EBSD analysis of microstructure changes in YSZ coatings during thermal cycling

Failure of atmospheric plasma sprayed (APS) yttria-stabilized zirconia (YSZ) coatings is complicated. Therefore, determining the microstructural changes that occur in the top coat during thermal cycling is beneficial to figure out the dominant failure mechanism in thermal barrier coatings. In this study, thermal cycling tests were conducted on YSZ coatings sprayed by APS. For comparison, coatings with the same spray parameters were held at 1100 degrees C for 25 h. Areas of interest in the cross-sections of the coatings were chosen prior to thermal cycling, in which ex-situ observation and analysis were performed via scanning electron microscope and electron backscatter diffraction. Residual stress values of coatings with different thermal cycling times were measured by X-ray diffraction. The growth of quasi-columnar crystals, which were related to a depletion in Al and an increase in stress, was observed. During thermal loading, the vertical cracks propagate comparatively easier leading to the spallation. The residual stress varies continually during the thermal cycling test and the changes are divided into several stages. The monoclinic phase transformation tends to occurred inside grains, creating compressive stress inside the coating, which improve the longevity of the coating.