Tribology and Surface Engineering for Lubricious Dry-Sliding Contacts of MoS2 Based Coatings

Molybdenum disulphide (MoS2) is a well-known solid lubricant with excellent lubricious and low wearing qualities when utilised in inert and high vacuum environments. However, the lubricity of MoS2 during room temperature sliding is drastically reduced in humid, terrestrial atmospheres. The reason behind this degradation is not fully understood, with multiple conflicting arguments as to why this may be the case. One leading hypothesis suggests that adsorbed water restricts easy lamellar shear and thus leads to higher coefficients of friction, with oxygen playing no part. An opposing hypothesis suggests that oxidation caused by water and oxygen in the atmosphere leads to a degradation of the coating – a theory which is often disputed, as it is claimed that the threshold temperature for oxidation of MoS2 is not reached. The purpose of this study was to elucidate the impact that water and oxygen have on the tribological and chemical degradation of MoS2 coatings. Friction tests of both aged and unaged MoS2 coatings were carried out using a micro-tribometer. The unaged coatings were tested in dry, 25% and 50% relative humidity (RH) air or nitrogen. Aged samples were aged in the same environment but returned to an ideal dry nitrogen environment for friction testing. Surface analysis techniques, such as Raman spectroscopy, Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy were used to analyse the chemical changes caused by ageing and friction testing. Results show that both air and water in the sliding environment lead to higher coefficients of friction of unaged MoS2. This is in line with the established literature. Higher coefficients of friction were also observed in air aged (N2 + O2) MoS2 compared to nitrogen (N2) aged MoS2. Raman spectra showed oxidation of air aged MoS2 in the wear scars at all humidity levels. Conversely, samples aged in nitrogen showed no oxidation at any humidity level, neither before nor after sliding. It was concluded that oxidation does not occur in MoS2 aged at room temperature when no sliding has occurred. However, samples aged in air displayed MoO2 and MoO3 in the wear scar after sliding, leading to the theory that gaseous oxygen is needed for the oxidation of MoS2 during sliding.