Timofey Frolov, William J. Boettinger, Yuri Mishin
The goal of this paper is to explore mechanisms of hillock and whisker growth in stressed poly-crystalline films by molecular dynamics simulations. Our stress-free model consists of three grains and a triple line aligned perpendicular to a free surface, plus a fourth wedge-shaped grain implanted between the triple line and the surface. This simulated grain geometry corresponds to that observed in experiments during hillock and whisker growth, with the fourth grain serving as a seed for hillock growth. The simulations under an applied in-plane biaxial compression reveal an upward motion and growth of the seed grain. The growth occurs by stress-driven grain boundry diffusion and accretion of material on some of the boundaries, pushing the seed grain upwards and sideways. The different diffusion and accretion rates at different boundaries give rise to internal stresses, which can be partially accommodated by grain boundary motion coupled to shear deformation. The hillock growth is countered by surface diffusion, which slow the growth down or even suppress it completely. Other mechanisms involved in the hillock growth are also discussed.