Molecular dynamics insights into nanoscale lubrication: a comparative study of regimes

This study utilizes molecular dynamics simulations to explore the tribological behavior of rough surfaces across various lubrication regimes—boundary, mixed, and elastohydrodynamic (EHL)—at the nanoscale. The analysis not only highlights the distinct interactions between asperity contacts and lubricant films but also introduces novel insights into how lubricant film thickness and surface roughness interplay to dictate frictional dynamics. The analysis reveals that in dry contact, surface roughness quickly stabilizes despite an initial increase in contact area, indicating a dominant role of asperity interlocking that influences frictional resistance. Transitioning to boundary lubrication, the presence of a lubricant layer modifies the contact mechanics, leading to a slower root mean square (RMS) reduction of surface roughness and a more gradual increase in friction force, emphasizing the lubricant’s protective effect against asperity flattening. Nevertheless, under lower loads, this protective effect can lead to increased frictional forces compared to dry contact due to higher final roughness. Mixed lubrication maintains a near-constant contact area after initial contact, with any variations in friction force primarily occurring in the early cycles due to rapid asperity flattening. In the EHL regime, the formation of a complete lubricant film between surfaces results in the lowest friction forces, dominated by viscous shear rather than direct asperity contact. Unique to this study is the quantification of these effects under realistic roughness conditions (RMS of 1.2 nm). These regime-specific behaviors underline the nuanced interplay between RMS, contact area, and friction forces, and enhance our understanding of the fundamental principles guiding the design of lubrication strategies for advanced mechanical systems.