From individual to equimolar binary transition metal diborides: The case of (Nb0.5M0.5)B2, M=Ti, Hf

(Nb 0.5 Ti 0.5 )B 2 and (Nb 0.5 Hf 0.5 )B 2 in bulk form were produced by Spark Plasma Sintering (SPS) from powders obtained by Self-propagating High-temperature Synthesis. The carbothermal reduction of oxide contaminants with graphite promoted the conversion of secondary phases into equimolar diborides, and improved samples densification, particularly for (Nb 0.5 Hf 0.5 )B 2 (from 89.7 % to 97.4 %). A single-phase solid solution was obtained for both systems at 2050 °C by SPS, while longer holding times (40 instead of 20 min) were needed for the Hf-containing ceramic. The latter system showed Vickers hardness (18.47 GPa) and Young’s modulus (586 GPa) slightly better than HfB 2 . NbB 2 and (Nb 0.5 Hf 0.5 )B 2 were the most sensitive systems to the oxidation environment, with the formed oxide scales detached from samples at 600 and 800 °C, respectively, due to volume changes accompanying phase transitions in the generated niobium oxides. Conversely, the formation of a stable mixed oxide (TiNb 2 O 7 ) made (Nb 0.5 Ti 0.5 )B 2 more resistant to oxidation, compared to TiB 2 and NbB 2 . • Homogeneous (Nb 0.5 Ti 0.5 )B 2 and (Nb 0.5 Hf 0.5 )B 2 solid solutions are synthesized by combining SHS and SPS techniques. • Graphite additive reduced oxide contaminants, improved sample densification, and increased the yield in the desired diborides. • Vickers hardness and Young’s modulus of (Nb 0.5 Hf 0.5 )B 2 better with respect to its individual constituents. • The formation of a stable mixed oxide (TiNb 2 O 7 ) makes (Nb 0.5 Ti 0.5 )B 2 more resistant to oxidation, compared to TiB 2 and NbB 2 .