HVOF Application of Nickel and Nickel Alloy to Tungsten 

Heavy Alloy for Jacketed Penetrators 

by John V. Kelley and Russell Kilbane 

ARL-TR-3095 November 2003 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3095 November 2003 

HVOF Application of Nickel and Nickel Alloy to Tungsten 

Heavy Alloy for Jacketed Penetrators 

John V. Kelley 

Weapons and Materials Research Directorate, ARL 

Russell Kilbane 

Independent Contractor 

Approved for public release; distribution is unlimited. 
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1. REPORT DATE (DD-MM-YYYY)

November 2003 

2. REPORT TYPE 

Final 

3. DATES COVERED (From - To) 

January–December 2000 

5a. CONTRACT NUMBER 

DAAD17-01-P-0066 

5b. GRANT NUMBER 

4. TITLE AND SUBTITLE 

HVOF Application of Nickel and Nickel Alloy to Tungsten Heavy Alloy for 

Jacketed Penetrators 

5c. PROGRAM ELEMENT NUMBER 

5d. PROJECT NUMBER 

18M43A 

5e. TASK NUMBER 

6. AUTHOR(S) 

John V. Kelley and Russell Kilbane* 

5f. WORK UNIT NUMBER 

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U.S. Army Research Laboratory 

ATTN: AMSRL-WM-MC 

Aberdeen Proving Ground, MD 21005-5069 

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ARL-TR-3095 

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Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES 

*Independent contractor. 

14. ABSTRACT 

In recent years, there has been an increased desire to replace depleted uranium (DU) anti-armor penetrators with tungsten heavy

alloy (WHA) penetrators. However, the ballistic performance of WHA does not compare with that of DU. Many methods of 

improving the ballistic properties of tungsten have been explored. One recent method includes jacketing a long thin core of 

WHA with a more ductile metal. This report examines the use of thermal-sprayed, high-velocity oxy-fuel coatings to apply the 

jacket material. However, applying thermal-sprayed coatings onto a tungsten substrate is a challenge. The differences in the 

coefficients of thermal expansion (CTE) make it difficult to achieve adequate adhesion of most coatings to tungsten. Further 

compounding the problem are residual stresses inherent in thermally sprayed coatings. The thicker the coating desired, the more 

likely disbonding will occur upon cooling of the substrate. Early attempts to apply an Ni coating (0.010–0.020 in thick) to a 

tungsten substrate yielded immediate disbonding and/or cracking of the coatings. The work presented is an investigation 

focusing on modifications to spray parameters and the use of multiple thin layers to minimize heat transfer and achieve better 

adhesion. The use of a material with a CTE nearer to that of tungsten as a bond coat and grading of coating materials was also

examined as a method for increasing the overall adhesion of the coating system. 

15. SUBJECT TERMS

outdoor exposure, marine corrosion, nonchromate, chromate, aluminum armor 

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John Kelley 

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iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental Procedure 3

2.1 Specimen Preparation......................................................................................................3

2.2 Coating Process ...............................................................................................................3 

2.3 Adhesion Testing.............................................................................................................5 

2.4 Visual Examination .........................................................................................................7 

3. Results and Discussion 7

4. Summary, Conclusions, and Recommendations 11

5. References 12

Distribution List 13 
iv

List of Figures 

Figure 1. Ni-jacketed tungsten penetrator.......................................................................................1 

Figure 2. Schematic of Sultzer Metco Diamond Jet (DJ) HVOF gun. ...........................................2 

Figure 3. The two sizes of cylindrical (subsized penetrator) samples. ...........................................4 

Figure 4. HVOF unit shown includes 9MP closed loop powder feeder. ........................................4 

Figure 5. Schematic of some intermediate layer combinations used..............................................6 

Figure 6. Schematic of the pull-off adhesion test. ..........................................................................6 

Figure 7. Radial cracking of an Ni-coated tungsten core penetrator using the continuous 

spray method..............................................................................................................................8 

Figure 8. Comparison of Ni coating on two different substrates using identical parameters.........8 

Figure 9. Cross section of specimen W2. .......................................................................................8 

Figure 10. The residual compressive stress is the result of coldworking done by the impact 

of subsequent “solid” particles at high velocity.........................................................................9 

Figure 11. The peened Ni splats of specimen W2. .........................................................................9 

Figure 12. Bond strength for HVOF-coated tungsten samples. Arrow for chrome coating 

indicates >3500 psi. .................................................................................................................10 

List of Tables 

Table 1. Coefficients of thermal expansion of materials used in this experiment..........................3 

Table 2. HVOF spray parameters using hydrogen as fuel gas........................................................5 

Table 3. Laboratory conditions for pull-off adhesion ASTM D 4541............................................6 
1

1. Introduction 

Since the end of the Gulf War, the North Atlantic Treaty Organization and the United States 

have been seeking alternatives to the use of both depleted uranium (DU) and lead munitions. 

One of the most viable alternatives to these materials is tungsten. Tungsten has been used in 

munitions as far back as the 1940s when Germany used it in WWII. Despite this long history, 

tungsten’s ballistic performance remains inferior to that of DU when used in armor-piercing 

applications. The unique adiabatic shear properties of DU provide a “self-sharpening” affect 

when penetrating passive armor systems. Tungsten, while similar to DU in density, tends to 

“mushroom” on impact. This behavior limits its ability to penetrate armor effectively. One way 

researchers are trying to enhance the ballistic performance of tungsten is by jacketing or coating 

a tungsten core with nickel (Ni) and other alloys (Figure 1). 

Figure 1. Ni-jacketed tungsten penetrator. 

A detailed explanation of how a Ni jacket enhances the ballistic properties of tungsten is beyond 

the scope of this report. However, one theory is that a thinner tungsten core with a lighter jacket 

material (1) may be accelerated to higher velocities than a solid tungsten penetrator, 

(2) concentrates the lethal mass of the penetrator into a smaller impact area, and (3) improves inflight stability of the round. Therefore, research into efficient and cost-effective methods of 

coating tungsten is being conducted. 

Several methods for applying a jacket to tungsten have been employed. Plating, cladding, and 

various welding techniques have been used with some success. Thermal spray coatings offer yet 

another alternative. The process of thermal spraying involves the transport of a coating material 

to a substrate at high velocities and elevated temperatures. Heated particles (in many cases, 

molten droplets) of coating materials are accelerated at high speeds and are made to impact a 

substrate. Successive particles then impact the surface where the high velocity causes the 

particles to deform and create a mechanical bond with the underlying surface. A coating forms 

by the layering of individual flattened particles on top of each other. The flattened particles form 

around surface features and solidify. The coatings often have fine microstructure due to rapid 

 

W

Ni
2

solidification of the particles. This gives the coatings unique mechanical properties. The bond 

strength of the droplets can depend upon the coating and substrate material, and the process 

parameters used to deliver the spray. When a coating material does not have the ability to make 

a chemical bond with the substrate, it is necessary to roughen the substrate surface in order to 

enhance the coating’s mechanical bond. 

One thermal spray technique is the high-velocity oxy-fuel (HVOF) system. This system uses an 

oxy fuel mixture of either hydrogen, propane, or propylene. A powdered coating material is fed 

through the back of the gun into an ignited gas mixture. The mixture heats the atomized powder 

and deposits it on a substrate at particle velocities approaching 3000 ft/s (gas velocities reach 

7000 ft/s), and flame temperatures reach 5000 °F (Figure 2). The system’s practical use is 

limited to metal and some ceramics coatings such as aluminum oxide. The HVOF process was 

chosen for this experiment because of its ability to apply higher density coatings (very low 

porosity <2%), with higher bond strength, and reduced residual stresses for coatings of greater 

thickness (1, 2).

Figure 2. Schematic of Sultzer Metco Diamond Jet*

(DJ) HVOF gun. 

Using tungsten as a substrate for thermal spray coating presents two challenges. First, tungsten 

is difficult to abrade, often resulting in profiles of less than the recommended 1 mil. However, 

the best that could be achieved on the tungsten samples presented in this report was an average 

surface roughness (RA ) of 9.57 μin. The low profile surface roughness makes it difficult to 

achieve a good mechanical bond. Second, tungsten is a refractory metal with a very high melting 

point and low coefficient of thermal expansion (CTE) relative to the coating material. The 

differences between the CTE of the materials coupled with the elevated temperatures causes 

significant thermal stresses at the interface. Table 1 shows the CTE of the materials used in this 

experiment and equation 1 shows a relationship between CTE and the residual thermal stress (3). 

 *

Sulzer Metco Diamond Jet is a registered trademark of Sulzer Metco Inc., 1101 Prospect Ave., Westbury, NY 11590. 
3

Table 1. Coefficients of thermal expansion of materials 

used in this experiment. 

Material CTE 

(in/in °C) 

Tungsten 4.6 

Nickel 13.3 

Chromium 6.2 

Ni

Ni W

W Ni W

Ni E

E E

E T ] * ( ) [

+

∆ − = α α

σ , (1) 

where α is the coefficient of thermal expansion, and E is the modulus of elasticity. From 

equation 1, the residual stress is directly proportional to the change in temperature (∆T). 

Finally, a literature search*

 revealed little in the way of coatings on tungsten alloys, and none 

were found that address thermal spray coatings. Therefore, the purpose of this experiment is to 

investigate ways to reduce the residual stress in the HVOF coating and thereby eliminate 

cracking of the coatings and maintain good adhesion to the substrate. 

2. Experimental Procedure 

2.1 Specimen Preparation 

Two specimen types were used in the experiment: a flat specimen, and a cylindrical specimen 

(Figure 3). The flat specimen was necessary to conduct pull off adhesion tests on the HVOF 

coatings. Because the initial objective of this effort was to examine the feasibility of using the 

HVOF coatings for jacketed penetrators, subsized tungsten and steel rods were coated and 

metallographically examined and compared. The two coating materials were Metco powders: 

Amdry Pure Chrome #05063, and pure Ni #56F-NS. Prior to the application of the coatings, 

tungsten substrate was grit blasted using a #20 grit aluminum oxide medium with the nozzle 

positioned ~90° to the sample surface. The samples were cleaned in acetone using an ultrasonic 

cleaner, then rinsed with deionized water, and immediately dried under a hot air blower. The 

final surface profile was then measured using a Rank Taylor Hobson Form Talysurf Series 

profilometer. 

2.2 Coating Process 

The HVOF system used in the experiment was a Sulzer Metco DJ. This system includes a robotmounted DJ gun, 9MP powder feeder, and DJ control unit. Figure 4 is a picture of the system 

used in this experiment. The spray parameters for each sample are given in Table 2. 

 *

Authors searched ARL Desktop Services/Technical Library Web of Knowledge and executed a general Internet search on 

Google. 
4

Figure 3. The two sizes of cylindrical (subsized penetrator) samples. 

Figure 4. HVOF unit shown includes 9MP closed loop powder 

feeder. 

All cylindrical specimens were sprayed while rotating at ~60 rpm. The flat specimens were 

secured in a vise with surface normal to the DJ gun nozzle. The cooling jets were positioned 

24 in from the specimen, blowing on the front and rear surfaces. Hydrogen was used as the fuel 

gas for all specimens. All specimens were labeled and identified as shown in Table 2. The spray 

parameters were controlled using a Sulzer Metco DJ control DJ Automatic Control System and 

the respective spray parameters for each specimen are also listed in Table 2. 

0.250 

inches

1.250 

inches

(A) Flat Adhesion Specimen

0.1735 

inches

6.3125 inches

0.3125 

inches

3.375 inches

(B) Geometry A (C) Geometry B 
5

Table 2. HVOF spray parameters using hydrogen as fuel gas. 

HVOF Coatings Spray Parameters 

Specimen ID # S1 W1 W2 W3 W4 W5 W6 W7 

Geometry CylA CylA CylB Flat Flat Flat Flat Flat 

Substrate Steel W W W W W W W 

Powder type Ni Ni Ni Ni Cr Cr/Ni Mixed Graded 

Hydrogen flow rate (FMR) 62 62 62 62 55 55 55 55/62 

Hydrogen pressure (psig) 140 140 125 125 125 125 125 125 

Oxygen flow rate (FMR) 32 32 32 32 42 42 42 42/32 

Oxygen pressure (psig) 170 170 170 170 170 170 170 170 

Air flow rate (FMR) 44 44 44 44 40 40 40 40/44 

Air pressure (psig) 100 105 105 105 105 105 105 105 

Carrier flow rate (FMR) 55 45 45 45 45 45 45 45 

Powder feed rate (lb/hr) 12 12 10 10 6 11 11 11 

Traverse speed (mm/s) 600 600 100 100 800 600 600 600 

Spray distance (in) 10 10 10 10 12 12 12 10–12 

Preheat passes 2 2 0 0 0 1 1 1 

In Table 2, the coatings for specimen S1 and W1 were applied using several consecutive spray 

passes. No cooling of the specimen was allowed between each layer. The coatings on samples 

W2–W7 were applied in a slightly different manor. An ~0.005-in layer was applied and allowed 

to cool to room temperature. The process was continued until the final coating thickness was 

achieved. 

Material modifications were also made in an attempt to increase the adhesion strength of the 

coatings. Intermediate layers with CTEs closer to that of the tungsten were introduced at the 

interface. These layers were intended to act as bond coats for the Ni. 

Three bond coat configurations were examined: (1) chromium (Cr), (2) 50-50 volume % of Crand Ni-mixed powder, and (3) a “graded” coating system consisting of a chrome underlayer, a 

50-50 volume % of Cr- and Ni-mixed powder and an Ni top layer. Figure 5 illustrates the 

intermediate layer configurations. 

2.3 Adhesion Testing 

Pull-off adhesion tests were performed using a hydraulic adhesion test apparatus (HATE) Mark 

VII coating adhesion tester (Figure 6) in accordance with American Society for Testing and 

Materials (ASTM) D 4541 (4). A loading fixture commonly referred to as a “dolly” was secured 

normal to the coating surface using a cyanoacrylate adhesive. After allowing the adhesive to 

cure for 24 hr in laboratory conditions (Table 3), the attached dolly was inserted into the test 

apparatus. The load applied by the apparatus was gradually increased and monitored on the 

gauge until a plug of coating was detached. The failure value in lb/in2

 (psi) was recorded and the 

failure mode characterized. 
6

Figure 5. Schematic of some intermediate layer combinations used. 

Figure 6. Schematic of the pull-off adhesion test. 

Table 3. Laboratory conditions for pull-off adhesion 

ASTM D 4541. 

Adhesive Type Cyanoacrylate 

Cure time (hr) 24 

Temperature (°C) 25 

Relative humidity (%) 31 

Substrate materials Tungsten heavy alloy 

Coating type HVOF 
7

Note: For this test to be accurate, the substrates must be of sufficient thickness to ensure that the 

coaxial load applied during the removal stage does not distort the substrate material and cause a 

bulging or a “trampolining effect.” On a thin specimen, the resultant bulge causes the coating to 

radially peel away outwards from the center instead of uniformly pulling away in pure tension 

and thus results in significantly lower readings. All of the samples substrates used in this study 

were made from ~0.250-in-thick tungsten and were of adequate thickness and stiffness for this 

pull-off test procedure (5). 

2.4 Visual Examination 

Throughout the spraying process all specimens were visually inspected after each layer was 

allowed to cool to determine if any cracking or debonding of the coating occurred. Visual 

examinations were also performed following the pull-off adhesion tests. The mode of failure for 

each coating was observed and recorded. 

3. Results and Discussion 

Initial coating attempts using uninterrupted deposition on flat tungsten substrates resulted in 

catastrophic failures. Because of the differences in CTE, the increased temperature and 

subsequent cooling of the sample to room temperature created a shear stress at the 

coating/substrate interface. As the sample cooled, the shear stress increased until it exceeded the 

coating bond strength, and the coating “popped” off. 

Similar attempts to spray cylindrical tungsten samples resulted in cracked coating upon cooling. 

Steel and tungsten penetrator cores were sprayed with Ni. These specimens were cross-sectioned 

and compared under an optical microscope. Figure 7 shows the radial cracking that formed in 

the Ni jacket surrounding the tungsten core. In Figure 8, a closer look at the interface shows the 

presence of gaps and voids between the Ni coating and tungsten substrate. However, when a 

steel cylindrical substrate with a CTE that is closer to Ni is coated using identical process 

parameters, there is no cracking, and an excellent bond is achieved (Figure 8b). It was evident 

that the CTE disparity is a factor and some process parameters needed to be adjusted. 

In order to reduce the maximum ∆T during spraying, the coatings were applied using several thin 

layers. Each layer was allowed to cool to room temperature before the next layer was applied. 

Subsequent layers were built up until the final coating thickness was achieved. All specimens 

produced using multiple interrupted layers were examined under 100× magnification for cracks 

or separation at the interface. In all cases, there was no significant cracking observed. Figure 9 

shows the cross section of specimen W2 (Ni on tungsten), which was produced using multiple 

interrupted layers. 
8

Figure 7. Radial cracking of an Ni-coated 

tungsten core penetrator using the 

continuous spray method. 

Figure 8. Comparison of Ni coating on two different substrates using identical parameters. 

Figure 9. Cross section of specimen W2. 

(a) Nickel on Tungsten Rod (b) Nickel on Steel Rod 
9

When the coating is allowed to cool to room temperature between layers, the HVOF coating 

maintains more of its compressive residual stress. The coldworking (peening) of the particles by 

the impact of subsequent particles causes the residual compressive stress, which somewhat 

offsets the tensile stresses induced by the different CTE (Figure 10). The peening effect can be 

seen by the more rounded droplet splats of the coating in Figure 11. 

Figure 10. The residual compressive stress is the 

result of coldworking done by the 

impact of subsequent “solid” particles at 

high velocity.*

Figure 11. The peened Ni splats of specimen W2. 

 *

Illustration courtesy of Sulzer Metco Inc., 1101 Prospect Ave., Westbury, NY 11590; www.sulzer.com. 

“Peened” Ni splats 

at interface. 
10

Conversely, the coatings applied without a cooling period between layers show a more 

elongated, flatter splat. This is most likely because the coating and substrate are under the flame 

and at higher temperature for a longer time. The splats become further softened by the elevated 

temperatures and flattened by the impact of subsequent layers. The initial residual compressive 

stress caused by the peening is relieved. Thus upon cooling, the residual tensile stress increases. 

The greater the ∆T of the substrate and coating “system,” the greater the residual tensile stress 

will be. The two tensile stresses are coupled together, and when they exceed either the adhesion 

strength of the coating or the tensile strength of the coating material, failure occurs. 

As mentioned earlier (Figure 5), three bond coat configurations were examined. Figure 12 

shows the results of the HATE adhesion tests of these coatings. Here it can be seen that the Cr 

(chrome) coating achieved the highest bond strength at >3500 psi. All specimens with a Cr 

“bond” coat had better adhesion strength than specimens with Ni applied directly to the tungsten 

substrate. The reason for this is that the CTE of the Cr is closer to that of the tungsten than any 

of the other coatings. This resulted in less shear stresses developing at the interface and lower 

tensile stresses in the coating material. 

Figure 12. Bond strength for HVOF-coated tungsten samples. Arrow 

for chrome coating indicates >3500 psi. 
11

4. Summary, Conclusions, and Recommendations 

The larger the CTE mismatch between substrate and coating, the greater the problem with 

residual stress and thus the more difficulty obtaining good adhesion. The cumulative residual 

stress is reduced when the ∆T of the coating and substrate during the application process is 

reduced. This was achieved by interrupting the coating process to allow for cooling between 

layers. The building up of thinner coating layers to reach the desired thickness is an effective 

way of minimizing ∆T. The addition of a bond coat can enhance coating adhesion. However, in 

this case, an intermediate layer of Cr only slightly increases the overall adhesion strength of the 

coating. 
12

5. References 

1. Compressive Stresses in JP-5000 Coatings; TAFA Technical Bulletin 22.1.0.3; Hobart Tafa 

Technologies Inc.: Concord, NH. 

2. Lau, M. L.; Jiang, H. G.; Nuchter, W.; Lavernia, E. J. Thermal Spraying of Nanocrystalline 

Ni Coatings. Solidi Status Physica 1998, 257–268.

3. Flinn, T.; Trojan, P. Engineering Materials and Their Applications, 4th Edition; Houghton 

Mifflin: NY, 1990; pp 141–148. 

4. ASTM D 4541. Standard Test Method for Pull-Off Strength of Coated Specimens Subjected 

to Corrosive Environments. Annu. Book ASTM Stand. 1989. 

5. Kelley, J. V.; Placzankis, B. E. Outdoor Exposure Results for Pretreated and Topcoated 

Aluminum Armor Alloys 2519, 5083, and 7039. Proceedings of the 2001 Triservice 

Corrosion Conference, San Antonio, TX, January 2001.
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