High-entropy pyrosilicate element selection is relatively blind, and the thermal expansion coefficient (CTE) of traditional β-type pyrosilicate is not adjustable, making it difficult to meet the requirements of various types of ceramic matrix composites (CMCs). The following study aimed to develop a universal rule for high-entropy pyrosilicate element selection and to achieve directional control of the thermal expansion coefficient of high-entropy pyrosilicate. The current study investigates a high-entropy design method for obtaining pyrosilicates with stable β-phase and γ-phase by introducing various rare-earth (RE) cations. The solid-phase method was used to create 12 different types of high-entropy pyrosilicates with 4–6 components. The high-entropy pyrosilicates gradually transformed from β-phase to γ-phase with an increase in the average radius of RE³⁺ ions ( $\overline{r}({\text{RE}}^{3+})$). The nine pyrosilicates with a small $\overline{r}({\text{RE}}^{3+})$ preserve β-phase or γ-phase stability at room temperature to the maximum of 1400 ℃. The intrinsic relationship between the thermal expansion coefficient, phase structure, and RE–O bond length has also been found. This study provides the theoretical background for designing high-entropy pyrosilicates from the perspective of $\overline{r}({\text{RE}}^{3+})$. The theoretical guidance makes it easier to synthesize high-entropy pyrosilicates with stable β-phase or γ-phase for the use in environmental barrier coatings (EBCs). The thermal expansion coefficient of γ-type high-entropy pyrosilicate can be altered through component design to match various types of CMCs.

Rare-earth zirconates with pyrochlore and fluorite structures have recently been identified as promising thermal barrier coating materials owing to their low thermal conductivities. In this study, six samples with the general formula (NdSmEuGd)_1-xDy_2xZr₂O₇ were synthesized to further reduce the thermal conductivity. X-ray diffraction and Raman spectroscopy showed that the transition from an ordered pyrochlore to a disordered fluorite structure is due to cation and anion disorder. Transmission electron microscopy showed that anion disorder occurred before cation disorder. A modified mass disorder parameter was introduced into this system, which can describe the change in thermal conductivity well. This parameter can be a basis for designing more complex materials with lower thermal conductivities.

In this study, the water vapor corrosion resistance of two types of high-entropy pyrosilicates ((Yb_0.2Y_0.2Lu_0.2Ho_0.2Er_0.2)₂Si₂O₇ ((5RE_1/5)₂Si₂O₇) and (Yb_0.25Lu_0.25Ho_0.25Er_0.25)₂Si₂O₇ ((4RE_1/4)₂Si₂O₇)) and two single-component pyrosilicates (Yb₂Si₂O₇ and Lu₂Si₂O₇) were evaluated at 1350 ℃ for 50–100 h, and the initial corrosion behaviors of these pyrosilicates were studied. The results showed that the final corrosion products of the four types of pyrosilicates were all X2-type monosilicates, exhibiting similar corrosion phenomena. However, (4RE_1/4)₂Si₂O₇ generated many nanoscale monosilicate grains during corrosion. The corrosion resistance of Lu₂Si₂O₇ was clearly better than those of the others, and (4RE_1/4)₂Si₂O₇ exhibited the worst corrosion resistance. The corrosion mechanism of the pyrosilicate blocks was analyzed from the perspectives of grain size, bulk hydrophobicity, and binding energy. This study potentially provides a theoretical basis for the preparation of high-entropy pyrosilicates with different atomic ratios according to the different properties of the various rare earth elements.

Yttria-stabilized zirconia (YSZ) coatings and Al₂O₃-YSZ coatings were prepared by atmospheric plasma spraying (APS). Their microstructural changes during thermal cycling were investigated via scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). It was found that the microstructure and microstructure changes of the two coatings were different, including crystallinity, grain orientation, phase, and phase transition. These differences are closely related to the thermal cycle life of the coatings. There is a relationship between crystallinity and crack size. Changes in grain orientation are related to microscopic strain and cracks. Phase transition is the direct cause of coating failure. In this study, the relationship between the changes in the coating microstructure and the thermal cycle life is discussed in detail. The failure mechanism of the coating was comprehensively analyzed from a microscopic perspective.