Phosphonium cation-based ionic liquids as neat lubricants: Physicochemical and tribological performance

This paper studies physicochemical properties (density, viscosity, corrosion, thermogravimetry, and conductivity) and tribological behavior of five phosphonium cation-based ionic liquids ([P66614][BEHP], [P66614][C8C8PO2], [P66614][NTf2], [P44414][DBS], and [P4442][DEP]) as neat lubricants within steel-steel contact. A reciprocating ball-on-disc configuration was used to perform tribological tests. Worn surfaces were analyzed by confocal microscopy, SEM, EDS and XPS. Main results showed that: [P66614][NTf2], [P44414][DBS], and [P4442][DEP] showed corrosion activity; the ionic liquids with more reactive phosphate type anion ([P66614][BEHP] and [P4442][DEP]) had the best tribological behavior; the best tribological results were related to the tribofilm formation of two phosphate species.