Tribology of tungsten disulfide films in humid environments: The role of atailored metal-matrix composite substrate

As a result of tribe-induced oxidation, tungsten disulfide (WS2) 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 i mparting oxidation resistance to WS2 films in sliding contact. The substrat e is an aluminum MMC disk reinforced with 20 vol.% silicon carbide (SiC) pa rticles. 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 b all. The wear scars and third-body transfer films were characterized by sca nning electron microscopy and Raman spechoscopy. In dry nitrogen, the frict ion coefficient of WS2 films on MMC substrates was low (0.035-0.050), indic ating that the carbide protrusions did not adversely affect the lubricating behavior. In humid air, the friction coefficient of WS2 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 cou nterface wear, or scratching of the steel ball by SiC, was practically abse nt. The role of carbide protrusions in controlling chemically assisted crac k propagation is discussed. The implications of this work as a model study for the design of thin film composite coatings is highlighted. (C) 1999 Els evier Science S.A. All rights reserved.