Role of temperature on tribological behaviour of Ti containing MoS2 coating against aluminum alloys

Friction and adhesion mechanisms of a Ti incorporated multilayer MoS2 (Ti-MoS2) coating, intended for use in elevated temperature engineering applications like warm forming or machining of lightweight Al alloys, were investigated. Pin-on-disk type tests were performed at temperatures between 25 °C and 500 °C to measure variations of the coefficient of friction (COF) values and wear rates of Ti-MoS2 sliding against Al-6.5% Si (319 Al) counterfaces. The steady state COF, μs, of Ti-MoS2 decreased from 0.11 at 25 °C to 0.05 at 200 °C. A μs of 0.13 was recorded at 350 °C. Then an increase in COF to 0.34 at 500 °C was observed. Progression of surface compositional changes during sliding at different temperatures was monitored using micro-Raman and X-ray photoelectron (XPS) spectroscopy. It was found that formation of a MoS2 rich transfer layer was responsible for maintaining low COF values up to 200 °C at which point MoO3 formation started. At T > 350 °C, MoO3 covered most of the contact surfaces resulting in an increase in the COF and volumetric wear losses accompanied by Al transfer to the coating surfaces. Accordingly, it can be suggested that the safe operation window for use of Ti-MoS2 coating against Al-Si alloys ranges between 25 °C and 350 °C.