A size effect in grain boundary migration: A molecular dynamics study of bicrystal thin films

Molecular dynamics simulations of stress-driven grain boundary migration in bicrystal thin films demonstrate that the grain boundary mobility decreases as the films are made thinner. Examination of the surface morphology proves that this effect is not associated with grain boundary grooving. The simulation data demonstrate that the grain boundary mobility is a linear function of the inverse thickness. We present a simple model to explain this effect based upon the fundamental mechanism of grain boundary migration: the collective rearrangement of a large group of atoms. Decreasing system size implies that more of the boundary is near the surface. The presence of the free surface interferes with the collective rearrangement of the atoms during boundary motion and hence slows the migration. A simple heuristic analysis, based on this effect, is consistent with the observed functional dependence of boundary mobility on bicrystal thickness.