Super strong ceramics for extreme environments

Ultra-high temperature ceramics (UHTCs) are candidate materials for use in extreme environment owing to their melting point exceeding 3000°C and excellent combination of thermo-mechanical properties. Boride-based ceramics have strengths of 500-600 MPa up to 1500°C, when doped with suitable secondary phases and densified using the proper sintering technique. However, upon testing at higher temperatures, strength generally collapses to 200 MPa. Here, the strength behavior of Zr/Hf-B2 based ceramics at temperatures up to 2100°C is presented and related to the microstructure tailoring in terms of secondary phases, grain morphology and grain size. Strengths over 1 GPa 1800°C were measured and fracture analysis and transmission electron microscopy showed this behavior to be due to a particular morphology of the grains, known as core-shell, which included a solid solution around the native boride grain. Densification and annealing treatment at high temperatures enabled to develop a hierarchical hybrid structure where metallic nanoparticles were homogeneously dispersed in micrometric ceramic grains resulting in unprecedented refractoriness.