Dynamical behavior of lubricant molecules under boundary lubrication explored via molecular dynamics simulations

Boundary lubrication with extremely thin films widely occurs in various situations, for instance, in micro-electromechanical system lubrication and hard disk drive lubrication. Lubrication performance is significantly affected by the surface layer properties and interactions between solids and liquids. However, the molecular dynamical behaviors are still unclear. Thus, our work considers the dynamical behaviors of molecules under boundary lubrication via molecular dynamics simulations. Different pressures and metal slab shapes are chosen as the variable conditions. The results indicate that a smooth metal slab model has a special conformation recovery process during compressing under medium pressures. After inducing shear velocity, the lubrication film exhibits sticky, stick-slip, or slip flows under different pressures. Sticky flow is accompanied by a conformation adjustment consisting of conformation recovery, chain alignment, and structure equilibrium, but there is no chain alignment step in the other two flow modes. The conformation recovery includes atomic adsorption onto the Fe wall under small and medium pressures. Under large pressures, the conformation recovery refers to atomic desorption phenomena. In addition, some properties, such as gyration and chain orientation, are strongly modified by the solid surface and show distinct differences along the pressing direction. Under the same simulation conditions, the rough wall model shows no slip behaviors attributed to the increased equivalent contact wall area and stronger pinning effect. Our work provides new insights into understanding the in-depth mechanism of boundary lubrication, providing theoretical guidance in developing advanced boundary lubrication techniques.