Inherent anisotropy in transition metal diborides and microstructure/property tailoring in ultra-high temperature ceramics—A review

This is the first comprehensive review on inherent anisotropic features of transition metal diboride (MB₂) and their implementation for tailoring the microstructure and properties of MB₂-based Ultra-high temperature Ceramics (UHTCs). The emphasis is on the processing approaches, microstructures, and properties of self-reinforced and/or textured MB₂-based composites with elongated MB₂ grains. The crystal structure characteristics and grain growth behaviour of MB₂ are also critically reviewed. Benefiting from the tailored microstructure, the MB₂-based ceramics exhibit some improved properties. Considering the success of Si₃N₄ ceramics in the field of structural ceramics, it is expected that the potential MB₂-based ceramic composites with abundant elongated MB₂ grains, textured structures, and controlled grain boundaries would possess improved fracture toughness, thermal shock resistance, and reliable high-temperature properties, which are desired for their practical applications. Accordingly, microstructure designing and tailoring provide an important perspective for the future development of UHTCs.