Effect of ionic liquids’ chemistry on their lubrication behaviour under various contact pressures

Abstract The promising properties of Ionic Liquids (ILs) such as their low vapour pressure, high thermal stability and high thermal conductivity make them strong candidates to be used as either neat lubricants or additives. The effect of ionic liquids' chemistry on their lubrication behaviour is examined in this study by comparing the performance of five neat ILs, i.e. Ethyl Methyl Imidazolium Ethyl Sulfate (EMIM ESU), Ethyl Methyl Imidazolium Octyl Sulfate (EMIM OSU), Butyl Methyl Imidazolium Octyl Sulfate (BMIM OSU), Ethyl Methyl Imidazolium Ethyl Phosphate (EMIM EP) and Butyl Methyl Phosphonium Methyl Sulfate (PSU). The first three are used to reveal the effects of both anion and cation chain length, while the other two are used to reveal the effect of anion and cation types. Tribological experiments are performed using a nanotribometer with a borosilicate ball rubbing against a silicon plate. 3D optical microscopy and X-ray Photoelectron Spectroscopy (XPS) are used to examine surface morphology and surface chemistry, respectively. The results showed that all parameters related to ILs’ chemistry have a strong influence on the tribological behaviour of the lubricated surfaces. The increase of anion/cation chain length leads to a significant decrease in wear and friction coefficients. The influence of anion and cation types appeared to vary depending on the contact pressure.