Friction and wear behaviors of ionic liquid of alkylimidazolium hexafluorophosphates as lubricants for steel/steel contact

Ionic liquids of the alkylimidazolium hexafluorophosphate type were prepared and evaluated as lubricants for the contact of steel/steel. Their tribological properties of the synthetic ionic liquid as the lubricants were investigated on an Optimol SRV oscillating friction and wear tester in ambient condition and on a CZM vacuum friction tester at 1×10^-3 Pa. These synthetic ionic liquids show excellent tribological performance and are superior to the conventional lubricant of liquid paraffin containing 2 wt.% of zinc dialkyldithiophosphate (ZDDP) in terms of the friction-reduction ability and load-carrying capacity. The elemental composition and chemical nature of the boundary films generated on the steel surface were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS). The SEM results showed light abrasion and phosphorus on the worn surface after lubrication with the ionic liquids of alkylimidazolium hexafluorophosphates, and the XPS results indicated the occurrence of the tribochemical reaction of ionic liquid with iron during the sliding process, with the formation of FePO₄ and FeF₂.