Processing and characterization of Ultra High Temperature High‐Entropy (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2-based Ceramics: Effect of W granulometry, graphite, and SiC addition

A highly dense and single phase (Ti 0.2 Zr 0.2 Hf 0.2 Mo 0.2 W 0.2 )B 2 ceramic product is obtained in this work at 1950°C (20 min, 20 MPa) by Spark Plasma Sintering (SPS) from powders prepared by Self-propagating High-temperature Synthesis (SHS). The formation of the (W,Mo)B 2 secondary phase is avoided using fine W precursors and adding 1 wt% graphite to the SHS powders before SPS. Kinetic limitations responsible for hindering the synthesis of the high entropy boride are correspondingly eliminated. The resulting 98.5 % dense sample exhibits a homogeneous microstructure, with Vickers hardness of 26.8 GPa. The introduction of 20 vol% SiC produces an increase of the K IC values from 2.32 to 5.11 MPa m 1/2 . Very relevant is that the volatilization of Mo- and W-oxides occurring during sample oxidation at high temperature, which leads to its rapid degradation with the formation of a very porous oxide scale, can be strongly inhibited by the silicate phases generated in the composite ceramic. • About 98.5 % dense High Entropy (Hf 0.2 Zr 0.2 Ti 0.2 Mo 0.2 W 0.2 )B 2 ceramic is fabricated by coupling SHS and SPS routes. • A marked and synergic effect on the solubilization of W-rich secondary phases is produced by the use of fine W precursor and graphite . • The low oxidization resistance, mostly due to the volatilization of Mo- and W-oxides, is enhanced with the introduction of SiC. • Benefits in fracture toughness are provided by SiC addition, with an increase of the K IC value from 2.32 to 5.11 MPa m 1/2 .