Research progress on ultra-high temperature ceramic composites

Abstract

Ultra-high temperature ceramics (UHTCs) are composed of ZrC, ZrB2, HfC, HfB2, HfN and TaC and their melting points are higher than 3000℃. They are a vital class of high temperature structural materials and have attracted a great deal of attention in the past two decades in fundamental research and in technological applications. The ZrB2-SiC and HfB2-SiC UHTC composites, in particular, have excellent oxidation/ablation resistance, moderate thermal shock resistance and high strength retention at elevated temperature. They can survive oxidizing environments at temperatures of 2000℃ and higher. These properties make them the most promising materials for use in extreme environments such as hypersonic flight, atmospheric re-entry, and rocket propulsion. ZrB2-based and HfB2-based UHTCs have similar oxidation and ablation behaviors however as ZrB2-based UHTCs are lighter and less expensive, they are used as aerospace materials. Here we provide a comprehensive review of UHTC composites including their preparation, mechanical properties, thermal shock resistance, oxidation/ablation properties and thermal response, with a particular focus on ZrB2-based UHTCs. UHTCs were generally fabricated by hot pressing, spark plasma sintering, pressureless sintering, reactive hot pressing and sinter forging. Hot pressing is the dominant densification technique for the preparation of UHTCs. The flexural strength of UHTCs decreases with an increase in grain size and it exhibits a strong correlation with the size of the SiC particulates. However, the fracture toughness trend is not consistent with that of strength and therefore the matching of grain size becomes important. UHTCs display plasticity at elevated temperature and their brittle-to-ductile transition temperature (1500℃) strongly depends on the grain size and the purity of the grain boundaries. Catastrophic failure occurs easily during the heating or cooling process. The thermal shock resistance of UHTCs is a major issue for future applications. The methods used to improve the thermal shock resistance of UHTCs are summarized from critical thermal shock temperature and strength retention rate after thermal shock testing points of view. The oxidation behavior of UHTCs significantly depends on the temperature and the temperature limits of the applications of these materials are analyzed. The effect of the additives on the overall performance of the resultant composites as well as the relationship between material composition, microstructure and performance are also discussed in detail. This provides useful insights into effective design principles to optimize the overall structural performance of ultra-high temperature ceramic composites according to special service environments. The remaining challenges and future outlook of this field are also addressed.