Multistep Locked-to-Sliding Transition in a Thin Lubricant Film

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Abstract
Using Langevin simulations, we study dynamical transitions in a model three-layer atomic film confined between two rigid substrates moving with respect to each other. With the increase of a dc force applied to the top substrate, first the middle layer of the lubricant film transitions from locked to sliding states; this regime shows a stick-slip behavior with a relatively high effective friction. Next, the layers closest to the substrates start to slide over the substrates, as well as with respect to the middle layer; the effective friction in this regime may be explained by energy losses due to the excitation of phonons in the lubricant. Finally, at high velocities the lubricant film decouples from the substrates and achieves a “flying” regime characterized by a very low friction coefficient.