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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.

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Mullite‐bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y2O3 and CaF2 were added in SiC and Al(OH)3 raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation‐derived SiO2 from SiC and Al2O3 decomposed from Al(OH)3 to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF2 addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF2 addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker‐like mullite reinforcement.

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.