Lubricity of Ionic Liquids Containing Phosphonate Functional Groups as Lubricants for Aluminum-on-Steel Contact

A series of novel 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 on an SRV test rig in a ball-on-disc configuration, using conventional ionic liquid 1-propyl-3-octylimidazolium hexafluorophosphate (coded as LP308) as a comparison. The worn Al alloy surfaces lubricated with different mediums 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 new ionic liquids showed higher viscosity than LP308. The synthetic ionic liquids had excellent friction-reducing and antiwear ability to the aluminum-on-steel contact and were superior to phosphazene (X-1P) in terms of the friction-reduction ability and load-carrying capacity, which was 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, showed the best lubricity and antiwear ability for the aluminum-on-steel system.