Friction and anti-wear properties of two tris(pentafluoroethyl)trifluorophosphate ionic liquids as neat lubricants

The tribological behaviour was evaluated for two ionic liquids (ILs) as neat lubricants for the sliding pair stainless steel AISI420 and 100Cr6 under four loads (14, 18, 22 and 26 N). The ILs contain the same anion, tris(pentafluoroethyl)trifluorophosphate [(C2F5)3PF3]− (also known as [FAP]−), combined with the cations 1-butyl-2,3-dimethylimidazolium [C4C1C1Im]+ or trihexyl(tetradecyl)phosphonium [P6,6,6,14]+. The IL [P6,6,6,14][(C2F5)3PF3] showed always better lubricating properties than [C4C1C1Im][(C2F5)3PF3] in terms of friction coefficient and wear volume. Under the same conditions, tribological tests were performed with two perfluoropolyethers as reference lubricants developing, in general, higher friction coefficients and also higher wear volumes than those obtained with both ILs. Surface interactions were studied by SEM observations and XPS analyses of the neat ILs and of the wear tracks.