  Recent highly idealized model studies of lubricated nanofriction for two
crystalline sliding surfaces with an interposed thin solid crystalline
lubricant layer showed that the overall relative velocity of the lubricant
$v_{\rm lub} / v_{\rm slider}$ depends only on the ratio of the lattice
spacings, and retains a strictly constant value even when system parameters are
varied within a wide range. This peculiar "quantized" dynamical locking was
understood as due to the sliding-induced motion of misfit dislocations, or
soliton structures. So far, the practical relevance of this concept to
realistic sliding three dimensional crystals has not been demonstrated. In this
work, by means of classical molecular dynamics simulations and theoretical
considerations, we realize a realistic three-dimensional
crystal-lubricant-crystal geometry. Results show that the flux of lubricant
particles associated to the advancing soliton lines gives rise here too to a
quantized velocity ratio. Moreover, depending on the interface lattice spacing
mismatch, both forward and backward quantized motion of the lubricant is
predicted. The persistence under realistic conditions of the dynamically pinned
state and quantized sliding is further investigated by varying sliding speed,
temperature, load, and lubricant film thickness. The possibilities of
experimental observation of quantized sliding are also discussed.
