Tribology of tungsten disulfide–nanocrystalline zinc oxide adaptive lubricant films from ambient to 500°C

Abstract

Tungsten disulfide (WS 2)–zinc oxide (ZnO) composite is a candidate material that exhibits adaptive lubricant behavior. Adaptive lubricants undergo chemical changes with changing environment to provide lubrication in extreme environments. In the current study, the tribological characteristics of WS 2–nanocrystalline ZnO films have been investigated from ambient to 500°C. The composite films were powder burnished on inconel substrates. Using a ball-on-flat tribometer, friction tests were conducted on WS 2–ZnO nanocomposite films containing 50% by weight of the oxide. For comparison, measurements were made on pure WS 2 films burnished under identical conditions. The room temperature tests were performed in dry nitrogen, while the elevated temperature tests were run in air. Wear scars and transfer films on the counterface were analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. Results showed that the nanocrystalline ZnO additive resulted in significant reduction in friction coefficient of WS 2 both at 300°C as well as at room temperature. Third-body analyses from 300°C tests revealed that tribooxidation is less prevalent in nanocomposite films. At 500°C, the friction coefficient of pure WS 2 films increased to 0.50 within the first 2000 cycles, whereas the nanocomposite films lasted the entire duration of 10,000 cycles with steady state friction coefficient of 0.22. Raman spectroscopy identified the formation of zinc tungstate (ZnWO 4) during the 500°C tribotests, confirming the adaptive lubricant concept in WS 2–ZnO nanocomposites.