Growth, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS2) nanocomposite thin films

This study describes the synthesis, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS2) nanocomposite solid lubricant thin films grown by cosputtering at room and 300 degrees C in situ substrate temperatures. The films were studied by focused ion beam (FIB) prepared cross-sectional scanning and transmission electron microscopies and X-ray diffraction (XRD) to determine the thin film structure and crystallinity as a function of varying titanium atomic percent and sputtering power. XRD confirmed that the pure WS2 thin films grown at room temperature (RT) and 300 degrees C were crystalline with hexagonal texture. Basal planes with c-axis orientated parallel to the substrate surface [(100) and (101) texture] were predominantly observed in all thin films. Co-sputtering at RT with any amount of Ti induced a dramatic change in the microstructure, i.e., Ti prevented the formation of crystalline WS2, making it amorphous with well-dispersed nanocrystalline (1-3 nm) precipitates. For RT friction tests, longer thin film lifetimes were exhibited when the thin films were doped with low amounts of Ti (similar to 5-14 at.%) in comparison to pure WS2 but there was no change in friction coefficient (similar to 0.1). For high temperature (500 degrees C) friction tests, slightly higher friction coefficients (0.2) but longer lifetimes were observed for the low at.% Ti doped thin films. Mechanisms of solid lubrication were studied by FIB prepared cross-sectional specimens and Raman spectroscopy wear maps inside the wear tracks to determine the sub-surface deformation behavior and formation of tribochemical products, respectively. It was determined that WS2 oxidized to form relatively low shear strength WO3 during wear (tribo-oxidation) and heating at 500 degrees C (thermal oxidation) as determined by Raman spectroscopy in the wear track and transfer film (third body) on the counterface. (C) 2009 Elsevier B.V. All rights reserved.