Effect of surface stiffness on the friction of sliding model hydroxylated α-alumina surfaces

Molecular dynamics simulations were performed to determine how interracial properties and friction forces of two model hydroxylated α-alumina surfaces are affected by softening or stiffening the surface potential, without changing the surface-surface intermolecular potential V_inter. The surface-surface vibrational spectrum for the soft surface, in absence of applied load, is influenced by coupling between V_inter and the surface vibrations. For the stiff surface, this coupling becomes important as the applied load is increased. Friction forces for the stiff and soft surfaces are similar at low loads, but at high loads the stiff surface has higher friction forces. This result mirrors the surface-surface vibrational spectra for the soft and stiff surfaces versus load. Increasing the load forces both systems into potential energy wells, requiring additional energy (or external force) to surmount potential energy barriers for sliding, leading to enhanced friction. At high sliding velocity, the instantaneous barriers and friction forces for sliding do not mirror the periodicity of the optimized interracial potential. As the sliding velocity is decreased, there is a longer time available for structural relaxations and energy transfer between the sliding coordinate and surface modes. As a result, the friction force versus sliding distance begins to exhibit the periodicity of the optimized interfacial potential. Knowledge of the optimized potential along the sliding coordinate (analogous to a reaction path potential) may assist in understanding tribological behavior under applied load and interracial sliding.