Tribological properties of ultra-thin ionic liquid films on single-crystal silicon wafers with functionalized surfaces

Two kinds of room temperature ionic liquid (RTIL) films carrying vinyl and hydroxyl functional groups were prepared on single-crystal Si wafers by spin coating. The tribological properties of the RTIL films sliding against AISI-52100 steel ball and Si₃N₄ ball in a ball-on-plate configuration were investigated on a dynamic–static friction coefficient measurement apparatus, using perfluoropolyether (PFPE) film as a comparison. The tribological behaviors of the ionic liquid films sliding against the same counterparts at extended test durations were also evaluated using a universal UMT-2MT test rig. The morphologies of the wear tracks of the RTIL films and the counterparts were examined using a scanning electron microscope equipped with an energy-dispersive X-ray analyzer attachment. It was found that the tribological performances of the ionic liquid films were closely related to the chemical structures of the RTILs and the chemical characteristics of the substrate surfaces. The films of vinyl group functionalized ionic liquids on hydroxylated substrate and vinyl group modified substrate exhibited very good friction-reduction and wear-resistant properties. It was assumed that there were enough strong forces between the films and substrate in these cases, and the ionic liquid molecules maintained good flexibility simultaneously. The films on hydrogen-terminated and methyl-terminated substrate showed poor tribological performance, which could be related to the relatively weak forces between the films and substrates. Moreover, the films on hydroxylated substrate showed lower friction at higher sliding velocities, which was assumed to be governed by the more rapid adsorption of the ionic liquid molecules on the steel ball at a higher sliding velocity. In addition, the ionic liquid films also had excellent tribological properties as they slid against silicon nitride ball. Therefore, it was supposed that the ionic liquid films could be used as a kind of universal lubricant for various combinations of the frictional pair.