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

Cappai L.

Primo

;Casu M.

Secondo

;Locci A. M.;Cao G.;Garroni S.;Cannillo V.;Bellucci D.;Orru' R.

2025-01-01

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Abstract

(Nb0.5Ti0.5)B2 and (Nb0.5Hf0.5)B2 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 (Nb0.5Hf0.5)B2 (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 HfB2. NbB2 and (Nb0.5Hf0.5)B2 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 (TiNb2O7) made (Nb0.5Ti0.5)B2 more resistant to oxidation, compared to TiB2 and NbB2.