Decohesion of a cut prestressed film on a substrate

Thin films or coatings on a substrate are often under a state of residual biaxial tension. This paper analyzes the cut test wherein a straight cut which is long compared to the film thickness is made through the film down to the substrate. Depending on the elastic properties of the materials, the residual stress level, and the toughness of the interface bond between the film and substrate, the cut may induce extensive decohesion, limited decohesion or no decohesion at all. The main thrust of this paper is concerned with the shape and extent of the zone of decohesion when extensive decohesion occurs. The interface crack front at the boundary of the decohered region is subject to mixed mode conditions involving modes 1, 2 and 3. The shape of the decohesion region depends sensitively on the way in which the mode 3 contribution enters the decohesion criterion. Observations of shapes for a polyimide film on a glass substrate permit the relative importance of the mode 3 contribution to be inferred when the decohered film makes no contact with the substrate. Some experimental observations indicate that the decohesion process is quite complex when the decohered film makes contact with the substrate. A prototypical criterion for mixed mode interfacial fracture is proposed for cases in which the interfacial crack remains open.