On the shear dilation of polycrystalline lubricant films in boundary lubricated contacts.

Shearing of a solidified polycrystalline lubricant film confined between two solid surfaces has been studied by molecular dynamics simulations. In the case of a perfect commensurate contact, we observe interlayer slips within the film and shear-induced order-to-disorder transition of lubricant molecules around grain boundaries. This process is accompanied by the nucleation, propagation, and annihilation of dislocations in the solidified film, resulting in repeated dilation and collapse of the lubricant film during the stick-slip motion. In the case of an incommensurate contact, only slips at the lubricant-solid interface happen and no dilation of the lubricant film is observed during the stick-slip friction. These observations are consistent with recent surface force balance experimental measurements. In combination with our recent work [R. G. Xu and Y. S. Leng, Proc. Natl. Acad. Sci. U. S. A. 115, 6560 (2018)], this study provides a renewed picture on the physical property of nanoconfined lubricant films in boundary lubrication.