Hafnia doping is expected to improve the performance of the silicon-bond layer of environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites. The optimal doping ratio, distribution of HfO 2 , and oxidation mechanism of the bond layer have not yet been fully addressed. A prototype Si-HfO 2 bond layer with a designed HfO 2 -rich area was used to examine its oxidation behavior. A random dispersion model was developed to calculate the optimal HfO 2 doping ratio and its appropriate distribution state. The simulation results recommended that 20-30 vol% is the optimal doping ratio, where HfO 2 is well dispersed inside Si without forming networks. This enables HfO 2 to react with and consume SiO 2 without accelerating oxygen diffusion inside the bond layer. This was confirmed by oxidation experiments on Si-xHfO 2 tablets, in which the thinnest thermally grown oxide was achieved for the 20 vol% HfO 2 -doped Si tablet. Both the microstructure design and material composition selection are highly important to further boost the performance of the EBCs.

Environmental barrier coatings (EBCs) greatly improve the service performance of SiC‐based ceramic matrix composites (CMCs) in high‐temperature combustion chambers. Working environments with physical ablation, high temperature, and chemical corrosion require the performance of designed EBC materials and/or structures to be properly evaluated before their real applications. In this paper, EBCs’ lifetime‐related phase stability, chemical compatibility, and microstructure retainability are discussed. And then, evaluation methods for basic and environmental properties of EBCs are thoroughly reviewed with newly proposed methods and improved techniques. Pros and cons of each method along with some potential strategies/techniques are also provided. We hope this article can give a timely and overall review for efficient and effective evaluation of EBCs and provide guidance not only for beginners but also for seasoned researchers when they design and develop high‐performance EBC systems.

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Bi2O3 is a promising sintering additive for YSZ that not only decreases its sintering temperature but also increases its ionic conductivity. However, Bi2O3 preferably grows into large-sized rods. Moreover, the addition of Bi2O3 induces phase instability of YSZ and the precipitation of monoclinic ZrO2, which is unfavorable for the electrical property. In order to precisely control the morphology and size of Bi2O3, a microemulsion method was introduced. Spherical Bi2O3 nanoparticles were obtained from the formation of microemulsion bubbles at the water–oil interface due to the interaction between the two surfactants. Nanosized Bi2O3–YSZ composite powders with good mixing uniformity dramatically decreased the sintering temperature of YSZ to 1000 °C. Y2O3-stabilized Bi2O3 (YSB)–YSZ composite powders were also fabricated, which did not affect the phase of YSZ but decreased its sintering temperature. Meanwhile, the oxygen vacancy concentration further increased to 64.9% of the total oxygen with the addition of 5 mol% YSB. In addition, its ionic conductivity reached 0.027 S·cm−1 at 800 °C, one order of magnitude higher than that of YSZ. This work provides a new strategy to simultaneously decrease the sintering temperature, stabilize the phase and increase the conductivity of YSZ electrolytes.

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