Optimization of High Entropy Borides for Advanced Applications

In the last decades, a steep increase in the demand of materials that can withstand extreme environments has been registered. The most representative field where these are encountered is the aerospace. For instance, wing lead edges, nosetips and propulsion systems are subjected to a combination of harsh conditions (high temperatures, oxidising environment, mechanical stresses). To this aim, a significant attention was focused by the scientific and industrial communities on the Ultra High Temperature Ceramic (UHTC) materials, based on transition metal borides and carbides, which are characterised by several advantageous physico-chemical properties, such as elevated melting temperatures, chemical inertness, good thermal and electrical conductivities, etc. In addition, UHTCs are also known to exhibit an intrinsic spectral selectivity and low thermal emittance, which makes them suitable for solar absorbers for applications in concentrating solar plants. In this context, the recently discovered High Entropy Borides (HEBs), where five or more metal cations are combined in near-equimolar percentages to generate a single phase crystalline solid solution, i.e. (Hf0.2Mo0.2Ti0.2Ta0.2Zr0.2)B2, were found to display peculiar properties that could provide a possible further advancement in this field. In particular, preliminary investigations evidenced that members of the HEB class showed better performances in terms of mechanical properties and oxidation resistance, when compared to the corresponding individual boride constituents. The first aim of the present thesis is to optimize the conditions for the fabrication of different HEB ceramics, namely (Hf0.2Mo0.2Ti0.2Nb0.2Ta0.2)B2, (Hf0.2Mo0.2Ti0.2Ta0.2Zr0.2)B2 and (Hf0.2Mo0.2Ti0.2Nb0.2Zr0.2)B2, in highly dense form. This will be conducted by taking advantage of the combination of two efficient synthesis and sintering techniques, i.e. Self-propagating High-temperature Synthesis (SHS) and Spark Plasma Sintering (SPS), respectively. The effect of the most important processing parameters, and additive introduction, in product characteristics will be systematically investigated to achieve nearly full dense andmonophasic ceramics.