Electrolytic hard chrome (EHC) plating is used extensively by aircraft original equipment manufacturers (OEM) to impart wear and corrosion resistance to many components. 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. 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 ceramic/metal or high-temperature, oxidation-resistant 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. The coatings can be applied in the same range of thicknesses that are used in chrome plating. A detailed technology assessment concluded that the optimum coatings for replacing EHC plating on propeller hub components were high-velocity oxygen-fuel (HVOF) Tribaloy 800, a cobalt (Co)-based alloy, and Co-cemented tungsten carbides (WC), WC (83%) / Co (17%) and WC (86%) / Co (10%) Cr (4%).
Steel (4340) coupons were coated with WC/17Co and WC/10Co4Cr at the Naval Aviation Depot, Cherry Point (NADEP-CP) in North Carolina. Axial fatigue, salt-fog corrosion and sliding wear tests showed that the performance of HVOF WC/17Co and WC/10Co4Cr was equal to, or superior to, EHC. WC/10Co4Cr also was considered acceptable to replace nickel-plated coatings. After a wear rig test for one standard overhaul life, a P3 aircraft low-pitch-stop lever sleeve coated with HVOF WC/17Co still looked pristine.
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 (i.e., 1.5 to 4 times longer cycle), reduced process turnaround time, and increased readiness. A detailed cost/benefit analysis estimated that a propeller hub overhaul facility that processes approximately 270 components per year would experience an annual cost increase of between $2,000 and $26,000, depending on the actual decreased repair frequency. However, if the avoidance of scrapping difficult- and expensive-to-replace propeller hub components and replacement of nickel plating on propeller hubs are considered along with conversion of other EHC processes at the facility to HVOF, a favorable positive payback period on the necessary capital investment would be realized. Based on the favorable materials and component tests, the U.S. Navy is proceeding to test HVOF-coated P3 aircraft propeller hub components under flight conditions. If successful, it is anticipated that both NADEP-CP and Warner-Robins Air Logistics Center will implement the technology for C-130 aircraft propeller hubs. (Project Completed - 2004)