Crystallization behavior of air-plasma-sprayed ytterbium-silicate-based environmental barrier coatings

A combination of characterization techniques has been used to provide new understanding of the complex crystallization behavior of as-sprayed amorphous Yb₂Si₂O₇-based air-plasma-sprayed environmental barrier coatings (EBCs). During crystallization heat-treatment, initially a mixture of metastable α-Yb₂Si₂O₇ and X1-Yb₂SiO₅ phases form, along with stable β-Yb₂Si₂O₇ and X2-Yb₂SiO₅ phases. Eventually the metastable phases transform to the stable β-Yb₂Si₂O₇ (major) and X2-Yb₂SiO₅ (minor) phases. The significant volume expansion associated with these transformations partially contributes towards the anomalous expansion measured in these EBCs after crystallization, but it does not account for all the measured expansion. In this context, in similar EBCs, it is also observed that the porosity increases upon crystallization heat-treatment, primarily in the form of thin, interconnected pores, which also contributes to the measured anomalous expansion. Based on this understanding, guidelines are provided for ‘near-net-shape’ crystallization of phase-pure, dense β-Yb₂Si₂O₇ EBCs that are free of vertical cracks.