Superhard mixed transition metal dodecaborides

Solid solutions of mixed metal dodecaborides of ZrB₁₂, YB₁₂, and ScB₁₂ were prepared by arc-melting and studied for their mechanical properties. Zr_1-xY_xB₁₂ formed an essentially perfect solid solution, closely following Vegard's law. Zr_1-xSc_xB₁₂ and Y_1-xSc_xB₁₂ 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_0.5Y_0.5B₁₂, Zr_0.5Sc_0.5B₁₂, and Y_0.5Sc_0.5B₁₂ 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 ZrB₁₂, YB₁₂, and ScB₁₂, respectively. In addition, Zr_0.5Y_0.5B₁₂, Zr_0.5Sc_0.5B₁₂, and Y_0.5Sc_0.5B₁₂ 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 WB₄) and their alloys and the traditional cutting tools material tungsten carbide (∼400 °C). Moreover, Zr_0.5Y_0.5B₁₂, Zr_0.5Sc_0.5B₁₂, and Y_0.5Sc_0.5B₁₂ have relatively low densities of 3.52, 3.32, and 3.18 g/cm³, respectively, comparable to or even lower than that of diamond (3.52 g/cm³) and significantly lower than those of other superhard borides such as ReB₂ (12.67 g/cm³) and WB₄ (8.40 g/cm³) and traditional cutting tools materials, e.g., WC (15.77 g/cm³), making them of potential interest for lightweight protective coatings and/or as materials for cutting and machining.