Ultra-high temperature ceramics for extreme environments

Modern demands in clean energy and space exploration require materials scientists to develop materials that perform in the most extreme conditions in our universe. Recent advances in hypersonic travel and nuclear technology have brought a family of refractory transition metal carbides, nitrides and diborides, known as ultra-high temperature ceramics (UHTCs) to the forefront. These materials have extremely high melting points (>4,000 °C), high thermal conductivity (>140 W m−1 K−1) and strong transition-metal-to-non-metal bonding (>600 GPa mechanical stiffness), which promise to enable their application in extreme environments. This Review covers the relation of metal–non-metal (M–X) chemistry to the high-temperature, thermal, mechanical and oxidation behaviour of UHTCs and discusses the effect of synthesis and potential additives on their properties. In addition, we present new areas of research, including advances in additive manufacturing, high-entropy compositions and 2D materials to improve the processing and performance of UHTCs. A focus on chemistry–synthesis–processing relationships will be key to enabling innovative designs to bring UHTCs to fruition as extreme environment materials. Ultra-high temperature ceramics (UHTCs), with their exceptionally high melting points and outstanding thermomechanical behaviour, are critical materials for extreme environment technologies. This Review establishes the key UHTC composition–synthesis–property relations and discusses the design of UHTCs for application in extreme environments.