As-Sprayed Highly Crystalline Yb₂Si₂O₇ Environmental Barrier Coatings (EBCs) by Atmospheric Plasma Spray (APS)

High amorphous phase formation tendency, a desirable microstructure and phase composition and silicon evaporation are the challenges of spraying Yb₂Si₂O₇ environmental barrier coatings (EBCs). This research addresses these issues by depositing as-sprayed high crystalline Yb₂Si₂O₇ using atmospheric plasma spray (APS) without any auxiliary heat-treating during spraying, vacuum chamber, or subsequent furnace heat treatment, leading to considerable cost, time, and energy savings. Yb₂Si₂O₇ powder was sprayed on SiC substrates with three different plasma powers of (90, 72 and 53 kW) and exceptional high crystallinity levels of up to ~91% and deposition efficiency of up to 85% were achieved. The silicon mass evaporation during spraying was controlled with a short stand-ff distance of 50 mm, and an optimum fraction of Yb₂SiO₅ secondary phases (<20 wt.%) was evenly distributed in the final deposits. The desirable microstructure, including a dense structure with uniform distribution of small porosities, was observed. The undesirable vertical crack formation and any interconnected discontinuities were prevented. Reducing the plasma power from 90 kW to 53 kW, while conducive for mitigating the silicon mass loss, was detrimental for microstructure by increasing the fraction of porosities and partially melted or unmelted fragments. The gradual decrease of the coating temperature after deposition alleviated microcracking but has an insignificant effect on the crystallinity level. Coatings annealed close to their operating temperature at 1300 °C for 24 hours demonstrated sintering and a crack healing effect, closing the tiny microcracks through the thickness. An improved coating composition was detected after annealing by the transformation of Yb₂SiO₅ to Yb₂Si₂O₇ (up to ~10 wt.%).