The multifunctional metastable SiBOCN system ceramics (including SiBN, SiCN, SiON, SiBC, SiBCN, SiBON, SiBOCN, etc.) are a new type of advanced structure–function integrated materials with unique structure and adjustable dielectric properties for high-temperature applications in thermal protection, communications, precise guidance, and microwave-absorption stealthy. These metastable materials generally require the rational co-design of multiscale structure and chemical composition to achieve desirable dielectric properties which induce interaction with incident electromagnetic wave. Herein, this review presents the latest development of metastable SiBOCN system ceramics, with the intent of summarising key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment focus on the main processing routes, basic mechanisms for microwave transmission and absorption, scientific basis for material selection in specific background, principles for multiscale structure design and chemistry optimisation, tunable microwave transparent or absorbing properties, and future challenges and prospects in this active research filed.

Thein situnano Ta₄HfC₅reinforced SiBCN-Ta₄HfC₅composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering. The microstructural evolution and mechanical properties of the resulting SiBCN-Ta₄HfC₅were studied. After the first-step milling of 30 h, the raw materials of TaC and HfC underwent crushing, cold sintering, and short-range interdiffusion to finally obtain the high pure nano Ta4HfC5. A hybrid structure of amorphous SiBCN and nano Ta₄HfC₅was obtained by adopting a second-step ball-milling. After reactive hot-pressing sintering, amorphous SiBCN has crystallized to 3C-SiC, 6H-SiC, and turbostratic BN(C) phases and Ta₄HfC₅retained the form of the nanostructure. With thein situgenerations of 2.5 wt% Ta₄HfC₅, Ta₄HfC₅is preferentially distributed within the turbostratic BN(C); however, as Ta₄HfC₅content further raised to 10 wt%, it mainly distributed in the grain-boundary of BN(C) and SiC. The introduction of Ta₄HfC₅nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics, reaching to 344.1 MPa and 4.52 MPa·m^1/2, respectively. This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix, which is beneficial to the real applications of SiBCN ceramics.

For the purpose of improving oxidation resistance of short carbon fiber (C_sf) reinforced mechanically alloyed SiBCN (MA-SiBCN) composites (C_sf/MA-SiBCN), the dense amorphous C_sf/SiBCN composites containing both MA-SiBCN and polymer-derived SiBCN (PDCs-SiBCN) matrices were prepared by repeated polymer infiltration and pyrolysis (PIP) process of layered C_sf/MA-SiBCN composites at 1100 ℃, and the oxidation behavior and damage mechanism of the as-prepared C_sf/SiBCN at 1300~1600 ℃ were compared and discussed with C_sf/MA-SiBCN. The C_sf/MA-SiBCN composites resist oxidation attack up to 1400 ℃ but fails at 1500 ℃ due to the collapse of the porous framework, while the PIP densified C_sf/SiBCN composites are resistant to statistic air up to 1600 ℃. During oxidation, oxygen diffuses through pre-existing pores and the pores left by oxidation of carbon fibers and pyrolytic carbon (PyC) to the interior of the matrix. Owning to the oxidative coupling effect of MA-SiBCN and PDCs-SiBCN matrices, a relatively continuous and dense oxide layer is formed on the sample surface, and the interfacial region between the oxide layer and the matrix of the as-prepared composite contains amorphous glassy structure mainly consisting of Si and O and incomplete oxidized but partially crystallized matrix which is primarily responsible for the improvement of the oxidation resistance.