Residual stresses as a factor in the selection of tungsten carbide coatings for a jet engine application

Tungsten carbide thermal spray coatings are important to the aerospace industry for the mitigation of midspan damper wear on jet engine fan and compressor blades. However, in some cases the coating can fail due to spallation and cracking, and in other situations the fatigue life of a fan or compressor blade is reduced when a coating is applied. Coating failures can result in decreased engine performance and costly maintenance time. A comprehensive experimental research program was conducted to evaluate coating crack resistance in bending, low-cycle fatigue properties of the coating and substrate (Ti-6Al-4V), coating performance in jet engine tests, and microstructures for a wide range of coating compositions (WC-Co, WC-Ni, WC-Co/Cr) and application processes. Coating residual stress distributions also were evaluated. Eleven coatings were ranked according to their performance relative to the other coatings in each evaluation category. Results from the bend and low-cycle fatigue evaluations were compared to the experimentally evaluated residual stresses. Comparisons of rankings indicate a strong correlation between performance and the residual stresses in the coatings. Results from the program were used to select a suitable coating system for final in-service use based on two important criteria: (1) the coating must not fail while in service, and (2) the coating must not induce crack propagation into the substrate of the midspan damper.