High temperature sliding wear behavior and mechanisms of cold-sprayed Ti and Ti–TiC composite coatings

Abstract

Ti and Ti-based alloys are used in many aerospace and automotive components due to their high strength-to-weight ratio and corrosion resistance. However, room and elevated temperature wear resistance remain an issue, thus requiring some form of secondary hard phase, e.g., refractory carbides, as well as solid lubrication to mitigate wear. In this study, cold spray Ti-xTiC (x = 14, 24 and 35 vol%) composite coatings were deposited on mild steel substrates with comparisons made to a baseline cold-sprayed Ti coating. The dry sliding friction and wear behavior were studied from 25 °C to 575 °C and during thermal cycling in this temperature range. While the room temperature friction coefficient of all the coatings remained relatively constant at ~0.5, the wear rate continually decreased from ~1 × 10−3 to ~2 × 10−5 mm3/N·m with increasing TiC loading. Raman spectroscopy measurements determined that the same TiO2 tribochemical phases (rutile and anatase) were present on the room temperature sliding wear surfaces, thus responsible for similar friction coefficients. With increasing sliding temperatures to 575 °C, the Ti–35%TiC composite coating exhibited the best overall tribological behavior, i.e., the friction coefficient decreased to ~0.3 along with a negative wear rate of −6.6 × 10−5 mm3/N·m (material gain on the wear track was recorded due to oxidation and transfer from the counterface). These friction and wear reductions were determined to be due to the formation of stable, low interfacial shear strength oxide glaze layers on the wear surfaces, composed of TiO2, WO3, and CoWO4 (transfer from WC-Co counterface). In addition, self-adaptive friction behavior was observed during thermal cycling as a result of the microstructural and tribochemical differences in the tribolayers.