Antiwear Properties of Binary Ashless Blend of Phosphonium Ionic Liquids and Borate Esters in Partially Formulated Oil (No Zn)

In this study, interaction of ionic liquid (IL) and borate esters (SBs) as antiwear (AW) additives with steel surfaces in tribological contacts was examined using blends which contained no prior AW additives but all the other ingredients present in a fully formulated engine oil. In detail, low phosphorus oil blends were prepared by adding trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (P_DEHP) at 700 ppm phosphorus and 2-methoxy-4,4,6-trimethyl-1,3,2-dioxaborinane or trimethoxyboroxine at 200 ppm boron treat rate to a partially formulated oil. The tribological properties of these novel ionic liquid (IL) additive and IL + SB additive mixtures were compared with those of zinc dialkyldithiophosphate (ZDDP) at equal phosphorus levels in the oil blends. Tribological experiments with a reciprocating cylinder on flat contact revealed that both P_DEHP and binary mixtures of P_DEHP + SB offer superior wear protection than ZDDP and the partially formulated oil without AW additives, expressed by a wear reduction of minimum 50%. X-ray absorption near edge structure spectroscopy (XANES) analysis revealed that tribologically formed films are primarily composed of calcium phosphate for oils with AW additives. The interaction of P_DEHP with SB results in additional boron oxide/boric acid and to some extent boron phosphate domains incorporated into the tribofilms.