  The structure and motion of carbon and h-BN nanotubes (NTs) deposited on
graphene is inquired theoretically by simulations based on state-of-the-art
interatomic force fields. Results show that any typical cylinder-over-surface
approximation is essentially inaccurate. NTs tend to flatten at the interface
with the substrate and upon driving they can either roll or slide depending on
their size and on their relative orientation with the substrate. In the
epitaxially aligned orientation we find that rolling is always the main
mechanism of motion, producing a kinetic friction linearly growing with the
number of walls, in turn causing an unprecedented supra-linear scaling with the
contact area. A 30 degrees misalignment raises superlubric effects, making
sliding favorable against rolling. The resulting rolling-to-sliding transition
in misaligned NTs is explained in terms of the faceting appearing in large
multi-wall tubes, which is responsible for the increased rotational stiffness.
Modifying the geometrical conditions provides an additional means of
drastically tailoring the frictional properties in this unique tribological
system.
