Peculiar Effect of Water on Tribological Properties of Natural Deep Eutectic Solvent

Deep eutectic solvents (DESs) have emerged as promising green liquid lubricants for solving tribological problems due to their economic and excellent physicochemical and lubrication properties. However, a trace amount of water would affect their structure, physicochemical properties, and tribological properties. The effect of water on the tribological properties of DES is still unclear and needs further investigate. Herein, we carried out a systematic investigation into the chemical structure, rheological properties, and tribological performance of DES-water (DES-W) binary systems constructed by combining DES with varying contents of water. The results revealed that low levels of water in DES had a minimal impact on its chemical structure but affected its fluidity and viscosity. Frictional experiments demonstrated that DES-W binary systems displayed a reduced coefficient of friction from 0.094 to 0.025 compared to pure DESs and manifested outstanding antiwear properties under a high-load condition. This was attributed to the formation of hydration layers, adsorption layers, and tribochemical films at the tribointerface through physicochemical adsorption and tribochemical reactions. Our findings not only foster the design and development of green lubricating materials but also expand the engineering applications of DESs to solve wear-related mechanical failures in practical application.