  High velocity impact between crystalline surfaces is important for a range of
material phenomena, yet a fundamental understanding of the effect of surface
structure, energetics and kinetics on the underlying thermo-mechanical response
remains elusive. Here, we employ non-equilibrium molecular dynamics (NEMD)
simulations to describe the nanoscale dynamics of the high velocity impact
between commensurate and incommensurate monocrystalline (001) copper surfaces.
For impact velocities in the range 100-1200 m/s, the kinetic energy dissipation
involves nucleation and emission of dislocation loops from defective sites
within the rapidly forming interface, well below the bulk single-crystal yield
point. At higher velocities, adiabatic dissipation occurs via
plasticity-induced heating as the interface structurally melts following the
impact. The adhesive strength of the reformed interface is controlled by the
formation and nucleation of dislocations and point defects as they modify the
interfacial energy relative to the deformed bulk. As confirmation, the excess
interface energy decreases monotonically with increasing impact velocity. The
relative crystal orientation of the surfaces equally important; the grain
boundaries formed following incommensurate impact exhibit higher impact
resistance, with smaller defect densities and interfacial enthalpies,
suggesting an enhanced ability of the grain boundaries to absorb the
non-equilibrium damage and therefore facilitate particle bonding. Our study
highlights the key role played by the atomic-scale surface structure in
determining the impact resistance and adhesion of crystalline surfaces.
