Phase stability and tensorial thermal expansion properties of single to high‐entropy rare‐earth disilicates

• Published in: Journal of the American Ceramic Society Vol. 106; no. 5; pp. 3228 - 3238
• Main Authors: Salanova, Alejandro, Brummel, Ian A., Yakovenko, Andrey A., Opila, Elizabeth J., Ihlefeld, Jon F.
• Format: Journal Article
• Language: English
• Published: Columbus Wiley Subscription Services, Inc 01-05-2023; American Ceramic Society
• Subjects: anisotropy; Ceramic matrix composites; Coatings; Composition; Crystallography; Dysprosium; Earth; Energy consumption; environmental barrier coatings (EBC); Gas turbines; Nickel base alloys; Operating temperature; Phase stability; Room temperature; Rotation; sol-gel; Sol-gel processes; Stability analysis; Superalloys; Synchrotrons; Temperature; Tensors; Thermal expansion; X-ray diffraction; Ytterbium
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.18986

Temperature limitations in nickel‐base superalloys have resulted in the emergence of SiC‐based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare‐earth disilicates (RE2Si2O7) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single‐ and multiple‐component rare‐earth disilicate powders were synthesized via a sol‐gel method with compositions selected to exist in the monoclinic C 2/m phase (β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P 1¯${\rm{\bar{1}}}$, phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X‐ray diffraction. The thermal expansion tensors for each composition were determined from high‐temperature synchrotron X‐ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium‐containing compositions had larger changes in the α31 shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo‐mechanical robustness of rare‐earth disilicate environmental barrier coatings.