Particle size effect on Gd₂O₃ preferential evaporation during plasma spraying of Gd₂Zr₂O₇

Rare-earth complex oxide ceramics are considered as promising candidate materials for next generation thermal barrier coatings (TBCs) due to its low thermal conductivity and phase stability. During plasma spraying, the component with a higher vapor pressure may experience severe preferential vaporization resulting in significant composition change from the starting powder. In this study, Gd₂Zr₂O₇ (GZO) with a hollow spherical structure is used as the feedstock material in a wide range of particle size to examine the vaporization behavior of Gd₂O3 during atmospheric plasma spraying (APS). Isolated Gd₂Zr₂O₇ splats with regular disc shape in different sizes were deposited on polished stainless steel substrates at the deposition temperature of ~300 °C to study the effects of particle sizes on vaporization loss of Gd₂O₃. The elemental compositions of each splat were analysed by EDS, and the ratio of Gd to Zr in different splats with different diameters was obtained. The results show that the vaporization loss of Gd increases remarkably with the decrease of the particle size. The size effect was correlated to the distribution of Gd in the deposited coating. It is clear that there exists evident size effect on Gd loss resulting from preferential vaporization of Gd₂O₃. Using Gd₂Zr₂O₇ powders with certain larger size the Gd loss can be significantly reduced. Moreover, powder structure study reveals that the size effect is determined by the molten droplet size rather than the apparent powder particle size.