✍ Scribed by J.M. Ambrico; M.R. Begley; E.H. Jordan
No coin nor oath required. For personal study only.
Initial geometric imperfections in oxide films on elastic-plastic substrates often develop local shape irregularities upon thermal cycling, even though the same system shows no such shape change under similar constant temperature exposure. This paper outlines a mechanics-based explanation of the evolution of such irregularities during thermal cycling, based on reversed plasticity in the material surrounding the irregularity and stress-dependent oxide film growth. Idealized models consisting of spherical or cylindrical elastic shells embedded in an elastic-plastic matrix are used to identify regimes where thermal strains, cyclic reversed plasticity, geometry and oxide growth combine to promote rapid evolution of irregularities. The entirely closed-form solutions for stress and displacements during temperature cycling allow for quick evaluation of the role of various parameters, including: the temperature amplitude, size of the irregularity, oxide thickness, yield stress of the substrate and stress-dependent oxide growth strains. The usefulness of the solutions is illustrated by comparing two oxide film-growth scenarios with preliminary experimental results on a representative thermal barrier system that exhibits significant feature growth. The comparison demonstrates that oxide growth enhanced by tensile stresses created during reversed plastic deformation may be a significant factor in accounting for the large geometry changes observed in experiments.
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