Review on ultra-high temperature boride ceramics

Ceramic materials having melting points higher than 3000 degrees C and suitable for structural applications at above 2000 degrees C are commonly known as Ultra-High Temperature Ceramics (UHTCs). Several transition metal di-borides, possessing the desired combinations of thermo-mechanical, physical and chemical properties, form an important sub-class of the UHTCs. Over the last couple of decades, there has been a growing interest for UHTCs in general, and for the transition metal di-borides in particular, due to the increasing demands in hypersonic aerospace vehicles, atmospheric re-entry vehicles and energy applications. However, problems pertaining to sintering, moderate fracture toughness and experimental challenges associated with reliably measuring the elevated temperature properties, as well as the properties that determine the performances at the actual service conditions, have limited their widespread applications. This paper comprehensively reviews the various routes/techniques, including the advanced ones, as adopted for the synthesis and densification of the di-borides. The effects of sinter-additives and reinforcements on the densification, microstructure and various properties, including elevated temperature properties have been discussed in critical terms. Due attention has been paid towards understanding the challenges associated with the experimental measurements of the high temperature properties under extreme environmental conditions and the very recently developed techniques for the same. Some of the existing and futuristic applications of transition metal di-borides have also been discussed. Finally, the review concludes with an outlook towards some of the outstanding issues.