Atomistic and continuum studies of crack-like diffusion wedges and associated dislocation mechanisms in thin films on substrates

Abstract

Large-scale atomistic simulations are performed to study plastic deformation in sub-micron polycrystalline thin films on substrates. The simulations reveal that stresses in the film are relaxed by mass diffusion from the surface into the grain boundary. This leads to formation of a novel material defect referred to as the diffusion wedge. A crack-like stress field is found to develop around the diffusion wedge as the traction along the grain boundary is relaxed and the adhesion between the film and the substrate prohibits strain relaxation close to the interface. The diffusion wedge causes nucleation of dislocations on slip planes parallel to the plane of the film. We find that nucleation of such parallel glide dislocations from a diffusion wedge can be described by a critical stress intensity factor similar to the case of a crack. Atomistic simulations of parallel glide dislocations associated with the crack-like grain boundary diffusion wedge represent a significant progress in the theory of diffusional creep in thin films on substrates.