Unravelling High-Load Superlubricity of Ionic Liquid Analogues by <i>In Situ</i> Raman: Incomplete Hydration Induced by Competitive Exchange of External Water with Crystalline Water
Hongyu Liang, Tianqiang Yin, Manqiang Liu, Caihong Fu, Xiaojie Xia, Shijing Zou, Xijun Hua, Yonghong Fu, Yongfeng Bu
Abstract
A high load-carrying capacity is the key to the practicality of liquid superlubricity, but it is difficult to achieve high load and low friction simultaneously by relying solely on a liquid film. Herein, a choline chloride-based ionic liquid analogue (ILA) macroscale superlubricant is first reported by tuning down strong hydrogen bonding in the ILA via introducing 2–10 wt % water, with a high load of 160 MPa and a low coefficient of friction of 0.006–0.008. In situ Raman reveals that competitive exchange between external water and crystalline water induces weak H-bond-dominated incomplete hydration, conferring a low-shear interface and considerable load-carrying capacity inside the lubricant. It is a hydrodynamic lubrication film rather than a tribochemical/physical adsorption film, allowing it to be applied to friction pairs of various materials. This study unveils the principle of water mediation of high-viscosity ILAs and also provides new insights into the design of practical ILA-based superlubrication materials with high load-carrying capacity.