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 ?-Yb2Si2O7 and X1Yb2SiO5 phases form, along with stable ?-Yb2Si2O7 and X2-Yb2SiO5 phases. Eventually the metastable phases transform to the stable ?-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 ?-Yb2Si2O7 EBCs that are free of vertical cracks.

Automated summary

A combination of characterization techniques was used to provide new understanding of the complex crystallization behavior of as-sprayed amorphous Yb2Si2O7-based EBCs. Different phases formed and transformed during crystallization, with an increase in porosity contributing to anomalous expansion. Guidelines for achieving phase-pure and dense EBCs free of vertical cracks are also provided based on this understanding.