The tribological properties of ionic liquids (ILs) have been widely investigated, and their excellent performance has been acknowledged by many researchers. However, their lubricity is closely linked to operating conditions. For wider applications, the complexity of their behavior must be understood. In this study, we compared the tribological properties of ILs at temperatures of 30 °C and 80 °C. Two ammonium ILs with different cation side-chain lengths, methyl-trioctylammonium [N1888] and tributylmethylammonium [N4441] and the same anion bis(trifluoromethylsulfonyl)imide [NTf2] were selected. Their tribological properties were investigated in the lubrication of three different friction pairs: bearing steel–bearing steel E 52100, bearing steel–stainless steel 304, and bearing steel–aluminum alloy 6082 T6. Tribotests were performed using a ball-on-plate reciprocating tribometer for 30 min at 4 N, 15 Hz, and a 1 mm stroke length. The worn surfaces were analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated diminished lubricity at higher test temperatures. The performance difference between the investigated temperatures depended on the friction pair, being almost negligible in bearing steel–bearing steel friction pairs and significantly different for bearing steel–aluminum friction pairs. The wear reduction ability was more sensitive to the test temperature than friction did. The investigated ILs were superior to the 5W–40 motor oil at both temperatures.
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