Ionic liquids as oil additives for lubricating oxygen-diffusion case-hardened titanium

This study attempts to use ionic liquids (ILs) as oil additives to lubricate oxygen-diffusion (OD) case-hardened titanium for improved friction and wear behavior. Four oil-soluble ILs were used as additives in a base oil for boundary lubrication of an OD-treated Ti-6Al-4V surface sliding against a bearing steel ball. The ILs improved the friction behavior to various extents and the two phosphate ILs clearly outperformed the conventional ZDDP. Unlike ZDDP that adversely caused a higher material loss, ILs exhibited excellent wear protection suggesting good material-chemical compatibility. Particularly, the carboxylate IL protected the OD-Ti surface from any measurable wear. The morphology and chemical composition of the worn surfaces and tribofilms were examined to gain insights of the anti-wear mechanism.