High-entropy fluorite oxides: Atomic stabiliser effects on thermal-mechanical properties

High-entropy oxides Hf0.25Zr0.25Ce0.25Gd0.125X0.125O2-δ (X = Ca, Ti or Si) have been fabricated via solid-state reactions, in which the co-stabiliser X was intentionally chosen for its significant atomic mass and size difference from Gd. The single phase of these cubic fluorite oxides with dense microstructures has been confirmed by XRD, EDS and TEM characterizations. These phases are thermally stable without the appearance of secondary phase and phase separation within the temperature range studied (up to 1200 °C). Compared to yttria-stabilised zirconia (YSZ), which is used in the current commercial thermal barrier coatings, these fluorite oxides have higher coefficients of thermal expansion and lower thermal conductivities. They also exhibit comparable Young's modulus and hardness with other reported high-entropy fluorite oxides. The fluorite oxides reported in this study are promising to improve the thermal expansion matching between ceramic topcoat and metal substrates for thermal barrier coating applications.