Virtually every military gas turbine engine (GTE) system in service depends on electrolytic hard chrome (EHC) plating in the engine overhaul process. Navy, Air Force, and Army depots generally use EHC in GTE maintenance to rebuild worn or corroded parts following machining to restore dimensional tolerance or to replace worn or damaged chrome previously applied. Hard chrome plating utilizes chromium in the hexavalent state (hex-Cr), which is a known carcinogen. The Environmental Protection Agency has issued fairly stringent air emission standards for hex-Cr, and the Occupational Safety and Health Administration has established a new permissible exposure limit (PEL) for hex-Cr in the workplace at a level of 5 µg/m³, which is substantially less than the previous PEL of 52 µg/m³. Because of increased operational requirements on GTEs, there are additional concerns related to the in-service performance of EHC. High-velocity oxygen-fuel (HVOF) thermal spray coatings are being validated by the Hard Chrome Alternatives Team (HCAT) and the Joint Group on Pollution Prevention with ESTCP support to replace EHC for line-of-sight applications on aircraft components such as landing gear, propeller hubs, hydraulic actuators, and helicopter dynamic components. Through collaboration between HCAT and the Propulsion Environmental Working Group (PEWG), this project validated HVOF coatings as a cost-effective, superior performance replacement for EHC plating on different types of components used in gas turbine engines.

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. The coating material is usually a metal or alloy (Tribaloy or stainless steel) or a cermet (WC/Co). This technology is used to deposit coatings 0.003-inch thick on original equipment manufacturer (OEM) parts and to rebuild worn components by depositing layers up to 0.015-inch thick. These thicknesses are in the same range as those being applied through chrome plating.

The HCAT and PEWG developed and executed Joint Test Protocols (JTP) to qualify HVOF and advanced plasma spray coatings as a replacement for EHC on GTE components. Partnerships were established between OEMs and the military, including General Electric Aircraft Engines with the Naval Aviation Depots in Jacksonville and Cherry Point for the TF34 engine, as well as Pratt & Whitney with Oklahoma City Air Logistics Center for the TF33 engine. The JTPs included extensive materials testing, such as fatigue, wear, and corrosion, and selected component and engine testing to verify the performance of the thermal spray coatings in comparison to EHC coatings. In addition, producibility testing and assessments were conducted, which included stripping, grinding, and assembly/disassembly operations. An Advanced Mission Test (AMT) was conducted on a TF33 engine in which seven components normally coated with EHC were instead coated with HVOF tungsten carbide/cobalt (WC/17Co). Inspection of the coatings subsequent to the test indicated performance superior to what would have been expected for EHC. A Joint Test Report was issued on the results of the testing, and cost/benefit analyses comparing thermal spray to EHC were prepared. As of the date of project completion, new repair procedures are being documented for the depots.

Insertion of the thermal spray coatings for the repair of gas turbine engines into Department of Defense depots will result in a significant reduction in toxic waste disposal and a safer working environment. Use of the thermal spray coatings also will result in a reduced turnaround time for repair and increased performance, leading to a reduction in life-cycle costs. A detailed cost/benefit analysis was conducted for replacement of EHC operations with HVOF at a military GTE overhaul facility. The 15-year net present value (cost savings) ranged from $500,000 to $3 million, depending on assumptions related to the hex-Cr PEL and whether the HVOF coatings would last longer than EHC in service. Based on these results, the Air Force is proceeding with implementation of HVOF coatings on their GTEs. The Navy implemented HVOF on the TF34 engine and is exploring qualification of the coatings on components from other engines. (Project Completed - 2006)