Effects of oxygen and water vapor on the formation and growth of silica layer beneath barium strontium aluminosilicate coatings

Barium strontium aluminosilicate (BSAS)-coated C/SiC composites were corroded under environments containing varying H₂O/O₂ ratios at elevated temperatures. After corrosion, continuous thermally grown silica oxide (silica-TGO) was formed at the BSAS coating/SiC bond coat interface, and the thickness of this layer increased with the corrosion time prolonged. The growth kinetics of the silica-TGO layer formed beneath the BSAS coating upon corrosion with a 50%H₂O–50%O₂ flowing gas mixture at elevated temperatures were examined. The results revealed that this silica-TGO layer beneath BSAS grew following a parabolic behavior, in line with a diffusion-controlled growth process. Based on the calculated growth rates and the activation energies of the silica-TGO layer, oxygen ion was identified as the main oxidant. The corrosion experiments under flowing gas mixtures containing varying H₂O/O₂ ratios revealed that water vapor was also involved in the growth of the silica-TGO layer.