Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb2Si2O2 coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO2 content with higher power and H-2 flow. Differences in microstructure and thermo-mechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb2SiO5 metastable phase was identified after thermal treatments at temperatures similar to 1000 degrees C that transformed to its stable isomorph at 1220 degrees C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the healing of microcracks present in the amorphous as-sprayed coating.