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

Abstract

AbstractFunctionally 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 SiCf/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.