Property-Targeted Compositional Design of RE₂O₃ High-Temperature Coatings

RE₂O₃ exhibit three crystal structures across the lanthanide series: hexagonal, monoclinic, and cubic, with all showing exceptionally high-melting temperatures (>2100°C) and excellent thermochemical stability. The cubic RE₂O₃, dysprosium through lutetium oxides, have isotropic thermal expansion with a reasonable match to Nb, making them suitable high temperature coatings for oxidation-prone refractory alloys. Multicomponent rare-earth oxides (MRO) allow the additional ability to target and optimize thermal expansion, resistance to molten deposits, and especially thermal conductivity, enabling their use as thermal barrier coatings in high-temperature, reactive environments such as turbine engines. Thermal conductivity of MROs has been shown to decrease with mixtures of RE₂O₃ with increasing mass and size variation. The larger, lighter, non-cubic lanthanide oxides, lanthanum through terbium oxides, mixed in a majority MRO cubic phase in non-equimolar proportions will precipitate as second phases once their solubility limit in the cubic RE₂O₃ is exceeded, enabling further reductions in thermal conductivity. In this work, MRO compositions are systematically varied to aid in achieving targeted thermal conductivity, thermal expansion, and resistance to molten deposits. Powder mixtures were combined, ball milled, and sintered via spark plasma sintering. Room temperature thermal conductivity was measured using the laser-based time domain thermoreflectance method. Thermal expansion was determined by dilatometry or lattice parameter measurements as a function of temperature. Resistance to molten CaO-MgO-Al₂O₃-SiO₂ was quantified after exposure at temperatures of 1300-1500°C for times between 1 and 96h. Alternative processing methods for controlling the second phase size and distribution with additional ability for microstructural design will be discussed.