Relationship between cracks and microstructures in APS YSZ coatings at elevated temperatures

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. Thermally grown oxide (TGO) was formed between the YSZ layer and the bond coat during heat treatment of the YSZ TBCs. The generation and extension behavior of cracks before and after heat treatment were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) combined with 3D reconstruction technology. The characterization results give a direct evidence that the generation and extension of cracks is closely related to other microstructures such as columnar grains, TGO morphology, TGO composition, and TGO orientation. For example, large interlaminar cracks in as-sprayed YSZ coatings often appeared along with columnar grains, which can be explained by their similar formation conditions. The formation of new cracks such as vertical cracks, plane cracks and other cracks along the TGO grain boundary after heat treatment often resulted from TGO growth stresses. Therefore, the generation and extension of cracks can be controlled by regulating the formation of columnar grains and TGO.