Atomic simulations of kinetic friction and its velocity dependence at Al/Al and α-Al2O3/α-Al2O3 interfaces

Kinetic friction during dry sliding along atomistic-scale Al(001)/Al(001) and α-Al2O3(0001)/α-Al2O3(0001) interfaces has been investigated using molecular dynamics (MD) with recently developed Reactive Force Fields (ReaxFF). It is of interest to determine if kinetic friction variations predicted with MD follow the macroscopic-scale friction laws known as Coulomb's law (for dry sliding) and Stokes' friction law (for lubricated sliding) over a wide range of sliding velocities. The effects of interfacial commensuration and roughness on kinetic friction have been studied. It is found that kinetic friction during sliding at commensurate α-Al2O3(0001)/α-Al2O3(0001) interfaces exceeds that due to sliding at an incommensurate α-Al2O3(0001)/ α-Al2O3(0001) interface. For both interfaces, kinetic friction at lower sliding velocities deviates minimally from Coulombic friction, whereas at higher sliding velocities, kinetic friction follows a viscous behavior with sliding damped by thermal phonons. For atomically smooth Al(001)/Al(001), only viscous friction is observed. Surface roughness tends to increase kinetic friction, and adhesive transfer causes kinetic friction to increase more rapidly at higher sliding velocities.