New Alkylether–Thiazolium Room-Temperature Ionic Liquid Lubricants: Surface Interactions and Tribological Performance

The use of newly synthesized alkylether-thiazolium ionic liquids as lubricants is described for the first time. Two ionic liquids composed of a thiazolium cation and a bis(trifluoromethanesulfonyl)amide ([Th][Tf2N]) or dicyanamide ([Th][(NC)2N]) anion have been studied, and their tribological behavior has been compared with that of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Im][Tf2N]) in pin-on-disk tests using sapphire balls against AISI 52100 or AISI 316L steels. All ionic liquids show higher contact angles on AISI 316L steel than on AISI 52100, the lowest values found for ([Im][Tf2N]) on both steel surfaces. AISI 52100 shows similar friction coefficients for all lubricants, and negligible wear rates for the ionic liquids containing the bis(trifluoromethanesulfonyl)amide anion. Immersion tests show no corrosion of AISI 52100 in imidazolium or thiazolium bis(trifluoromethanesulfonyl)amide ionic liquids. AISI 316L shows similar friction coefficients for both bis(trifluoromethanesulfonyl)amide ionic liquids, but the lowest wear rate is obtained for [Th][Tf2N]. An increase in friction coefficient and wear rate is observed for thiazolium dicyanamide. This increase is related to a tribocorrosion process due to decomposition of the thiazolium cation. XPS shows the formation of iron sulfide on the wear track on AISI 316L after lubrication with thiazolium dicyanamide. No tribocorrosion processes take place for the [Tf2N] ionic liquids.