Superhard Mixed Transition Metal Dodecaborides

Solid solutions of mixed metal dodecaborides of ZrB 12, YB 12, and ScB 12 were prepared by arc-melting and studied for their mechanical properties. Zr 1– x Y x B 12 formed an essentially perfect solid solution, closely following Vegard’s law. Zr 1– x Sc x B 12 and Y 1– x Sc x B 12 undergo a face centered-cubic to body-centered tetragonal transition at 90–95 at. % Sc as determined by powder X-ray diffraction and transmission electron microscopy. The compounds Zr 0.5 Y 0.5 B 12, Zr 0.5 Sc 0.5 B 12, and Y 0.5 Sc 0.5 B 12 are superhard (Vickers hardness ≥ 40 GPa) and demonstrate an increase in hardness to 45.8 ± 1.3, 48.0 ± 2.1, and 45.2 ± 2.1 GPa under a load of 0.49 N, respectively, compared to 40.4 ± 1.8, 40.9 ± 1.6, and 41.7 ± 2.2 GPa for pure ZrB 12, YB 12, and ScB 12, respectively. In addition, Zr 0.5 Y 0.5 B 12, Zr 0.5 Sc 0.5 B 12, and Y 0.5 Sc 0.5 B 12 solid solutions show a substantial increase in oxidation resistance to approximately 630, 685, and 695 °C, respectively, when compared to other superhard metal borides (e.g., ∼400 °C for WB 4 ) and their alloys and the traditional cutting tools material tungsten carbide (∼400 °C). Moreover, Zr 0.5 Y 0.5 B 12, Zr 0.5 Sc 0.5 B 12, and Y 0.5 Sc 0.5 B 12 have relatively low densities of 3.52, 3.32, and 3.18 g/cm 3, respectively, comparable to or even lower than that of diamond (3.52 g/cm 3 ) and significantly lower than those of other superhard borides such as ReB 2 (12.67 g/cm 3 ) and WB 4 (8.40 g/cm 3 ) and traditional cutting tools materials, e.g., WC (15.77 g/cm 3 ), making them of potential interest for lightweight protective coatings and/or as materials for cutting and machining.