Toward Highly Dense Yb-Silicate Microstructures Deposited by Air Plasma Spray for Environmental Barrier Coating Applications I: Influence of Local Deposition Rate

Environmental barrier coatings (EBCs) are used to shield high-temperature Si-based ceramic matrix composite gas turbine components from the harsh, water vapor-rich operating environment. The success of this application is directly correlated to the intrinsic gas tightness or hermeticity of the EBC volatilization barrier, which are typically rare-earth disilicates. For the air plasma-sprayed (APS) EBCs, the hermeticity is directly related to the processing parameters during deposition. In this work, the effect of surface speed and feeding rate on the microstructural evolution of plasma-sprayed Yb₂Si₂O₇ coatings was studied. A qualitative assessment by means of microstructure analysis of hermeticity after crystallization heat treatment and the influence of the aforementioned processing parameters is discussed. It was found that utilizing lower feeding rates can minimize certain types of disadvantageous cracking in the as-deposited Yb₂Si₂O₇ coatings. Moreover, at these low feeding rates (~1-5.4 g/min) regardless of the selected surface speed (250-1250 mm/s), highly dense Yb₂Si₂O₇ microstructures could be obtained in the as-sprayed state. Contrastingly at higher feeding rates (~46.2 g/min), deleterious Yb₂SiO₅ band formation was observed at the top of each spray pass, which correlated to worsened cracking in the microstructures, particularly at low surface speed (250 mm/s). The monosilicate band formation was linked to the fine particle fraction of the feedstock (i.e., particles < 15 µm), and by sieving out this fraction the band formation could be eliminated when spraying at the higher feeding rate.