  In light of the race towards macroscale superlubricity of graphitic contacts,
the effect of grain boundaries on their frictional properties becomes of
central importance. Here, we elucidate the unique frictional mechanisms
characterizing topological defects along typical grain boundaries that can vary
from being nearly flat to highly corrugated, depending on the boundary misfit
angle. We find that frictional energy dissipation over grain boundaries can
originate from variations of compressibility along the surface, heat produced
during defect (un)buckling events, and elastic energy storage in irreversible
buckling processes. These may lead to atypical non-monotonic dependence of the
averaged friction on the normal load. The knowledge gained in the present study
constitutes an important step towards the realization of superlubricity in
macroscopic graphitic contacts.
