Viability of Xenotime-type Rare Earth Orthophosphates as High-Temperature Environmental Barrier Coatings
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Department of Materials Science and Engineering
Committee:
Dr. Patrick Hopkins (UVA MAE)
Dr. Jon Ihlefeld (UVA MSE)
Dr. Kenneth Kane (JHU APL)
Dr. Bi-Cheng Zhou (UVA MSE)
Dr. Elizabeth Opila (UVA MSE, Advisor)
ABSTRACT
Recent work demonstrates that multicomponent rare-earth (RE) disilicates offer promise as thermal/environmental barrier coatings (T/EBCs) for SiC-based composites for aero turbine engines. Reduced thermal conductivity is attributed to increased phonon scattering caused by variations in RE mass and size, enabling T/EBC capability. However, exposure of RE disilicates to molten calcium-magnesium-alumino-silicates (CMAS) results in rapid degradation of the RE disilicate and exposures to high-temperature steam results in the formation of RE monosilicates leading to shorter lifespan. Additionally, separation of rare earths, only to recombine them for improved thermal properties, makes this an expensive concept. Naturally occurring xenotime (REPO4) minerals containing mixed (Y,Gd,Dy,Er,Yb)PO4 offer promise as a more economical T/EBC if rare earth separation is not required. This proposal seeks to study CMAS resistance properties, high-velocity high-temperature steam resistance, thermophysical properties, and oxygen diffusivity for individual and synthetic mineral xenotime rare earth phosphates synthesized from RE2O3 and H3PO4 and densified by spark plasma sintering. Viability of xenotime minerals as a T/EBC is observed to be an improvement over state-of-the-art RE disilicates.
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