Imidazolium hexafluorophosphate ionic liquids as high temperature lubricants for steel–steel contacts

A series of long-chain 1,3-dialkyl imidazolium ionic liquids (ILs) were synthesized and evaluated as lubricants for steel–steel contacts both at room temperature and 150 °C. Relationship between the alkyl chain length and the tribological properties of the ILs was investigated in detail. The results indicated that the ILs bearing long alkyl side chains have excellent friction-reducing and anti-wear properties, especially at high temperatures and high loads. The worn surfaces of steel discs were observed and analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The friction reduction and load carrying capacity were supposed to be due to the formation of high quality boundary films consisting of both tribochemical reaction film and the ordered absorption film.