Ionic liquids as lubricants for steel–aluminum contacts at low and elevated temperatures

Room temperature ionic liquids (ILs) are high performance fluids with a wide thermal stability range. In this work we present the first study of ILs as lubricants under a wide range of temperature conditions (−30, 100, and 200 °C). The tribological performance of the imidazolium ionic liquids 1-hexyl, 3-methyl (L106) and 1-octyl, 3-methyl (L108) imidazolium tetrafluoroborates have been compared with that of a mineral oil (MO) and the synthetic ester propylene glycol dioleate (PGDO) in pin-on-disk aluminum–steel contacts. ILs show lower friction and wear values than conventional oils at all temperatures. The lubricating performance depends on thermal stability, polarity of the molecules, their ability to form ordered adsorbed layers and the tribocorrosion processes, which take place at the interface. While the conventional oils MO and PGDO fail above 150 °C due to thermal decomposition, the longer alkyl chain L108 provides an effective surface separation at all temperatures. L108 only shows friction and wear increments at −30 °C in the presence of water, due to severe abrasion. While the more polar, shorter alkyl chain L106 shows severe wear at 200 °C due to aluminum fluoride wear debris formation by tribocorrosion reactions. The time for tribocorrosion to take place has been determined from friction increments and wear debris generation. Wear mechanisms are discussed on the basis of SEM, EDS, and XPS results.