The interactions of two potential topcoat materials for environmental barrier coatings, Yb 2 SiO 5 and Yb 2 Si 2 O 7 , with calcium-magnesium-aluminosilicate (CMAS) engine deposits were studied. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron diffraction were used to investigate the phase transformation associated with the exposure of Yb 2 SiO 5 and Yb 2 Si 2 O 7 to CMAS at 1300°C. It was found that Yb 2 SiO 5 strongly reacts with CMAS to completely dissolve the Yb 2 SiO 5 and form hexagonal Ca 2 Yb 8 (SiO 4) 6 O 2 deposits. In contrast, no discernable reaction between CMAS and Yb 2 Si 2 O 7 , was observed over the 96-hour exposure.

The interaction of calcium-magnesium-aluminosilicates (CMAS) with Yb 2 Si 2 O 7-based multilayer environmental barrier coatings was studied using a combination of high-energy synchrotron X-ray techniques. Changes in the phase-specific stresses and lattice spacings as a function of temperature were investigated with wide-angle X-ray scattering. Small-angle X-ray scattering was used to study the interaction of CMAS with nanoporosity, while X-ray computed tomography was used to study the formation of cracks in the coating. It was found that CMAS interacts strongly with the Yb 2 Si 2 O 7 topcoat, leading to changes in lattice spacing, alterations of the ambient temperature stresses, and large cracks in the CMAS that extend through the topcoat.

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