Metastable Si-B-C-N ceramics and their matrix composites developed by inorganic route based on mechanical alloying: Fabrication, microstructures, properties and their relevant basic scientific issues

The development of novel high-temperature structural and multifunctional thermal protection materials for harsh environment applications, such as high-temperature oxidation, severe thermal shock, ablation by combustion gas flow etc., is one of the urgent needs of the modern aerospace industry. Ceramic matrix composites such as C-f/(C, SiC, Si3N4), SiCf/ZrB2, SiCp/(Si3N4,HfB2) have received much attention in recent years. Coincidently, metastable silicoboron carbonitride (Si-B-C-N) ceramics and corresponding matrix composites stand out from all recent materials offering great potential at high temperatures due to their high microstructural stability and excellent high-temperature properties including resistance to oxidation, thermal shock and ablation. Using inorganic powders (such as Si, C, B, BN, etc.) instead of organic precursor as raw materials, the inorganic processing route based on mechanical alloying (MA), one of the non equilibrium processing technique, coupled with sequential sintering, although apparently very'hard' compared to the 'soft' polymer precursor method, is actually a simple and effective way to prepare monoliths with the uniform microstructures and superior properties. It has been used to obtain dense Si-B-C-N monoliths and structural parts stable at high temperatures providing new experimental data and therefrom a more detailed understanding of the intrinsic properties of metastable Si-B-C-N materials, benefitting progress towards engineering applications. This review summarizes the state-of-the-art research in Si-B-C-N ceramics and their matrix composites obtained by the inorganic processing route in the last decade compared with those of precursor derived counterparts, including material design and preparation, microstructural features and evolutionary process, mechanical and thermophysical properties, resistance to oxidation, thermal shock and ablation, and the mechanisms of oxidation, ablation and crystallization of amorphous Si-B-C-N ceramics. Future trends for Si-B-C-N relevant materials are also pointed out.