Growth and Failure of Oxide Irregularities During Thermal Cycling: Interactions Between Stress, Geometry and Oxide Formation

Abstract

In some oxide thin film systems, such thermal barrier coating systems (TBCs), thermal cycling leads to the development of geometric irregularities in the film. The evolution of these irregularities involves very large changes in aspect-ratio and often occurs rapidly over several hundreds of cycles. A key aspect of this behavior is the development of tensile stresses in the irregularity due to plastic yielding of the surrounding metal. These stresses can accelerate the elongation of the oxide (which translates into shape evolution of the irregularity) by various mechanisms, including enhanced oxide formation, inelastic stretching (creep) and failure of the oxide. An idealized analytical model consisting of a thin elastic shell embedded in an elastic-plastic matrix is used to explore the interactions between geometry, thermal strains, plasticity and oxide growth. Boundaries between purely elastic deformation, uni-directional yielding and reversed plasticity are shown to have a strong dependence on the size of the irregularity relative to the oxide thickness. For any given thermal strain, there is a critical aspect ratio of the irregularity that leads to maximum tensile stress in the oxide. The resulting closed-form solutions allow for quick and easy evaluations of various oxide-growth scenarios, including stress-dependent oxide formation. This talk will present the application of these models to TBC thermal cycling experiments, and discuss how stress-dependent oxide formation plays a role in the rapid evolution of these irregularities. A variety of oxide growth scenarios will be illustrated, and used to demonstrate that oxide failure and subsequent oxide formation in the cracked region is the most likely explanation for the rapid shape evolution seen in the experiments.