Characteristics of thick columnar YSZ coatings fabricated by plasma spray-physical vapor deposition

Columnar-structure is favourable to the durability of thermal barrier coatings (TBCs), and the efficacy of TBCs increases with the thickness of the TBCs. In this study, thick columnar YSZ coatings (similar to 1.5 mm) were deposited by multiple PS-PVD processes. The phase compositions, microstructures and mechanical properties were examined with various techniques including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nanoindentation. Results show that there are only tetragonal phase on the top surface while both monoclinic and tetragonal zirconia are present on the bottom surface. The first layer of the coatings is composed of randomly oriented grains. Immediately after the quasi-equiaxed zone, columns start to grow. The top coatings exhibit looser microstructure compared with the middle and bottom regions, and thus have lower values of hardness and Young's modulus.