Tribological behavior of nanocrystalline zinc oxide films

This paper describes the room temperature tribological characteristics of nanocrystalline zinc oxide (ZnO) films. The films were grown onto 440C stainless steel substrates by pulsed laser deposition (PLD) using the 248 nm line from an excimer laser to ablate a hot pressed hexagonal ZnO target. The substrate temperature and oxygen partial pressure were varied to grow films with different stoichiometries and grain sizes. Friction measurements were made against a 440C steel counterface in a ball-on-flat configuration. The friction coefficient of a conventional hot pressed ZnO disk was high, 0.65; the steel counterface after wear testing was full of scratches. By contrast, the friction coefficients of PLD ZnO films were between 0.16 and 0.34; the steel counterface after testing was practically free of scratches. Extensive plastic deformation of ZnO was observed on the wear scars. The friction behavior of ZnO films was sensitive to Hertzian stress and sliding speed, but once a smooth wear scar was formed, the test conditions had no significant effect on the friction. The improved tribological performance of the PLD ZnO films was attributed to their nanocrystalline grain structure and oxygen substoichiometry. (C) 1997 Elsevier Science S.A.