High‐entropy (Ho _1/4 Er _1/4 Dy _1/4 Gd _1/4 ) ₂ Si ₂ O ₇ : A promising thermal environmental barrier coating material

Abstract This work presents the exploration of a promising thermal environmental barrier coating material by engineering the compositions of (Ho 1/4 Er 1/4 Dy 1/4 X 1/4 ) 2 Si 2 O 7 high‐entropy rare‐earth disilicates (HERED‐X, X = Gd, Tm, Yb, Lu, Sc, Y). Among all the samples, the as‐fabricated HERED‐Gd samples possess the best CMAS corrosion resistance with a corrosion rate of 8.2 ± 0.14 µm h −1 at 1673 K, which is attributed to the synergistic effects of high chemical reactivity of HERED‐Gd and good stability of the formed apatite in CMAS melt. Moreover, they demonstrate a matched coefficient of thermal expansion (4.6 × 10 −6 K −1 ) with SiC f /SiC composites (4.5–5.5 × 10 −6 K −1 ) and a low thermal conductivity (1.89 W m −1 K −1 at room temperature).