Environmental influence on the surface chemistry of ionic-liquid-mediated lubrication in a silica / silicon tribopair

In this study, 1-ethyl-3-methyl imidazolium trifluoro tris(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate [EMIM] FAP and 1-hexyl-3-methyl imidazolium tris(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate [HMIM] FAP were selected as lubricants for silica/silicon surfaces. Pin-on-disk tribometry was used to test the performance of these lubricants under two different environmental conditions (humid air and a nitrogen atmosphere). The surface reactivity of the ionic liquids under mechanical stress was investigated ex situ by X-ray photoelectron spectroscopy. Environmental conditions appear to affect the mechanism of boundary lubrication in different ways, depending on the contact pressure. Tests carried out at 0.5 N applied load showed low friction and no detectable wear in a nitrogen atmosphere, and a substantial increase in both wear and friction in humid air. It is proposed that the presence of water in the IL induces a change in the structure of the confined lubricant film, leading to contact between the sliding surfaces. At higher load (4.5 N), the observation of wear, both under nitrogen and in humid air, reveals that the film is no longer able to prevent contact between asperities, which now dominates the observed tribological behavior. XP-spectra acquired on samples tribostressed at high load, under the two environmental conditions, reveal evidence for the formation of a reaction layer that is hydrolyzed or oxidized in the presence of water and oxygen, suggesting that the variation of wear with the environment is related to changes in the tribochemical reactions involving the silicon surface.