Lubricity of ionic liquids containing phosphonate functional groups as lubricants for aluminum-on-steel contact

A series of functional room-temperature ionic liquids (RTILs) of 1-(O,O-diethyl phosphonyl-n-propyl)-3-alkylimidazolium hexafluorophosphate was prepared and evaluated as a kind of promising lubricant for aluminum-on-steel contact. The viscosity and density of the synthetic RTILs were measured. The glass transition temperature and the heat capacity jump associated with the transition from the non-equilibrium glass to the metastable supercooled liquid were analyzed. The lubricity of the synthetic RTILs as the lubricants for a aluminum-on-steel contact was comparatively investigated by SRV test rig in a ball-on-disc configuration, using conventional ionic liquid l-propyl-3-octylimidazolium hexafluorophosphate (coded as LP308) to compare. The worn Al alloy surfaces lubricated with different media were observed on a scanning electron microscope, while the chemical states of some typical elements in the neat synthetic ionic liquids and on the worn Al surface lubricated therewith were analyzed by means of X-ray photoelectron spectroscopy. It was found that the synthetic ionic liquids show higher viscosity than LP308. The synthetic ionic liquids have excellent friction-reducing and antiwear ability to the aluminum-on-steel contact and are superior to phosphazene (X-1P) in terms of the friction-reduction ability and load-carrying capacity, attributed to the formation of chemical adsorption and tribo-chemical reaction boundary film on the worn surface during sliding process. Of the three synthetic ionic liquids, [DPPOIM] [PF₆] shows the best lubricity and antiwear ability for the aluminum-on-steel system.