  Nanomechanical devices can operate at much higher speeds than their
macroscopic analogues, due to low inertia. For example, peak speeds >100m/s
have been predicted for carbon nanotube devices. This stimulates our interest
in the atomic-scale physics of friction at high velocity. Here we study a model
nanosystem consisting of a graphene flake moving freely on a graphite substrate
at >100m/s. Using molecular dynamics we discover that ultra-low friction, or
superlubricity, is punctuated by high-friction transients as the flake rotates
through successive crystallographic alignments with the substrate. We term this
phenomenon frictional scattering and show that it is mathematically analogous
to Bragg scattering. We also show that frictional scattering can be eliminated
by using graphitic nanoribbons as frictional waveguides to constrain the flake
rotation, thus achieving persistent superlubricity. Finally, we propose an
experimental method to study nanoscale high-velocity friction. These results
may guide the design of efficient high-frequency nanomechanical devices.
