Mechanics of thermally driven buckling-induced debonding in thin films
Abstract
Compressed thin films bonded to a substrate have a tendency to buckle and subsequently debond from the substrate. In this thesis, first an elastic analysis pertaining to the mechanics of buckling and debond initiation in films bonded to a substrate is described. The film is modeled by a column with fixed ends whereas the interface between the film and the substrate is described by a cohesive zone model. The cohesive zone is a region within which the interfacial forces are effective. The nature of these forces is assumed to be described by a bilinear traction separation law. The film-substrate system is assumed to be thermally loaded to induce compression within the film. The analytical solution describing the mechanics of buckling and debond initiation is obtained by applying the principle of minimum free energy. The outcome of the analysis are critical non-dimensional parameters describing the mechanics of buckling-induced debonding. The most important of these critical parameters is the foundation compliance ratio which separates film-interface systems with different kinds of post-buckling responses. The importance of foundation compliance ratio in wrinkled films is also discussed. It is shown that films wrinkle when the foundation compliance ratio is unity. The elastic analysis is also extended to study buckling and post-buckling behavior in elastic-plastic films. The elastic-plastic analysis assumes the film to be in a state of uniform plasticity before debonding. The elastic-plastic solutions are obtained for the following path of temperature excursion: precompression of the film by a sudden temperature excursion followed by a monotonic increase in temperature. It is assumed that there is no further plastic deformation after buckling. This assumption is valid under certain conditions and is described in terms of foundation compliance ratio. The elastic-plastic analysis is also extended to study debond propagation in a partially debonded film. Expressions to evaluate energy release rate at the tip of the propagating debond are obtained. A thermally driven buckling test to estimate interfacial fracture toughness in layered structures of thin films is also described. The test offers several advantages over the existing test methods to characterize interfacial failure. The test is demonstrated on a model system consisting of aluminum and SU8 films on a silicon substrate wherein interfacial fracture is induced at the interface between aluminum and SU8 film. The temperature at which debonding occurs is high enough to cause plasticity in the aluminum film before debonding. The elastic-plastic analysis of debond initiation and propagation is applied in the thermally driven buckling test to estimate the interfacial fracture toughness. Since aluminum films debond in a state of uniform plasticity, the thermally driven buckling test is also used to estimate the yield strength of aluminum films.
Degree
Ph.D.
Advisors
Subbarayan, Purdue University.
Subject Area
Mechanical engineering
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