The evolution of fracture process zones in as-received and oxidized air plasma sprayed TBCs

Abstract

The evolution of the fracture process zones as precursor to fracture was elucidated in as-received and oxidized air plasma sprayed (APS) Thermal Barrier Coating (TBC) specimens. Multiple techniques of Digital Image Correlation (DIC), Acoustic Emission (AE), Finite Element Analysis (FEA) and Scanning Electron Microscope (SEM) micrographs were combined together to obtain in-depth understanding of the failure mechanisms involved in a TBC component under uniform bending stress field. Both sides of specimens were monitored simultaneously during loading and it is shown that within a uniform applied strain, the localized zones of non-symmetric strains with the different lengths were initiated and growth within the ceramic top coat perpendicular to the principle stresses. The process zones were initiated in both as-received and oxidized specimens upon yielding the substrate. However, different strain localization areas were observed on two sides of each specimen. It was observed that separations in each localized zone were resisted by cohesive tractions controlled by the material microstructural features and not the applied external load. One of these localized strains was then rotated parallel to the interface propagating until the final delamination. In the oxidized specimen, the delamination was faster and spallation of top coat ceramic layer followed immediately after cracks reached the bond coat interface. The cohesive properties were determined using a hybrid experimental-numerical approach.