Interface shear stresses induced by non-uniform heating of a film on a substrate

When a thin film bonded to a substrate is heated, normal stresses in the film and shear stresses at the interface develop that may lead to debonding or fracture of the film. The thermal stresses in such a situation are analyzed here using a shear lag model. Both the thin film and substrate are modeled as linearly elastic, isotropic, homogeneous materials and are in the plane strain condition. Results from the shear lag model are compared with 2D finite element results in order to validate the approximations inherent in the shear lag model. The shear lag model is found to be valid as long as the film is thin compared with the dimension that characterizes the spatial variation of the temperature field. Studies of the shear stresses along the interface of the film and substrate using the model indicate that the maximum shear stress may be as high as 20% of the maximum in-plane normal stress in the film and hence exceed the shear strength of the interface. The work is motivated by possible applications in optical disc technology, materials processing, etc.