Microstructural changes and phase transformations in a plasma-sprayed zirconia-yttria-titania thermal barrier coating

A thermal barrier coating comprising a zirconia-yttria-titania ceramic coat deposited by plasma spraying on an alloy steel substrate using an Ni-Cr-Al-Co-Y intermediate bond coat has been soaked at various temperatures in the range 700-1400 degrees C and cooled to room temperature in static air. Some thermal treatments were also carried out in an argon atmosphere. Scanning electron microscopy (SEM) studies show that a discontinuous thickness of the bond coat is the reason for the failure of the coating due to thermal treatment. The phase transformations and microstructural changes due to thermal treatment have been investigated using X-ray diffraction and SEM. Thermal treatment did not affect the monoclinic phase of ZrO2 content present in the ceramic coat. In the case of thermal treatment in air and argon (below 800 degrees C) the cubic --> tetragonal phase transformation occurred. The appearance of the fracture surfaces of the ceramic coat suggests that interlamellar fracture in the as-sprayed condition changed to intralamellar fracture after thermal treatment below 1200 degrees. At higher temperatures the fracture were intergranular in appearance. Reasons for these changes in the appearance of the fracture surfaces are suggested with reference to the microstructure. Microstructural investigations also show some changes in the area of porosity distribution of the ceramic coat. Thermal treatment heals some pores, but other voids appear (cracking of the coating).