Binary to Quaternary Rare-Earth Phosphates: Compositional Effects on Thermal Properties and CMAS Corrosion Resistance...

Equimolar multicomponent xenotime rare-earth phosphates (REPO₄), ranging from binary to quaternary REPO₄, 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 REPO₄ 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_1/4Lu_1/4Er_1/4Y_1/4)PO₄ exhibits minimal thermal conductivity because of its higher degree of size disorder compared to (Lu_1/4Yb_1/4Er_1/4Y_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 Ca₈MgRE(PO₄)₇ and RE₂Si₂O₇ for all REPO₄. However, at 1400 °C, the CMAS corrosion behavior of REPO₄ differs significantly. Binary and ternary REPO₄ exhibit direct CMAS infiltration along the grain boundaries without creating a protective reaction layer, whereas quaternary (4RE_0.25)PO₄ 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_1/4Yb_1/4Er_1/4Y_1/4)PO₄, compared to their binary (Lu_1/2Y_1/2)PO₄ and ternary (Lu_1/3Er_1/3Y_1/3)PO₄ counterparts, imply the potential of quaternary phosphates as promising EBC candidates for SiC/SiC CMCs in high-temperature environments.