Protic ammonium carboxylate ionic liquid lubricants of OFHC copper

Abstract

The protic ionic liquids (PILs) di-[bis(2-hydroxyethyl)ammonium] adipate (DAd), bis(2-hydroxyethyl)ammonium salicylate (DSa) and bis(2-hydroxyethyl)ammonium oleate (DO), composed of organic anions and cations, were selected for pin-on-disc tribological tests as lubricants of OFHC copper discs sliding against OFHC copper pins, after corrosion tests. The tribological performance of the new PILs was compared with that of the non-polar synthetic lubricant PAO 6 and with the aprotic ionic liquid (APIL) 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM](CF3SO2)2N). The order of coefficients of friction and wear rates was: DAd<DSa<([HMIM](CF3SO2)2N)<PAO 6. The fatty acid derivative DO, with a long lateral chain shows a transition from low to high friction and wear after the running-in period. The best performance is observed for the di(secondary ammonium) dicarboxylate DAd, with a friction coefficient one order of magnitude lower than that of PAO 6 and the absence of wear on the copper surfaces. DSa, with only one ammonium cation and one anion containing both carboxylate and hydroxyl groups, also shows outstanding tribological performance with a friction reduction of 70% and a wear rate reduction of 94% with respect to PAO 6. These results are interpreted in terms of the formation of hydrogen bonds and stable boundary films due to the higher number of ammonium protons and carboxylate and hydroxy groups present in DAd with respect to the rest of the lubricants. When DAd is added to PAO in a 1wt% proportion, a friction reduction of 20% and a wear rate reduction of 34% with respect to PAO 6 take place. Wear mechanisms and surface interactions are discussed.