Effects of Relative Humidity on Lubricating Properties of Ionic Liquids

Minimizing friction losses in mechanical systems is essential for realizing a low-carbon society, as it will result in energy and resource conservation. In this study, we focused on the suitability of ionic liquids that form an electric double layer for use as liquid lubricants. Ionic liquids have been reported to exhibit ultralow friction with friction coefficients of less than 0.01. However, it is necessary to elucidate the effects of water on their lubricating properties, as water can cause corrosive wear on metallic sliding materials during lubrication using fluorine-based ionic liquids. In addition, water destroys the layer formed at the friction interface and adversely affects the lubricating properties. Here, we investigated the lubricating properties and lubricating mechanisms of both hydrophilic (1-butyl-3-methylimidazolium dicyanamide [BMIM][DCN] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]), and hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] and 1butyl-3-methylimidazolium tris (perfluoroalkyl) trifluorophosphate [BMIM][FAP]) ionic liquids at different relative humidity (RH) levels (15, 50, and 80%). [BMIM][DCN] exhibited good lubricating properties at 15% RH because of the formation of an adsorption layer derived from both types of ions. However, the adsorption layer was destroyed by water molecules as the humidity was increased. [BMIM][BF4] also exhibited good lubricating properties at 15% RH. This ionic liquid formed a chemical layer as well as an adsorption layer. However, these layers were destroyed by water molecules at 50 and 80% RH. In the case of [BMIM][PF6] too, a chemical layer and an adsorption layer were formed at 15 and 50% RH, and these layers exhibited the best lubricating properties. On the other hand, the layers were destroyed by water molecules at 80% RH. Finally, [BMIM][FAP] exhibited good lubricating properties even under high-RH conditions because this ionic liquid was the most hydrophobic.