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

Molecular dynamics simulations were performed to determine how interfacial properties and friction forces of two model hydroxylated alpha -alumina sur faces are affected by softening or stiffening the surface potential, withou t changing the surface-surface intermolecular potential V-inter. The surfac e-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 force s. This result mirrors the surface-surface vibrational spectra for the soft and stiff surfaces versus load. Increasing the load forces both systems in to potential energy wells, requiring additional energy (or external force) to surmount potential energy barriers for sliding, leading to enhanced fric tion, At high sliding velocity, the instantaneous barriers and friction for ces for sliding do not mirror the periodicity of the optimized interfacial potential. As the sliding velocity is decreased, there is a longer time ava ilable for structural relaxations and energy transfer between the sliding c oordinate and surface modes. As a result, the friction force versus sliding distance begins to exhibit the periodicity of the optimized interfacial po tential. Knowledge of the optimized potential along the sliding coordinate (analogous to a reaction path potential) may assist in understanding tribol ogical behavior under applied load and interfacial sliding.