HVOF sprayed WC-Co-Cr as a generic coating type for replacement of hard chrome plating

C. Reignier, A. Sturgeon/Cambridge UK, D. Lee/Indiana USA, D. De Wet, Swindon/UK

Paper 99 presented at ITSC 2002 International Thermal Spray Conference, 4-6 March 2002, Essen, Germany

HVOF sprayed cermets are increasingly considered as an alternative to electrolytic hard chrome plating for several application types, particular those experiencing severe wear combined with corrosion. WC-10Co-4Cr is one candidate coating and can be deposited using several variants of the HVOF spraying process. This work has considered HVOF sprayed WC-10Co-4Cr as a generic coating type for replacing hard chrome. Coatings were prepared using the Jet-Kote ^®, Diamond Jet ^® and JP5000 ^® HVOF systems. With all three systems dense, low porosity coatings were prepared with good coating adhesion. The friction, wear and corrosion performances of these HVOF coatings were compared against the performance of a commercial hard chrome coating system. The WC-10Co-4Cr coatings displayed similar values for friction coefficient to the hard chrome plating, while exhibiting better resistance to abrasive wear. In addition, salt spray tests confirmed that the HVOF coatings at least matched the corrosion resistance of hard chrome plating over a period of 240 hours. These results also demonstrated that similar performance was obtained with all three HVOF systems, indicating that HVOF sprayed WC-10Cr-4Co as a generic coating type can be considered as an alternative to hard chrome.

1 Introduction

Environmental issues ^[1] over treatment of the toxic hexavalent chrome produced during the process are driving research for candidate coatings to replace hard chrome plating. ^[2,3] Over the past decade cermet coatings sprayed by the high velocity oxyfuel process (HVOF) have been increasingly considered as alternative to electrolytic hard chrome plating for several applications and in particular those experiencing severe wear combined with corrosion. ^[4] HVOF sprayed cermet coatings are dense, have high bond strength and retain a high level of carbide. HVOF sprayed cermets like WC-Co, WC-10Co-4Cr, Cr ₃C ₂-NiCr, WC-NiCr and WC-Ni are being investigated to replace hard chrome plating for applications that include ball valves, corrugating rolls, printing rolls ^[3] and aircraft landing gear. ^[5]

This work has considered HVOF sprayed WC-10Co-4Cr as a generic coating type for replacing hard chrome plating. WC-10Co-4Cr coating has been deposited using three variants of the HVOF spraying process ^®, Diamond Jet ^® and JP5000 ^® systems. The HVOF sprayed WC-10Co-4Cr coating needs to demonstrate adequate corrosion resistance, adhesion and wear performance, ^[6] to be considered as a promising alternative to hard chrome plating. In this work, the friction, wear, adhesion and corrosion performance of the HVOF WC-10Co-4Cr coatings were compared against the performance of a commercial hard chrome coating system. Effect on the coating adhesion of corrosion through the coating and penetrating along the interface at the coating edge was also evaluated.

2 Coating preparation

WC-10Co-4Cr coatings were prepared onto low carbon (0.14%) steel using three commercially available HVOF systems, ^[7] the Jet Kote ^® IIA system (Deloro Stellite), the Diamond Jet ^® 2600 (Sulzer Metco) and the JP5000 ^® (Praxair Surface Technologies Inc). Powder type and spraying conditions recommended by the gun manufacturer were used to prepare the WC-10Co-4Cr coatings with each of the HVOF system. Nominal coating thickness was 350µm. Powder details are reported in Table 1 and spraying conditions are given in Table 2. The test piece surfaces were first grit-blasted using alumina grit of mesh size 60 (BS410) and then degreased immediately prior to coating. The hard chrome plating, APTICOTE 100N, was prepared by Poeton Ltd UK to a thickness of 200µm.

Table 1. Powder details

Table 2. Spraying Parameters

3 Experimental procedure

3.1 Wear testing

Abrasive wear was measured following the ASTM standard G65-91. The abrasive was 200µm rounded quartz with a flow rate of 150.min ^-1. The applied force was 130N and the test duration was 1200s. The weight loss was measured at four intervals during the test. The test was carried out on the WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® HVOF system as well as on the hard chrome plating. Wear rate is reported as a volume loss, calculated from the measured weight loss and a measured coating density. For each coating type three wear tests were undertaken.

3.2 Friction coefficient

Friction coefficient was measured continuously during a 1000m long sliding test described by the ASTM standard G99-90. A 10N force was applied to a ball with an 850 to 900HV hardness. The ball had a relative speed to the disc of 0.1m.s ^-1. The test was carried out on the WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® HVOF system as well as on the hard chrome plating. For each coating three wear tests were undertaken.

3.3 Corrosion test

An electrochemical test was carried out on the WC-10Co-4Cr coatings prepared with the 3 HVOF systems and on the hard chrome plating. The electrochemical test consists of anodic polarisation of the coating in de-aerated seawater at 25°C. The test method followed the guidelines described in the ASTM standard G61-86. The test was carried out in an Avesta cell. The coated test piece was anodically polarised from the rest potential at a rate of 10 mV.min ^-1. On reaching a corrosion current of 10 mA.cm ^-2 the scan was reversed and decreased at 10 mV.min ^-1. A plot of the corrosion current density to a platinum counter electrode as a function of the polarisation potential was recorded. The thermal sprayed coatings were tested after being ground and vacuum sealed with a resin, while the hard chrome coatings were tested as-prepared without sealing.

3.4 Adhesion test

Coating adhesion was measured at ambient temperature on the WC-10Co-4Cr coatings prepared with the 3 HVOF systems and on the hard chrome plating. The adhesion test used is described by the ASTM standard C633-79. It consists of coating one face of a substrate fixture, bonding this coating to the face of an uncoated loading fixture, with a high strength structural adhesive (trade name FM1000). The assembly was placed in a tensile loading machine with self-aligning devices. Tensile load was increased at 1mm.min ^-1 and the load at failure recorded. For each coating type, the adhesion of five test pieces was measured.

3.5 Measurement of coating adhesion after salt spray test

Salt spray exposure was carried out on the WC-10Co-4Cr coatings prepared with the 3 HVOF systems and on the hard chrome plating as described in the ASTM standard B117-97. In this test a 5wt% NaCl solution was atomised to create a fog within an enclosed chamber holding the coated fixtures. The position of the coatings was such that they were supported 15 to 30° from the vertical and parallel to the principle direction of flow of the fog through the chamber. The exposure duration was 10 days with the temperature maintained at 35°C. The WC-10Co-4Cr coatings and hard chrome plating were tested in the as-sprayed condition. The coating edge was protected against the fog using a suitable masking tape, as shown by the schematic in Fig.1.

After the salt spray test, adhesion of the coating was measured as described in 3.4 for the WC-10Co-4Cr coatings prepared with the 3 HVOF systems and for the hard chrome plating.

4 Results and discussion

4.1 Metallography

Cross section images of the WC-10Co-4Cr coatings prepared with the three HVOF systems are presented in Fig.2 to 4. The WC-10Co-4Cr coatings prepared with the three HVOF systems had similar microstructure, they are dense with few pores and no apparent un-melted particles. Cross section of the hard chrome plating is presented in Fig.5.

The WC-10Co-4Cr coatings tested showed friction characteristics similar to the hard chrome plating in dry sliding wear conditions. Indeed the WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® HVOF systems and the hard chrome plating had a friction coefficient of about 0.8. Hard chrome plating and the WC-10Co-4Cr coatings prepared with the Jet Kote had an initial friction coefficient of about 0.8 and gradually dropping down to about 0.5 after a period of 1000s. The WC-10Co-4Cr coating prepared with the JP5000 ^® showed a continuous increase in friction as the test progressed, reaching a friction coefficient value of 0.8 after 1000s. Friction coefficient of the Diamond Jet ^® coatings was not evaluated during this work.

4.4 Corrosion test

Plots of corrosion current density versus the applied potential are reported in Fig.7 for the WC-10Co-4Cr coatings prepared with the 3 HVOF systems as well as for the hard chrome plating. The WC-10Co-4Cr coatings prepared with the three HVOF systems and the hard chrome plating had a similar rest potential of about -200/-250mV _sce. This indicates similar activity of the coatings and chrome plating in de-areated seawater relative to the saturated calomel electrode. Corrosion current densities were measured for the WC-10Co-4Cr thermal spray coatings at below 0.1 mA/cm ² for a potential of 100mV _sce. This indicates low level of dissolution or attack of the coating and underlying substrate. The corrosion performance of the WC-10Co-4Cr thermal spray coatings did not match that of the hard chrome plating, which had a lower corrosion current density, measured below 0.01mA/cm ² at a potential of 100mV _sce.

After the test, evidence of pitting was observed on the hard chrome plating, while the WC-10Co-4Cr coating surface was uniformly discoloured. Pitting of the hard chrome plating indicates that the hard chrome plating had been locally attacked during the accelerated test.

4.5 Adhesion test

In the as-spray condition, the WC-10Co-4Cr coatings prepared with the three systems and the hard chrome plating all had higher bond strength and cohesive strength than the FM1000 adhesive. The FM1000 adhesive bond strength was measured at about 80MPa.

4.6 Measurement of coating adhesion after salt spray test

Coating adhesion was measured after the WC-10Co-4Cr coatings and hard chrome plating were exposed to a salt spray environment to evaluate the impact of corrosion on the coating adhesion. The WC-10Co-4Cr coatings prepared with the 3 HVOF systems and the hard chrome plating had similar corrosion performance in the salt spray environment. Indeed after the 10 days immersion in the salt spray test, no evidence of significant corrosion, or apparent breakdown was observed on the WC-10Co-4Cr coatings and the hard chrome plating. The WC-10Co-4Cr HVOF coatings were slightly discoloured, as indicated by the image of the surface of the Jet Kote ^® WC-10Co-4Cr coating after the salt spray test in Fig.8. The hard chrome plating did not show any evidence of corrosion after the salt spray test. No evidence of corrosion was observed in the WC-10Co-4Cr coatings and in the substrate as shown by a cross section of the middle of the sample, Fig.9.

After exposure to the salt spray test, the hard chrome plating and the WC-10Co-4Cr coatings prepared with the 3 systems retained higher bond strength than the adhesive above 80MPa. This indicates that although discoloured by the salt spray test, the HVOF WC-10Co-4Cr coatings retained high cohesive and bond strength.

Measurement of the coating bond strength after exposure to the salt spray environment was repeated for the WC-10Co-4Cr coated samples in the as-spray condition, with their edges un-protected against the salt spray environment. Bond strength was measured between 30 to 60MPa for these coatings. Evidence of corrosion was observed on the substrate at the specimen edges as shown in Fig.10 and 11. For each of the WC-10Co-4Cr coatings prepared with the three HVOF systems, failure was initiated at the coating edge and propagated through the coating.

Fig.11. Cross section through the WC-10Co-4Cr coating exposed to the salt spray test with its edge un-protected

5 Discussion

The exposure of the WC-10Co-4Cr coatings and hard chrome plating to the salt spray corrosive atmosphere did not jeopardise the coating and plating bond strength when the coating edges were protected. When neither the substrate nor the WC-10Co-4Cr coating edges were protected against the salt spray corrosive atmosphere, corrosion took place at the edge and propagated along the interface between the coating and the substrate. It seems that corrosion took place mainly on the substrate. Corrosion at the edge reduced considerably the adhesion of the coating at these locations. Such corrosion at the edge was not observed for the WC-10Co-4Cr coatings prepared onto stainless steel, and high adhesion was retained after salt spray test. This seems to indicate that corrosion of the substrate was the main reason for the loss of coating bond strength. The interface between the coating and the substrate seems to be a weak point for the propagation of corrosion. When using WC-10Co-4Cr coatings onto steel, the substrate at the coating edge should not be exposed directly to the corrosive atmosphere. At this point the coating does not protect the substrate against the corrosive atmosphere. Hard chrome plating was tested only with its edges protected against the salt spray environment.

6 Summary and conclusion

• Similar performance was measured for the WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® and the Diamond Jet ^® HVOF systems during the corrosion and adhesion tests.
• WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® had wear only 20 to 25% that measured for the hard chrome plating in dry abrasion wear conditions.
• The WC-10Co-4Cr coatings prepared with the Jet Kote ^® and the JP5000 ^® showed friction characteristics similar to the hard chrome plating in dry sliding wear conditions, with a friction coefficient of about 0.8.
• The WC-10Co-4Cr coatings prepared with the 3 systems show low resistance to corrosion in seawater compared to hard chrome plating.
• WC-10Co-4Cr coatings prepared with the 3 systems and the hard chrome plating had high adhesion to carbon steel substrate, with bond strengths in excess of that of the adhesive.
• The WC-10Co-4Cr coatings prepared with the 3 systems performed well in the salt spray environment, matching the performance of the hard chrome plating with no visual indications of corrosion attack after 240 hours.
• Salt spray exposure did not degrade the adhesion of the hard chrome plating and WC-10Co-4Cr HVOF coatings when the coating edges were protected against the salt spray environment.
• WC-10Co-4Cr HVOF coating edge is a weak point where corrosion can be initiated and developed through the interface, degrading coating adhesion.

7 Acknowledgements

The authors wish to thank Kathy Schlegelmilch and the other technicians at Deloro Stellite and TWI for their services in making this body of work possible.

8 References

1. Bodger, B.E., Emery, W. A., Sommerville, D.A. 'An Example of the Decision Making Process for the Evaluation of Tungsten Carbide Thermal Spray Process as Replacements for Electrodeposited Chrome Plating' inProceedings of the American Electroplaters and Surface Finishers Society (AESF) 33 ^rd Aerospace/Airline Plating and Metal Finishing Forum, held March 24-27, 1997 in Burlingame, CA.
2. C. Wasserman, J. Burmann, S. Gustafsson, R. Bocking. 'Replacement of hard chrome palings by HVOF-Coatings' Proceedings of the 5 ^th HVOF conference, Erding, 16-17 November 200. pp109-114
3. B. D Sartwell, P. E. Bretz 'HVOF thermal spray coatings replace hard chrome' Advanced Materials and Processes 8 (1999) pp 25-28
4. U. Erning, D. C. Nestler, G. Tauchert. 'HVOF coatings for hard chrome replacement- properties and applications' Proceedings of the United Thermal spray Conference 9917-19 March 1999 Dusseldorfpp462-467
5. Startwell, Bruce D. 'Thermal Spray Coatings As Alternative to Hard Chrome Plating' in Welding Journal, July 2000, pages 39-43.
6. Savarimutthu, A.C, Taber, H. F., Megat, I., Shadley, J. R., Rybicki, E.F., Cornell, W.C., Emery, W.A. Somerville, D.A., Nuse, J.D. 'Sliding Wear Behavior of Tungsten Carbide Thermal Spray Coatings forReplacement of Chromium Electroplate in Aircraft Applications' in Journal of Thermal Spay Technology, Vol. 10 No 3, September 2001, pages 502-510.
7. Raghu, D., 'Developments in HVOF Processes and Materials', Proceedings of Thermal Spray Coatings 97, Gorham Advanced Materials, Atlanta, GA, April 1997.