Electrolytic hard chrome (EHC) plating is used extensively by aircraft manufacturers and maintenance facilities to provide wear and/or corrosion resistance or to restore dimensional tolerance to many different types of components, including hydraulic actuator pistons. EHC plating baths contain chromic acid, in which the chromium is in the hexavalent state, with hexavalent chromium (hex-Cr) being a known carcinogen. Wastes generated from plating operations must be disposed of as hazardous waste and plating operations must abide by U.S. Environmental Protection Agency emissions standards and a hex-Cr permissible exposure limit (PEL) as specified by the Occupational Safety and Health Administration (OSHA). In February 2006 OSHA promulgated a new hex-Cr PEL of 5 µg/m3, with an action level of 2.5 µg/m3, an order of magnitude below the previous PEL of 52 µg/m3. It is anticipated that the new hex-Cr PEL will significantly increase the cost of chrome plating to manufacturers and Department of Defense (DoD) facilities. The objectives of this project were to demonstrate, through coupon materials testing, functional rig testing, and delta qualification testing of actual hydraulic actuators, that high-velocity oxygen-fuel (HVOF) coatings have equivalent or better performance than EHC coatings.
A tri-service/original equipment manufacturer (OEM)/private-sector group, designated the Hard Chrome Alternatives Team (HCAT), was formed to demonstrate and validate thermal spray and other types of coatings as environmentally acceptable, superior-performance alternatives to EHC on many different types of aircraft components. A detailed technology assessment concluded that the optimum coatings for replacing EHC plating on hydraulic actuator components were HVOF WC(86%)/Co(10%)Cr(4%), Cr3C2(80%)/NiCr(20%), and Tribaloy 400, a cobalt-based alloy. HVOF is a standard commercial thermal spray process in which the coating to be applied is injected, as a powder, into a supersonic flame of a fuel (usually hydrogen, propylene, or kerosene). The powder particles are accelerated to high speed and soften in the flame, forming a dense, well-adhered coating on the substrate. Through consultation with cognizant stakeholders, a Materials Joint Test Protocol (JTP) was developed for materials testing on coupons, including extensive fatigue, corrosion, fluid immersion, and environmental embrittlement testing. A functional rod/seal test plan was developed that included evaluation of the performance of the HVOF coatings sliding against actual seals used in actuators. Qualification testing plans on actuators containing HVOF WC/CoCr-coated pistons also were developed. This included the evaluation of both utility and flight-control actuators. A cost/benefit analysis for implementation of HVOF thermal spray coatings to replace EHC on actuators was performed.
For materials testing, substrates were 4340 high strength steel (180-200 ksi ultimate tensile strength [UTS]), PH15-5 stainless steel (155 ksi UTS), and Ti-6Al-4V (130 ksi UTS). HVOF coatings were those indicated above. Axial fatigue testing indicated that the performance of the HVOF-coated materials was either equivalent or superior to that of EHC-coated materials. None of the HVOF coatings caused environmental embrittlement in either deionized water or a 5% NaCl solution. The salt-fog corrosion performance of the HVOF coatings was somewhat inferior to that of EHC. Fluid immersion tests in commonly used cleaners, etchants, hydraulic fluids, and fuels showed no attack on the HVOF coatings. The one exception was that the cobalt-containing coatings were strongly attacked by bleach. Functional rod/seal testing was performed by Naval Air Systems Command (NAVAIR) Patuxent River using HVOF WC/CoCr-coated piston rods and actuator speeds and temperatures intended to simulate service conditions. Several different types of seals were used with the result that in almost all cases the HVOF-coated rods produced significantly less fluid leakage and less seal wear than EHC-coated rods. Qualification testing on a number of flight-control and utility actuators containing HVOF-coated piston rods was performed by the Oklahoma City Air Logistics Center. Overall, actuators with HVOF-coated rods were found to perform as well as or better than those with EHC-coated rods, although in some cases different seals were required. The cost/benefit analysis indicated that repair facilities could realize substantial savings by converting from EHC to HVOF for overhaul of actuators.
Implementation of this technology will result in a reduction of hex-Cr emissions, leading to reduced toxic waste disposal and a safer working environment. The superior performance of the HVOF coatings will lead to decreased component repair frequency, reduced process turnaround time, and increased readiness. Taken together, these benefits should result in reduced life-cycle costs on DoD aircraft. (Project Completed – 2007)