The effect of chemical structure on the tribological performance of perfluorosulfonate ILs as lubricants for Ti-6Al-4V tribopairs

The lubrication of Ti-6Al-4V tribopairs with various tetraalkyl phosphonium perfluorosulfonate ionic liquids (TPP ILs, C6F13SO3P4444, C8F17SO3P4444 and C8F17SO3P8888) was systematically investigated and compared with conventional ILs lubricants based on BF4− or PF6− and TFSI− anions. The results show that the TPP ILs have superior lubricating properties to existing IL lubricants on Ti-6Al-4V. Analysis of the lubrication mechanism showed that both physicochemically adsorbed films and tribochemical reaction films were produced on Ti alloy tribopair surfaces during sliding when lubricated with the TPP ILs. The formation of physicochemically adsorbed films between the TPP ILs and the friction surfaces were mainly responsible for the superior tribological properties compared to the reference samples. By comparing the tribological properties of TPP ILs with those of the tetrabutyl ammonium perfluorosulfonate IL C8F17SO3N4444 and tetrabutyl phosphonium carboxylate IL C8F15O2P4444, the chemical structures of the perfluorosulfonate ILs was found to have significant influence on the lubricating ability. The introduction of the sulfonate group and P-containing functional group effectively improved the lubricating properties of the TPP ILs to lubricate Ti alloy tribopairs. This work provides insight into the behavior of IL lubricants on Ti alloy surfaces, and will inspire further studies into use of liquid lubricants for titanium alloys.