Molecular dynamics simulations of dynamic friction and mixing at rapidly moving material interfaces

Friction studies are important to applications of high-speed machining and ballistic penetration modelling because of the need to understand the behaviour of rapidly moving interfaces. Of particular interest is the velocity dependence of the effective tangential force. The aim of this work is to investigate the relation between velocity weakening and structural transformation of nano-crystalline materials. To this end, molecular dynamics simulations of Cu/Ag tribopair have been performed at a high normal pressure of 5.1 GPa and sliding friction with velocities ranging from 0.025 km/s to 1 km/s. Although at low sliding speeds the frictional force shows a linear dependency on the sliding speed, at higher speeds (above a critical speed) this dependency is reversed and the frictional force decreases with increasing sliding speed, approaching a plateau. Plastic deformations start to occur above a sliding speed of 0.05 km/s, originating from the interface and moving outward into the material. The study also investigates the heat dissipation in the proximity of the interface and its relationship with atomic diffusion. For sliding velocities greater than 200 m/s, the temperature near the interface of the two materials exceeds the melting point of Ag. Mixing of the two materials is observed at the sliding interface, with the mixing layer width increasing when sliding speed increasing. Finally, modelling issues that result in underestimating the thermodynamic properties of Cu and Ag and modelling artifacts due to reservoir boundary conditions are discussed.