Robust Macroscale Superlubricity Enabled by the Protic Ionic Liquid Polyol Aqueous Solution: From Interface Adsorption to Tribofilm Formation

Achieving a robust macroscale liquid superlubricity on an engineering steel interface through the use of waterborne green lubricants has gained growing attention, given the substantial potential in reducing energy consumption, carbon emissions, and equipment failure. However, maintaining superlubricity for a prolonged duration under various conditions is a major obstacle for practical applications, which is mainly limited by insufficient recognition about the superlubricity. Herein, a robust and durable macroscale liquid superlubricity enabled by the unique lubricant based on the protic ionic liquid as an additive, which consists of dodecylbenzenesulfonic acid as anions and dimethyl alkyl amines as cations, in aqueous 1,2-propanediol triggers at the self-mated steel interface. The lubricant provides an average coefficient of friction of around 0.007 under a high application load (200 N) and a high sliding speed (0.557 m/s), resulting in the maintenance of superlubricity at a real contact pressure of about 400 MPa, which increases the upper limit of the sliding velocity and contact pressure compared to current water-based lubricants between steel contacts. A specialized intermittent test was designed to reveal the evolution process of superlubricity, which involves the action of the running-in period, the change of lubricant chemical characteristics, the adsorption of additives at the friction interface, and the evolution law of the tribofilm. The superlubricity mechanism was enabled by the chemical adsorption of the ionic liquid, especially anions, onto the worn metal surface, coupled with the tribo-induced chemical reactions that generate an adaptable low-shear interface, where various ions and hydrophilic functional groups in the lubricant will form a protective layer in a certain sequence, reducing shear stress and preventing direct contact with the friction pair. This work provides specific insights into the role of chemical adsorption and the tribochemical film induced by the ionic liquid additive in achieving superlubricity and is an important step forward for the industrial applications of green water-based superlubricity technologies.