Thermal Spray for Hard Chrome Replacement
Criteria limiting the exposure to hexavalent chromium were first established by the National Institute for Occupational Safety and Health in 1975 due to its negative environmental and human health effects. Since then, there has been a goal to eliminate its use completely and the Department of Defense (DoD) has made a significant investment into developing alternative technologies.
Research and development efforts aimed at improving the environmental footprint of a process or a technology area often lead to improvements in performance as well. High velocity oxygen fuel (HVOF) is such an example. An initial Defense Advanced Research Projects Agency-funded project had shown that the best alternative for chrome in most aircraft components was a HVOF thermal spray coating. As a result, in 1996, SERDP and ESTCP launched one of its first major programs – the Hard Chrome Alternatives Team (HCAT), with the goal to find a chrome alternative of equivalent performance and acceptable cost. The HCAT program was a partnership between the DoD, Industry Canada, and the Canadian Department of National Defence, with team members spanning defense organizations, the aerospace industry, suppliers, test laboratories, national laboratories, and universities in the U.S. and Canada. Prior to its ending in 2007, the HCAT program developed a series of reports on chrome replacement for landing gear, engines, helicopter dynamic components, propeller hubs, and hydraulic actuators. The HCAT program also paved the way for installation of HVOF systems at original equipment manufacturers, DoD depots, and commercial overhaul shops.
The result of the HCAT program was elimination of hard chrome plating, a major hexavalent chromium process, from its biggest use in aerospace. HVOF is now used throughout the aircraft industry and in military depots where aircraft are repaired. Every new aircraft landing gear is now designed with an HVOF coating on the landing gear cylinder. Many other major components including aircraft hydraulic actuators, engine components, and helicopter components also use the same technology. Not only does an HVOF coating eliminate hexavalent chromium, but it lasts longer, wears less, corrodes less, does not embrittle steel or reduce its fatigue life. HVOF has eliminated a major application of, and the hazards associated with, hexavalent chromium. And while it is not an answer for every use of hard chrome plating, DoD is continuing to develop and implement other technologies to eliminate it completely.