A Promising High-Entropy Thermal Barrier Material with the Formula (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12

YSZ has been widely used as a TBC material, but its phase change at high temperatures limits its development, thus the need for developing new thermal barrier materials resistant to high temperatures. Rare-earth aluminate ceramics with a garnet structure (Yb3Al5O12) have been considered as a potential thermal barrier material. The melting point of Yb3Al5O12 is 2000 °C, which has a potential high temperature application prospect. However, Yb3Al5O12 has lower thermal expansion and higher thermal conductivity than YSZ, which is a widely employed thermal barrier coating (TBC) material. To overcome these obstacles, (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12, a high-entropy ceramic, was prepared by a solid-state reaction and pressureless sintering. The thermal conductivity of the (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12 was 3.48 W/(m·K) at 300 K, approximately 25.48% lower than that of the Yb3Al5O12 (4.67 W/(m·K)). The thermal expansion coefficient of the (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12 was 9.28 × 10-6 K-1 at 673-1273 K, approximately 18.52% higher than that of the Yb3Al5O12 (7.83 × 10-6 K-1, 673-1273 K). When the (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12 was annealed at 1550 °C for 7 days, its average grain size only increased from 0.7 μm to 1.3 μm. Moreover, the (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12 exhibited better chemical stability and a lower grain growth rate than the Yb3Al5O12. This study reveals that (Y0.2Dy0.2Ho0.2Er0.2Yb0.2)3Al5O12 is a promising candidate for the future generation of thermal barrier materials.