High-Temperature Environmental Degradation Behavior of Ultrahigh-Temperature Ceramic Composites

In recent years, there is a very strong interest for the development of zirconium and hafnium diboride-based ultrahigh-temperature composites (UHTCs) for use in nose cones and leading edges of hypersonic vehicles, which are subjected to high temperatures and ablative environment during reentry into the earth atmosphere. An overview of the literature on high-temperature environmental degradation behavior of zirconium and hafnium diboride-based UHTCs has been presented, with emphasis on their resistance to oxidation under non-isothermal, isothermal, and cyclic conditions, as well as under ablative conditions during reentry at ~2000 °C. It has been observed that using SiC and Si-bearing reinforcements such as Si3N4 and MoSi2 aids in the formation of a borosilicate scale on the surfaces, which is capable of protecting partially or fully against further damage under extreme environments, depending on the temperature. Formation of oxidation products at grain boundaries and interfaces during creep contributes to damage by grain boundary sliding and intergranular cracking. Both nature of oxidation products and mechanisms of their formation leading to degradation are found to vary significantly with the temperature regimes of exposure. On subjecting to ablative exposure at temperatures close to 2000 °C, active oxidation of SiC along with vaporization of B2O3 influences the kinetics and mechanisms of degradation. Formation of ZrO2-rich oxide scale at such temperatures is believed to play the role of an in situ formed thermal barrier coating, which protects the composite underneath from damage. The effects of reinforcements and their volume fractions on oxidation and ablative behavior have been discussed.