Ultra-high temperature ceramic nano-composites with hierarchical structures for enhanced properties

Ultra-high temperature ceramics (UHTCs) are candidate materials for use in extreme environment owing to their melting point exceeding 3000°C and excellent ablation resistance. Despite the interesting combination of thermo-mechanical properties, they remain however failure sensitive materials. One route to mitigate the strength-ductility paradox is the creation of hierarchical structures, where several mechanisms can act in synergy on different spatial variations. Here we explore how to promote and tailor a multi-scale microstructure arrangement in ZrB2 materials sintered in presence of transition metals (TM), leading to particular morphology of the grains, known as core-shell, which includes a (Zr,TM)B2 solid solution around the native boride grain. Super-saturated solid solution leads to the precipitation of nano-inclusions within micron-sized boride grain matrix and phase stability diagrams enabled to define the conditions of partial pressure within the sintering chamber that drive precipitation of nano-inclusions in the form of either metal or carbide. Besides, the strength behavior of these core-shelled ceramics at temperatures up to 2100°C is presented and related to the microstructural features. Strengths over 1 GPa 1800°C were measured and fracture analysis and transmission electron microscopy proved this behavior to be due to the hierarchical hybrid structure with nanoparticles homogeneously dispersed in micrometric ceramic grains.