Abstract
The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) include good tolerance to harsh environments, thermal expansion matches with the interlayer mullite, good high-temperature phase stability, and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE4Al2O9 have been considered as candidates of EBCs for their superior mechanical and thermal properties, but the phase transition at high temperatures is a notable drawback of these materials. To suppress the phase transition and improve the phase stability, a novel cuspidine-structured rare-earth aluminate solid solution (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9 was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics (HECs). The as-synthesized HE (Nd0.2Sm0.2Eu0.2Y0.2Yb0.2)4Al2O9 exhibits a close thermal expansion coefficient (6.96×10-6 K-1 at 300–1473 K) to that of mullite, good phase stability from 300 to 1473 K, and low thermal conductivity (1.50 W·m–1·K–1 at room temperature). In addition, strong anisotropic thermal expansion has been observed compared to Y4Al2O9 and Yb4Al2O9. The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms, and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.
Highlights
Silicon-based ceramics such as Si3N4 and SiCf/SiC ceramic matrix composites (CMCs) have been considered as candidates to replace superalloys to manufacture the hot section structural components in gas turbine engines due to their superior high-temperature mechanical properties, good durability, and excellent creep- and oxidation-resistance [1,2,3,4,5,6]
J Adv Ceram 2020, 9(5): 595–605 components is the lack of environmental durability in combustion environments because the protective silica scale on the surface of silicon-based ceramics can be corroded by high-temperature water vapor and yields volatile products, such as Si(OH)4, which will result in the rapid recession [7,8,9,10,11]
Cuspidinetype rare-earth aluminates RE4Al2O9 are a class of ceramics that have been considered as potential materials for environmental barrier coating (EBC) of silicon-based ceramics owing to their high melting point, low Young’s modulus, good damage tolerance, close thermal expansion coefficients to that of mullite, low thermal conductivity, and good resistance to the hot gas atmosphere [15,18,19,20,21]
Summary
J Adv Ceram 2020, 9(5): 595–605 components is the lack of environmental durability in combustion environments because the protective silica scale on the surface of silicon-based ceramics can be corroded by high-temperature water vapor and yields volatile products, such as Si(OH), which will result in the rapid recession [7,8,9,10,11]. A new type of ceramics consisting of multi-principal component elements in equal molar or near equal molar fractions but form single phase solid solutions have triggered much attention, which are referred to as high-entropy ceramics (HECs) [26,27,28]. These materials exhibit many intriguing properties, such as higher hardness, sluggish grain growth rate, lower thermal conductivity, strong microwave absorption capability, and better water-vapor resistance, etc. The mechanisms for low thermal conductivity and anisotropic thermal expansion will be presented
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