The properties of an atomic lubricant confined between two approaching soli
ds are investigated by a model that accounts for the curvature and elastic
properties of the solid surfaces. Well defined atomic layers develop in the
lubricant film when the width of the film is of the order of a few atomic
diameters. An external squeezing-pressure induces discontinuous, thermally
activated changes in the number n of lubricant layers. The precise mechanis
m for these layering transitions depends on n, and on the lubricant-surface
pinning barriers. Thus, in the absence of sliding, unpinned or weakly pinn
ed incommensurate lubricant layers give rise to fast and complete layering
transitions. Strongly pinned incommensurate and commensurate layers give ri
se to sluggish and incomplete transformations, resulting in trapped islands
. In particular, for commensurate layers it is often not possible to squeez
e out the last few lubricant layers. However, lateral sliding of the two so
lid surfaces breaks down the pinned structures, greatly enhancing the rate
of the layering transitions. In the case of sliding, an important parameter
is the barrier for sliding one lubricant layer with respect to the others.
When this barrier is larger than the lubricant-surface pinning barrier, th
e lubricant film tends to move like a rigid body with respect to the solid
surface. In the opposite case, slip events may occur both within the lubric
ant film and at the lubricant-solid interface, making the squeeze-out proce
ss much more complex. In some of the simulations we observe an intermediate
phase, forming immediately before the layering transition. This transient
structure has a lower 2D density than the initial phase, and allows the sys
tem to release elastic energy, which is the driving force for the phase tra
nsformation. (C) 2000 American Institute of Physics. [S0021-9606(00)70421-1
].