Processing of ZrB2- and HfB2-Based Ultra-High Temperature Ceramic Materials: A Review

ABSTRACT ZrB2 and HfB2 are leading materials in the category of ultra-high temperature ceramics (UHTCs). UHTCs are a group of ceramic materials that can withstand ultra-high temperature (>2,000°C) in oxidizing conditions. Such a high temperature will be encountered by future hypersonic reentry vehicles. Future reentry vehicles will have sharp edges to improve aerodynamic performance. The sharp leading edges will cause higher surface temperature than that of the actual blunt-edged vehicles. The sharp edges have less surface area to dissipate heat and thus the temperature gets increased. To withstand the intense heat generated when these vehicles reenter the earth’s upper atmosphere, UHTC materials are needed. UHTC materials are composed of borides of early transition metals. From the larger list of borides, ZrB2 and HfB2 have received the most attention as potential candidates for leading edge materials because their oxidation resistance is superior to that of other borides because of the stability of the ZrO2 and HfO2 scales that form on these materials at elevated temperatures in oxidizing environments. Processing of these materials is very difficult because these materials are very refractory in nature. Powder synthesis usually involves high-temperature solid state reactions. Dense bodies are generally produced by hot pressing and spark plasma sintering. For coating preparation, plasma spray, chemical vapor deposition, and physical vapor deposition is used. All these processing methods for ZrB2 and HfB2 are difficult because of the involvement of very high temperatures. In this review, the present state of the knowledge on processes employed for powder synthesis, fabrication of dense bodies, and coating processes is discussed. The advantages, limitations, and challenges in the different processing methods are addressed.