High-velocity oxygen-fuel (HVOF) thermal spray technology using tungsten carbide/cobalt (WC/Co) and tungsten carbide/cobalt-chromium (WC/CoCr) currently is being validated by the U.S./Canadian Hard Chrome Alternatives Team (HCAT) for hard chrome plating replacement on external surfaces and outer diameters (OD), which comprise about 75 percent of all hard chrome applications in the Department of Defense. However, there are no similar dry coating technologies for internal surfaces and internal diameters (ID) that are ready for demonstration/validation. (Current HVOF spray guns are limited to IDs greater than 11 inches). This SERDP project is being executed by the HCAT but is concentrating on plasma spray technology for which much smaller guns, suitable for ID applications, have been developed. However, conventional plasma spray coatings such as WC/Co are too porous for direct replacement of hard chrome and, therefore, research must be conducted on developing new powders and deposition parameters so that the ID plasma spray coatings can achieve properties comparable to those for the HVOF coatings used on external surfaces.
The objective of this project is to develop an ID coating technology for hard chrome plating replacement that is clean, useable for rebuilds, environmentally acceptable, and amenable to both the original equipment manufacturer and the depot maintenance production environments.
The National Research Council (NRC) of Montreal, Canada, and the Naval Research Laboratory will draw on the capabilities of the two largest thermal spray equipment manufacturers (Praxair and SulzerMetco) to execute this project. The NRC will perform measurements of particle velocities and temperatures and analyze the coating structure. SulzerMetco will develop the use of WC composites and alloy coatings using a standard ID gun capable of spraying IDs down to about a 3-inch diameter (suitable for landing gear and large hydraulic actuators). Using a miniature ID plasma spray gun, Praxair will develop the use of small particle WC-Co (in the range 1- 20 microns) that is now commercially available. They will compare the results with the Tribaloy-400 powder that they are currently using for some ID coating applications. This small gun should be capable of operating down to a 1.5-inch ID (primarily for hydraulic actuators for flight surfaces, engine controls, etc.) and may also be an excellent way of spraying small particles.
Coatings of WC composites and Tribaloy 400 have been deposited inside 3-inch ID tubes. The quality of these initial coatings appears quite good based on observations of low porosity, low stress, and high deposition rate.
The immediate environmental benefit of the thermal spray approach is the complete elimination of hexavalent chrome mist and the chrome-contaminated toxic wastes associated with chrome plating, stripping, and masking operations. The work will lead directly to ID coating methods that are clean and produce a higher quality, longer lasting product. This coating method has the potential for significant cost reduction in both production and sustainment. In general, WC-Co coatings last at least two to three times longer than hard chrome. This leads to lower frequency-of-repair, better mission-readiness, and the ability to reduce spare parts inventory. The much reduced production time over chrome plating gives faster turnaround in overhaul operations while enhancing mission-readiness and reducing inventory requirements. (Project Completed – 2005)