Effective superlubricity behavior triggered by interfacial tribochemical reactions from simple protic ionic liquids aqueous

Superlubricity technology is one of the fastest growing fields of tribology in recent years because it can fundamentally reduce friction and wear. However, the current challenges for liquid superlubricity are the bearing capacity of lubricants and the stable superlubricity behavior at high shear rates. In this work, four imidazole-phosphate ionic liquids (ILs) with different chain lengths were prepared and used as pure water additives to achieve stable superlubricity with the friction coefficient of 0.008 under high loading (300 N) and high speed (0.461 m/s) between steel interfaces. The evoluation progress and tribochemical reaction of superlubricity is explored by the staged experiment. The results showed that a key factor of achieving superlubricity is the running-in period, in which, on the one hand, the contact pressure greatly reduce to maintain a suitable bearing range. On the other hand, the ordered accumulation of hydrogen ions, organic anions and cations, and ion of hydration formed a multi-layer electrostatic layer at the interface of the friction pair, which not only provides a protective layer to avoid serious corrosion, but also enhances the bearing capacity of the lubricating film attributed to the electrostatic repulsion. In addition, the tribochemical reaction between ILs and the surface of metal forms a functional tribofilm composed of carbides, iron oxides, nitrides and phosphides, which prevents the direct contact of the friction pair and reduces wear. Consequently, this research offers valuable insights into the optimal IL structure, revealing the tribochemical reaction of the superlubricity process and holding the significant potential of water-based superlubricity systems for practical applications.