Edge effects in thin film delamination

Abstract

Thin films bonded to a substrate often sustain large in-plane residual stresses that are transferred to the film via shear stresses on the interface near their edges. These edge zones play a significant role in film delamination. A new method is introduced to analyze both the residual stress distribution in a film near its edge and the energy release rate and mode mix for an interface delamination crack emerging from, or converging upon, an edge. Two two-dimensional configurations are considered: (a) a film whose edge lies in the interior of the substrate and (b) a film whose edge is aligned with the edge of the substrate (i.e. the film/substrate geometry is a quarter-plane). There are significant differences between the two cases. For the former, (a), the energy release rate approaches the steady-state, limiting rate for a long interface crack when the crack has extended less than one film thickness. By contrast, the energy release rate in case (b) remains far below the steady-state rate until the crack extends to ten or more film thicknesses from the edge. In case (b), the edge effect provides a significant protection against edge delamination, whereas in case (a) it does not. Elastic mismatch between the film and the substrate is significant in case (b), but not in case (a). A second set of behaviors is investigated wherein the interface crack approaches the edge of the film from the interior. For both types of edges, the energy release rate drops well below the steady-state rate at remaining ligament lengths that are very large compared to the film thickness, approaching zero as the delamination converges on the edge. Analytic features which account for the various behaviors will be highlighted, and practical implications for thin film delamination will be discussed.