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

• Published in: Journal of the American Ceramic Society Vol. 104; no. 1; pp. 448 - 462
• Main Authors: Wright, Andrew J., Huang, Chuying, Walock, Michael J., Ghoshal, Anindya, Murugan, Muthuvel, Luo, Jian
• Format: Journal Article
• Language: English
• Published: Columbus Wiley Subscription Services, Inc 01-01-2021
• Subjects: Ablation; Chemical precipitation; Clustering; CMAS; Corrosion; Entropy; Fluorite; Grain boundaries; high entropy ceramic; Nickel base alloys; Protective coatings; Sand; sand infiltration; Superalloys; thermal barrier coating; Thermal expansion; Yttria-stabilized zirconia; Yttrium oxide; Zirconium dioxide
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.17448

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(1‐2x)/3Zr(1‐2x)/3Ce(1‐2x)/3YxYbx]O2‐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 (Hf0.2Zr0.2Ce0.2Y0.2Yb0.2)O2‐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 (Hf0.284Zr0.284Ce0.284Y0.074Yb0.074)O2‐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.
Optical images highlighting the process of the molten sand and ablation experiments.