Superlubricity and Antiwear Properties of in Situ-Formed Ionic Liquids at Ceramic Interfaces Induced by Tribochemical Reactions

Several ionic liquids (ILs) are formed in situ with monovalent metal salts and ethylene glycol (EG). The macroscale superlubricity and antiwear properties of the ILs were studied between ceramic materials. Superlow coefficients of friction of less than 0.01 could be obtained for all ILs at silicon nitride (Si ₃ N ₄ ) interfaces induced by tribochemical reactions. Notably, the IL ([Li(EG)]PF ₆ ) formed with LiPF ₆ and EG exhibited the greatest superlubricity and antiwear properties. The results of film thickness calculations and surface analysis showed that the lubrication regime during the superlubricity period was the mixed lubrication, and a composite tribochemical layer (composed of phosphates, fluorides, silica (SiO ₂ ), and ammonia-containing compounds), hydration layer, and fluid film contributed to superlubricity and wear protection. It was found that the small size of metal cations was beneficial for alleviating wear, and PF ₆ ^- anions exhibited the smallest friction and best antiwear performance at Si ₃ N ₄ interfaces. This work studied the lubricity and antiwear properties of ILs with different cations and anions, enriching the range of alternative ILs for macroscale superlubricity and low wear, and is of importance to engineering applications.