Mechanical properties and oxidation behaviour of plasma sprayed functionally graded zirconia–alumina thermal barrier coatings

The microstructures of thermally sprayed coatings usually incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects various coating properties such as elastic modulus, thermal conductivity and dielectric behaviour. This study is conducted to improve the image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for TBCs deposited with partially stabilized zirconia (PSZ), alumina and zirconia–alumina composite coatings by gas tunnel type plasma spraying. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for porosity analysis agreed with coating qualities and the influences of the Vickers hardness. The paper also discusses the thermal behaviour and high temperature oxidation resistance of the Al2O3 coatings as compared to composite coatings at the interface. This interlayer is preferred to minimize the detrimental effect of phase transformation of γ-Al2O3 to α-Al2O3.