Optimal design of multiphase interface synergistic lubrication: molecular dynamics insights into DLC films under oil lubrication

Abstract This study employed molecular dynamics (MD) simulations to systematically investigate the microscopic friction mechanisms of a sealing pair consisting of diamond-like carbon (DLC) coated plungers and nitrile butadiene rubber (NBR) under oil-lubricated conditions. A friction model comprising base oil and the DLC–NBR interface was first constructed, and four different quantities of oil molecules (Qoil) were set to analyze the effects of lubrication status on friction behavior, structural evolution, stress transmission, and temperature evolution of the system. The results indicate that the interfacial friction behavior initially improves and deteriorates with increasing oil content. When the Qoil is just sufficient (15CH), the oil lubrication effect reaches its optimal state. At this point, the oil molecule adsorption layer transforms the friction interface from solid-solid contact to solid-liquid contact, inhibiting the migration of molecular chains on the NBR surface. The oil molecule adsorption layer acts as a stress buffering layer, reducing the shear stress and normal stress of the friction pair by approximately 50% respectively, thereby lowering the friction force to 28.6 nN. Meanwhile, the oil film blocks the transfer of interfacial frictional heat to the NBR substrate and absorbs heat continuously, reducing the temperature of NBR from 380 K to 340 K. When the oil content is too high (25CH), not only will the adsorption strength at the interface be enhanced, and the available space for oil molecules is compressed, raising the short-range repulsive force. Eventually, this leads to an increase in the sliding resistance. However, an excessively high oil content (25CH) will increase the viscosity of the adsorption layer and the adsorption strength at the solid-liquid interface, thereby reducing the lubrication performance and consequently increasing the sliding resistance. This study elucidates the oil-content-dominated multiphase interfacial lubrication mechanism and provides a molecular-level theoretical basis for the optimized design of DLC-coated plunger seals.