Two-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate were used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling was analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density was found close to the impenetrable film-substrate interface. There was a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state were independent of film thickness. In such films the stress outside the boundary layer was on average very low, so that the film-thickness-independent boundary layer was responsible for the size effect. A larger size effect was found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreased while the stress in the film increased. The size dependence was only described by a Hall–Petch type relation for films thicker than the threshold value.

Two Hardening Mechanisms in Single Crystal Thin Films Studied by Discrete Dislocation Plasticity. L.Nicola, E.Van der Giessen, A.Needleman: Philosophical Magazine, 2005, 85[14], 1507-18