Finite size effects at high speed frictional interfaces

Non-Equilibrium Molecular Dynamics simulations have exhibited characteristic velocity weakening for the tangential frictional force at smooth single crystal interfaces for velocities greater than a critical velocity, v$_{c}$. This behavior has been seen in a number of material pairs including Cu-Ag, Ta-Al and Al-Al. Expressions for v$_{c}$ that characterize this behavior depend on system size. We discuss the size dependence for Al-Al single crystal interfaces for two cases: an Al(111)/Al(001) interface sliding along [1-10],N=1.5M, and an Al(110)[001]/Al(110)[1-10] interface sliding along [001], N=7.5x10$^{6}$ corresponding to a three-fold increase in system size normal to the sliding direction. We find agreement with an inverse size scaling for v$_{c}$. We discuss the similarities in behavior for a highly defective plastically deformed sample with Al(110)[001]/Al(110)[1-10] orientation having the same normal dimension and N= 16.0x10$^{6}$.