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After doing several miscibility essays with eight ionic liquids (ILs) and four base oils, the ILs tri(butyl)ethylphosphonium diethylphosphate P4,4,4,2C2C2PO4 and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate P6,6,6,14(C2F5)3PF3 were selected to be studied as lubricant additives. The neat IL P4,4,4,2C2C2PO4, the base oils, and several blends were characterized in terms of density, viscosity, and thermal stability. The tribological performance of the miscible base oil/IL blends (1 wt %) and the neat P4,4,4,2C2C2PO4 were evaluated for the lubrication of an AISI 420 steel-100Cr6 steel contact pair. The friction coefficients and wear volumes obtained are also compared with those corresponding to the pure base oils and their mixtures with conventional additive zinc dialkyldithiophosphate (ZDDP). As neat lubricants, P4,4,4,2C2C2PO4 showed the best antifriction ability, whereas in terms of wear, better results were obtained with P6,6,6,14(C2F5)3PF3. However, higher improvements in both friction and wear were found for blends containing P4,4,4,2C2C2PO4. XPS analyses of the worn surfaces lubricated with these mixtures indicated the presence of phosphorus in the tribofilm formed on the wear track. However, this compound was slightly detected on tribosamples lubricated with blends containing P6,6,6,14(C2F5)3PF3.
Gómez‐Otero et al. (Mon,) studied this question.