Experimental and Molecular Dynamics Study on the Friction and Film Formation of Water-Containing Synthetic Ester Lubricants

The involvement of water in the tribochemical interaction between lubricants and metal alters the interfacial chemical composition and tribological behavior. However, the complete mechanisms underlying this lubrication process have not been definitively elucidated. This study investigates the effect of water content and additives on the frictional performance of ester-based lubricants through molecular dynamics (MD) simulations and experiments. A lubrication MD model based on the ReaxFF reactive force field was established to simulate the influence of different shear velocities, loads, and water contents on the lubricating performance of synthetic ester lubricants between two solid surfaces. Concurrently, optical microscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were employed to examine the tribological behavior and composition of wear tracks of synthetic ester lubricants with varying water contents during line contact friction tests. Experimentally, it was found that a higher water content leads to increased wear, while detergent and antiwear additives help remove wear debris from the contact zone under high-water conditions, thereby maintaining smoother surfaces. The MD simulation results reveal that small water molecules disrupt the long-chain oil molecules in the lubricant layer, suppressing their lateral sliding and increasing internal shear, thereby deteriorating lubrication. The main contribution of this work lies in combining MD simulations with experiments to establish a link between nanoscale structures and macroscopic performance, elucidating the mechanisms by which water content affects frictional behavior, and providing a theoretical basis for the development of lubricants suitable for water-containing conditions.