Interaction of ytterbium pyrosilicate environmental-barrier-coating ceramics with molten calcia-magnesia-aluminosilicate glass: Part I, Microstructures

Ytterbium pyrosilicate (or disilicate; β-Yb ₂Si ₂O ₇ ) is a promising ceramic for environmental barrier coatings (EBCs) for ceramic-matrix composite (CMC) components in the hot-section of gas-turbine engines. In addition to the various requirements an EBC must satisfy, it must be resistant to high-temperature attack by calcia-magnesia-aluminosilicates (CMASs) ingested by the engine in the form of sand, dust, ash, etc . Here, the microstructures of dense, polycrystalline β-Yb ₂Si ₂O ₇ EBC ceramics, before and after high-temperature (1500 °C) interaction with a representative CMAS glass, are characterized, primarily using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Since actual EBCs are dense and polycrystalline, sintered pellets of EBC ceramics serve as acceptable proxies for such basic studies. It is confirmed that the CMAS glass wets certain grain boundaries in β-Yb ₂Si ₂O ₇ ceramics, and the mechanisms associated with the penetration of the CMAS glass are elucidated. It is also found that secondary-phase inclusions, that are ubiquitously present in these ceramics, but rarely characterized, are either rounded or faceted. The relatively large, partially-faceted ones are invariably attached to the grain boundaries, and can contain crystalline X2-Yb ₂SiO ₅ and Yb ₂O ₃ , and amorphous SiO ₂ secondary phases primarily. However, after high-temperature CMAS interaction, these inclusions become occluded within β-Yb ₂Si ₂O ₇ grains, and are filled with Yb-containing CMAS glass. All these new findings contribute towards the broader understanding of β-Yb ₂Si ₂O ₇ /CMAS interactions, with implications for the design of EBC microstructures and approaches to mitigate CMAS-induced degradation in EBCs. The accompanying Part II paper presents results from extensive characterization at the atomistic level, and addresses the anisotropic nature of the interfaces in the system, as well as defects along the interfaces, before and after the interaction with the CMAS glass.