Tribo-chemical reactions of anion in pyrrolidinium salts for steel-steel contact

The tribological performance of three ionic liquids (ILs) containing the same cation, 1-butyl-1-methylpyrrolidinium [C4C1Pyrr]+, and the anions tetracyanoborate [B(CN)4]−, trifluoromethanesulphonate [CF3SO3]− and tris(pentafluoroethyl)trifluorophosphate [(C2F5)3PF3]− were analyzed. A reciprocating ball-on-plate tribometer was used at room temperature and under loads of 14, 18, 22 and 26 N. Two perfluoropolyethers were chosen as reference oils. [C4C1Pyrr][CF3SO3] showed the most significant friction reduction. Nevertheless, its anti-wear performance was, in general, worse than that of [C4C1Pyrr][(C2F5)3PF3], which led to comparable or lower wear than the corresponding reference oil. [C4C1Pyrr][B(CN)4] showed poorer tribological properties than the other ILs. XPS analyses on the worn surfaces lubricated with [C4C1Pyrr][CF3SO3] or [C4C1Pyrr][(C2F5)3PF3] revealed that nitrates and nitrogen oxides were tribochemically formed from the cation, whereas the anion was mainly adsorbed.