Effects of Powder Structure and Size on Gd2O3 Preferential Vaporization During Plasma Spraying of Gd2Zr2O7

Abstract

Complex rare-earth oxide ceramics with pyrochlore structure (A2B2O7) are considered as promising candidate materials for next generation thermal barrier coatings (TBCs) due to low thermal conductivity and high phase stability. During plasma spraying, the component with a higher vapor pressure would experience preferential vaporization. In this study, Gd2Zr2O7 (GZO) powders with a hollow spherical structure in a wide range of particle size distribution were used to examine the preferential vaporization behavior of Gd2O3 during plasma spraying. Individual GZO splats were deposited to study the effect of particle size on Gd vaporization loss. The results show that almost all particles experienced Gd preferential vaporization loss that increased remarkably with the decrease in particle size. The Gd vaporization loss was significantly suppressed only when the particle size of molten droplet was larger than about 35 μm. The relationship between the Gd/Zr ratio and the droplet size reveals the significant effects of particle size and powder structure on Gd vaporization loss. It is clear that the vaporization behavior of in-flight GZO particles is dominated by molten droplet size rather than the size of feedstock powders with a porous hollow structure. The particle size effect on Gd vaporization resulted in wide distributions of Gd/Zr ratio in plasma-sprayed GZO coatings. Furthermore, it is found that after annealing at 1300 °C, the vaporization loss of Gd led to the formation of monoclinic ZrO2 from metastable pyrochlore phase in GZO coatings with inhomogeneous chemical composition.