Molten calcium-magnesium-aluminosilicate interactions with ytterbium disilicate environmental barrier coating

摘要

Thermochemical interactions between calcium-magnesium-aluminosilicate (CMAS) glass and an environmental barrier coating of ytterbium disilicate (Yb_2Si_2O_7) and ytterbium monosilicate (Yb_2SiO_5) were investigated. Top coats were deposited by plasma spray-physical vapor deposition onto silicon carbide substrates. CMAS powder was prepared as a glass and cast into a tape to yield a CMAS loading of ～29 mg/cm~2. Samples were heat treated with CMAS at 1300 °C for 1-10 h or at 1400 °C for 1 h in air. Polished specimen cross-sections were characterized using scanning electron microscopy, X-ray diffraction, X-ray energy-dispersive spectroscopy, and transmission electron microscopy to evaluate resulting microstructures, phases, and compositions at CMAS/Yb_2Si_2O_7 interfaces. Coatings exposed at 1300 °C-10 h and 1400 °C- 1 h were fully infiltrated and compromised by CMAS. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of cyclosilicate, silicocarnotite, and Yb_2Si_2O_7 at 1300 °C and Yb_2Si_2O_7 at 1400 °C enabled CMAS to effectively infiltrate top coats, rendering the predominantly Yb_2Si_2O_7 coating ineffective at arresting molten CMAS degradation.