Influence of concentration and anion alkyl chain length on tribological properties of imidazolium sulfate ionic liquids as additives to glycerol in steel-steel contact lubrication

Commercially available ionic liquids were used as additives in the model lubricant fluid glycerol. The stability of the mixtures was controlled by measuring of turbidity. Tribological experiments have been performed on a reciprocating sliding tribometer in the boundary lubrication regime with three ionic liquid concentrations, as well as with neat glycerol and the neat ionic liquids at 100 °C. Wear and friction were measured, and the worn surfaces were examined with optical microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The results show the influence of the ionic liquid concentration and the anion alkyl chain length on the tribological behavior. Significant improvement in friction and wear reduction at low ionic liquid concentrations was detected and attributed to sulfur species in tribofilm.