How anion and cation species influence the tribology of a green lubricant based on ionic liquids

A group of halide-free ionic liquids involving two different anions (methyl sulfate and methyl sulfonate) and four types of cations (short-chain tetraalkylammonium, dialkylpyrrolidinium, choline, and methoxycholine) were investigated as 2.5 wt% additives in glycerol as a model base fluid, yielding highly biodegradable polar lubricants for study of ionic liquid interaction with a substrate. The results were compared to the behavior of conventional bis(trifluoromethylsulfonyl)imide ([Tf2N]) ionic liquids with identical counter-ions. The neat ionic liquids (100 wt%) were tested in identical manner and compared to the behavior when they operate as additives. Tribotests were performed in a ball-on-disc configuration under boundary conditions, by lubricating steel-steel couples at room temperature and at 100 degrees C. Wear reduction was achieved for all temperatures, and the results were strongly anion-dominated, with good results for methyl sulfates and the [Tf2N] references. Particularly for higher temperatures, ionic liquids were also able to reduce friction by a substantial amount, with a clear order between the individual anions, and the lowest values were again obtained for methyl sulfates. Cationic influence on the test results was found to be subordinate for both temperatures. It could be recognized that at elevated temperatures, the newly formulated lubricants containing an ionic liquid as an additive behaved similarly to neat ionic liquids in terms of friction and wear reduction. Using X-ray photoelectron spectroscopy analyses, the formation of a beneficial iron sulfide film was detected, with the sulfur originating from the sulfate of the ionic liquid, presumably as a result of a redox reaction with metallic iron. For this mechanism, a hypothesis for possible reaction pathways was developed.