Boundary lubricity of phosphonium bisoxalatoborate ionic liquids

Abstract The lubricating performance of trihexyl(tetradecyl)phosphonium bis-(oxalato)-orthoborate (P-BOB) ionic liquid is analysed at 80°C and 140°C, together with an ionic liquid containing partially hydrated version of the anion. The reduction of the anions produces oxalate complexes that contribute synergistically to lower friction. To confirm the role of oxalate in enhancing lubricity, two ionic liquids, at different stages of the synthesis were used. One of the batches contained precursor orthoborate anions complexed with oxalic acid and consequently showed the lowest friction at 80°C. Upon heating the precursor was converted into [BOB]- and displayed the same friction at 140°C as the fully synthesised species. The mechanisms of the breakdown of the [BOB]- anion and formation of the tribofilm are elucidated.