A Comparison of the Tribological Properties of Two Phosphonium Ionic Liquids

For over two decades, ionic liquids have been among the most exciting lubrication topics. Ionic liquids were investigated by using them as neat lubricants and lubricity-enhancing additives. However, new and unique features were revealed by introducing new ionic liquids. This paper compares the tribological properties of two ionic liquids with the same trihexyltetradecylphosphonium [P 6,6,6,14] cation and different anions—dicyanamide [DCN] and bis(2,4,4-trimethylpentyl)phosphinate. The widely investigated 1-Butyl-3-methylimidazolium hexafluorophosphate [BMIM] [PF6] ionic liquid was used as a reference. The lubricity was comprehensively investigated using two testing modes: reciprocation in a ball-on-plate tribometer and continuous sliding in a ball-on-disc tribometer. The tests were performed at temperatures of 30 and 80 °C. The friction, wear, and film thickness were evaluated, and a worn surface analysis was conducted. It was found that in the case of reciprocation, anion has a significant effect on the lubricity. The difference was particularly evident when the results at two temperatures were compared. The ability to build a low-friction tribo-film was suggested as the primer source of lubricity. In the case of continuous sliding, the differences were not as noticeable. In this case, viscosity was assigned to be the leading property.