This report summarizes the results of a numerical investigation
into the spallation mechanism in plasma-sprayed thermal barrier
coatings observed under spatially-uniform cyclic thermal loading.
The analysis focuses on the evolution of local stress and inelastic
strain fields in the vicinity of the rough top/bond coat interface
during thermal cycling, and how these fields are influenced by the
presence of an oxide film and spatially uniform and graded
distributions of alumina particles in the metallic bond coat aimed
at reducing the top/bond coat thermal expansion mismatch. The
impact of these factors on the potential growth of a local
horizontal delamination at the rough interface's crest is included.
The analysis is conducted using the Higher-Order Theory for
Functionally Graded Materials with creep/relaxation constituent
modeling capabilities. For two-phase bond coat microstructures,
both the actual and homogenized properties are employed in the
analysis. The results reveal the important contributions of both
the normal and shear stress components to the delamination growth
potential in the presence of an oxide film, and suggest mixed-mode
crack propagation. The use of bond coats with uniform or graded
microstructures is shown to increase the potential for delamination
growth by increasing the magnitude of the crack-tip shear stress
component.
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