Assessing boundary film forming behavior of phosphonium ionic liquids as engine lubricant additives

The reduction of friction and wear losses in boundary lubrication regime of a piston ring-cylinder liner tribo-system has always been a challenge for engine and lubricant manufacturers. One way is to use lubricant additives, which can form boundary film quickly and reduce the direct contact between asperities. This article focuses on the assessment of boundary film forming behavior of two phosphonium-based ionic liquids (ILs) as additives in engine-aged lubricant to further improve its film forming capabilities and hence reduce friction and wear of contacting surfaces. A reciprocating piston ring segment-on-flat coupon under fully flooded lubrication conditions at room temperature (approx. 25 °C) was employed. The trihexyltetradecyl phosphonium bis(2-ethylhexyl) phosphate and trihexyltetradecyl phosphonium bis(2,4,4-tri-methylpentyl) phosphinate ionic liquids were used as additives in 6 vol. % quantity. Benchmark tests were conducted using fully formulated new lubricant of same grade (with and without ILs). Results revealed that the addition of phosphonium ILs to engine-aged lubricant led to quicker initiation of boundary film forming process. In addition, friction and wear performance of engine-aged lubricant improved by the addition of both ILs and these mixtures outperformed the fresh fully formulated oil. Chemical analysis showed higher concentration of phosphorus element on the worn surface indicating presence of ILs in the formed tribofilms