  Molecular dynamics simulations of frictional sliding in an Atomic Force
Microscope (AFM) show a clear dependence of superlubricity between
incommensurate surfaces on tip compliance and applied normal force. While the
kinetic friction vanishes for rigid tips and low normal force, superlubric
behavior breaks down for softer tips and high normal force. The simulations
provide evidence that the Frenkel-Kontorova-Tomlinson (FKT) scaling applies
equally to a more realistic 3-D incommensurate AFM model except in the limit of
very low stiffness and high normal load limit. Unlike the FKT model in which
the breakdown of superlubricity coincides to the emergence of the meta-stable
states, in the 3-D model some meta-stable states appear to reduce frictional
force leading to non-monotonic dependence of force on normal load and tip
compliance. Meta-stable states vary with the slider positions, and the relative
stabilities of these meta-stable states result in varying transition mechanisms
depending on sliding velocity.
