Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid in Nanoscale Single-Asperity Sliding Contacts

While phosphonium phosphate ionic liquids (ILs) have been evaluated as additives for engine oils owing to their excellent physico-chemical properties, miscibility with hydrocarbon fluids, and promising tribological properties, their lubrication mechanism is still not established. Here, atomic force microscopy (AFM) nanotribological experiments are performed using diamond-like carbon-coated silicon tips sliding on air-oxidized steel in neat trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate IL. The AFM results indicate a reduction in friction only after the removal of the native oxide layer from steel. Laterally resolved analyses of the steel surface chemistry reveal a higher concentration of bis(2-ethylhexyl)phosphate ions adsorbed on regions where the native oxide is mechanically removed together with a change in surface electrostatic potential. These surface modifications are proposed to be induced by a change in adsorption configuration of bis(2-ethylhexyl)phosphate anions on metallic iron compared to their configuration on iron oxide together with a reduction of surface roughness, which lead to the formation of a densely packed, lubricious boundary layer only on metallic iron.