Analysis of the t′ + c Phase Content from ZrO2-16 Mol.% Y2O3-16 Mol.% Ta2O5 Mechanoactivated Powder Mixtures in Coatings Deposited by Suspension Plasma Spraying

Abstract

Gas turbines or power generation engines used in transportation, defense, and energy sectors use thermal barrier coatings (TBCs) to protect metallic components exposed to high-temperature conditions. This work focuses on the in situ synthesis during coating deposition by suspension plasma spraying (SPS) from powders mechanoactivated by high-energy ball milling of ZrO₂-16 mol.% YO_1.5-16 mol.% TaO_2.5 (16YTZ) for applications in TBCs. This composition is expected to present a non-transformable tetragonal phase (t′ phase), suitable to overcome the thermodynamic limits of the mostly used conventional 6-8 wt.% yttria stabilized zirconia (YSZ). The 16YTZ powder mixtures were mechanoactivated by either planetary ball milling (240 RPM) and high-energy ball milling (1700 RPM). These mechanoactivated powders were used to obtain densified samples through sintering at 1500 °C for 2 and 20 h, as well as to deposit coatings by SPS at a stand-off distance between 60 and 80 mm using Ar/He as the plasma forming gas. The ceramics and coatings were characterized by XRD, SEM, and Raman spectroscopy. The densified ceramics showed a high t′ phase content, i.e., > 98% of the 16YTZ system, with an average tetragonality of 1.0267. Coatings deposited at a stand-off distance of 60 mm, and a deposition rate of 33 µm/min, present a porosity of less than 5%, and a tetragonality of 1.0187, which is higher than that of standard YSZ coatings. Phase quantification by Rietveld refinement revealed a multiphase condition as result of the heat transfer in the plasma plume leading to in situ synthesis of the ZrO₂ base solid solutions, formed by 89.2 wt.% of t′ + c phases (48.4 and 40.8 wt.% of t′ and c-ZrO₂, respectively) 8.1% monoclinic ZrO₂, 2.4% cubic Y₂O₃ and 0.4% orthorhombic Ta₂O₅ on average. Achieving a high percentage of t' + c phases in the coating with a porosity of less than 5% is a significant accomplishment. This success is closely linked to the fact that tetragonality exceeds that of YSZ. These results provide a solid foundation for future experimental designs aimed at optimizing spray parameters.