Simulations of lubricated contacts:

Contribution from B. Persson (Germany)

We have performed computer simulations of the squeezing of thin lubrication films between elastic solids with curved surfaces. Here we present some results for CH_4 lubricants between a flat elastic substrate (Gold) and a block (also Gold) with a Gaussian tip. We show some results related to the squeeze out transition from two monolayer of lubricant to one single monolayer, the n=2–&ndash>1 transition. Well before the onset of squeeze out the lubricant bilayer form a hexagonal structure at the interface. Detailed analysis of the n=2–&ndash>1 transition reveals that the expulsion process is proceeded by a structural change (yield). Simulation snapshots (Fig. a+b) shows the formation of a more open structure during the yield, where the lubrication molecules in contact with the tip form a c(2x2) (approximately commensurate) structure and the second layer (in contact with the substrate) a similar structure, thus allowing the second layer to relax slightly towards the first layer, which relief elastic energy in the block. A similar effect was observed for other lubricants. The final state of the squeeze-out is a nearly perfect commensurate (relative to the substrate) c(2x2) structure, shown in Fig. c from the direction of the substrate (with the top layer of substrate atoms included in the picture).

Snapshot pictures of the CH_4 atoms in the contact area during the yield'' or rearrangement transition preceeding the n=2––>1 layering transition (solid Au atoms are excluded). The figure shows the central 30 Angstrom x 30 Angstrom section (in the xy-plane) viewed from the substrate side. The color scheme represents the vertical z-coordinate, changing from dark blue at the first layer next to the substrate, to red far away. (a) shows the system just before the yield'' onset and (b) just before the layering onset.

Snapshot picture of the final system configuration for the squeeze-out of CH_4, with solid Au atoms included. The figure shows the central 30 Angstrom x 30 Angstrom section (in the xy-plane) viewed from the substrate side. The color scheme represents the vertical z-coordinate, changing from dark blue at the substrates layer, to red far away.