Nonequilibrium energy dissipation at the interface of sliding model hydroxylated alpha-alumina surfaces

Nonequilibrium molecular dynamics simulations were performed to study the dynamics of energy transfer at the interface of a small nanoscale hydroxylated alpha-alumina surface sliding across a much larger surface of the same material. Sliding velocities of 0.05, 0.5, 5, and 50 ms and loads of 0, 0.0625, 5, 15, 25, and 100 nN were considered. Nonequilibrium energy distributions were found at the interface for each of these conditions. The velocity distribution P(v) for the atoms in a sublayer of the smaller surface oscillates during the sliding, reflecting the periodicity of the interfacial intermolecular potential. When averaged over the sliding, this P(v) for each of the sublayers is bimodal with Boltzmann and non-Boltzmann components. The non-Boltzmann component, with temperatures in excess of 1000 K and as high as 2500 K, is most important for the interfacial H-atom sublayer and becomes less important in moving to a sublayer further from the interface. Similarly, the temperature of the Boltzmann component decreases for sublayers further from the interface and approaches the 300 K temperature of the boundary. The temperature of the Boltzmann component decreases, but the importance of the non-Boltzmann component increases, as the sliding velocity is decreased. The temperature of the non-Boltzmann component is relatively insensitive to the sliding velocity. Friction forces are determined by calculating the energy dissipation during the sliding, and different regimes are found for variation in the friction force versus sliding velocity v(s) and applied load. For v(s) of 0.05, 0.5, and 5 ms, the friction force is inversely proportional to v(s) reflecting the increased time for energy dissipation as v(s) is decreased.