Synthesis and consolidation of high entropy ceramics

The synthesis and consolidation of High Entropy-Ultra-High Temperature Ceramics (HE-UHTCs) based on transition metal borides is addressed in this work. This emerging class of ceramics is based on the combination, in near equimolar proportions, of at least four constituents to generate single-phase crystalline solid solutions with maximum configurational entropy, i.e. (Hf0.2Ti0.2Ta0.2Nb0.2Mo0.2)B2, (Hf0.2Ti0.2Ta0.2Zr0.2Mo0.2)B2, etc. The studies conducted so far evidenced that HE-UHTCs often exhibit superior oxidation resistance and mechanical properties, with respect to their individual constituents. Therefore, these materials have gained a significant interest for their high-temperature applications in several innovative and traditional industrial fields. Unfortunately, the obtainment of dense single phase HE-UHTCs represents quite a difficult goal. In this work, a two-steps process, consisting in the combination of the SHS technique with Spark Plasma Sintering (SPS), is successfully adopted for the preparation of various HE-UHTCs. In this regard, the use of the SHS method is found to highly promote the obtainment of the single-phase HE ceramic during the subsequent SPS stage. The introduction of different additives on SHS powder prior the SPS step is also considered.