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, as well as silicon evaporation, are the key challenges of spraying Yb2Si2O7 environmental barrier coatings. This research addresses these issues by depositing as-sprayed high crystalline Yb2Si2O7 using atmospheric plasma spray without any auxiliary heat-treating during spraying, vacuum chamber, or subsequent furnace heat treatment, leading to considerable cost, time, and energy savings. The Yb2Si2O7 powder was sprayed on SiC substrates with three different plasma powers of 90, 72, and 53 kW. Exceptional high crystallinity levels of up to similar to 91% and deposition efficiency of up to 85% were achieved. The silicon mass evaporation during spraying was controlled with a short stand-off distance of 50 mm, and an optimum fraction of the Yb2SiO5 secondary phase (< 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 to 53 kW, while conducive to mitigating the silicon mass loss (up to 4%), was detrimental to the microstructure by increasing the fraction of porosities (up to similar to 15%) 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 degrees C for 24 h 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 similar to 10 wt.%).

Automated summary

This research addresses the challenges of spraying Yb2Si2O7 environmental barrier coatings, including high amorphous phase formation tendency, desirable microstructure and phase composition, and silicon evaporation. By depositing high crystalline Yb2Si2O7 using atmospheric plasma spray without auxiliary heat-treating, vacuum chamber, or subsequent furnace heat treatment, cost, time, and energy savings are achieved. The spraying process with controlled silicon evaporation and an optimum fraction of the Yb2SiO5 secondary phase leads to exceptional crystallinity levels and a desirable microstructure. Decreasing the plasma power from 90 to 53 kW reduces silicon mass loss but increases porosity fraction.