A molecular dynamics study of the transition from ultra-thin film lubrication towards local film breakdown

The transition from ultra-thin lubrication to dry friction under high pressure and shear is studied using molecular dynamics: the quantity of lubricant in the confined film is progressively reduced toward solid-body contact. A quantized layer structure is observed for n-alkanes confined between smooth, wettable walls, featuring an alternation of well-layered, low friction configurations, and disordered ones, characterized by high friction, and heat generation. The molecular structure influences the ordering of the fluid and the resulting shear stress. In fact, Lennard-Jones fluids are characterized by low friction due to the absence of interlayer bridges, opposed to the always entangled states and high shear stresses for branched molecules. Surface geometry and wettability also affect the behavior of the confined lubricant. The presence of nanometer-scale roughness frustrates the ordering of the fluid molecules, leading to high friction states. Furthermore, local film breakdown can be observed when the asperities come into contact, with strong wall–wall interactions causing the maximum in shear stress. Finally, friction is limited to a small, constant value by the presence of smooth, non-wettable surfaces in the system due to the occurrence of wall slip.