Molecular structure insight into the tribological behavior of sulfonate ionic liquids as lubricants for titanium alloys

The wide industrial application of titanium alloys requires lubricating materials with superior performance. Although ionic liquids have been recognized as potential choices for excellent lubricants, research on their tribological properties as applied to titanium alloys is not mature and systematic. In this work, a series of halogen-free sulfonate IL lubricants with different anions and cations were designed, synthesized, and applied to the titanium alloy (Ti-6Al-4V) friction pairs. From the perspective of molecular structure, the influencing factors on their physicochemical properties (thermal stability, viscosity) and tribological performances (friction-reducing and anti-wear performances) were explored, including the influences derived from their active element, chain length, molecular rigidity and branched chain structure. The lubricating mechanism contributed by the IL molecules and the Ti surfaces during sliding were analyzed by means of SEM, XPS, ECR and QCM. The results showed that physical adsorption and tribo-chemical reactions were involved during the sliding process. And the lubricating performances of the synthesized ILs may largely depend on their ability to form physical/chemical adsorption films on the titanium interfaces. The rigid benzene ring structure and flexible alkyl chain are conducive to strengthening the stability and efficiency of the formed interfacial tribo-film.