Simulation of thin film delamination under thermal loading

The conventional approach to analysis of thin film delamination is based on the consideration of the film, which is subjected to residual stresses arising from the thermal mismatch between the film and the substrate, within the framework of the classical fracture mechanics and the structural buckling theories. Such concepts as the energy release rate and the stress intensity factors are crucial in this case. A different approach to analysis of thin film delamination considers the effect of the compliant interface between the film and the substrate. This compliant interface is described by the traction-separation constitutive law. In the present work, a framework for modeling delamination of thin films is described in accordance with the latter 'decohesion' approach. The film is modeled as a geometrically nonlinear elastic beam attached to a rigid substrate with a cohesive layer of zero thickness. The cohesive layer is described by normal and tangential tractions and corresponding displacement jumps. An exponential 'softening' constitutive law relates tractions and displacements of the cohesive layer. Such formulation allows for studying nucleation, propagation and arrest of local delaminations - edge cracks and blisters. This is in contrast to the traditional approach of the classical fracture mechanics where stress analysis is separated from a description of the actual process of material failure. Finite element analyses are carried out for the qualitative study of the influence of different parameters of the thin film and the cohesive layer as well as different thermal loads on delamination behavior of the film-substrate system. The results of the analysis show that the stability of the delamination propagation depends mainly on the shape of the thermal load and less on the distribution of the cohesive surface strength along the film. The location of the nucleation of the film separation is essentially sensitive to the combination of the cohesion properties of the film/substrate interface and the thermal load shape and fewer to the film geometrical defects. It is found that the interior blisters - the arrested delamination processes - are created only for special cases of the film and cohesion property combinations, otherwise the unstable interior delaminations or edge cracks take place. The material imperfection of the film - the inhomogeneous thermal expansion coefficient distribution along the film increases or decreases the effect of the thermal load on the film response and has a little effect on the film delamination process.