Effect of Water Content on the Tribological Properties of Grease

ABSTRACT In mechanical lubrication systems, grease can become contaminated with water due to seal failure and high ambient humidity. Water can alter the fibrous network of the thickener in grease, leading to base‐oil separation, reduced consistency and abnormal oil bleeding. In this study, experiments were combined with molecular dynamics (MD) simulations to investigate the tribological behaviour of lubricating systems with different water contents, together with changes in parameters such as density distribution and system height. The aim was to clarify the short‐term effects of water on grease structure and lubrication performance. The results show that the system with 5% water exhibits the best lubrication performance. Compared with the anhydrous system, the maximum wear‐scar depth decreased by 0.142 μm and the maximum stress decreased by 29%. These findings suggest that a moderate amount of water forms hydrogen bonds with the polar groups of the lithium soap thickener, strengthening the soap‐fibre network. This interaction stabilizes the emulsion formed by water, base oil and thickener, resulting in the highest resistance to compression. In contrast, excessive water increases the intermolecular spacing between thickener molecules, weakens the structural integrity of the thickener network, promotes lubricant‐film rupture and reduces load‐carrying capacity. The experimental results are consistent with the MD simulations.