Interaction of phosphonium ionic liquids with borate esters at tribological interfaces

Abstract

Synergistic interaction between ionic liquids (ILs) and soluble borate (SB) esters was examined in the context of tribofilm formation for antiwear applications. ILs composed of phosphonium cation and phosphate anion (P_DEHP) and dithiophosphate anion (P_DEPDT), respectively, were mixed with borate esters in group I base oil at 1000 P ppm and 200 B ppm and their tribological properties were evaluated using a cylinder on reciprocating flat and ball on rotating disc contact tribometers. Friction and wear results were compared with those of a reference oil containing zinc dialkyl dithiophosphate (ZDDP). Results indicate that the addition of SB to ILs results in a shortened incubation time for tribofilm formation and significantly better wear protection in comparison to ZDDP. The chemical compositions of the interfacial tribofilms examined with X-ray absorption near edge structure (XANES) spectroscopy suggest that interaction between IL + SB results in incorporation of boron as boron oxides and boron phosphate in the bulk of the tribofilms while the surface is largely composed of short chain iron polyphosphates. On the other hand, tribofilms derived from only ILs are composed of short chain iron polyphosphate and tribofilms derived from SB1 and SB2 exhibit trigonal B chemistry.