Sand corrosion, thermal expansion, and ablation of medium‐ and high‐entropy compositionally complex fluorite oxides

Abstract

Abstract Sand corrosion, thermal expansion, and ablation properties of a new class of medium‐ and high‐entropy compositionally complex fluorite oxides (CCFOs) are examined as potential protective coating materials. Five binary oxides were mixed and sintered into dense, single‐phase CCFOs of the general formula: [Hf (1‐2 x )/3 Zr (1‐2 x )/3 Ce (1‐2 x )/3 Y x Yb x ]O 2‐δ ( x = 0.2, 0.074, and 0.029). These CCFOs exhibit decreased molten sand infiltration and interaction at intermediate temperatures (1200‐1300°C) in comparison with a cubic yttria‐stabilized zirconia (YSZ) reference; however, at higher temperatures, the trend is reversed due to the increased chemical reactivity. The equimolar high‐entropy (Hf 0.2 Zr 0.2 Ce 0.2 Y 0.2 Yb 0.2 )O 2‐δ exhibits no grain boundary penetration by molten sand at all examined temperatures (1200°C‐1500°C), although reaction and precipitation are significant. Moreover, these CCFOs exhibit higher intrinsic thermal expansion coefficients (CTE) than the YSZ reference, thereby being more compatible with Ni‐based superalloys. The 8YSZ‐like (Hf 0.284 Zr 0.284 Ce 0.284 Y 0.074 Yb 0.074 )O 2‐δ exhibits the highest CTE in this series of CCFOs due to oxygen clustering effects. Finally, these CCFOs also exhibit lower emissivities and form unique faceted microstructures in ablative environments.