This paper studies the tribological performance of two phosphonium cation-based ionic liquids: trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P6,6,6,14][(iC8)2PO2]) and trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P6,6,6,14][BEHP]), as lubricant additive at 1 wt% to a polyalphaolefin. A comparison of their tribological behavior was made to that of one type of zinc dialkyldithiophosphate (ZDDP). Stribeck curve tests were made in a Mini Traction Machine (MTM) tribometer at entrainment speeds ranging from 2000 to 10 mm/s, 30 N-load, slide-to-roll ratio (SRR) of 50%, and temperatures of 40, 60, 80 and 100 °C. Tribofilm formation tests were also conducted in the MTM at 100 °C, load of 50 N, entrainment speed of 150 mm/s, SRR of 50%, and duration of 60 min. Additionally, reciprocating 60-min wear tests at 60 N-load, frequency of 15 Hz, stroke length of 4 mm and at room temperature were performed with IL-containing mixtures at 0.5 and 1 wt%. Coefficient of friction was recorded during the tests, and wear on the discs surface was measured using confocal microscopy. SEM-EDS and XPS were also used for studying the lubricant-surface interactions after these tests. Lubricants including [P6,6,6,14][(iC8)2PO2] exhibited better tribological performance than [P6,6,6,14][BEHP] ones and close to the ZDDP results. SEM images of worn surfaces exhibited evidence of plastic deformation and adhesive wear and EDS analysis showed that only active elements (P, S, Zn) were detected for mixtures containing ZDDP. XPS analysis indicated a different lubrication mechanism of the blends with ionic liquids compared with the ZDDP ones.
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