Halogen-free borate ionic liquids as novel lubricants for tribological applications

The unique properties of ionic liquids favour their applications in diverse fields, such as synthesis, catalysis, electrochemistry and nanotechnology. Their application as lubricants in several systems has found that these substances are able to provide remarkable protection against wear and significantly reduce friction whether they are used as additives or in the neat form. Therefore, in the present work, a further approach to provide halogen-free ionic liquids as lubricants for steel–steel contacts is discussed. The special chemical compositions of two imidazolium borane ionic liquids selected allowed the replacement of hetero-elements such as fluorine and sulphur that are usually found in ionic liquids. Their tribological properties were evaluated with a Schwing–Reib–Verschleiss (SRV) tribometer using an oscillating steel–steel contact with ball-on-disc geometry under boundary conditions. The addition of a phosphate based ionic liquid significantly improved the tribological properties of the imidazolium borane ionic liquids in comparison to a reference halogen containing ionic liquid (used as neat and lubricant additive). XPS analyses of the wear scars confirmed the formation of a phosphate based tribofilm that significantly improved the friction reducing properties and anti-wear performance of the lubricants.