Imidazolium ionic liquids as additives of the synthetic ester propylene glycol dioleate in aluminium–steel lubrication

Four room-temperature ionic liquids (ILs), 1-CnH2n+1-3-CH3-imidazolium X− [X = PF6; n = 6 (L-P106). X = BF4; n = 2 (L102), 6 (L106). X = CF3SO3; n = 2 (L-T102)], have been studied as 1 wt.% additives of the synthetic ester propylene glycol dioleate (PGDO) in pin-on-disk tests for AISI 52100 steel–ASTM 2011 aluminium contacts at 25 and 100 °C. At room temperature, friction values for PGDO + 1 wt.% IL are similar or higher than that of the base oil, and only the more polar additives L102 and L-T102 reduce wear rates. At 100 °C, all additives reduce both friction and wear with respect to the base oil. The results are discussed in terms of the molecular structure and properties of the lubricants. Wear mechanisms and surface interactions have been studied by energy dispersive (EDS) and X-ray photoelectron (XPS) spectroscopies.