Lubricity and corrosiveness of ionic liquids for steel-on-steel sliding contacts

Ionic liquids are expected to be used as new high-performance lubricants because of their low volatility, high thermal stability, and high oxidation stability. It is well known that some ionic liquids exhibit excellent lubricity for metals. On the other hand, there is concern about the corrosiveness of ionic liquids caused by tribo-chemical reactions. In this study, the lubricating properties of seven kinds of commercially available ionic liquids were evaluated under steel-on-steel sliding contacts using an SRV oscillating sliding tester. The worn surfaces were analyzed by optical microscopy, confocal laser scanning microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. The halogen-containing ionic liquids exhibited excellent lubricity for steel compared with the halogen-free ionic liquids. The X-ray photoelectron spectroscopy analytical results showed the formation of iron fluoride on sliding surfaces lubricated with the fluorine-containing ionic liquids. Formation of this iron-fluoride boundary layer reduced friction; however, corrosion occurred on the worn surfaces after the sliding. On the other hand, less corrosive damage was observed on the worn surfaces that slid under the dry-nitrogen atmospheric condition. These results suggested that the hydrofluoric acid, which was produced by the reaction between iron fluoride and contaminated water from the atmosphere, caused the corrosion on the worn surface. The halogen-free ionic liquids showed inferior lubricity compared with the halogen-containing ionic liquids, though they did not cause any remarkable corrosion. The corrosion-inhibition effect of the phosphorus element was shown in the halogen-containing ionic liquids composed of a phosphate anion or phosphonium cation. The friction-reduction effect was, however, inferior to that of the phosphorus-free ionic liquids. The tribo-chemical reaction of the phosphorus contained in the ionic liquids yielded a phosphate boundary layer that prevented the formation of iron halide on the sliding surface and enhanced the wear resistance.