Coatings on 4340 Steel Af ter 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 434 0 Steel With: a) Ni Sublayer, b) Sealer, and c) EHC Coating Only. After B117 Testing....................................................... 80 Figure 3-39. HVOF Coatings on 4340 Steel Af ter the B117 Test: a) EHC Coating Only b) EHC With Sealer, c) EHC With Ni S ublayer, and d) 0.010”-thick WC-CoCr. ... 81 Figure 3-40. 0.003”-thick HVOF WC-Co Co ating on 4340 Steel with Sealer After Blister has been Removed......................................................................................... 81 Figure 3-41. Protection Ratings on 3 00M After ASTM B117 Testing. .......................... 82 Figure 3-42. Radar Plot of the Protecti on 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 Ae rmet 100 After ASTM B117 Testing.................. 83x i Figure 3-45. Radar Plot of the Protecti on Ratings on Aermet 100 After ASTM B117 Testing....................................................................................................................... 8 4 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 Ae rmet 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 S upplemental 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, Gr ound, 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, Groun d. 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, Groun d. 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 Ra tings for HVOF and EHC Coatings on 4340 Rod Tested at NRL and KSC.................................................................................... 97 Figure 3-56. Comparison of Appearance Ra tings 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 Bl ock; b) Fretting Shoe. .......................................................... 105x ii Figure 3-65. Wear Test Rods and Bushings - a) Rod, b) Straight Bushing, c) Seal Groove Bushing. .................................................................................................................. 107 Figure 3-66. Cross-sectional Sche matic of Fretting Wear Test..................................... 109 Figure 3-67. Cross-sectional Schematic of Pi ston 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; Bl ue: Wear of Nitrile Seal.................................... 118 Figure 3-72. Sliding Wear – L12 data. Vi sual 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 Bu shing; 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 – Wei ght 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 a nd 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 (coa ting 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 Re plicate – 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 Re plicate – Weight Loss. EHC and WC-Co on Metal Bushings and Nitrile Seals............................................................................ 129 Figure 3-81. L8 DOE Data for 5th Half Re plicate – Visual Ranking for Rod Wear. EHC and WC-Co on AlNiBronze Bushin gs and Karon B Liners. .................................. 130 Figure 3-82. L8 DOE Data for 5th Half Re plicate – 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 Equipm ent, Showing Ball Delivery Tube, Samplex ii i 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 r ods; e) and f) 0.010”-thick on flats. ..... 138 Figure 3-88. Surfaces of WC-Co Coatings Af ter 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. Failure s Occurred on Shank at Button................... 153 Figure 3-100. Sequence 3 Average Hours to Fa ilure with No Notch Scribing. (Numbers are verage time-to-failure data values – not e 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-fa ilure data values – note log scale. Error bars indicate high and low values.) (N ote 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 Ti me to Failure for Scribed and Un-scribed Specimens - DI Water............................................................................................. 157 Figure 3-104. Sequence 3 – Comparison of Ti me to Failure for Scribed and Un-scribed Specimens - 5% NaCl Solution............................................................................... 157x iv Figure 3-105. Open Circuit Potential for Co ated 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 Integrit y Big Bar Design (Metcut). ....................................... 167 Figure 4-2. NAVAIR "Big Bar" Axial Integr ity Test Specimen (Eui Lee, NAVAIR). 168 Figure 4-3. Axial Big Bar Integr ity 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 Cy linder Under Strain-Control. 0.010” WC-Co, R=-0.33 (OO-ALC) ...................................................................... 170 Figure 4-6. Spallation of a 0.015” WC/C oCr Coating From a Test Cylinder (OO-ALC).

............................................................................................................................... .. 171 Figure 4-7. Same as Figur e 4-6 , but Under FPI............................................................ 171 Figure 5-1. Location of HVOF-coated Pi ns 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 HVO F WC/17Co-coated Piston Mounted on P3C Test Article at Lockheed-M artin in Mariet ta, Georgia ..................................... 178 Figure 5-5. Close-up of HVOF WC /17Co-coated Piston in MLG ................................ 178 Figure 5-6. Navy E-6A R econnaissance Aircraft .......................................................... 179 Figure 5-7. E-6A Uplock Hook Shaft............................................................................ 180 Figure 5-8. P-3C Main Landing Gear w ith 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/C oCr 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/CoC r Coating. Left: Standard; Right: FPI (Heroux Devtek). .................................................................................................... 195 Figure 6-7. FPI of HVOF-coated Landing G ear After Fatigue to Stress Sufficient tox v Cause Coating Delamination, Showing Circumferential Coating Cracks (C. Edwards, OO-ALC). ............................................................................................... 195 Figure 6-8. Ultrasonic De tection of Crack Beneath HVOF WC/CoCr Coatings (Boeing)

............................................................................................................................... .. 197 Figure 6-9. Ultrasonic Measurement of De lamination 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 Trans itioned from Hard Chrome Electroplating to HVOF Thermal Spraying........................................................................................ 204



# L IST OF TABLES

Table 1-1. Landing Gear Aircraft Applications.................................................................. 4 Table 1-2. Defense System Pr ograms Potentially Affected............................................... 4 Table 1-3. Applicable Mate rials Processing, Coating Depos ition and Test Standards. .... 7
Table 1-4. Commercial Pr ograms Affected and Potentia lly Affected by HVOF Chrome Replacement................................................................................................................ 8 Table 2-1. Optimized Deposition Cond itions 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 Cond itions for WC-17Co - DJ 2600 and JP 5000 HVOF Guns. ............................................................................................................. 16 Table 3-3. Inputs and Outputs for Desi gn 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........................................................ 25xv i 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 De terminants 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 Procedur es for Fatigue Specimens............................................. 42
Table 3-22. DiamondJet 2600 Process Para meters 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 Protect ion Rating for Samples that had Undergone 1,000 Hours of ASTM B117 Testing........................................................................ 66 Table 3-27. Average Value of the Protecti on 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 Prot ection 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 Supplem ental Corrosion Testing – 300M Rods............. 88 Table 3-37. Sample Matrix for Supplem ental Corrosion Testing – 4340 Plates............. 89 Table 3-38. Failed Coatings on 4340 Substrates. ............................................................ 91
Table 3-39. Failed Coati ngs on 300M Substrates............................................................ 91xv ii Table 3-40. Quick Data Locator. (Click on item number to view.).............................. 104
Table 3-41. Wear Test Materials and Coati ngs - Fretting. (Fretting blocks uncoated, shoe 4340 coated) ................................................................................................... 106
Table 3-42. Wear test Materi als 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 (D esign of Experiment) - Sliding.............................. 113 Table 3-46. L-12 Design of Expe riment Matrix for Fretting......................................... 114 Table 3-47. L-12 Design of Experime nt Matrix for Bushings/liners............................. 114
Table 3-48. L-8 Wear Test Ma trix. (Note: each test condi tion 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 - 1s t and 2nd Half Replicates. ............................................. 122
Table 3-53. L8 DOE Data - 3r d and 4th Half Replicates............................................... 123
Table 3-54. L8 DOE Data - 5t h 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 Im pact 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 Embritt lement Test Matrix. ....................................................... 148
Table 3-62. F-519 Test Specimen Hardness.................................................................. 148 Table 3-63. F-519 Test Specime n Notch Tensile Strength............................................ 148 Table 3-64. Summary of Crack Initiation Points........................................................... 151 Table 3-65. F-519 Hydrogen Embrittlement Te sting – 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......................... 184xv iii 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 Substr ate Cracks (Boeing St Louis).................................... 196

Table 7-1. Annual Number of C-130 Landi ng 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 Ev aluation for Direct Implementation......................... 212
Table 7-6. Net Present Value by Scenario...................................................................... 213 Table 8-1. SAE Aerospace Materi al Specifications for HVOF..................................... 217 Table 9-1. Components Appr oved for HVOF at OO-ALC. .......................................... 255xi x 

### L IST OF 

### A CRONYMS
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 manufacturerx x 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 averagexx i 

### A CKNOWLEDGMENTS
The financial and programmatic support of th e Department of Defense Environmental Security Technology Certifica tion Program, under the direction of Dr. Jeffrey Marqusee, Director, and Mr. Charles Pellerin, Progr am Manager for Pollution Prevention, is gratefully acknowledged. In addition, the financial and programmatic support of the Joint Group on Pollution Prevention and the Ai r Force Materiel Command, coordinated by Ms. Debbie Meredith, is also gratefully acknowledged. The authors would also like to express tha nks to the following individuals who made substantial contributions to the execution of the project:
Mr. Warren Assink and Mr. Gene Jeune lot, 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, AFRLxx ii This page intentionally left blank1 Background 1 

# 1. Background
The replacement of hard chrome plating in aircraft manufact uring activities and maintenance depots is a high priority for the U.S. Depa rtment of Defense and the Canadian Department of National Defence. Ha rd chrome plating is a technique that has been in commercial production for over 50 years and is a critical proces s that is used both for applying hard coatings to a variet y of aircraft components in manufacturing operations and for general rebuild of worn or corroded components that have been removed from aircraft during overhaul. In pa rticular, 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 chromi um (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 emi ssions 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 coordina tion 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 μ g/cm 3 to between 0.5 and 5.0 μ g/cm 3 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, othe r weapons systems and facilities where worker exposure to hex-Cr would be expected

• Developed data on current worker exposur e levels to hex-Cr using OSHA Method 215

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

• Identified future actions required to co mply with the anticipated PEL reductions
The following operations were identified as ha ving the potential for exposing workers to hex-Cr:

• Metal cleaning (including abrasive blas ting and grinding) of chromate-coated materials

• Electroplating of chromium

• Painting and application of ch romate paints and coatings

• Welding, thermal spraying and thermal cutting
The following conclusions were reached by the task group:

__________________ Manuscript approved February 27, 20042 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 pres ently available engineering control, is not completely effective in reducing exposure to below 0.5 ∝ g/cm 3 for many operations or even below 5 ∝ g/cm 3 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 faci lities, housekeeping and maintenance of equipment. There will also be costs du e to reduced efficiency of not only the operations involving hex-Cr but adjacen t operations and personnel as well. The estimated costs for compliance with a PEL of 0.5 ∝ g/cm 3 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/cm 3 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 he x-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/m 3 for hex-Cr emissions in plati ng facilities. For example, the August 2000 issue of the American Journal of Industrial Medici ne contained a report on a study of 2,357 workers over a 30-year period wh ich 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 th eir conclusion that the study appeared to support a lowering of the PEL to less than 0.001 mg/m 3 . 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 avai lable thermal spray systems HVOF thermal spraying can be used to deposit both meta l 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 applicatio ns for these coatings, their qualification as an acceptable replacement for hard chrome plating has not been adequately demonstrated, particularly for fatigue-sensi tive aircraft components. The Environmental Security Technology Certif ication Program (EST CP) was established as a program of the U. S. Department of Defense (DOD) in December 1993. The ESTCP, which is managed by the Deputy Unde r Secretary of Defens e for Installations and Environment, demonstrates and validates lab-proven technologies that target the most urgent DOD environmental needs. These technologies provide a return on3 investment through reduced environmental, safety, and occupational health (ESOH) risks; cost savings; and improve d efficiency. The new technologies typically have broad application both to the DOD sustai nment community and industry. In order to conduct the advanced developm ent work required for qualification of the HVOF coatings, a project entitled, "Tri-Ser vice Dem/Val of Chromium Electroplating Replacements,” principally sponsored by ESTCP, was established in March 1996. A project team, designated the Ha rd Chrome Alternatives Team (HCAT) was established to execute the project. From 1996 to early 1998, the HCAT acquire d and installed HVOF thermal spray systems at the Naval Aviation Depot in Cherry Point, North Carolina (NADEP-CP) and the Corpus Christi Army De pot (CCAD). It also performed some generic fatigue and corrosion testing on HVO F 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) coatin gs. Substrate materials included 4340 steel, 7075 aluminum alloy, and PH13-8 stainless steel. From a fatigue standpoint the HVOF coatings generally performed better than th e 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 corro sion studies, the perfor mance of the WC/Co was comparable to the EHC, with the Trib aloy 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 valuab le, it was realized in early 1998 that because hard chrome plating was being used on such a wide vari ety of aircraft components, it would be impossible to develop one test plan or conduct one series of tests th at would address all materials and component qualification requirement s. It was therefore decided to develop separate projects related to categories of aircraft compone nts onto which hard chromewas 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 ac tivities during the acquisition and sust ainment of weapons systems. It was jointly determined by the HC AT and JG-PP that the first projects to be executed would be on landing gear and prope ller 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 de velopment 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 materi al, 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 al so explore what avenues were4 available for component testing. It was s ubsequently 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 Depa rtment of National Defence (DND) and Industry Canada (IC), facing environmental rest rictions 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 ma intenance operations. The Canadian Government formed their own project team , designated the Canadian Hard Chrome Alternatives Team (C-HCAT), and a partners hip was formed between both projects. A formal Project Arrangement, conducted unde r the auspices of the U.S.-Canadian Research and Technology Projects Me morandum of Understanding (MOU), was

Table 1-1. Landing Gear Aircraft Applications Target HazMat Current Process Application Current Specifications Candidate Parts/ Substrates

Hexavalent Chromium Hard ChromiumElectroplating 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-155 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 supplie d 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/C o (83%/17%) HVOF coatings whereas the CHCAT would evaluate WC/CoC r (86%/10%4%) HVOF coatings. It was further decided that all materials testing would be incorporat ed into one document constituting Part I of the JTP, but that there would be a clear de lineation 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- J acksonville (NADEP-JAX)
 Naval Aviation Depot- Cherry Point (NADEP-CP)
 Ogden Air Logistics Center (OO-ALC)
 Naval Air Warfare Center, Aircra ft 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 principa l organizations that const itute the C-HCAT and which participated in the development of the Ca nadian 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 orga nized in sections, with each section devoted6 to the type of test being conducted. Most se ctions were divided in to 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 th e two different HVOF coatings was meant to be very similar, but there were some di fferences. 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 reproduc ing the Materials Joint Test Report (MJTR) (without appendices), provides th e results of all of the te sting 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 Join t Test Protocol, there were a number of component rig and flight tests conducted on landing gear com ponents 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 f acilities, it was necessary to address several producibility issues including stripping and grinding of the WC/Co coatings, determining their compatibility with different fluids used fo r cleaning and other ope rations, and performing non-destructive inspection of the WC/Co coatings. The resu lts of investigations in these areas are described in Section 1.
Another issue related to successful tran sitioning 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 charact eristics 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 referen ces to specific standard s related to materials processing, coating deposition, and testi ng. These are summarized in Table 1-3.7 Table 1-3. Applicable Materials Processi ng, Coating Deposition and Test Standards
ASTM Standards: ASTM E466: Standard Pr actice for Fatigue testing ASTM B117: Standard Practice for Sa lt Spray (fog) Apparatus, Operating ASTM G85: Standard Practice for Modified Salt Spray (FOG) ASTM B537: Standard Practice for Ranking El ectroplated 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 re sin 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 Plati ng, Black (Electrodeposited) MIL-C-20218: Chromium Plati ng, Electrodeposited, Porous MIL-H-83282: Hydraulic Fluid, Fire Re sistant, 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 Flan ged, Self-lubricating, General Spec for AMS-4640: Aluminum Bronze, Bars, Rods, Sh apes, 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 test8 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 component s. For OEM applications, Boeing has specified WC/CoCr on the landing gear for th e 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 certifi cation. Delta Airlines is one of several U.S. airlines that have aggressively move d 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 Internationa l Corporation under Government Contract F09603-90- D2215, Oklahoma City Air Logistics Cent er, Tinker Air Force Base, May 25, 1994 1.2 “Hard Chrome Coatings: Advanced Tec hnology for Waste Elimination,” Final Report issued by Northwestern University, Ev anston, 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 La nding Gear, Part I: Materials Testing.” Prepared by Hard Chro me Alternatives Team for Environmental Security Technology Certification Pr ogram, 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 tracks9 

# 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 so ften 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 stai nless 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) pa rts, and to rebuild worn components by depositing layers up to 0.015” thick.

Applicability : HVOF was originally developed prim arily for gas turbine engine (GTE) applications. The primary thermal spray proc esses are Flame Spray, Plasma Spray, Arc Spray, HVOF and the recently-developed cold sp ray. The original high velocity spray technology was the pulsed deposition det onation gun (D-gun) developed by Union Carbide (later Praxair). The quality of th e 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 fo r 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 ot her types of coatings and appl ications, including a variety of aircraft components such as flap and sl at tracks, landing ge ar and hydraulics for commercial aircraft. It is now being used in many applications outside the aircraft industry, such as industrial rolls and vehicl e hydraulics. The orig inal 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 va riety of alloys and cermets. It is limited for high temperature materials such as oxide s, most of which cannot be melted in the flame. The areas to be coated must be acces sible 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 te mperature oxidation-resistant Tribaloys). Figure 2-1. Schematic of HVOF Gun and Process (Sulzer Metco DiamondJet)10 The combination of HVOF NiAl with an overlay er 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 ai r and dust out of the booth through a louvered opening (shown in Figure 2-2).

. Gun and control panel. The gun burns th e fuel and oxygen inside its combustion chamber and injects the powde r axially into the flame. The gas exits the gun at supersonic speed, while the pa rticles are accelerated to high velocity but usually remain subsonic. The control panel contro ls 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 f eeders are commonly used to permit changeover from one coating to another without interrupting the spraying.

. 6-axis industrial robot and c ontroller. Most in stallations use an industrial robot to manipulate the gun and ensure even sprayi ng. 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-bottl e setup lasts only a Figure 2-2. HVOF Spray of Landing Gear Inner Cylinder11 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 pressu res 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 coa ting, 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

Flow rate 5.3-5.7 gph (factory set) 8.3-8.7 gph Gun Cooling Water Flow Water Temperature to Gun: 65-80 o F typical (ground water, temp varies) 64-72 o F 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 area12 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 ge ar (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 aer ospace specifications were those issued by prime contractors such as Boeing, whose BAC 5851 thermal spray specification, supported by BMS 10-67G powde r 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 sp raying 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 fo r HVOF shops to have 3 or 4 technicians dedicated to masking and spraying. HVOF tr aining is essential and is usually provided by equipment vendors such as Praxair and Sul zer Metco. Training is also available through the Thermal Spray Society. Depot pe rsonnel taking part in the HCAT program have been trained by Jerry Schell, thermal spra y coatings expert at GE Aircraft Engines. Since thermal spray is a more complex t echnology than electropl ating, plating line personnel cannot be transfe rred 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 Res earch on Cancer (IARC) Group 2B material, which means that “The agent (mixture) is possibly carcinogenic to humans”, whereas Cr 6+ 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)/m 3 , is lower than the 1 mg(Cr)/m 3 for metallic chrome, and is the same as the 0.1 mg(Cr)/m 3 for Cr 6+ . 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. Ne vertheless personnel should wear a dust respirator when handling the powder, worki ng 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 Ameri can Welding Society recommends the use of a respirator complying with American National Standards Institute (ANSI) Z88.213 Ease of operation : Since in commercial systems the entire system is programmable, including the gun control and robot, it is gene rally 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 op timized experimentally for new materials and powders, and must be adjusted for different components to ensure proper coating speed and gun traverse rate. Small diameter compone nts, 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 technol ogy had been successfully used by Boeing for a number of years for their commercial airc raft 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 cl adding, and concluded that HVOF was the best overall alternative for use in depots a nd most OEM aircraft applications. At the beginning of the HCAT program, Lufthansa succ essfully 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 onl y 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 limitati ons of HVOF are summarized in Table 2-2.


# 2.5. References

2.1 “Hard Chrome Coatings: Advanced Tec hnology for Waste Elimination,” Final Report issued by Northwestern University, Ev anston, IL, under DARPA Contract MDA972- 93-1-0006, 199614 Table 2-2. Advantages and Limitation s 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 toxicity15 

# 3. Materials Testing

3.1. Summary of Tests Conducted As indicated in Section 1, this section re produces the Materials Joint Test Report, but without the appendices. The en tire report [3.1] is available from the HCAT web site at www.hcat.org . Table 3-1 summarizes the tests c onducted 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 Generic16 

# 3.2. Coating optimization and characterization The optimized deposition conditions for th e hydrogen-fueled Sulzer Metco Diamond Jet DJ 2600 HVOF gun used at most of the depot s 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 Condition s 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 Mo del 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 65-80 o F typical (ground water, temp varies) 64-72 o F

Out 77-100 o F typical 117-125 o F
Delta 12-20 o F (~ 14 o F typical) 51-54 o F 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 area17 

### 3.2.1. Rationale/Background

3.2.1.1. HVOF Application As in all coating methods, th e properties and performance of the coating depends on both the coating material and the deposition condi tions. Optimal coating properties can therefore only be obtained when the critic al deposition parameters are in the proper range. In chrome plating the coating pr operties are primarily governed by solution chemistry, temperature, and current dens ity. 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 es pecially true for thermal spray coatings, where it has been found, for example, that a coat ing 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 compone nts and gas turbine engine parts was quickly identified as the major life limiting ch aracteristic. This does not eliminate the need to evaluate other characteristics such as corrosion, wear, hydrogen embrittlement, etc. but coating optimization initially c oncentrated on fatigue performance, with modifications for other properties as necessa ry. 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 me mbers 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 chos en as the mechanism to provide an optimized coating deposit. The variables in the pro cess were identified a nd experiments conducted to determine the best parameter set for optimum results. Work was required for the varied commercial systems to demonstrate cap ability across the available equipment sets. Thus, data summarized in this section cove rs both the JP-5000 (Praxair TAFA) and the DJ 2600 (Sulzer Metco) systems, and represen ts 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 carrie d 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 manufacturi ng 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 pa rameters are identified as significant, these variables will be the first areas of investigation in problem solving. In optimizing the HVOF process, it is importa nt to understand the difference between the general output of the process and the characteristics/prope rties of the final coating deposit. As stated earlier, the final goal of the coating optimization was maximized fatigue performance with close emphasis on othe r 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 measuremen t 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 tech nical sense that characteristics such as microstructure and hardness will ultimately determine coating performance in areas such as wear or corrosion resistance, while residu al stress and substrate temperature are known to strongly influence fatigue. Thus, even though the ultimate goal is enhanced fatigue performance, that performance can be ensu red indirectly by meas uring 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 fa bricated prior to depos ition (for daily spray booth qualification), or sprayed during coa ting 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 th e process and issues are sometimes identified that are unexpected from previous experience and planning. This was the situation in developing preci se testing documentation for measurement of the Almen Strip and Temperature during spra ying (procedures detailed later in this section). Another major property issue identified duri ng 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 ba sed upon the Q.C. tests listed earlier and19 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 aggressi ve 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 cla ssification 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 op timization 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 para meter set. This method is used in many manufacturing environments when numerous va riables exist and there is insufficient time and financial resources to analyze each indi vidual process input (i.e. to carry out a full matrix test). Pre-DOE experiments are usua lly run on a iterative basis to determine the limits of the various parameters and which ha ve the most significant effect on the output of the process. A DOE matrix is then designed using standard experimental design protocols in which the variable s selected are usually assigned high and low values for the numerous DOE test runs. Sta tistical analysis is applied an d 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 pr ogram 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 speed20 As stated earlier, even with the DOE met hodology, there must be a general starting or reference point. This was provided by th e 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 ga ge 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 involve d 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 effi cient manner or provides no Almen Strip response. This initial set id entifies some reasonable responses for the actual DOE experimentation.
Actual DOE – When a reasonable set of pro cess inputs and ranges have been identified, a number of runs/experi ments 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 se t, and repeat spray cycles are conducted to establish consistency.

3.2.2.3. General Example: CCAD Analysis A typical example of this process is repr esented 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 e nd 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 co atings involved the sy stems, locations, and time frames as listed in Table 3-6. In disc ussing each location, the general matrix design and effect on the three major outputs of Alme n, Temperature and Microhardness will be presented. No pre DOE or fi nal optimization fine tuning will be summarized but the final spray parameter set is listed. Table 3-4 Optimization Paramete rs 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 N 2 mass flow for N 2 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 Amp526.0 6 2005NS 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, HV 300 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/cohesion22 Jerry Schell, an HCAT team member fr om GEAE, was selected to manage the optimization procedure based upon his previous experience with HVOF and his extensive knowledge of the DOE methodology with spec ial 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 1998-923 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, HV 300 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

Eff ec t s R ange Thk/ P ass CY6 T emp Al men HV300

A

verage 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 15.724 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 di stance are the major factors in the spray process. The data for microhardness, al men strip values, and substrate temperature identifies these variables as the critical pa rameters for control and the obvious areas to investigate in future problem troubleshooti ng. There is also a degree of process robustness with the analysis indi cating that a fairly wide range of values can be used to achieve a consistent end result.
Three other areas shou ld also be noted:
 Deposition rate of the coating is obvious ly controlled not only by powder feed rate but traverse speed of the part being sp rayed. 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 transf erred to the part as the gun sweeps across it. It is therefore critical to keep de position rate constant in spraying test bars, almen strips, or pa rts to best approximate a consistent and repeatable process
 For the JP 5000 analysis, the nozzle lengt h was a significant f actor 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 3925  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. No n-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 assi st 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 consol e supply pressure reading) Gun Compressed Air Gun Cooling Water Flow 8.3-8.7 gph 64-72 o F
117-125 o F
51-54 o F 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 area26 systems, there is enough difference in the desi gn 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 N 2 mass flow for N 2 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 Amp526.0 6 2005NS 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, HV 300 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 1036227 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 DOE28 3.2.3.3.2. General Discussion As expected, combustion gas and stand-off di stance are the major factors in the spray process. The data for microhardness, alme n strip values, and substrate temperature identifies these variables as the critical pa rameters 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 shou ld also be noted:

 Deposition rate of the coating is obvious ly 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 crit ical to keep deposition rate constant in spraying test bars, Almen strips, or part s 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 varia tion in the residual stress state of the coating. This variable can be contro lled 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 DOE29 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 binde r, 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 investiga tion was performed to verify the outcome of the DOE at CCAD and to finalize the process pr ior to test bar coating deposition. Table 3-14 presents the DOE matrix. Table 3-13. Final Summary of Factors in CCAD DOE

% RESPO N SE S 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 Eff ec t s R ange Thk/ P ass CY6 T em p Al m en HV300 Lower 0.168 325 7.2 1024 Upper 0.746 496 21.5 1203 Mean +/ - X % 63.2 20.8 49.8 8.030 Table 3-15 represents the random run sequence for the Hiemco DOE.

Figure 3-4., Figure 3-5. and Figur e 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 5448 0 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 N 2 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, HV 300 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 facto r Std.Ord Turn Table Robot Trav Hydrogen Oxygen Ai r 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 11.531 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 Temp erature Response for Hitemco DOE32 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 DOE33 3.2.3.4.1. General Discussion As expected, combustion gas and stand-off di stance are the major factors in the spray process. The data for microhardness, Alme n strip values, and substrate temperature identifies these variables as the critical pa rameters for control and the obvious areas to investigate in future problem troubleshooti ng. 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 65-80 o F typical (ground water, temp varies)

Out 77-100 o F typical Delta 12-20 o F (~ 14 o F 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 area34 robustness with the analysis indi cating 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 dependenc y 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 cr itical 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 contro l 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 inconsis tencies. Subsequent experience has shown that this is not the case. Almen strip results were found to be str ongly 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 th ickness, many initial values have been reported for the .003-.005” thic kness range due to interest in those areas. However, values for Almen respons e 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 e nd of the spray run. This appro ach provides no information on the true temperature excursions that occur dur ing 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 100 o 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 bi gger than the specimen diameter. This requires a compensation factor when sprayi ng 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 100 o F hotter with instantaneous measurement (IR pyrometer) vs. touch probe after all spraying is complete. The IR unit must therefore be calibrated to de fine 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 guide line to control the process. A complete guideline for Almen Strip/Substr ate 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 une xpected 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) wi ll have a substantial e ffect 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 duri ng the LG JTP evaluation. Additional work performed as part of HCAT by the Air For ce 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 Nozzle36 coating parameters were chosen that were optimized for wear performance rather than fatigue, HVOF cyclic fatigue lives were lowe r than chrome. Temperature control was also suspect. Respraying of bars at the same location with HCAT-optimized parameters and better temperature contro l resulted in satisfactory fa tigue 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 repor t, and will be the subject of a subsequent report), and the need for process optimizati on 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 velo city/temperature measurements that can be obtained with systems like the DPV 2000. It will be critic al to investigate and understand these issues to produce an optimized coating which meets co ating integrity criteria. This work, in conjunction with the present information, wi ll 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 vari ables 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 understandi ng 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 success fully 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 i ssue of coating inte grity (spalling and delamination) and process understanding conc erning 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 driv en 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 preven t loss of mechanical properties due to overheating, and
2. deposit thermal spray coatings with compre ssive 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 th e process and tensile coating stresses. The commercial development of the HVOF process, which reli es 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 define d 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 establis hed. There are established methods and Table 3-18. Quick Reference to Primary Da ta (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 3-2438 specifications for determining fatigue propertie s. However, with coatings now applied, some of the guidelines used for bul k 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 desi gning a fatigue testing protocol, some areas re quiring definition are:
1. What is the load carrying capability of th e 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 repeat able on the fatigue bars to produce a consistent thickness and surface for testing? As with all fatigue evaluations, consider ations must be given to the following: Frequency or speed of testing –will dete rmine 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 Fi gure 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 separa tion of curves between coating conditions. Some tests with smooth bars were include d 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 condi tions for military landing gear components. Some testing was done at R=0.1 (tension-tens ion) 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 Bar40 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, gi ven 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 exposu re to the environment in question. When defining the protocol for testing, the fr equency 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) intensity41 The two smaller specimens (Fi gure 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 step s: 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. Gri nding was performed to IAW MIL-STD-866 and Figure 3-9. Hourglass Configur ation 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 steel42 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 secti on surfaces in the polishing step of the specimens. Nital etching IAW MIL-ST D-867 was conducted on all specimens to examine for grinding burns. All specimens we re 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 specifi ed 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 electr oplated 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 pa tch ends to limit stress concentrations. The coatings were deposited around the entire ci rcumference to a thickness of 0.002 – 0.003” over the final diameter to allow for grinding after coating. The fatigue specimens for hard chrome we re prepared for plating by lightly hand abrading the areas to be coated with a Scot chbrite 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 gr ind 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 gr it 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 centere d in the gage section and ensured proper feathering of the patch ends. Tape maski ng restricted grit blas ting 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 Materials43 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 Pa rameters 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: N itrogen 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- 72 o F Out, 117-125 o F Delta, 51- 54 o F 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” po inted at coating area, 90-110 psi44 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 ambien t temperature. The procedure for conducting the corrosion fatigue te sts 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 th e fatigue-testing machine in a cell (See Figure 3-10., Figure 3-11. a nd 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 consid ered 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 Hourglass45 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 Specimen46 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 72047 

### 3.3.6. Test results The US Landing Gear JTP concentrated on fa tigue of WC-Co coatin gs at both a .003” and .010” thickness. Substrate materi als 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 coa ting integrity (cracking/spalling) were not anticipated by the JTP, and the issue did not arise until the US JT P 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 configurati on is quite sensitive in differentiating chrome vs. HVOF in the unpeened condition.
 Figure 3-14. shows the comparison of bar c onfigurations. As experienced in other protocols, the smooth bar configuration m oves 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 shif ted 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, N fENGINEERING 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 = -149 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, N fENGINEERING 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, N fENGINEERING 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 = -150 4340, R = -1, AIR LARGE (0.010"CTNG) VS. SM ALL (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, N fENGINEERING 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” S p ecimens , R es p ectivel y

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, N fENGINEERING 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” Thickness51 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 di fferentiation between chrome and HVOF. Again, the hourglass configuration a ppears to show more sensitivity in differentiating chrome vs. HVOF in the unp eened 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 perf ormance is better than the smooth configuration. Again, as with previous protocol obser vations, 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 degrad ation 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” co ating thickness, th ese data not only represent a thicker deposit but also a larger diameter bar. Since the bar diameters are different, the comparison can be somewh at skewed since larger diameter bars sometimes show reduced fatigue performance, as noted above. We cannot therefore separate the effect s 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 thicke r coating does not pr ovide 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, N fENGINEERING 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, N fENGINEERING 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” Thickness53 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, N fENGINEERING 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, N fENGINEERING 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, Respectively54 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 c onfigurations. 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 degr adation in fatigue performance but the differences between chrome and HVOF ar e 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, N fENGINEERING 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= -155 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, N fENGINEERING 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, N fENGINEERING 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= -156 

### 3.3.7. Coati n g integrity As noted above, the coating integrity analysis (cracking and spalling) performed in this testing was primarily post-test since this issu e was highlighted as a major concern only as the final bars were being fi nished 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 summari zed in Table 3-24. For the small hourglass configuration a nd the .003” thickness, the majo rity 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 especia lly 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, N fENGINEERING 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= -157 Figure 3-26. Cracking of HVOF Coat ing in Central Region of Hourglass Specimen

Figure 3-27. Failed 300M Specimens Showing Spalling in Small Zone Near Fracture Surface58 Figure 3-28. Failed 300M Specimens Showing Total Spallation on Fracture59 

### 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 speci mens 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 differe nt 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 EHC No spalling N/A -1 NaCl 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/A60  Surprisingly, there was little difference in hourglass vs. smooth configurations, in contrast to earlier protocols where th e 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 fr acture, 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 o ccur 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 co ating. 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 coati ng as it becomes thicker. This results in improved fatigue performance of the substrate and higher co mpressive 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-ba sed to carrier-based situations, environmental factors can influence performance. The fatigue testing is significant in establishing that HVOF coatings do not degrade substrat e fatigue, with or without corrosion. However, the data established in this protoc ol and subsequent eval uations have identified the need for coating integrity (cracking and spalling) meas urements to optimize coating performance and establish the load-limiting factors. From initial additional testing, coating integrity appears to be a concern primarily for thick co atings (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 perfor mance characteristic, and the results will be presented in a subsequent report.



### 3.3.10. Conclusions The WC-Co coatings meet the JTP accepta nce criterion of fatigue performance equal to or better than chrome platin g 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 b een identified and is being addressed in parallel programs to estab lish application and lo ad 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 repo rt, some generic fatigue and corrosion testing were conducted prior to the ini tiation of the landing gear pr oject [3.2]. ASTM B117 salt fog testing and atmospheric co rrosion 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 demons trating slightly inferior performance [3.2, 3.3]. Although it was intended that this JT R 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 ju st for the same substrate material as used in the JTP would also be presented in this re port. 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 3-5862 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 fo r 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 depos ited at the Naval Aviation Depot in Jacksonville, Florida using a Sulzer Me tco 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 deposite d at Southwest Aeroservice, Inc. in Tulsa, Oklahoma using a Stellite Jet Ko te II HVOF system with hydrogen as the fuel gas. The Tribaloy 400 powder ma terial was designated as T400 and was manufactured by Stellite with a compositi on 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-d eposited 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 be ing 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 atmospheri c 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 rati ng 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 clean ing, respectively, of the coatings on 4340 steel after th e three-year atmospheric exposure. Once the samples were cleaned, it was possible to identify surface de fects such as blisters or pits. Removing the blisters and portions of the coating that we re undercut by corrosion provided a better representation of the a ffected 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 protec tion 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 fr om 10 to 50. The prot ection 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 signi ficant amount of undercutting of the coating. The average protection rating for the edges wa s 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 indi vidual samples ranged from 0 to 2, with an average of 0.5. The edges were also heavily corroded with significan t undercutting. The average protection rating was 0.2 for the edge s 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 indivi dual 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 50 064 binder. The HVOF WC-17Co coating clea rly outperformed the EHC and HVOF T400 coatings in terms of providing prot ection 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 we re previously reported [3.2]. Table 3-27 provides a summary of the B117 results whic h 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 discu ssed 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 Exposu re (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 Exposure66 

### 3.4.3. Rationale for JTP testing With concurrence of the stakeholders the co rrosion, a non-standard specimen shape was used in an attempt to better represent th e typical rod-shaped components on which HVOF would most likely be employed. The speci men shape was a 1” diameter round rod 6” long, scribed when necessary. Both Do D standard ASTM B117 and GM 9540 cyclic tests were done to simulate service conditi ons. Because Boeing usually specifies a Ni sublayer beneath the EHC, and a sealant appl ied 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 th e 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 th e 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 0.267 

### 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 recei ved 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 specifi cation MIL-STD-866. Subsequent to grinding, on some of the EHC coati ngs (see test matrix, Table 3-31), a polystyrene resin impregnation sealer wa s 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 Diamondj et hybrid gun with hydrogen as the fuel gas. The WC-Co powder material wa s 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 350 o F. The thicknesses was 0.003” and 0.010” (+/- 0.0005”) subsequent to gr inding (i.e., coatings were deposited approximately 0.002” to 0.003” thicker th an specified and then ground to final dimension). All HVOF coatings were depos ited to an Almen number of 3-12N. Subsequent to deposition, each coati ng was ground to a surface finish of 8 microinches (+0/-2 microinches) in acco rdance 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 approx 3/4” Mask 1.25” + top”

Coat 5” length+ bottom Grit blast 5” 1/4” + bottom 4.75”

Figure 3-32. Corrosion Coupon, 1” Diameter Bar68 stock removal (per side, or 0.004” on diameter) was required for finish grinding. The finishing in-feeds di d 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 cr oss 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 th e coating into the substrate. Two scratches were made around the circumference of the specimen 3-in ches apart and a linear scratch was made 3.5” inches in length, crossing both circumfe rential 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 fo r corrosion resistance.) Sample types: 1” dia rods, 6” long, Circumfere nce ground to a surface finish of 32-64 microinches prior to coating. Figure 3-33. Illustration of Scratched Corrosion Test Specimen69 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 surface s 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 180-22 0 grit aluminum oxide or glass beads in accordance with standa rd 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 th e ends always pointed vertically to keep the scribe uppermost.

3.4.4.2.1. Test Description Three types of corrosion te sts were conducted.
1. Salt Fog Corrosion Test. This test was conducted in accordance with ASTM B117-94. In this test, the sa mples 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 35 o C. The specimens were visually inspected for surf ace 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 indicate d in Table 3-29. At each 500 hour interval, the specimens were removed from the test chamber, inspected, and photographed.
3. SO 2 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 app lied 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 wa s 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% CaCl 2 and 0.25% NaHCO 3 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
>50 071 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 572 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

SO 2 4340 Uncoated N/A N/A N/A 5
SO 2 4340 EHC 3 no no 5
SO 2 4340 EHC 3 no yes 5
SO 2 4340 EHC 3 yes no 5
SO 2 4340 EHC 10 no no 5
SO 2 4340 WC/Co 3 no no 5
SO 2 4340 WC/Co 3 no yes 5
SO 2 4340 WC/Co 10 no no 5
SO 2 4340 WC/Co-Cr 3 no no 5
SO 2 4340 WC/Co-Cr 10 no no 5

SO 2 300M EHC 3 no no 5
SO 2 300M EHC 3 no yes 5
SO 2 300M EHC 3 yes no 5
SO 2 300M WC/Co 3 no no 5
SO 2 300M WC/Co 3 no yes 5

SO 2 Aermet 100 EHC 3 no no 5
SO 2 Aermet 100 EHC 3 no yes 5
SO 2 Aermet 100 EHC 3 yes no 5
SO 2 Aermet 100 WC/Co 3 no no 5
SO 2 Aermet 100 WC/Co 3 no yes 5

Total 27573 3.4.4.3. Test results As of the date of writing this report, the SO 2 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 de termined at 125 hour intervals during the B117 test. After exposure to th e accelerated tests, all specimens were removed from the salt fog cabinet and were cleaned with a Sc otch 3M abrasive pad to remove loosely adherent corrosion products then dried w ith 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 pos sible to identify surface defects such as blisters or pits (see Figure 3-34 as an exam ple). Removing the blisters and portions of the coating that were undercut by corrosion pr ovided a better representation of the area that was affected by corrosion. A protect ion 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 subs trate and the corrosion of the coating were considered when assigning a protection rati ng 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 re plicate samples were examined and given a protection rating (0-10) in accordance w ith 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 differen tly; 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 horiz ontal 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 th e 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 electr on microscopy were performed on these samples. The examination of the samples following th e 1,000-hour exposure clearly showed that the base alloy influenced the co rrosion 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 sc ribe creep values for all samples.

3.4.4.3.1. 4340 Steel in ASTM B-117 The experimental results for these samples ar e 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 di rect comparison. Figure 3-38 shows the 4340 steel with the various 0.010” hard chro me coatings. The overall best performance for a coating to protect this base metal wa s the EHC with a nickel sublayer. This combination achieved the highest protection ra tings of any coating. The use of an74 organic sealer generally enha nced 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 coa ting composition at 0.003” in thickness. The 0.003”-thick WC-Co coatings without seal er performed poorly, receiving a rating of 1. The coating was cracked and severely und ercut. 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. Howeve r, 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 mi croscope, the attack in the corroded regions appeared to be a generalized di ssolution. This type of attack on the coatings was also seen with the other unsealed HVOF coating/substr ate combinations. A possible opportunity for improvement of the WC-Co was seen by th e addition of chromium to the composition (see Figure 3-35, Figure 3-36, and Figur e 3-37). The 0.003”-thick WC-CoCr had portions of the coating that were remove d and some undercutting. Corrosion of the coating also occurred and th e average protection rating was 3.75. The 0.010”-thick WCCoCr without sealer had a prot ection rating of 7.75. In this series, three of the samples showed cracking of the coating in isolated ar eas and some undercutti ng of the coating in these areas. In two of the latter cases, the e poxy mask at the top of the rod appeared to be compromised and this was the site that appear ed to be the origin of the undercutting and cracking. The corrosion of the 0.010”-thi ck WC-CoCr coatings was light. The B117 results clearly demonstrate the performance enhancement of the chromium-containing alloy without relying on an orga nic 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 co atings 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 se aler coating provided a protection rating which equaled the general performance of th e EHC without nickel. This result needs confirmation however because the 0.010”-thi ck WC-Co coating with organic sealer showed poor performance. Certainly this is counterintuitive; a 0.010”-thick coating of the equivalent composition ought to outperf orm 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 bliste ring, which was much more severe in the sample with the 0.010”-thick coating. The da mage 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 be tween 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 ve rsions. Figure 3-40 shows th e corrosion that has occurred beneath a removed blister. The coating adjacen t 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 scribe75 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 cr eepage from the horizon tal scribes, running practically the entire length of the sample. The vertical scribe creepage values were 11 mm and 18 mm while the maximum creepage fr om 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 B1 17 test chamber as controls. These rods experienced severe corrosion and were remove d 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 experime ntal 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. A ll EHC coatings achieved protection ratings of 8 or above, with the EHC with a nickel s ublayer achieving an al most 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 thic kness 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 de gree 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 prot ection 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 additi onal 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 remain s 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 co atings 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 exposi ng 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 pres ent bar and radar charts that summarize the results for this base metal test series. The corrosion testi ng 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 coati ngs 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 co rrosion 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 protecti on 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 (Coat ing 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 1.077 removed and showed that some undercut ting had occurred. The coating was not compromised in the other two samples in this group. All of the uns ealed WC-Co coatings showed a degree of corrosion and the corro sion product was removed from the samples during the cleaning process. The 0.010”-t hick 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 re moved 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 represen tative samples of the 0.003”-t hick WC-Co HVOF coatings without sealer on 4340 steel, 300M, a nd 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 creep age results. Overall, there was almost no scribe creepage for the Aermet 100/coating co mbinations. 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 se ries. This supports th e theory that intact organic sealers act as a barrier, but once da maged, they may enhance the corrosion at localized points. As with the 300M metal sa mples, 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 exposi ng 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 result s 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 corro sion 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 9 078 coatings. The unsealed coatings on the 4340 steel and Aermet 100 were completely intact, i.e, there were no cracks and/or def ects 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 co ating 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 Coatin g + Sealer on 4340 Steel After B117 Test: a) Before and b) After Cleaning79 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 Protect ion Ratings on 4340 Steel After ASTM B117 Testing80 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 Testing81 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 Coatin g on 4340 Steel with Sealer After Blister has been Removed82 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

10mil Cr, No Ni, No Sealer 3mil WC/Co, 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 Testing83 Figure 3-43. 0.003”-thick Coatin gs 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 Testing84 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

10m il Cr, No Ni, No Sealer 3m il WC/Co, No Sealer 3mil WC /CO, Sealer 10mil WC/Co, no Sealer

Figure 3-45. Radar Plot of the Protectio n Ratings on Aermet 100 After ASTM B177 Testing

Figure 3-46. 0.003”-thick WC-Co HVOF c oating on 4340 Steel, 300M, and Aermet 100 After the B117 Test85 

### 3.4.5. Additional B117 Corrosion Testing

3.4.5.1. Background It was apparent that there were significan t differences between the results that had previously been obtained for B117 tes ting 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 performa nce 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-C o coatings performed substantially better than the EHC coatings after exposure for three years. In order to attempt to determine the reas on 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-inc h-diameter rods was a deviation from the protocol. Another difference between the ea rlier B117 and atmospheric tests and those conducted for the Landing Gear JTP was that th e former coatings were tested as-sprayed and the latter were ground to a surface fini sh 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 materi al, leading to corrosi on proceeding underneath the WC-Co and subsequent fracturin g and spallation of the coatings. Figure 3-47. 0.010”-thick HVOF Coatin gs: 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 Test86 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-i nch-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 peen ing, grit blasting) and coating depositionparameters 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 fr om the end. At the other end, the coating terminated one-quarter-inch from the end. T hus, 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 coa tings 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 te sting required a total of 84 samples.

Table 3-35. Sample Matrix fo r 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
Y = Yes H = Hitemco Notes:
N = No S = Southwest Aeroservice88 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 289 As can be seen from Table 3-35 to Tabl e 3-37, there were two samples for each condition. One complete set of samples was in serted 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 corrosi on 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 l oosely adherent corrosi on 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 def ects 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 290 3.4.5.3. Results and Discussion In the following discussion, those samples pr eviously tested under the Landing Gear JTP will be referred to as the “LGJTP” samp les and those samples tested under this supplemental corrosion study will be referred to as the “Supplemental” samples. A general observation regarding the previous a nd 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 expos ure. 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 consiste d 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 resu lts. The protection rating details how well the coating protected the base metal from co rrosive 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 corr osion of the base metal. However, the appearance rating gives a relative scale to th e level of corrosive attack on the coating itself. A specimen might have a protection ra ting of 10 but an a ppearance 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 resi st corrosion does impact its service life. Coatings which undergo significant surface corros ion in salt fog are more likely to allow substrate corrosion in time compared to t hose coatings more impervious to attack. Appendix 3 in the JTR [3.1] provides the prot ection and appearance ratings for all of the tested samples.

3.4.5.3.1. Protection Ratings As mentioned previously, th e corrosion levels observed on the Supplemental samples were not as severe as those on the LGJTP samp les. 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 botto m 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 th e 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 subs trate metal while the samples were in a condensing, electrolytic salt atmo sphere. 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 corro sion performance in the ASTM B117 test.91 During the rating of the samples, the comme nt 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 cl arity to this study and the remaining analysis will concentrate on the rod geometry samples. For the 4340 metal rods, two of the NRL expos ed samples had breached coatings as well as three of the KSC exposed samples. Thos e samples with failed coatings are indicated in Table 3-38, while the full da ta 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, wh ile the full data are shown in Figure 3-58.

The tables presented above show roughly equi valent 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 sample s 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. In ferentially, 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 sample s that failed to protect the substrate where ground but, as will be discussed below, genera l corrosion of the coating was worse with unpolished surface samples. Figure 3-51 show s 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 SWAero92 compared to the ground samples. Note th at one of these ground samples has had a coating failure. While a few more Southwest-Aeroservice-applie d coatings failed relative to those applied by Hitemco, there was no definitive differen ce between the coatings deposited by the two vendors. A final note on substrate pr otection is the repeat finding that the EHC samples outperformed all others. No EHC coati ng 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 bette r than either of the HVOF coatings.

3.4.5.3.2. Appearance Ratings The appearance rating is an important attrib ute in gauging the exte nt of corrosion on the coating. In most of the samples exposed in this series, the underl ying metal substrate was protected but the coating itself was degraded to varying degrees. For instance, the EHC coatings outperformed all other alloys and rece ived the highest appearance rating (10) for seven of the eight EHC samples. The eight h sample received a 9 rating owing to small staining of the surface probably derived fr om Cd corrosion products running onto the surface. The HVOF-coated samples received va rying 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 com positions. Figure 3-55 and Figure 3-56 show the relative appearance rankings for the coat ings 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 aggres sive corrosion environment. Overall, the relative rankings are similar for either samp le 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, a nd 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 co atings to be worse. However, as before with the 4340 samples, one of the two ground sa mples 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 th e 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-depos ited coatings were tested, no definitive conclusion is able to be drawn from this data set when accounti ng 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 prot ecting the substrate. See Figure 3-60 and Figure 3-61 for a comparison of the WC-Co coatings.
2. There was no definitive difference betw een the coatings applied by the two93 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 im plies WC-CoCr coatings show improved performance over WC-Co coatings. Add itional 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 thei r ability to protect the substrate steel. While 0.003”-thick EHC in this data set di d protect the substrate (one sample only was tested in each location), the LGJTP re sults 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, a nd the Supplemental Tests, Right94 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 Sup plemental Tests, Right95 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 Substrates96 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, NRL97 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 KSC98 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 KSC99 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: Ground100 Figure 3-60. WC-Co Coatings on 4340 steel. Left: 0.003”-thick, Ground. Right: 0.010”-thick, Ground101 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 Steel102 

### 3.4.6. Discussion of overall corrosion results The data show that the overall corrosion pe rformance 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 observa tions 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 te sting conducted by others on EHC of equivalent thicknesses (see, for exampl e, 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 corro sion 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 expl ain 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 Steel103 protection to the steel substrates evaluate d in this study as ev idenced 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 coa ting 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 testi ng 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 seri es of tests, corrosion engineers at NRL indicated that steel such as 4340 would be substa ntially 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-C o coatings. However, in terms of undercutting and breaching, there was e ssentially no difference between the coatings, again indicating the importance of ensuring the corrosive medium cannot have access to the coating term ination areas on the base material.



### 3.4.7. Significance It is difficult to determine the true signi ficance 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 Southwes t Aeroservice hard chrome appeared to be exceptionally good. However, some significant findings should be noted:
 Clearly, HVOF coatings provide corrosi on resistance compared with uncoated steel. However, given the wide dispar ity in results between the various tests reported here it is not possible to be su re how they compare with hard chrome coatings typical of those deposited in depots.
 Since WC-CoCr appears to provide bette r 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 high104 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 industrie s) 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 th at companies such as Boeing have substituted these HVOF coatings in place of hard ch rome 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. R ecognizing this, true measurements of performance were designed to be made in Pa rt 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 mate rial, surface finish, test conditions, etc.) have the most impact on wear performan ce 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 3-78105 Landing gear applications of hard chrome include both long-stroke sliding wear experienced by landing gear utility actuato rs and inner cylinders, and short stroke, oscillating wear (dithering and fretting) experienced by pins and hydraulic cylinders during ground operations. Testing was designe d 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 Shoe106 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 specime ns 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 st eel 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” thicke r 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, 0.010107 a)

b)

c)

Figure 3-65. Wear Test Rods and Bushings - a) Rod, b) Straight Bushing, c) Seal Groove Bushing108 

### 3.5.4. Coating deposition methodology

Coating materials and thicknesses test ed 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 test s were made in the form of Design of Experiment (DOE) protocols, with most of th e testing being done fo r sliding wear, which has the broadest application. Test conditions were designed to obtain measurable results in a short time rather than to duplicate se rvice 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 Coat ings - 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 seal109 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 pot ential 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 cont ributions to sliding wear performance of the major wear factors identified in th e 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 bl ock was modified to incorporate a section of seal material. Significance: Short-stroke, unlubricated w ear is typical of the conditions experienced by pins and hydrauli cs 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 hold er on a standard closed-loop MTS machine. The shoe was pressed against the block with a normal load, P N , applied by a dead weight system. Lubrication, where needed, was supplied by dripping from above the fretting region. Test conditions (loa ds, 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 Test110 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 tr eated to 160-180 ksi; AMS 4640 AlNi-bronze rod in the TQ50 or e quivalent 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 te st rod was oscillated on four bearings, driven by a hydraulic actuator via a lo ad cell to measure the drive load, P drive . 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. Th e lubricant was supplied at the bushing from a suspended bag through a small tube and a c onstricting valve that controlled the drip rate

. Test description: For both sliding wear and fr etting the coefficient of friction was measured as

∝

= [P drive(max) - P drive(min) ]/2 P N Test conditions were as defined in the test matrices. Hydraulic fluid was added to permit fully-lubricated, starved-lubricated, or unlubr icated testing. Under “Full” lubrication fluid was added throughout the test, while unde r “partial” lubricati on 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 Test111 For the nitrile seal rod/bushi ng 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 sm all orifice in the bushing. For the partial lubrication tests in the L 12 and L8 phases, the test was stopped and depressurized after 1 hour and disassembled to drain the lubr icant. The rod and bushing cavity were wiped with a clean, dry rag to re move any excess lubricant, reassembled, and repressurized with dry n itrogen 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 cente r 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 desc ribed 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 Equipment112 Table 3-43. Description of Visual Wear Ranking
Rank Description
1 No visible wear scar 2 Slight burnishing of circum ferential 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; circ umferential 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 fr om approximately 10:00 to 2:00 on the rod 9 Rating 8 plus some longitudinal grooving 10 Rating 8 plus extensive, deep l ongitudinal grooving, metal transfer. Figure 3-69. Visual Ranking Standards for Rods, VR 01 to VR 10113 3.5.5.3. Test matrices

Table 3-44. Matrix for P re-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 no114 *Originally assigned at 288 lb P N . 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 200115 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 Bronze116 

### 3.5.6. Test results

3.5.6.1. Pre-DOE The pre-DOE matrix was not a design-of-expe riment 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 coat ing behavior could be drawn from the preDOE data. However, test conditions were determined:
 For seal tests it was found th at a load of 40 lb compre ssed the seal, leaving only about 0.001” between the rod (or in fr etting tests the shoe) surface and the bushing (or block in fretting te sts). Above this load the rod (or shoe) touched the bushing (or block). Therefore 40 lb was the maximu m test load used in all pre-DOE seal tests (see Ta ble 3-44 and Table 3-45).
 The 288 lb. load resulted in greate r wear than did the 72 lb. load.
 Unlike the rod/bushing tests, it was possi ble 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 se vere than the lubricated test, even at low load. This test did not appear to distinguish between th e 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 si gnificant 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 d ecimal 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 subs equent 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 Fi gure 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 – Sho e; L12 DOE Matrix (Red against metal bushings; blue against nitrile seals)118 The L-12 fretting data showed very little di fferentiation 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 6BWEIGHT CHANGE (GRAMS)

Avg. block wt. = 60 gm Avg. seal wt. = 0.84 gm

Figure 3-71. Fretting Wear - L12 DOE Matr ix; Yellow Data Wear of Coated Shoe; Red Data Wear of Uncoated 4340 Block; Blue Data Wear of Nitrile Seal119 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 107A7B8A8B9A9B10A-L10AR10B-L10BR11A-L11AR11B-L11BR12A-L12AR12B-L12BRRANKING

Figure 3-72. Sliding Wear – L12 Data. Vi sual Wear Ranking – Rod. (Red against bushings; blue against seals)120 From the L12 matrix allowed us to draw th e following conclusions for design of the final L8 DOE matrix:
1. Rod coating and bushing type were statis tically significant design factors, as expected.
2. Normal load, P N , was a significant variable us ing 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 0.10007A8A9A10A-L10B-L11A-L11B-L12A-L12B-LGRAMS 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. Weig ht Change – Bushing/Seal (Yellow data wear of coated rod; red data wear of uncoat ed bushing; blue data wear of nitrile seal)121  Lubrication – partial (1 st 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 we ight 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 3 rd and 5 th 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) 9122 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 1123 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 (0.007 3) (0.0073) (0.4355) (0.2570) 6 66B 66B-EHC/NITRILE/240K/72 (0.007 1) (0.0044) (0.2583) (0.2772) 9

67A 67A-WCCo/NITRILE/80K/72 (0.000 9) (0.0124) (0.0779) (0.1863) 1 67B 67B-WCCo/NITRILE/80K/72 (0.003 5) (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 1124 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 (0.001 6) (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 (0.002 1) (0.0040) (0.1539) (0.1256) 8 80B 80B-EHC/NITRILE/240K/288 (0.016 6) (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) 1125 0 1 2 3 4 5 6 7 8 9 10

49A - EHC/4340/80K/72

49B-E HC/4 3 4 0 /8 0 K /7

2
50A-E H C/

4 3 4 0/

2 4 0 K/2

8 8

5 0B

-E

H C/

4340 / 2 4 0K/

2 8 8

5 1 A-EHC/A l NiB

r/8

0 K /2

8 8
51B-E H C/

A lNi

B r/8

0 K/288

5 2A

-EHC/A l N i B r/2

4 0 K/

7 2 52B-EHC/AlNi B r/240K/72

53

A -WCCo / 4340/80K/2 8 8

5 3B

-W

CCo/43 4 0 /8

0 K /288

54

A -WCCo / 4340/240K/ 7 2

5 4B

-W

CCo/ 4 3 4 0 /2

4 0 K/72

55A - WCCo/AlNiB r /80K/7 2

5 5 B

- W

CCo

/ Al

NiB

r/8

0 K/

7 2

5 6 A-W

CCo

/ Al

NiBr

/2

4 0 K/288

5 6B

-W

CCo/AlNi Br

/240K/ 2 8 8 VISUAL



### EHC WC-Co

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

(0.0750) (0.0600) (0.0450) (0.0300) (0.0150) 0.0000 0.0150

4 9A-EHC / 4340/80K/72

49B-

E HC/4340/80K/72

50 A -EHC/4340/ 2 40K/288 50B-EHC/4340/240K/288

5 1A-E

H C / Al N iBr/8

0 K /2 8 8

51 B -EH

C /A

lN i B r/ 8 0K

/ 28

8

52A

- EHC/

A lNiB

r /240K

/ 72 52B-EH C /A

l Ni

B r/ 2 40

K /72 53A-WCCo/4340/80K/288 53B-WCCo / 43 4 0/80K/288

5 4A-WCCo/4340/240K / 72

5 4 B -WC

C o /4 3 40 /2 4 0K

/ 7 2

5 5 A-

W CCo

/ Al

N iBr/8

0 K /7 2

5 5 B- W C C o / Al

N iB

r /8 0 K /7 2

5 6A-WCCo/AlN i Br / 24

0 K /2 88

5 6 B -W

CCo/A l Ni

B r/ 24 0 K /2 8 8 ROD BUSHING EHC WC-Co

Figure 3-75. Sliding Wear, L8 DOE – Weight Loss for Baseline Metal Bushings126 0 1 2 3 4 5 6 7 A C

B A B

C A D

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 Visualiz ation of the Major Effects in the L8 DOE Based on Visual Wear Rati ng; Only Effects D and A (normal load and number of cyles) are Statistically Significant127 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 B A D A B A C

C A D Pareto Chart of the Standardized Effects (response is Rod Wear Volume, A lpha = .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 Visualizat ion of the Major Effects in the L8 DOE Based on Rod Wear Volume; Only Effect C (coating material) is Statistically Significant128 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
Coatings Cycles 0 1 2 B D A B A D

C A C

A Pareto Chart of the Standardized Effects (response is Bushing Wear Volume, A lpha = .10)
A : A B: B C: C D: D

Figure 3-78. Pareto Chart and Visualizatio n 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 St atistically Significant129 0 1 2 3 4 5 6 7 8 9 10

66A-EHC/NITRILE/240 K /72 66B-EHC/NITRI L E/240 K /72 65A-EHC/N IT R IL E/80K/ 2 88 65B-EHC/N IT R IL E/80K/ 2 88

6 8 A-

WC

Co

/NI

T RI

L E / 2 40K/2 8 8

6 8 B- WCCo/NI T RI

L E / 2 40K/2 8 8

6 7 A-

WCCo/NI T RI

L E / 80

K /7 2

6 7 B- WCCo/NI T RI

L E / 80

K /7 2

5 0 A-

EHC/434 0/ 2 40K/2 8 8

5 0 B-

EHC/434 0/ 2 40K/2 8 8

4 9 A-

EH

C /4 3 4 0/ 80

K /7 2

4 9 B-

EH

C /4 3 40

/ 80

K /7 2

5 3 A-

WC

Co

/ 43

4 0 /8 0 K/ 28

8

5 3 B- WC

Co

/ 43

4 0 /8 0 K/ 288

5 4 A-

WC

Co

/ 43

4 0 /2 40K/72

5 4 B- WC

Co

/ 43

4 0 /2 40K/72 Visual Nitrile Seals Metal Bushing EHC

EHC WC-Co
WC-Co

Figure 3-79. L8 DOE Data for 3rd Half Re plicate – 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-

E HC/N I T R ILE

/ 2 4 0K/

7 2

6 5A-

E HC/N I T RILE/ 8 0 K / 2 8 8

6 5B

-E

H C /N

I T RILE/ 8 0K/

2 88

68A-

W C C o/N

I TRI

LE/

2 4 0K/2

8 8

6 8B-

W C C o/

NI TR

ILE/

2 4 0K

/ 2 8 8
67A-W C Co/NI TR

ILE/

8 0 K /72

6 7 B-WCC o /NITRILE/80K/ 7 2

5 0A

-E

H C /43

4 0 / 2 4 0K

/ 2 8 8

5 0B

- EH

C /43

4 0/240K / 2 8 8

4 9A-EHC/4340/80K/72 49B-EHC/4340/80K/72

53A

- WCCo/4340/80K/288 53B-WCCo/4340/80K/288

5 4A

-W

C C o/ 4 3 40

/24

0 K /72

5 4B

-W

CCo/

4 3 40

/24

0 K/7

2 ROD BUSHING

Nitrile Seals Metal Bushing EHC EHC WC-Co 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

7 3A-EHC /K

ARON /8 0K

/ 7 2 73B-EHC / KA

R ON

/ 8 0 K / 7 2

74A

- EHC/KARO N /24

0 K/2

8 8

7 4B-EH C/

KA

R ON

/ 2 4 0K/

2 88

7 5 A-WCC o Cr/KAR ON

/80K/ 2 88 75B-WCCoCr/KARON/80K/288

7 6A-WCCoCr/K A RON/24 0 K / 7 2

7 6B-WCC o Cr/KARO N /24

0 K / 7 2 77A-EHC/AlNiBr/80K/288

7 7B-EH C /Al

N iBr

/ 8 0 K / 2 8 8

7 8 A -E

H C /A

lN

i B r / 2 4 0K

/72

7 8 B -EH

C/

Al

Ni

B r/24

0 K / 7 2 59A-WCCoCr/AlNiBr/80K/72

5 9B-WCC o Cr/A

l Ni

B r/80

K /72

60

A-WCC o Cr/A

l Ni

B r/24

0 K / 2 8 8

60

B -WCCoCr/Al N iBr

/ 2 4 0K/

2 88 Karon B Seals AlNiBronze Bushing EHC EHC WC-Co,Cr WC-Co,Cr

Figure 3-81. L8 DOE Data for 5th Half Re plicate – 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/ K ARON/ 8 0K/7

2

73B

- EHC/K A RO

N /8 0K

/72

74A

- EH

C /K

A RO

N /2 40K/

28

8

7 4B

-E

H C/KAR O N/ 2 40K/

28

8

75A

- WC

Co

C r/K

A RO

N /80K

/ 288

75B-W C C oC

r/ KA

R ON

/ 80K

/ 288

76A

- WCC o Cr/

K AR

O N/ 240

K/ 72

76B

- W

C C oC

r/ KA

R ON

/ 240K / 72 77A-EHC / A l NiBr/ 80K

/ 288 77B-EHC / A l Ni

B r/8

0K/

288

78A

- EH

C/

A l N i Br/

240K / 72

78B

- EH

C / A lN

i Br/

2 40K/

72 59A-WC C oC

r/ AlNi

B r/80K / 72

59B-W C C oC

r/ A l Ni

B r / 80

K/

72

60A

- WC

C oC

r/ AlNi

B r / 240

K/ 288

6 0B

-WC

C oC

r/ AlNiBr/240 K /288Weight 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 Repl icate – 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 agai nst 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 replicateVisual 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 replicateWeightloss

Chrome rod Chrome bushing/seal HVOF rod HVOF bushing/seal

Figure 3-84. Average Rod and Bushing/Se al Wear for L8 Matrix - Weight Loss132 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. Statis tically 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 co atings. The only real surprise here is that surface finish did not discriminate, wh ereas in flight test ing and rig testing, surface finish has been found to strongly a ffect seal wear an d seal life, with smooth HVOF coatings giving much better performance than rough coatings 1,2

. Once the detailed statistical analysis is comple te it will be possible to analyze the data more thoroughly. However, some obse rvations can be made at this time:
 In general, the two tests at each condition were reasonably consistent, especially in visual ranking. Weight change wa s 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 ch ange was less than 0.01 grams in 880 grams, or 0.001%, and small amounts of tr apped 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 (Figur e 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 af ter being tested against these liners. Weight loss measurements were not nearly as clear.



### 3.5.8. Significance These wear tests generally b ear out the findings of rig a nd flight tests. HVOF WC-Co and WC-CoCr both experience less wear than EH C. However, they tend to cause more wear in adjacent seals and bus hings. In order to counterac t this tendency users have successfully used two approach es – 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 differe nt coatings and seal materials. Overall, HVOF coatings show less wear than EHC, but tend to cause more wear on adjacent components. Passes acceptance criteria134 

# 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 si mulating runway debris (evaluated by Gravelometry), and
 low velocity large item impact simulati ng 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 st eel 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 3-92135 were deposited 0.002-0.003” thicke r 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-approvedspecifications 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, a nd 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 sa mples 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 sp ecifications for EHC, WC-Co, and WCCoCr. Flat specimens coated independen tly 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)
 R a measurement by profilometry The JTP required only visual ranking of co ated round rods. However, the additional ranking methods were added by Boeing for be tter 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 eval uate impact within coated area Vibrating gravel hopper
Door Plexiglass 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 Vise138 

### 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 (Ran kings from 1 (best) to 3 (worst) for different evaluation methods)
0.003-0.004” 0.010-0.011” Evaluation 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 profilome ter 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 af ter 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 Flats139 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 flats140 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. Micr oscopic observation showed this area to be bounded by circumfere ntial 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 microgr aphs 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 a nd fine circumferential cracks visible in the micrographs around the impact were counted sepa rately, as well as the number of radial cracks at the 3 and 9 o’clock positions (i.e. pa rallel 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 circum ferential 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)141Cr 24WC-Co 24WC-CoCr 24Cr 60WC-Co 60WC-CoCr 60Cr 102WC-Co 102WC-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 HeightsCr 24WC-Co 24WC-CoCr 24Cr 60WC-Co 60WC-CoCr 60Cr 102WC-Co 102WC-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 Heights142 

### 3.6.7. Discussion of impact results In both types of impact tests, results are de termined 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 repr esent selected areas, c hosen 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 wa s 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 pr event 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 he ight (i.e. with impact velocity or energy). The increase was more signifi cant in chrome than in HVOF.
 Chrome shows some radial cracking along the rod axis and the amount of cracking increases somewhat with drop he ight. HVOF coatings show no radial cracking,.
 There is no evident dependence of damage on coating thickness.
 WC-CoCr may show somewhat less cr acking than WC-Co, but if so the difference is marginal.



### 3.6.8. Significance While gravel impact performance is very si milar, HVOF coatings are likely to be less sensitive than chrome to FOD by dropped tool s 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 dama ge 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 plat ing methods, the chrome plating process generates hydrogen at the surf ace 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 re quire 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 coa ting, during service, or even during later overhaul cycles. The hydrogen produced dur ing the corrosion process can also cause hydrogen embrittlement and failure under static lo ad. 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 3-104144 HVOF and EHC coatings on the susceptibil ity 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 ch rome 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 specime n 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 Specime n 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-pla ting 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 se ction 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 Figur e 3-94, and the coating ground to a 16 ∝ ” (Ra) finish. The coating in the notch was not ground. During HVOF spraying, the overspray materi al, which would normally bounce back off the surface, tended to become trapped in th e notch, producing a thicker and more porous coating. This entrapment was minimized by di recting 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 Coating146 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 not ch is to cut through the coating with a diamond file. This works well for soft coa tings but was not possible with hard HVOF coatings or hard chrome plating. Ther efore 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 cu t just into the under lying 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 relia ble 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 Notc h (exposing the substrate) on a CNC Milling Machine (Left – schematic of cutting blad e 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 R c Coatings: EHC, WC-Co, WC-CoC r, 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 speci men 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 Se quence 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 H 2 O DI H 2 O 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 168149 hardness and notch tensile strength results.



### 3.7.8. Coating structure Chrome plating into the not ch 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 poros ity. 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 th at 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 Specimens150 

### 3.7.9. Test results

3.7.9.1. Cracking of coatings

On stressing to 45% NTS both the chrome a nd HVOF coatings cracked (Figure 3-97). The chrome plate exhibited many fine crack s around the notch radius, but the WC-Co showed only a few cracks penetrating to the subs trate. After stressi ng, liquid penetration to the substrate would be e xpected 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 th e 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 excep tion 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 me tallographically. Specimens that failed on loading showed no sign of embrittlement (i.e. they failed by overload instead of 

# cracks

Figure 3-97. Metallographic Cross Sectio ns 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 diam eter 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 diam eter 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 produ ced 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 specimens152 average Time-to-failure values. Because ther e 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 4.8NoneEHC, 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 Ho urs to Failure (NF = No failure in 200hrs), With and Without Hydrogen Bake (All failures in notch)153NoneEHC, 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 (Fa ilures occurred on shank at button)154 4.8 8.3

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 l og 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-t o-failure data values – note log scale; Error bars indicate high and low values) (Note that these te sts were run for 0.003”- thick coatings only)156NoneEHC, 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 Summary157 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 Solution158 3.7.9.4. Open circuit potential Open circuit potentials were also measured as a function of time fo r 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 C oated Sheet Specimens in 5% NaCl Solution159 

### 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 bake d, 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 mos tly passed when baked. (Note that this Sequence did not include chrome plated sp ecimens since it is already known that hydrogen baking after chrome plating is requ ired 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 week s 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 Solution160 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 th e notch). This means either
1. that the notch K t 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 cr acks at 45% NTS, preventing its supporting the load and reducing the K t . 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 entir ely 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 sa mples that still failed did not have the hydrogen fully removed, probably because of in sufficient bakeout time (i.e. the long time required to diffuse hydrogen either along th e long path between the button and the HVOF-coated region or to diffuse it through th e bright Cd). Note that the loss of hydrogen from the button/shank join on baki ng 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. Theref ore, 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 show s time to failure with the notch partially exposed (i.e. scribed). All of the data ar e summarized in Figure 3-102 for comparison. Figure 3-100 (unscribed coa tings) shows the following:
 While the specimens failed sooner in Na Cl 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 pe rformance of the HVOF coatings and chrome plating can be understood in terms of the open circuit potential differen ce between the substrate and161 the coating. Figure 3-105 s hows 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 l og 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 fa r 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 diffi cult 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 cathodi c coating area to the anodic substrate area is higher, and the anode and cathode are in very close proximity. This creates higher potential gradie nts with higher corrosion curre nt densities and hence more local hydrogen. In contrast the scribed ar eas 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 th e combination of the thick coating in the notch that will lower the K t , and the high elastic modulus of the coating will lead to much longe r time-to-failure. On the othe r hand we might well expect that a porous coating in the notch will make corrosive attack eas ier and reduce time-tofailure. We can draw some inferences from our data:
 As noted above, the thickne ss of the HVOF coatings in the notch could have reduced the K t 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 K

t was fairly small.
 When the coating was scribed, the true K t 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 expecton sharpening the notch, the time-tofailure of notched specimens actually in creased, again showing that the thick HVOF coating in the notch did not lower K t . Furthermore, cutting through the coating would have eliminated any eff ect due to elastic modulus, because the coating was no longer continuous and therefor e no longer able to carry load. If the elastic modulus of the coating had c ontributed to increased time-to-failure162 then failure time should have dr opped on scribing, not risen.
 We would expect a similar electrochemi cal potential between the substrate and either good quality or poor quality HVOF coatings, since the coating chemistry is the same. Therefore we would not exp ect any effect due to electrochemical potential, as seen between HVOF and ha rd 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 th rough 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 K t 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 pr ior to coating (which is done to prevent HE during plating) as well as for hydroge n baking after (to prevent subsequent HE failure).
 Since hydrogen can diffuse through HVO F 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 s ubsequent stress corr osion failure in service. According to OO-ALC and th e Air Force Aging Landing Gear Life Extension Program, these are two comm on 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 adja cent to them without tr apping hydrogen and causing embrittlement. However, diffusion through HVOF coatings may occur at a slower rate than diffusion through EHC, and may theref ore require a longer hydrogen bakout of any non-porous coating electroplated contiguous with an existi ng 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 st eels coated with HVOF WC-17Co are in all cases equal to or better than those coated with EHC.

o The HVOF coatings develop a circumfe rential crack pattern, and in some cases spall at high loads.

 This coating integrity issue of cr acking, delamination, and spalling is presently under detailed investig ation, 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 yiel d (220ksi) at R= -1, while 0.012”- thick coatings have spalled. U nder 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 relia ble repair schemes involving monolithic or duplex coating structures.
 Corrosion

o Corrosion results are highly variable . HVOF coatings clearly provide significant protection agai nst 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 coa tings do not corrode, but permit substrate corrosion through crack s 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 us eful discriminants between coatings, but sliding rod/bushing wear tests could discriminate.

o Although detailed statistical analys is has not been done, the overall conclusion is that HVOF WC-Co and WC-CoCr coatings show less wear165 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 (usua lly 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 embritt lement during deposition

o Hydrogen can diffuse through HVOF co atings to permit embrittlementrelief heat treatment of electroplating or stripping operati ons 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 La nding Gear, Part I: Materials Testing,” Prepared by Hard Chro me Alternatives Team for Environmental Security Technology Certif ication Program, November 2002 (available from www.hcat.org) 3.2 P. M. Natishan, S. H. Lawrence, R. L. Fo ster, J. Lewis and B. D. Sartwell, “Salt Fog Corrosion Behavior of High-Velocity Oxygen-Fuel Thermal Spray Coatings Compared to Electrodeposited Hard Chro mium,” Surface and Coatings Technology, v. 130, pp. 218-223 (2000). 3.3 Jay Randolph, Delta Airlines, “Service Evaluation Status a nd Impact of HVOF Coatings on Landin Gear at Delta Airlines ,” Gorham Advanced Materials Conference on Advanced Coating Systems for Gas Tu rbine Engines and Aircraft Components, San Antonio, March 2000 3.4 Tony DeGennaro, Green, Tweed & Co., “Evaluation of Chrome Rod Alternative Coatings,” Report #GTE0644, September 1999166 3.5 Naval Air Warfare Center Aircra ft Division Report No. NAWC-ADLKE-DDR481400-0001167 

# 4. Coating Integrity and Spalling
The coating integrity issue was first reporte d by C-HCAT when fatigue testing WC/CoCr specimens. They found that above a certain stre ss coatings tended to delaminate and that the stress at which this occurred was lower for thicker coatings. The finding led to extensive testing to understa nd and quantify the phenomenon.
The Canadian team carried out extensive mate rials 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 th e interface with the underlying steel, where they would bifurcate and spread through the coa ting parallel to the in terface. 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 sm all 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 Integr ity Big Bar Design (Metcut)168 In general it has been found that the spalli ng 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 curr ently engaged on a Process Mapping program funded by HCAT, ALGLE, and the thermal sp ray 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 se t 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 servi ce 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 threshol ds for 0.010”-thick WC-Co coatings are shown in Figure 4-5. Most tests were carri ed out at R= -0.33, which the Air Force calculates is their expected R ratio for defl ection and spring back. Only the brown point was taken at R= -1. Testing was done in a se ries of steps of increasing strain, using 50 cycles per step to 190 ksi, and 10 cycles pe r step thereafter until co ating failure. The force at the outer end of the bushing at the top of the bar wa s computed by the standard equation for the maximum stress at the outer fiber of a beam:

I Mc

= max σ , 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 expe cted, they actually fell somewhat to the left of the Mc/I curve due to the small but finite contribution of the hi gh modulus coating to Figure 4-4. Hill AFB Bend Test Fixture170 the stiffness of the tube.) Since a struct ural 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 ar e displayed on the Mc/I line. The stresses for these points can be read from the verti cal axis, while the co rresponding strains are indicated by the positions of the equivalent color points on the lower horizontal axis. The stress/strain curve computed by finite element an alysis for this test cylinder, as well as the standard reference stress/strain curve (app licable 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 30 0M 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, unde r tensile stress in a stress-controlled test rig (s uch 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 lim it 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 lStress (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 thecoating 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 with in the bushing shows some de lamination and peeling. A view of the same area under FPI clearly shows th e crack array (Figure 4-7). Note that the cracks are only found on the maximum tensile st ress 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 Rese arch 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 FPI173 

# 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-sc ale 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 me t 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) pres ented in Table 5-1 without failure. Failure was defined as any deformation or crack th at 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 re presented 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 Requirements174 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 actua lly 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 a ssess 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 Bo eing 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 350 o F during coating depositi on. 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 vi sual 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-c oated Pins in Boeing F/A-18E/F Main Landing Gear Fatigue Test175 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 atleast 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 airc raft around the globe generally for antisubmarine warfare, search and rescue, mini ng and anti-surface warfare, and over-land intelligence, surveillance and reconnaissance. The Navy and its Foreign Military Salespartners 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 requir ements through at least the year 2015. Phase II of the SLAP included the design of stru ctural parts/assemblies to be included in Service Life Extension Program (SLEP) modifi cation 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 unde r 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 Te st 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 29,753177 It was decided that as part of the test, th e piston on the left-hand MLG would be coated with HVOF WC/17Co instead of EHC. A pi ston was shipped to Naval Aviation Depot Jacksonville where the hard chrome was rem oved using standard st ripping techniques. Then the piston was sent to Southwest Ae roservice where it was grit blasted and WC/17Co coatings were applied in acco rdance 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 350 o 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 Piston178 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/Cocoating successfully completed the fullscale fatigue test and therefore is qualified for use on operational aircraft. Figure 5-4. Left-hand MLG Containi ng HVOF WC/17Co-coated Piston Mounted on P-3C Test Article at Lo ckheed-Martin in Marietta, Georgia

Figure 5-5. Close-up of HVOF WC/17Co-coated Piston in MLG179 

# 5.2. Flight Testing

5.2.1. E-6A Uplock Hook Shaft The E-6A is a Navy reconnaissance aircraft si milar to the Boeing 707 (see Figure 5-6.). In February 1999 Naval Aviation Depot Jack sonville 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 opera ting out of Tinker Air Force Base. NADEP Jacksonville obtained two of th e shafts and stripped the EH C 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 us ing 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 350 o F during coating deposition. The coatings were deposit ed 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 we re 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 20 00. Periodic inspections of the shafts were conducted. As of 28 February 2 003, Aircraft 164388 had 3446 flight hours and 2731 landings, and Aircraft 162784 had 3871 fli ght 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 Aircraft180 

### 5.2.2. P-3C Main Landing Gear Components In early 1999, Naval Aviation Depot Jack sonville obtained approval from NAVAIR to conduct flight testing of HVO F coatings on a P-3C landing gear. NADEP Jacksonville obtained a MLG Piston Assembly (Part Nu mber 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 acco rdance 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 350 o 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 NADE P Jacksonville where it was installed on Aircraft Number 156522. The aircraft co mpleted overhaul in December 1999 and was returned to Squadron VP-30 operating out of J acksonville 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 th ey are obscured by the wheels. Figure 5-7. E-6A Uplock Hook Shaft WC/Co WC/Co181 The Navy P-3 Flight Support Team (FST) i ssued a Maintenance E ngineering Advisory message to VP-30 indicating that this was a special test article. The P-3 FST and NADEP Jacksonville Material Engineering Labo ratory coordinated periodic inspections in conjunction with normal squadron inspecti on intervals (normally 224 days) to monitor and record the performance of the WC/Co coa ting. Inspections were both visual and fluorescent penetrant. In August 2000 the comp onent had been subjected to 850 landings and it was removed from service due to an in ternal oil leak not related to the WC/Co coating. The component was repaired, return ed to VP-30, and installed on Aircraft 160284 in April 2001. As of 29 March 2003 the co mponent had been subjected to a total of 2133 landings combined on both aircraft. Visual and fluorescent penetrant inspection indicated no evidence of coating wear, delamina tion, 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 Cyli nder (indicated) and Axle Journals WC/Co183 

# 6. Producibility

6.1. Stripping of HVOF coatings Stripping is a very important process for de pot overhaul, since old coatings must be removed in order to rebuild worn or dama ged components. Alt hough 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 Kestle r 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, si ze 0.9 1.0” diameter x 5- 6”L, 1 each Southwest Aeroservices in Tulsa, OK app lied 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 DPH 300 >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 <1Roughness (assprayed) Microinch Ra 150-250 111 150

Porosity (void volume) Vol. % <1 <1 <1

Microstructural uniformity, 200x field Unmelted particle <3 0 0185 6.1.1.1. Masking, fixturing, and positioning specimens Masking specimens. All four sides of the pa nels were masked off by a 1/8-inch band of Military Specification MIL-P-23377 epoxy prim er to preclude possible solution entry from the panel edges into the coating and subs trate interfaces. Two co ats of primer were applied, air dried for over 72 hours, and baked at 325 o - 330 o F 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 positione d horizontally and alternated at a 90 o 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: 130 o – 150 o F Current: 40 – 80 A/ft 2 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: 130 o – 150 o F 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 t op 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 150 o F at potential of 6 volts and electrolytically strip for one hour . (Note: Since the titanium fixture was a poor electrical conductor, the cu rrent density was about 20-25 A/ft 2 which was below the 40-80 A/ft 2 requirement. Therefore, th e stripping rate was relatively slow.)
2. Remove the fixture from the process solu tion tank. Rinse in two different cold water tanks for 5 minutes in each ta nk. Rinse in a hot water tank (140 o 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 coating187 was removed near the sides than in the center, apparently due to higher current density near the edges. There was disc oloration 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 ta nk 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 pane ls were stripped fast er 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 characteri stic 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 <100188 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 co ating residue left on the panel; whereas, “<100%” means small amount s 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 <100189 6.1.1.5. Post Stripping Inspections and Cross-Sectional Examination Visual Inspection: The coatings on the th inly 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 strippi ng solution. The epoxy primer maskant was mostly intact with minor chipping. Microscopic Inspection: Afte r stripping the panels showed staining but no etching.

6.1.1.6. Conclusions and Recommendations Proposed chemical or electrochemical stripp ing 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 af ter 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 pr ocessing, 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 TimePercent 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 Time190 able to comply with the hydrogen embr ittlement requirements of ASTM 519.

. The process solution does not contai n known carcinogens or extremely toxic chemicals.

. A standard process in a conventional wast ewater 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. Th e substrate weight loss due to prolong etching was inconclusive. However, visual and microscopic (20X) examinations showed no evidence of depletion or pitting. The stri pping rates for WC-Co and WC-Co-Cr coatings are comparable. The team recommended that a stripping test pro cedure 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 pr oduction parts are cylindr ical. Because of non-uniform stripping due to edge effects us e flat panels for st ripping tests only if actual production parts to be HVOF-coate d 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 sp ecimens at the following times: before coating, after coating, at several intervals during stri pping, 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 mo re 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 corro sion protective oil to strippe d low alloy steel parts to prevent rusting.



### 6.1.2. Stripping of HVOF WC/CoCr – Heroux Devtek, Canada As part of their producibili ty 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 cyc lic voltammetry Tafel curves, weight loss, and hydrogen embrittlement. Heroux checked several chemistr ies (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 solu tion 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 + Na 2 CO 3 (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 Na 2 CO 3 + 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/hr192 Ultrasonic agitation was found to greatly improve the stri pping rate (Figure 6-5), presumably by removing stripped material (e specially the carbide particles) from the surface. Ultrasonic agitators can be i mmersed in a standard stripping bath. As we would expect from Figure 6-2, th ere 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 subj ect 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 overheati ng (grind burning) the substrat e steel. Grind burning is quite common with EHC and many manufacturers check for it as a matter of course with Barkhausen Noise (Rollscan) equipment. Prio r 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 impr oves 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 subs trate when grinding the HVOF coating, although it is not as easy to do as with EHC.

. Abusive grinding is not necessa rily 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. (N ote 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 compatibil ity through a combination of Tafel plots of dissolution rate, immersion test, and F-519 hydrog en embrittlement testing [6.2]. A wide range of production, overhaul, and service flui ds were tested, with the results outlined in Table 6-6. The only serious issues were possibl e 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 bur ns on steel after grin ding and prior to coating) – Since by definition the soluti on 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, sugges ting that hydrogen cannot migrate through HVOF WC/CoCr as readily as through EHC. A 12 hour bake did eliminate failure (Note: Slower hydrogen evol ution 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 chan ge, no dissolution or embrittlement Degreasers No weight or finish ch ange, 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 e xposure; 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 embrittlement195 with the results of the HCAT embr ittlement testing, see Section 3.7). The only serious concern is with Nital etchi ng, 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 proces s, 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 f ound to cause embrittlement it is possible that the cleaning contributed to th e hydrogen level and th erefore 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 La nding Gear After Fatigue to Stress Sufficient to Cause Coating Delaminati on, Showing Circumferential Coating Cracks (C. Edwards, OO-ALC)196 out damage in a coated surface. FPI is also capable of detecti ng cracks in the coated surface ca used 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 s hows both a major delami nation and cracking of the coating.



### 6.4.2. Magnetic particle inspection (MPI) MPI has been found to be not very useful fo r detection of cracks and damage in HVOF coatings (see Table 6-7).



### 6.4.3. Ultrasonic inspection Ultrasonic inspection is not widely used in th e 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 coa tings [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 overc oating. 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 we ll 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 ultras onic crack response is illustrated in Figure 6-8 for thin and thick HVOF coatings. Table 6-7. NDI Testing of Substr ate 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 detec tion beneath coating; excellent detection when crack propagated through coating197 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 cy cles than an equivalent WC/Co coating. This method has not been used for depot evalua tion as depots generally do not have test equipment of the required sophistication. Figure 6-8. Ultrasonic De tection 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 (no delamination) “good” 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

0.009198 

### 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 signa l drops off more slowly with HVOF than with EHC coatings, permitting somewhat greater sensitivity with HVOF coated components. Figure 6-11 shows that the Ba rkhausen method can readily detect shot peening and grind burns beneath an HVOF coati ng. 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 Sign al 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
MAGNMP 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 re sults 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 electr oplating processes were developed (refer to Figure 7-1). This generic flow diagra m is based on information provided by the company. In general, each of the repair ed 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 coll ect information on the ba seline 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 engineer s, plating operators, plating supervisors, chemists, and other employees throughout the facility. The information gathered during the site visit was supplemented with corres pondence 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, va lues were assumed based on data from other facilities and using engi neering 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 asso ciated with hard chrome electroplating and from the use of personal protective equipment (PPE)

. Maintaining an accumulation point for waste

. Purchasing and maintaining PPE202 . Purchasing and storing drums, labels, a nd 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 appli cation 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 co atings 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 De-Mask Rinse Strip (chemical)

Abrasive Blast

InspectRepeat as needed

Acceptable PartsRework as needed

Unacceptable Parts Parts Inspect Detailed Inspection Mask De-mask

Shot Peen

Bake Dry Plate Req’d Plate Not Req’d

Figure 7-1. EHC Process Flow at Commercial MRO Shop203 

# 7.3. Data and assumptions The approximate number of U.S. and Cana dian C-130 landing gear components that are hard chrome electroplated annually at the M RO facility is shown in Table 7-1. This number does not include rework of compone nts; 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 chro mic acid each year for hard chrome electroplating the affected ar eas on the C-130 landing gear co mponents. 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 co mponents and areas to receive HVOF during fabrication and repair/overhaul. These pa rts were selected as likely components for transitioning because they have the largest ex ternal chrome plated surfaces, which would be suitable for transitioning to HVOF co atings. Additionally, these components frequently require stripping and re plating during the overhaul process. HVOF Spray Mask

Grind/ Polish De-Mask Strip
(electrochemical)

Abrasive

Blast

InspectRepeatas needed

Acceptable PartsRework 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 250204 Due to sensitivity of operating data, remain ing detailed data and calculations are not shown in this report. However, all data colle cted are on file. Type s 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. Assump tions made to complete the analysis are described in Section 7.3.3. 

### NLG Cylinder NLG Piston MLG Piston

Figure 7-3. Areas Expected to be Transition ed from Hard Chrome Electroplating to HVOF Thermal Spraying205 

### 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 com ponents 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 es timated 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 step s (refer to Figure 7-1). H. Labor requirements for maintenance of the chrome plating baths, including cleaning, make-ups, and a ddition of chemicals to the tanks. I. Types of inputs (i.e., materials, en ergy, and labor) and outputs (e.g., air emissions, wastewater, and hazardous waste) associated with the hard chrome electroplating process st eps 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 th ickness 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, vi nyl tape, lead tape, wax, and deionized water. N. Energy usage related to baking th e three affected components was provided. Bath heating costs were difficult to evaluate since they are hidden in a larger central heating sy stem 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 gri nding 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 proc ess 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 wastewat er treatment process was provided. W. The annual costs associated with trea tment 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 Electro plating 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/C oCr material to be implemented. B. Affected C-130 components and the ar eas 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 la rge 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 aff ect 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 effl uents, and hazardous waste Record keeping and reporting re lated 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 employees207 Providing medical exams (same requirements regardless of location at facility).

I. The company is currently in complia nce 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 spra y equipment were evaluated: Diamond Jet 2600, Diamond Jet 2700, and TAFA JP5000. For each type of equipment, estimates ofeffects 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 averag e 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 averag e 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 appl ying 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 fo r setting up the part and spray pattern.



### 7.3.3. Assumptions About Curren t 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 Canadi an 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 ar e 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 ar ea 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 st eps 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 fo r diamond grinding wheels (the chrome plating process uses aluminum oxide wh eels). The diamond grinding wheels are expected to last longer than conventiona l grinding wheels, even when grinding the HVOF coatings. The additional useful lif etime of the diamond grinding wheels is expected to offset the higher purch ase 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 estimate d 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 wi ll 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 st ripping tanks for removing HVOF coatings was not included in capital costs. Th e annual material and utility costs for stripping WC/Co or WC/CoCr we re 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. Th is increase in landing gear lifetime is based on results of current ESTCP testi ng, published literatu re, and engineering judgement; some recent testing has shown wear resistance between 2.5 and 4 times as great as that of electroplated ch rome. 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, wast e disposal, and wo rker health and safety costs. U. An estimate of approximately 1,200 landi ng gear components processed per year was used for this analysis; this number will remain the same after HVOF implementation. The coating thickne ss 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 s ite issues, 12% of the total component surface area will continue to be elect roplated after HVOF implementation.

Benefits from decreased turn-around time (TAT ) of landing gear was al so included in this CBA. Because the company has reported no di fficulty in meeting required schedules for processing aircraft C-130 landing gear compone nts, 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 g ear components using the baseline hard chrome electroplating process. These data and assumptions were also used to estimate the annual operating costs for fabrica ting and overhauling C-130 landing gear components with HVOF. The annual operating co st avoidances reporte d in this section were derived from comparing the operat ing 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 averag e 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 durabi lity of WC/Co coatings applied by HVOF. However, Heroux did not want to include th ese 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 ope rating data to determine the cost avoidance that may result from transitioning from ha rd chrome electroplating to HVOF thermal Table 7-2. Estimated Annual Operating Cost Avoidance Annual Operating Cost Avoidance a

Labor $113,540 Materials $75,520
Utilities b $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 in 3 of the three components electroplated; after HVOF im plementation assumes 1500 in 3 HVOF coated and 200 in 3 electroplated.

b Average of equipment and material types211 spraying, quotes from shops were obtaine d for chrome plating and HVOF spraying the three affected components. Quotes for hard chrome plating the components were obtained from Southwest Aeroservices in Tu lsa, 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 pa rts 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 presen t 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. Fo r 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 perf ormance 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-$1,012212 A summary of the financial evaluation for im plementing HVOF to replace hard chrome electroplating of C-130 landing ge ar components is shown in Ta ble 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. Th e 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 calculat ed 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 be ing investigated as an alternative to hard chrome electroplating for fabricating a nd repairing/overhauling ai rcraft landing gear. A CBA was performed to identify the potenti al financial impact of implementing these HVOF coatings for application to C-130 aircra ft 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 implementa tion 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 chro me 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, great er 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 co atings due to increa sed 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 a nd 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 stri pping 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 s cenario 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 i ssues 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 Ma rch; Paul Willis; and Herbert Herman. Construction Productivity Advancement Rese arch (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. Engineeri ng 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 Corpor ation, Johnstown, PA. November 29, 1999. Environmental Cost Analysis Methodology (ECAM) Handbook. Environmental Security Technology Certifica tion 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; Pa ul 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), Guidelin es 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 corresponde nce between March and October 2001. Peter Longobardi. Praxair. Email and phone correspon dence March 2001. Richard Thorpe.217 

# 8. Specifications and Standards
In order to use the HVOF process in manufactur ing or repair it is e ssential for engineers to be able to refer to specifications fo r HVOF deposition and processing. To facilitate this HCAT has developed specifications fo r both HVOF materials and deposition on high strength steel and for grinding. Prior to the HCAT program only the Boei ng 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 fo r aerospace specifications is the Society of Automotive Engineers (SAE), who issue Aerospace Materials Specifications (AMS specs). Obtaining AMS specifications invol ves drafting the specifi cations and working closely with the SAE AMEC committees over a period of one to thr ee years to create a document that is satisfactory for all comm ittee 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 thei r status are shown in Table 8-1. The specifications developed in this program are reproduced below.
Table 8-1. SAE Aerospace Ma terial 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 Po wder 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 Deveraux218 

# 8.1. Proposed AMS 2448
APPLICATION OF TUNGSTE N 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 requir ements for applying tungsten carbide thermal spray coatings to ultra high strength steels (220 ksi and above) utilizing high velocity oxygen fuel (HVOF) combustion driven processe s and the properties for such coatings. The processes and procedures herein apply on ly to the properties of the as-deposited coating. 1.2 Application This process has been used ty pically to provide coatings of lower porosity and higher adhesive, and/or cohesive, strength than gene rally 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, a nd processes described or referenced in this specification may involve the use of hazar dous 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 a nd 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 th is specification to the extent specified herein. The latest issue of SAE publications sha ll apply. The applicable issue of other ublications shall be the issue in eff ect 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 Powder219 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 Rockwe ll 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 micropro cessor 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 utili zed 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 metallurgi cal processes such as heat treating, shot peening, or similar should be complete d 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 c ontamination on surfaces that are to be coated. 3.3.3 Masking: Parts shall be masked by an ap propriate means to protect all surfaces that are220 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 desi red 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 prepar ed and handled in the same manner as the area on which coating is required a nd 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 mechani cal 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 appli cation 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 ro tating components shall be sprayed at the same rotational speed and increm ental step rate as the component. If there are multiple application angles on the same component , each of the deposition angles shall be221 evaluated with coupons for compliance w ith minimum mechanical property limits. Coupons shall be evaluated prior to coa ting application on production components.

3.4.6 Specimen Material: Bond strength speci mens shall be manufactured from AMS 6484, heat-treated to produce a hardness Rockwe ll C 40 minimum and no higher than the hardness of the component substrate. Meta llographic 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 ca n be made from the same material, in the same condition as the component. 3.5 Surface Finishing: Procedures for fini shing shall be in accordance with purchaser’s specifications. 3.6 Properties
3.6.1 Adhesion 3.6.1.1 Bend Test: Specimens prepared and t ested in accordance with 3.7.1 shall not show separation of the coating from the substr ate, when examined visually without magnification. Cracking of the coating a nd 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 HV 300g 950 minimum. 3.6.3 Microstructure: A detailed standard pro cedure for metallographic specimen preparation and examination shall be used to ensure cons istent results as in 4.4.1. Examination of a suitably prepared cross-sectioned specimen sha ll 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) le ngth) when examined at 400x magnification on the cross sectioned specimen. Any single voi d 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 coati ng / 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 th an 0.002 inch in length following directly along the bond line. Any length less than th is 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 Stress222 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 f ashion 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 co ating application (second reading). The first reading and subsequent coating application sh all 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 de position. The measured change in deflection of the Almen strip resulting from the applica tion of the coating shall be reported as the difference of the two readings, second reading mi nus first reading, and is indicative of the desired compressive coating stresses if the si gn of the difference is positive. Acceptable values for Almen type “N” strip arc height s 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 in ches +/- 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 appr oximately ten degrees per second. Panels shall be bent to obtain a 90-degree permanent set. 3.7.2 Bond Strength: Test specimens, approxima tely 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 pr epared 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 mini mum 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 fro m spalling, chipping, flaking, and other imperfections detrimental to usage of the coating.
3.9 Tolerances: Unless otherwise specifie d 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 simila r 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 Removal223 3.12.1 Equipment 3.12.1.1 Tank: A container of adequate volum e 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 u se 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: Indi cating 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 th eir 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 104 o – 150 o F (140 o – 150 o Optimum) (40 o – 65 0 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 Solution224 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 120 o – 150 o F (50 o – 65 0 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 co rrected when the following has occurred: a) A non-removable smut is formed or fore ign 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 a pply 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. Remo val rate should be approximately 2-6 mils of coating per hour. Step 4 – When coating is adequately strippe d, 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, o il free air and apply any necessary in process corrosion preventative compound to prevent corro sion 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 sm ut. Removal rate should be225 approximately 1 mil of coating per hour. Step 3 – When coating is adequately stri pped, 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. Ther efore, 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 Toleranc es (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 requi rements 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: Samp le 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 pr ocess description of prep aration, application, and inspection for each part number. The d escription shall include control factors and parameters that provide coated parts meeting specified requirements. Control factors considered proprietary by the processor sh all be assigned codes within the process description. The processor shall maintain a co mplete record of proprietary factors and codes. These factors shall include sufficien t 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, shall226 be approved by the cognizant engineering orga nization 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 contro ls 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 accordan ce 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, quan tity 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 with in the same lot. Ex cept 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 speci fied 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 alternat e method acceptable to the purchaser. The parts may then be re-coated and re-tested. Whiche ver 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 nonc onforming 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 no tified 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 pack aged to ensure that the required physical characteristics and properties of the coating are preserved. 5.2 Packages of parts shall be prepared fo r shipment in accordance with commercial practice and in compliance with applicable rules a nd regulations pertaining to the handling, packaging, and transportation of the part s to ensure carrier acceptance and safe delivery.

6.0 ACKNOWLEDGMENT 6.1 Processor shall mention this specification numbe r 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/pou nd 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 sp ecify 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 Restoration228 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 & A CCESSORY 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 JURISDIC TION 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 car bide-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 th is 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 Stre et, 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 18.5231 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 sha ll 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 capab le 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 t horoughly blended, uniform in color and quality, dry, free flowing and free from fore ign 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 fo r vendor’s tests and shall be responsible for performance of all required tests. Purchaser re serves 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 acceptanc e 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 re quires approval by the cognizant engineering organization (see 4.4.2), and when purchaser de ems confirmatory testing to be required. 4.2.1 For direct U.S. Military procurement, s ubstantiating 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 pr eproduction sample, or both, whichever is specified, shall be approved by the cognizan t engineering organization before production powder is supplied. 4.4.2 The supplier shall make no significant cha nge in ingredients, processes, or controls from those on which the approval was based, unl ess the change is approved by the cognizant engineering organization. A significant cha nge 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 par ticle 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 cont ainers 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 legi bly identified w ith 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 acco rdance with commercial practice and in compliance with applicable rules and regulations pertaining to the handling, packaging, and transportation of th e 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 numbe r 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 b een 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 specification234 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 sp ecification has been classified under Federal Standardization Area Symbol “MFFP” 8.5 Key Words: Thermal spray coatings, wear resistant coatings235 

# 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 Stre et, 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 Weight236 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 the237 requirements of AMEC 00AB. Purchaser and vendor shall agree upon standards for acceptance. 3.4 Quality: Powder, as received by purchaser, shall be t horoughly blended, uniform in color and quality, dry, free flowing and free from fore ign 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 test s. Purchaser reserves the right to sample and to perform any confirmatory testing d eemed necessary to ensure that the powder conforms to the requirements of this specification.
4.2 Classification of Tests: Tests for all technical requirements are accep tance tests and pre-production tests and shall be performed prior to or on the initial sh ipment 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, subs tantiating 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 pr eproduction sample, or both, whichever is specified, shall be approved by the cognizan t 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 cha nge 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 cont ainers 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 legi bly identified w ith 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 acco rdance with commercial practice and in compliance with applicable rules and regulations pertaining to the handling, packaging, and transportation of th e 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 numbe r 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 technical239 revisions, not editorial changes, have b een 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 coatings240 

# 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 requir ements 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 ty pically 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 limite d to such applications. This process specification does not cover superfini shing of HVOF applied coatings. 1.3 Safety - Hazardous Materials While the materials, methods, applications, a nd processes described or referenced in this specification may involve the use of hazar dous 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 a nd 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 th is specification to the extent specified herein. The latest issue of SAE publications sha ll 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 19428-2959241 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 degradati on of the part. Provisions shall be made to supply a constant applicati on 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 specifie d, 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 gr inding 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 othe r 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 pa rts. All speeds, feeds, and stock removal parameters are actual and not necessarily machine or indicator readings. Prior to grinding, clean all surfaces to be gro und 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 shou ld be re-balanced at the next mounting. True the wheel face so that it is geometri cally correct for the application and runs concentric with the centerline of the ar bor 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 eac h 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 traver se 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 feed s shall not exceed 0.080 inch per pass.
3.5.2.5 For surface grinding, the roughing infeed (d epth 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 (dep th 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 ra nges 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 surface s 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 whic h 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 fo r heat checks and pull-outs visually using up to 40X magnification. Penetrant inspection, in acco rdance with ASTM E 1417, shall be used to check for cracking caused by, or rev ealed 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 contro l factors, a ground part, or both, whichever is specified, shall be appr oved by the cognizant engineer ing organization before production finish ground parts are supplied.

4.5 The processor of parts ground in accordan ce 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 Figur e 1) unless the change is approved by the244 cognizant engineering organization. A signi ficant 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 no tified 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 prope rties of the ground coating are preserved.

5.2 Packages of parts shall be prepared fo r shipment in accordance with commercial practice and in compliance with applicable rules a nd 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 numbe r 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 sp ecify 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 rotating245 assembly; grinding wheel, mounting arbor, and the machine spindle. Depending on the balancing machine, th e 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 mater ial 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 cm 3 (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 cont act 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 aw ay 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 e xpose the grain. This is accomplished by using an aluminum oxide or silicone car bide 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 determin ed 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 wh ich corresponds to the U.S. Standard wire mesh size screen used for si zing 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 su rface 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 th e 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-b onded 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, Corro sion 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 R a SPECIAL MACHINING INSTRUCTIONS: Check wheel speed before starting._____________________________ _Verify roughing traverse rate (xxx Seconds/xxx inches length of roug hing traverse) prior to grinding.

____________ Verify finishing traverse rate (xxx Seconds/xxx inches le ngth 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 t ypically 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 dependi ng on the size of the O.D. grinding machine. The conventional abrasive wheel mounted on th is arbor/flange fixtur e 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 wheel s, the diamond wheel that needs truing and the conventional abrasive truing wheel, the prefer red 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 dire ction of rotation of the workpiece). 4. The diamond wheel should be running at sp indle speed. Usually on O.D. grinders this is about 5600 SFPM (28 m/s). 5. Adjust the conventional abrasive wheel s mounted in the work position to 1500 SFPM (7 m/s) or about 1/3 the surf ace footage of the diamond wheel.
6. Traverse the diamond wheel across the conve ntional abrasive wheel s 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 wh eels face to sharpen the wheel for grinding. The recommended dressing stick specification is 38A (one or two grit sizes finer than the249 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 fl uid should be equal to or slig htly greater th an peripheral velocity of the grinding wheel. An exampl e 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 su rface 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. Therefor e, for a one inch wide wheel, select nozzle width of 1.25”.

Area (of nozzle opening) = 1.25 x d in 2

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 in 3 /U.S. gallon = (1.25 x d in 2 ) (72,000 in/min) 1 1⁄4” wide 

### d251 11,550 in 3 /min = 90,000 x d in 3 /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 st akeholder community from the outset and identify important ar eas of concern. Contributions from program offices, system support offices, de pot engineers, and OEMs were made toward development of the JTP and all results, positive and negative, were presented to them for evaluation and c onsideration. 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 f acilities to acquire data, develop specifications, and carry out other tasks cr itical to the success of the overall project. Although commercial competit ors were involved in the team, they exhibited a remarkable degree of coop eration in developing a common technology base.

. Data acquisition – The whole team has been kept aware of all significant data through biannual program review meetings a nd 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 th at 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. E quipment was installed by the program in several depots and team experts from i ndustry 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 existen ce of standards and specifications to which engineers could refer. For this reason the team assigned specific members to develop specifications and shephe rd 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 c oncluded that on high-stre ngth steels that are used to fabricate landing gear components, the fatigue, wear and impact-resistant properties of the HVOF WC/17Co and WC/10Co4C r 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 perf ormance of WC/17Co coa tings in service on the P3 aircraft, lead to the conclusion that the HVOF coati ngs should perform at least as well as EHC coatings. There are no hydrogen embrittlement issues associated with HVOF coatings and the testing i ndicated that re-embrittlement is less of an issue with HVOF coatings than with EHC. All of the ri g and flight testing performed to date has indicated excellent performa nce of the WC/17Co and WC/10Co4Cr coatings. This is true even of components in s ituations where materials testi ng had raised coating integrity and delamination concerns. The one issue that still can impact implemen tation of the HVOF coatings on landing gear components is delamination at high stresses a nd 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 s hown 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 stre ss 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 land ing 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-rever sed 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 implem entation 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-c oated landing gear inner cylinder it will rapidly degrade the seal and cause excessive l eakage. 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 su rface over time. It has been found that it is no longer sufficient simply to specify an av erage roughness (Ra), or a root mean square roughness (RMS), surface finish, as is tr aditionally done with EHC. For many applications users are specifying more deta iled surface parameters that can be achieved with superfinishing equipment, such as bear ing 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 “lifetim e coating” that will not need to be reworked for the life of the component (or at least for many maintena nce cycles). However, this requires a255 recognition that the maintenance methods comm only used for chrome plated parts can no longer apply. For example, many plated pa rts are assumed always to deteriorate in service and are therefore always stripped and the substrate examined by NDI (magnetic particle, MPI, or fluorescent penetrant inspectio n, 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 sin ce 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 end eavoring to develop better through-coating inspection procedures.
The corrosion data is another area in whic h HVOF coatings perform in a different way from EHC coatings. EHC-plated parts tend to corrode by liquid penetration followed by substrate corrosion and underc utting 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, expe rience corrosion of the matrix alloy. This could lead to gradual dissolution and roughening of the surf ace, leading to seal damage and a gradual increase of seal leakage. Neither of thes e failure mechanisms is necessarily always “better” than the other, but the differe nce needs to be rec ognized in planning implementation and maintenance.


# 9.3. Implementation The Air Force is proceeding with implemen tation of HVOF coatings on landing gear components at Ogden ALC, the primary overhau l 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 th e 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. Thes e spray booths would be used for processing more than just landing gear components as mo st 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 manipul ation of various type s 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 strips256 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 la nding gear Goodrich has specified HVOF WC/CoCr as the baseline coati ng 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 landi ng gear hydraulic actuators. As a result of the program Messier-Dowty ha s 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 carri er-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 procedur es for the coatings, and grinding procedures for the coati ngs. The HCAT has worked with the SAE Aerospace Metals Engineering Committee to devel op four separate specifications in these areas (see Section 8). Those related to pow der 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 Carb ide 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 sp ecifications can now be utilized by any manufacturing or overhaul depot and their use wi ll 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.257 . 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 ope rating cost savings at a major commercial landing gear overhaul facility for replacing 88 % of their chrome pl ating 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 E HC and HVOF would be different at other facilities based on labor rates, types of equipment used a nd other factors, it is still anticipated that there would be substantial sa vings by replacing as mu ch of their chrome plating operations as possible with HVOF. Additional cost savings that would be realized are associated with the estimate d 50-100% extension of service life for the WC/Co or WC/CoCr coatings over EHC coa tings. In the future, as HVOF-coated landing gear components come into the fac ility 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 cost s 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 desc ribed 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 th e HVOF coating operations to a job shop. The results showed that it would be less co stly 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 repa ir shop to process landing gear components.


# 9.6. Regulatory Compliance and Acceptance The principal environmental and worker safe ty 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 se rious regulatory issu es 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 fi lter system. The deposition is carried out in a well-controlled manner using robotic sy stems 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 spray258 equipment in operation, such as flame or plasma spray, and therefore they had the appropriate air handling equipment (e.g., e xhaust hoods, bag houses) available and also had the appropriate air permits to co ver operation of the HVOF systems.