Growth, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS 2) nanocomposite thin films

This study describes the synthesis, structure and friction behavior of titanium doped tungsten disulphide (Ti-WS 2) nanocomposite solid lubricant thin films grown by cosputtering at room and 300 °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 WS 2 thin films grown at room temperature (RT) and 300 °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 WS 2, 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 (∼ 5–14 at.%) in comparison to pure WS 2 but there was no change in friction coefficient (∼ 0.1). For high temperature (500 °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 WS 2 oxidized to form relatively low shear strength WO 3 during wear (tribo-oxidation) and heating at 500 °C (thermal oxidation) as determined by Raman spectroscopy in the wear track and transfer film (third body) on the counterface.