Functionally multilayered Yb-silicate EBCs fabricated using slurry spraying-reactive sintering technique

Abstract

Functionally multilayered environmental barrier coatings (EBCs) with the desired layer sequence of Yb₂O₃/Yb₂SiO₅/Yb₂Si₂O₇/SiO₂/Si were obtained using an unconventional yet simple technique of reactive sintering. Yb₂O₃ slurry was sprayed on polycrystalline silicon (used as a bond coat for SiC_f/SiC composites) and reactive sintered to generate graded layers that provide a combination of mitigated thermal expansion mismatch as well as silica activity. The Yb₂O₃-terpineol slurry produced a uniform deposit of the Yb₂O₃ topcoat. Reactive sintering of exploratory coatings (t ∼ 10–20 µm) was carried out in air by varying the temperature (1100–1400°C) and time (2–24 h). X-ray diffraction and scanning electron microscopy-energy-dispersive spectroscopy technique confirmed the in situ formation of X₂-Yb₂SiO₅ and β-Yb₂Si₂O₇. Rietveld refinement was performed to know the volume fractions of the respective phases. The set of process parameters identified from the preliminary work was employed to fabricate EBCs with thickness ∼ 250 µm. The X₂-Yb₂SiO₅ and β-Yb₂Si₂O₇ phases formed next to the Si/SiO₂ interface as two separate layers. Post-sintering heat treatments in air at 1300, 1350, and 1400°C for 3–50 h elucidated the growth characteristics of the oxide layers to be a function of both temperature and time. The theoretical considerations of diffusional reactive phase formation of the EBC multilayer growth rates are discussed.