Electrolytic hard chrome (EHC) plating is used extensively by helicopter original equipment manufacturers (OEM) to impart wear and corrosion resistance to many components, including pistons, bearing joints, and support pins. Hard chrome plating utilizes chromium in the hexavalent state (hex-Cr), which is a known carcinogen. As a result, the Environmental Protection Agency has issued air and water emission standards for chromium, and recent epidemiological studies support lowering the permissible exposure limit (PEL) for hex-Cr, established by the Occupational Safety and Health Administration, by as much as two orders of magnitude, which would greatly increase the cost of chrome plating to OEMs and Department of Defense (DoD) facilities. A tri-service/OEM/private-sector group, designated the Hard Chrome Alternatives Team (HCAT), was formed to validate thermal spray coatings as an environmentally-acceptable, superior-performance alternative to EHC on many different types of aircraft components.
HVOF is a standard commercial thermal spray process in which a powder of cobalt (Co)-cemented tungsten carbide (WC) or a Tribaloy is injected into a supersonic flame of a fuel, usually hydrogen, propylene or kerosene. The particles of powder soften in the flame and form a dense, well-adhered coating on the substrate. A detailed technology assessment concluded that the optimum coatings for replacing EHC plating on helicopter dynamic components were high-velocity oxygen-fuel (HVOF) Tribaloy 400 (a Co-based alloy), WC (83%) / Co (17%), and WC (86%) / Co (10%) Cr (4%). The technology can be used to deposit 0.003 inch coatings on new OEM parts or to rebuild 0.015 inch layers on worn components.
Various steel and other alloy landing gear components were coated with WC/17Co and WC/10Co4Cr at the Ogden Air Logistics Center (OO-ALC) at Hill Air Force Base in Utah, where aircraft landing gear are overhauled, and also at the Naval Aviation Depot Jacksonville in Florida, where fixed-wing aircraft are overhauled. Materials and component testing, including axial fatigue, salt-fog and cyclic corrosion, sliding wear, impact, and hydrogen embrittlement, showed that the performance of HVOF WC/17Co and WC/10Co4Cr was equal to, or superior to, EHC coatings. Visual inspection of an F/A-18 E/F model nose landing gear after rig tests and a P3 aircraft main landing gear after flight tests showed no evidence of coating delamination or wear.
This technology will result in the elimination of hex-Cr emissions, leading to reduced toxic waste disposal and a safer working environment. HVOF guns can coat 900 square inches per hour, which is fifty times faster than chrome plating. The superior performance of the HVOF coatings will lead to decreased component repair frequency, reduced process turnaround time, and increased readiness. A detailed cost/benefit analysis estimated that the annual cost avoidance for a landing gear overhaul facility that processes more than 1,000 components per year would be approximately $200,000, resulting in a 3.5 year payback period on the capital investment. HVOF coating is being implemented at OO-ALC, where the application of WC/17Co coatings up to a thickness of 0.010 inches has been approved on 12 different landing gear components. Commercial airlines also have successfully completed tests on HVOF coatings under flight conditions.