Tribology of tungsten disulfide films in humid environments
Abstract As a result of tribo-induced oxidation, tungsten disulfide (WS 2 ) loses its lubricating behavior in humid environments. The purpose of this study is to explore the role of a tailored metal-matrix composite (MMC) substrate in imparting oxidation resistance to WS 2 films in sliding contact. The substrate is an aluminum MMC disk reinforced with 20 vol.% silicon carbide (SiC) particles. The MMC disk was metallographically polished and etched to create SiC particle protrusions. The films were grown on the MMC substrates using a pulsed laser. Friction and wear tests were performed in dry nitrogen and in humid air with 90% relative humidity. The counterface was a 440C steel ball. The wear scars and third-body transfer films were characterized by scanning electron microscopy and Raman spectroscopy. In dry nitrogen, the friction coefficient of WS 2 films on MMC substrates was low (0.035–0.050), indicating that the carbide protrusions did not adversely affect the lubricating behavior. In humid air, the friction coefficient of WS 2 films on polished steel substrates increased to 0.4 during the first 1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of 50,000-cycle tests, with friction coefficients ranging from 0.15 to 0.22. The counterface wear, or scratching of the steel ball by SiC, was practically absent. The role of carbide protrusions in controlling chemically assisted crack propagation is discussed. The implications of this work as a model study for the design of thin film composite coatings is highlighted.