Squeezing and stick-slip friction behaviors of lubricants in boundary lubrication.

The fundamental questions of how lubricant molecules organize into a layered structure under nanometers confinement and what is the interplay between layering and friction are still not well answered in the field of nanotribology. While the phase transition of lubricants during a squeeze-out process under compression is a long-standing controversial debate (i.e., liquid-like to solid-like phase transition versus amorphous glass-like transition), recent different interpretations to the stick-slip friction of lubricants in boundary lubrication present new challenges in this field. We carry out molecular dynamics simulations of a model lubricant film (cyclohexane) confined between molecularly smooth surfaces (mica)--a prototypical model system studied in surface force apparatus or surface force balance experiments. Through fully atomistic simulations, we find that repulsive force between two solid surfaces starts at about seven lubricant layers (n = 7) and the lubricant film undergoes a sudden liquid-like to solid-like phase transition at n < 6 monolayers thickness. Shear of solidified lubricant films at three- or four-monolayer thickness results in stick-slip friction. The sliding friction simulation shows that instead of shear melting of the film during the slip of the surface, boundary slips at solid-lubricant interfaces happen, while the solidified structure of the lubricant film is well maintained during repeated stick-slip friction cycles. Moreover, no dilation of the lubricant film during the slip is observed, which is surprisingly consistent with recent surface force balance experimental measurements.