Toward resolving anxiety about the accelerated corrosive wear of steel lubricated with the fluorine-containing ionic liquids at elevated temperature

Ionic liquids (ILs) have potential as high-temperature liquid lubricants based on their extremely low vapor pressure and chemical stability. There are many ILs that contain a fluorine atom in their anion moiety. However, since fluorine-containing liquid lubricants such as perfluoropolyether (PFPE) sometimes show very large wear of steel due to an excessive chemical reaction between their fluorine atom and the iron, similar excessive corrosive wear of steel is anticipated when fluorine-containing ILs are used at high temperature. Eight types of ILs with the same bis(trifluoromethanesulfonyl)amide (TFSA)-anion moiety were used as sample oils. Tribotests were carried out with a ball-on-disk-type reciprocating motion tribometer by changing the testing oil temperature from room temperature (RT) to 250 °C. X-ray photoelectron spectroscopy (XPS) analyses were carried out to determine the extent of chemical reaction. All eight of the sample ILs tested showed increase in friction and wear with the rise in temperature, and decreasing friction and wear after that. A very marked increase in wear was observed at 250 °C for all of the sample ILs, due to the deterioration of the ILs. We also found that the wear increase with increasing temperature was derived not from the excessive corrosive wear but from the reduction in the load-bearing capability of fluids due to the decrease in viscosity. In this region, the formation of metal fluoride and sulfide increased very slightly with the temperature. In the temperature range in which the friction and wear were decreased with temperature, the formation of metal fluoride and sulfide increased. A high degree of fluoride formation was detected by XPS with the PFPE lubricant, with which the level of wear was very much larger than that with the TFSA-based ILs. It can be concluded that there is no need to worry about the accelerated corrosive wear of the TFSA-based ILs up to 200 °C.