Compatibility between Various Ionic Liquids and an Organic Friction Modifier as Lubricant Additives

Tribological performance of a boundary lubrication contact is largely dominated by the friction modifier (FM) and anti-wear additive (AW) in the lubricant. While oil-soluble ionic liquids (ILs) have recently demonstrated promising AW functionality, their compatibility with FMs is little known and even less understood for non-ferrous alloys. Here we report the latest results for several selected ILs when used together with an organic FM (OFM) in lubricating a steel-bronze contact. Depending on the IL chemistry, either synergistic or antagonistic effects were observed. The three aprotic ILs ([P8888][DEHP], [P66614][BTMPP] and [P66614][C17H35COO]) seemed to degrade the OFM’s lubricating performance. In contrast, the protic IL [N888H][DEHP] exhibited a strong synergistic effect with the OFM, yielding ultra-low steady-state friction coefficient (0.02) and wear rate (<10-8 mm3/N-m), which significantly outperformed the IL or the OFM alone. Surface characterization found no chemically-reacted tribofilm on the br...