On the oxygen vacancy annealing in air plasma sprayed yttria-stabilized zirconia thermal barrier coatings

Yttria-stabilized zirconia (t'-YSZ) thermal barrier coatings (TBCs) were deposited by air plasma spraying (APS) using commercial purity and high-purity feedstock powders. The coatings were subjected to short-term, low-temperature annealing (500-1200 degrees C for 30 min). Significant structural changes were observed by X-ray diffraction and Raman spectroscopy. These changes were attributed to annihilation of excess oxygen vacancies created by the process. APS induces partial reduction of YSZ due to the reducing potential of the high-temperature and hydrogen-rich plasma environment. Laser flash thermal conductivity, impulse excitation elastic modulus, and dilatometric measurements show significant changes in the YSZ during the short-term low-temperature thermal exposure, concurrent to the vacancy annealing. The results were analyzed in terms of the coating chemistry and the processing environment. High-purity coatings exhibited larger increment in thermal conductivity and elastic modulus than commercial purity coatings.