Room temperature ionic liquids as lubricant additives in steel-aluminium contacts: Influence of sliding velocity, normal load and temperature

1-n-Alkyl-3-methylimidazolium X− [X = PF6; n = 6 (L-P106). X = BF4; n = 2 (L102), 6 (L106), 8 (L108). X = CF3SO3; n = 2 (L-T102). X = (4-CH3C6H4SO3); n = 2 (L-To102)] and 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (L-PY104) have been studied as 1 wt.% base oil additives in variable conditions pin-on-disk tests for AISI 52100 steel-ASTM 2011 aluminium contacts. Friction coefficients and wear rates increase under increasing normal loads. Effective lubrication is obtained for a 0.15-0.20 ms−1 sliding velocity. Low friction and minimum wear rates are achieved for all additives at 25 °C, 2.45 N and 0.15 ms−1. A transition to an abrasive wear mechanism is observed at 0.06 ms−1, both at room temperature and at 100 °C. Friction coefficients for IL addtives are similar or lower than for neat ILs, while wear rates for 1 wt.% ILs can be several orders of magnitude lower than those for neat ILs. The exception is the long alkyl chain L108 which always shows better lubricating ability as pure lubricant, probably due to its lower miscibility with the base oil. Energy dispersive (EDS) and X-ray photoelectron (XPS) spectroscopies show surface interactions but, in contrast with neat ILs, no tribocorrosion processes are observed for the IL additives. Aluminium corrosion is only observed after 100 h immersion in water with 1 wt.% L102.