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Replacement of Chromium Electroplating on Helicopter Dynamic Components Using HVOF Thermal Spray Technology
Bruce Sartwell | SERDP/ESTCP Program Office
Electrolytic hard chrome (EHC) plating is a technique that has been in commercial production for more than 50 years. It is a critical process that is used both for applying hard coatings to a variety of aircraft components in manufacturing operations and for general rebuild of worn or corroded components that have been removed from aircraft during overhaul. Chromium plating baths contain chromic acid, in which the chromium is in the hexavalent state, with hexavalent chromium (Cr6+) being a known carcinogen. During operation, chrome plating tanks emit a Cr6+ mist into the air, which must be ducted away and removed by scrubbers. Wastes generated from plating operations must be disposed of as hazardous waste, and plating operations must abide by U.S. Environmental Protection Agency (EPA) emissions standards and the Occupational Health and Safety Administration (OSHA) permissible exposure limit (PEL). In February 2006, OSHA reduced the PEL for worker exposure to Cr6+ from 52 μg/m3 of Cr6+ to 5 μg/m3.
Previous research and development efforts established that high-velocity oxygen-fuel (HVOF) thermal spray coatings are the leading candidates for replacement of hard chrome. HVOF thermal spraying can be used to deposit both metal alloy and ceramic/metal (cermet) such as tungsten carbide/cobalt (WC/Co) coatings that are dense and highly adherent to the base material. They also can be applied to thicknesses in the same range as what is currently being used for chrome plating. There are HVOF thermal spray systems commercially available. Although there are a wide number of applications for these coatings, their qualification as an acceptable replacement for hard chrome plating has not been adequately demonstrated, particularly for fatigue-sensitive aircraft components. The Hard Chrome Alternatives Team (HCAT) was formed to perform the demonstration/validation for the HVOF coatings. After successfully demonstrating HVOF coatings on landing gear components, hydraulic actuators, propeller hubs, and gas turbine engines (GTE), this project demonstrated HVOF coatings on helicopter dynamic components (HDC).
Objectives of the Demonstration
The objectives of this project were to demonstrate through materials and rig testing that the performance of HVOF WC-17Co (83wt% WC particles in a 17wt% Co matrix), WC-10Co4Cr, Tribaloy 400 (T400), and duplex T400/WC-Co coatings on HDCs is equal or superior to that of EHC coatings. Materials testing included axial fatigue, fretting fatigue, and salt-fog corrosion. Rig tests were carried out on helicopter drive system and rotor system components.
A tri-service/OEM/private-sector group, designated the Hard Chrome Alternatives Team (HCAT), was formed to demonstrate and validate thermal spray coatings as an environmentally-acceptable, superior-performance alternative to EHC on many different types of aircraft components. 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 cobalt-based alloy), WC(83%)/Co(17%), and WC(86%)/Co(10%)Cr(4%). With stakeholder input, a Joint Test Protocol (JTP) was developed that was divided into two parts, with the first part addressing materials testing on coupons, including extensive fatigue, fretting and sliding wear, and corrosion testing. The second part of the JTP addressed rig and flight testing on actual coated components. In addition, producibility testing and assessments were conducted that included optimum methods for stripping and grinding of the HVOF coatings.
Significant demonstration results included the following:
- Fatigue: Cycles-to-failure at different stress levels were measured for fatigue specimens fabricated from 4340 steel (150-170ksi), PH3-8Mo stainless steel, carburized 9310 gear steel, and 7075Al, and coated with EHC (baseline), WC-17Co, WC-10Co-4Cr, Tribaloy 400, and duplex T400/WC-Co. In all cases, the fatigue of the HVOF specimens was equal or superior to the EHC. The most significant data was for 7075Al substrates, where both WC-CoCr and duplex T400/WC-Co improved fatigue significantly over EHC.
- Fretting Fatigue: Fretting fatigue was tested for WC-Co on each substrate against 52100 bearing steel. In all cases, the fretting fatigue performance of HVOF coating, while worse than standard fatigue, was superior to EHC. Therefore, the HVOF coatings passed the acceptance criteria.
- Corrosion: ASTM B117 salt fog exposure tests were conducted on each of the substrate/coating combinations. HVOF performance was lower than that of hard chrome, just as it is in most other cabinet corrosion testing (B117 and G85). Therefore, the HVOF coatings failed the acceptance criteria. As demonstrated in HCAT beach exposure testing as well as widespread service experience, however, actual performance of HVOF coatings is usually superior to that of hard chrome. Cabinet tests, which were developed for testing paint systems and keep the surface constantly exposed to the corrodant, are known to be very poor predictors of the actual performance of these hard coatings.
- Rig Testing: Bench-top rig testing was carried out by Bell Helicopter for H-1 drive system and rotor system components coated with HVOF WC-Co in place of EHC—rotor brake disc adapter flanges, tail rotor drive quill spacers, collective scissors and sleeve, and control rods. In all cases, the HVOF coatings met or exceeded EHC performance, qualifying HVOF WC-Co for these components.
- Cost Assessment: Cost assessment was carried out using the Environmental Cost Analysis Methodology (ECAM) model, for HDCs at Fleet Readiness Center-East (FRC-E), as well for all components at FRC-E. Improved performance was taken into account, and the costs of the lower Cr6+ PEL were also considered. Even without considering increased service life, cost savings for HDCs alone were predicted to have a 15-year net present value (NPV) of $3.2 million, with a payback period of 3 years, while all components had a 15-year NPV of $7.8 million and a payback period of 2 years. When performance improvements were included, the 15-year NPV for all components increased to $10 million.
The use of HVOF in place of hard chrome is now being realized. The CH-53 main rotor damper now uses HVOF WC-Co on the piston, HVOF T400 on the housing internal diameter (ID), and plasma spray WC-Co on a cylinder housing land. H-1 drive and rotor system components have been qualified with HVOF WC-Co in place of hard chrome through bench testing of coated systems at Bell Helicopter. The sleeve used on the AH-1W is already HVOF WC-Co coated by the original equipment manufacturer (OEM). Lead-the-fleet testing of H-60 dampers using HVOF coatings is under way that is expected to result in HVOF coating of all of these dampers in the fleet to improve their mean time between failure (MTBF). FRC-E is currently carrying out qualification testing on a set of HVOF-coated H-46 components in order to qualify HVOF for depot use.