Lubricant film formation properties of alkyl imidazolium tetrafluoroborate and hexafluorophosphate ionic liquids

The film thickness and friction properties of four imidazolium ionic liquids (1-butyl-3-methyl tetrafluoroborate and hexafluorophosphate and 1-hexyl-3-methyl tetrafluoroborate and hexafluorophosphate) were measured for mixed rolling-sliding conditions and the results compared to an additised mineral oil. Film thickness results showed that three of the fluids demonstrated classical EHL behaviour; however, the 1-butyl-3-methyl fluids gave anomalously thick, time-dependent films at low speeds (<0.3 m/s). Post-test inspection of the specimens revealed a loosely bound brown film deposited in the track. Film formation appeared to originate in the bulk fluid where brown “fibrous” agglomerations were observed. These were flocculated by shear flow and deposited in the track after passing through the contact. Overall the RTIL friction coefficients were less than the mineral oil for all conditions investigated. In the absence of thick film formation all RTILs gave a similar friction coefficient of 0.03 in the boundary regime, which is thought to be due to electrical double layer formation. In the fluid film regime traction was determined by the nature of the anion.