Insight into macroscale superlubricity of polyol aqueous solution induced by protic ionic liquid

Currently, macroscale liquid superlubricity remains limited to low applied loads and typical ceramic friction pairs. In this study, a robust macroscale superlubricity with a coefficient of friction (COF) of approximately 0.006 is realized at the bearing steel interface induced by protic ionic liquids (ILs) in propylene glycol aqueous solution, and the lubrication system exhibits excellent anti-corrosion properties. Results show that superlubricity can be achieved by employing ILs with longer alkyl chains over a wide load (< 350 N) and speed (> 700 r/min) range. By systematically investigating factors affecting superlubricity, including the IL structure, ionization degree, test conditions, polyol, water-to-alcohol ratio, and lubrication state, the superlubricity mechanism is discussed. Notably, a thicker and denser stern layer can be formed using ILs with longer alkyl chains, which participates in the tribochemical reaction with the metal substrate to form a tribofilm during rubbing. The hydrogen bond network layer formed by the hydrogen ion and polycol aqueous solution can withstand high applied loads. Water can be used to reduce the shear stress of polyols, and enable superlubricity to be achieved under high-speed rotations. Moreover, an inevitable running-in period serves as a dispersing contact stress and dynamically forms a lubricating film, where the lubrication state locates mixed lubrication and then transforms into boundary lubrication as the roughness of the contact surface increases. This study is expected to significantly promote the development and application of superlubricity in the engineering field.