Hybrid Suspension/Solution Precursor Plasma Spraying of a Complex Ban (Mg_1/3Ta_2/3)O₃ Perovskite: Effects of Processing Parameters and Precursor Chemistry on Phase Formation and Decomposition

Ba(Mg_1/3Ta_2/3)O₃ (BMT), a high melting point refractory oxide, is envisioned as a thermal barrier coating material. In this study, six chemical reagents combinations are investigated as BMT coating precursors: one BMT powder suspension and five Ta₂O₅ suspensions in nitrate solutions or acetate solutions. A hybrid suspension / sol plasma spray process is designed to axially inject these precursors into a RF thermal plasma torch to synthesize BMT and to deposit nanostructured coatings. X-ray photoelectron spectroscopy (XPS) was used to evaluate the element evaporation during plasma spraying. Thermogravimetric analysis and differential thermal analysis (TG/DTA) are applied to investigate the BMT formation. Parameters such as precursor chemistry and proportion, plasma power, spray distance and substrate preheating are studied with regards to the coating phase structure. The results indicate that the combination of twice the Mg stoichiometric amount with a power of 50 kW shows the best results when using nanocrystalline Ta₂O₅ as Ta precursor. When choosing nitrates as Ba and Mg precursors, predominant crystalized BMT can be obtained at lower plasma power (45 kW) when compared to acetates (50 kW). BaTa₂O₆, Ba₃Ta₅O₁₅, Ba₄Ta₂O₉, Mg₄Ta₂O₉ are the main secondary phases during BMT preparation process. Because of the complicated acetate decomposition, the coating deposition rate from nitrate precursors is higher than that from acetate ones.