Surface chemistry and tribological behavior of ionic liquid boundary lubrication additives in water

Ionic liquids have been intensely studied regarding their properties as an environmentally friendly solvent. Conversely, little work in the field of tribology has been accomplished to date. Ionic liquids posses many favorable properties that would suggest that they have significant potential for use in the tribology field. One area that is discussed here is the use of ionic liquids as a boundary lubricant additive for water. The chemical and tribochemical reactions that govern their behavior were evaluated for two ionic liquids. Under water lubricated conditions, silicon nitride ceramics exhibit a characteristic running in period of high friction, through which surface smoothing and tribochemical reactions lead to low friction coefficients. The running-in period of high friction is the period where the majority of the wear occurs. The use of a suitable boundary lubricant to reduce the running-in period could be of significant importance. A promising candidate for this application is ionic liquids. We present the affects that ionic liquids have on the friction and wear properties Of Si3N4, in particular the affects on the running-in period. Tribological properties were evaluated using a pin-on-disk and reciprocating tribometers. Solutions containing 2 wt % ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with x-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). The test specimens were 1" diameter Si3N4 disks sliding against (1/4)" Si3N4 balls. The use of ionic liquids as a boundary lubricant additive for water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms controlling the friction and wear include the formation of a transfer film and the inception of an electric double layer.