  The increase of sliding friction upon increasing load is a classic in the
macroscopic world. Here we discuss the possibility that friction rise might
sometimes turn into a drop when, at the mesoscale and nanoscale, a confined
lubricant film separating crystalline sliders undergoes strong layering and
solidification. Under pressure, transitions from N to N-1 layers may imply a
change of lateral periodicity of the crystallized lubricant sufficient to alter
the matching of crystal structures, influencing the ensuing friction jump. A
pressure-induced friction drop may occur as the shear gradient maximum switches
from the lubricant middle, marked by strong stick-slip with or without shear
melting, to the crystalline slider-lubricant interface, characterized by smooth
superlubric sliding. We present high pressure sliding simulations to display
examples of frictional drops, suggesting their possible relevance to the local
behavior in boundary lubrication.
