Microstructural and crystallization behaviour of as-sprayed and heat-treated Yb2Si2O7 environmental barrier coatings (EBCs) deposited by atmospheric plasma spray (APS): axial and radial injection

The state-of-the-art material for environmental barrier coatings (EBCs) is Yb₂Si₂O₇. To be effective, EBCs require a dense microstructure free of cracks, minimum silicon evaporation, and stoichiometric compositions, while avoiding amorphous phase formation. This study compares the microstructure, crystallinity level, and phase composition of Yb₂Si₂O₇ EBCs produced by two inherently different plasma torches before and after heat treatment. One torch benefitted from lower powers and radial injection of the powder (3MB, Oerlikon metco), while the other was capable of working at higher powers with the axial injection of powder particles (Mettech Axial III). High plasma enthalpies in Axial III produced as-sprayed fully crystalline coatings (up to 92%), but fully amorphous coatings (~95%) were generated with the 3MB. The microstructure of both coatings was relatively dense and free from wide vertical cracks, but severe microcracking was observed in the fully amorphous coating. Due to short stand-off distances and particle dwelling times in the plasma, silicon mass loss was mitigated with Axial III. Significant silicon evaporation was observed among the splats with 3MB, despite lower torch powers, stemming from longer trajectories and dwelling times of particles through the plasma. After heat treating the coatings for 24 hours at 1300°C, the amorphous coatings were almost fully crystallized (up to 94%), and high Yb₂SiO₅ contents (> 30 wt.%) after annealing supported severe silicon evaporation during spraying with 3MB. The microcracks were healed after the annealing treatment in both coatings.