| Abstract: |
The atmospheric plasma spray (APS) has been recognized as a suitable coating process to deposit environmental barrier coatings (EBCs) to protect silicon-based ceramic matrix composites (CMCs) against high-temperature water vapor in gas turbines. Nevertheless, high crystallinity, dense and crack-free microstructures with a desirable phase composition and distribution in the coatings are vital to meet the expectations. To investigate the effect of APS process parameters on the as-sprayed microstructure and phase composition of Yb2Si2O7 EBCs, three different plasma torch powers, and two nozzle diameters (3/8” and 5/16”), combined with adding a shroud attachment, were employed. High levels of crystallinity were reported by XRD analysis for all the as-sprayed coating conditions. In addition, excellent inter-splat adhesion was observed with higher plasma powers indicating a complete melting of splats upon impact. The coatings deposited with a 3/8” nozzle at maximum power, was dense and free from through-thickness cracks and interconnected pores or channels. However, decreasing the torch power resulted in a considerable deposition efficiency reduction (up to 20%) and a less dense structure due to a higher portion of unmelted or semi-melted particles during spraying. A smaller nozzle diameter led to higher particle velocities and, in consequence, denser coating microstructures. However, some cracking was observed in such a condition due to a higher modulus of elasticity of the deposited coatings. Si-depleted secondary phases of Yb2SiO5 and Yb2O3 were formed due to the silicon evaporation during spraying. While small Yb2SiO5 inclusions were uniformly distributed through the coatings in all the conditions, the Yb2O3 phase was only present in coatings deposited using the shroud because of the higher heat transfer to the in-flight particles in such conditions.
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