Boundary lubrication friction and wear are largely determined by the friction modifier (FM) and anti-wear additive (AW) in the lubricant. While ionic liquids (ILs) have recently demonstrated promising AW functionality, their compatibility with FMs is little known. Here we report the latest results for selected oil-soluble ILs when used together with either an organic FM (OFM) or a Mo-based FM. Both synergistic or antagonistic effects were observed depending on the IL chemistry. Specifically, a protic ammonium-phosphate IL exhibited a strong synergistic effect with both FMs, yielding ultra-low steady-state friction coefficient (~0.02) and wear rate (~10<sup>-9 </sup>mm<sup>3</sup>/N-m), which significantly outperformed the IL or either FM alone. Surface characterization discovered distinct friction reducing and wear protection mechanisms for the synergistic IL+FM pairs in lubricating a cast iron or bronze surface sliding against a steel ball. The cast iron surface benefited from a chemically-reacted tribofilm as a result of interactions among the IL molecules, wear debris, and contact area. In constrast, the bronze surface relied on a unique physically-adsorbed surface film as a result of interconnected IL and FM molecules by hydrogen bonding.

Using ionic liquids as lubricant additives together with friction modifiers

2019 Fall National Meeting

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