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 antiwear (AW) additives 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 nonferrous 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 an ultralow steady-state friction coefficient (0.02) and a low 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 bronze worn surface. On the other hand, a unique physically adsorbed surface film as a result of interconnection between the IL and OFM molecules by hydrogen bonds is proposed on the basis of chemical analysis. Such an adsorption surface film is expected to be difficult to compress vertically but easy to shear horizontally, leading to low friction and wear.