Thin Film Properties of Thermally Stable Protic Ionic Liquids

Ionic liquids (ILs) have aroused particular interest as potential lubricants because their thermal stabilities are quite superior to those of conventional lubricants, however, their thin film properties have not been investigated very much. Newly synthesized diazabicycloundecenium (DBU) and imidazolium (IM) sulfonates were evaluated as a potential molecularly thin film lubricant for carbon-coated media. The ILs have steady low friction coefficients even after a heat treatment at 250 °C at which those of the conventional lubricant, Z-DOL, steeply increased. A systematic analysis of the thin films was performed using FTIR-RAS and their thermal degradation kinetics were examined. The thermal degradation process of the IL thin films could be described by a zero-order kinetics mechanism. The calculated activation energies show that the ILs physiosorb on the surface and the thermal stabilities of the IL films are significantly lower than that of the bulk. The FTIR measurements indicated two friction reduction regimes for the ILs; i.e., the perfluoroalkylsulfonate molecules, which seem to be a thermally decomposed product from the IL molecules, still remained and oriented parallel to the surface in the temperature range of 200 °C or higher and reduced the friction.