Phase purity and evolution in sol–gel derived single component and multicomponent rare-earth disilicates

Abstract

Rare-earth disilicates are a focus of study for use as environmental barrier coatings in gas-turbine engines. These coatings require thermomechanical and thermochemical stability at elevated temperatures and properties can be tailored through the use of multicomponent rare-earth disilicates. Producing rare-earth disilicates via sol–gel is documented in literature, but there are differing procedures with varying phase purities. This work establishes trends that dictate the effects of water content, pH, and heat treatment conditions that determine the final phase purity of Yb, Er, Lu, Sc, and Y disilicate powders made via sol–gel. The phase(s) of the powders were identified and quantified using X-ray diffraction (XRD) to extract weight fractions. In situ XRD during heating from room temperature to 1200°C was used to observe the crystallization and phase evolution of the sol–gel-based powders, allowing for the identification of a rarely reported low temperature triclinic phase in ytterbium-, erbium-, and lutetium-based disilicate sol–gels that forms prior to transformation into a monoclinic phase. Ex situ XRD allowed for the phase identification of sol–gels processed at 1400°C. These experiments demonstrated that phase-pure disilicates could be formed under conditions with no intentional water additions, a target pH of 2, and long heat treatment times at high temperatures (e.g., 1400°C). These conditions remain valid for not only single-cation rare-earth disilicates of Yb, Er, Lu, Sc, and Y but also a multicomponent disilicate containing equimolar concentrations of all of these cations.