Ultra-high temperature ceramics with exceptional strength at elevated temperature

Ultra-high temperature ceramics (UHTCs) are candidate materials for use in extreme environments owing to their melting point exceeding 3000°C and excellent ablation resistance. Despite the interesting combination of thermo-mechanical properties, they remain susceptible to brittle failure. One route to mitigate the strength-ductility paradox is the formation of hierarchical structures, where several mechanisms can act synergystically on different length scales. Here we explore how to promote and tailor a multi-scale microstructure arrangement in ZrB2 materials sintered in the presence of transition metals (TM), leading to particular morphology of the grains, known as core-shell microstructures. These materials are composed of a (Zr,TM)B2 solid solution shell around a nominally pure boride grain core. Super-saturated solid solutions lead to the precipitation of nano-inclusions within a micron-sized boride grain matrix. Phase stability diagrams enabled identification of the conditions of oxygen activity that drive precipitation of either metallic or carbide nano-inclusions, Figure 1. The strength behavior of these core-shelled ceramics at temperatures up to 2100°C will be presented and related to the microstructural features. Strengths over 1 GPa 1800°C were measured. Fracture analysis and transmission electron microscopy proved this behavior was due to the hierarchical hybrid structure with nanoparticles homogeneously dispersed in micrometric ceramic grains.