Binary to Quaternary Rare-Earth Phosphates: Compositional Effects on Thermal Properties and CMAS Corrosion Resistance of Environmental Barrier Coatings

Equimolar multicomponent xenotime rare-earth phosphates (REPO 4 ), ranging from binary to quaternary REPO 4, were synthesized via chemical coprecipitation. The synthesized powders were densified via spark plasma sintering (SPS) at 1500 °C. Their thermal conductivity, coefficients of thermal expansion (CTEs), and resistance to calcium–magnesium–alumina–silicate (CMAS) corrosion were systematically characterized to evaluate their potential as environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites (SiC/SiC CMCs). All REPO 4 compositions studied here exhibit CTEs close to SiC/SiC-CMCs. Thermal conductivity depends on cation-size disorder and compositional complexity. Notably, at a constant number of constituent elements, increased cation-size disorder reduces thermal conductivity. Among the quaternary compositions, (Sc 1/4 Lu 1/4 Er 1/4 Y 1/4 )PO 4 exhibits minimal thermal conductivity because of its higher degree of size disorder compared to (Lu 1/4 Yb 1/4 Er 1/4 Y 1/4 )PO 4. The CMAS corrosion test at 1300 °C for 96 h resulted in the formation of a uniform, dense, and continuous reaction layer mainly consisting of Ca 8 MgRE(PO 4 ) 7 and RE 2 Si 2 O 7 for all REPO 4 . However, at 1400 °C, the CMAS corrosion behavior of REPO 4 differs significantly. Binary and ternary REPO 4 exhibit direct CMAS infiltration along the grain boundaries without creating a protective reaction layer, whereas quaternary (4RE 0.25 )PO 4 develops a discontinuous reaction layer and halts the CMAS penetration to some extent. The reduced thermal conductivity and enhanced CMAS corrosion resistance of the quaternary compositions, in particular (Lu 1/4 Yb 1/4 Er 1/4 Y 1/4 )PO 4, compared to their binary (Lu 1/2 Y 1/2 )PO 4 and ternary (Lu 1/3 Er 1/3 Y 1/3 )PO 4 counterparts, imply the potential of quaternary phosphates as promising EBC candidates for SiC/SiC CMCs in high-temperature environments.