Ultra-high temperature ceramics: Microstructure control and properties improvement related to materials design and processing procedures

The principal scientific and technological barriers to the fabrication of ultra-high temperature ceramics (in particular borides and carbides of zirconium and hafnium) are investigated. Generally these ceramics exhibit difficult sinterability, poor toughness and low thermal stability at high temperatures under oxidizing atmospheres. Different fabrication routes are presented, aiming at improving the performances of these ceramics: i) the use of sintering aids, ii) reactive hot pressing starting from solid precursors; iii) innovative densification method like spark plasma sintering (SPS); iv) the introduction of second phases (SiC, MoSi) suitable to improve oxidation resistance and mechanical properties. Examples are shown for each of the mentioned routes, pointing out the correlations among starting composition, processing procedures, microstructure and properties.