Xenotime‐Type Rare‐Earth Orthophosphates REPO 4 for Use as High‐Temperature Environmental Barrier Coatings

ABSTRACT Rare‐earth phosphates (REPO 4 ) have been proposed as a next‐generation of environmental barrier coatings (EBCs) for SiC‐based ceramic matrix composites in aero turbine engines. In an effort to make more inexpensive and more sustainable coatings, REPO 4 containing mineral, xenotime, is being investigated. Using four naturally occurring minerals, the composition of the mineral was determined to be (Y 0.80 Gd 0.04 Dy 0.07 Er 0.05 Yb 0.04 )PO 4. A phase pure mineral‐synthetic REPO 4 of the same composition was synthesized using RE 2 O 3 and H 3 PO 4 and was densified using spark plasma sintering. Results are compared to state‐of‐the‐art Yb 2 Si 2 O 7 (YbDS). The coefficient of thermal expansion (CTE) was observed to be slightly anisotropic due to the tetragonal crystal structure but the a and b lattice directions were in good agreement with β‐SiC. Thermal conductivity determined via time domain‐thermoreflectance (TDTR) was observed to be approximately equivalent to YbDS. Exposure to molten calcium‐magnesium‐aluminosilicates (CMAS) showed the formation of a multilayer reaction product which inhibited infiltration better than conventional YbDS. Additionally, high‐temperature high‐velocity steam exposures showed parabolic growth rates of reaction products similar to YbDS, but with a significantly longer incubation time. With the determination of important thermophysical and thermochemical properties in this work, xenotime minerals are demonstrated to be viable candidates for EBC applications.