| Superhard materials which are a material with Vickers hardness HV above 40 GPa, with high hardness, corrosion resistance, abrasion resistance and many other excellent properties, are widely used in geological exploration, oil drilling, metallurgy, etc. Diamond and cubic boron nitride(c BN) are the most representative superhard materials. Unfortunately the thermal stability of diamond is poor, and diamond is easy to react with iron-based materials. On the other hand, c BN is lower that of diamond. Furthermore, the single crystal of c BN is prone to damage of cleavage. Therefore, the search for alternative superhard materials has been focused for a long time. Theoretic prediction reported that the transition-metal borides(TMBs), carbides(TMCs), nitrides(TMNs) with high valence electron density, strong covalent bond and 3D spatial arrangement of atomic are the potential superhard materials. So, TMBs, TMCs, and TMNs have attracted much attention.In general, it is effective approach to enhance the hardness of TMBs by improving the boron content and construction of three-dimension network structure. Zirconium borides have been focused due to its unique combination of physical and chemical properties. Therefore, we chose the high boron content of zirconium dodecaboride(Zr B12) as the object of our study. Zirconium dodecaboride was prepared by high temperature and high pressure(HTHP) and arc melting. The physical properties including the Vickers hardess and high pressure behavior of Zr B12 were characterized.ZrB12 have successfully been synthesized using both HTHP and arc melting. The high purity powder of Zr and B were used as the start materials. The high pressure synthetic experiments are carried out on the cubic anvil apparatus(SPD6×600T) with temperature of 2000 ℃ and pressure of 5.0 GPa. With the holding time of 15 min, the bulk materials of pure Zr B12 were obtained. The basic method of arc melting is to adjust the output energy of arc to about 3000 ℃ of arc, melt the start materials. The bulk materials of pure Zr B12 were also obtained.The Vickers microhardness measurements of as-synthesized Zr B12 were performed by Micro-Hardness Tester(HV-1000). The results show that the convergence of hardness value of Zr B12 is 26.4 GPa. Compared with Zr B2, the hardness value of Zr B12 is increased 34.7%. This result shows that the hardness of the transition metal borides can be effectively enhanced by improving boron content in the transition metal borides, construction of 3D network structure. To further explore the intrinsic hardness of Zr B12, we tested the hardness of different crystal planes of Zr B12 single crystal samples, and obtained the hardness value of different crystal planes. The Vickers hardness value of {100} crystal plane, {110} crystal plane, {111} crystal plane is 25.5 GPa, 26.4 GPa, 25.1 GPa, respectively. The results of hardness tests show that the mechanical property of Zr B12 crystal is isotropic.The high-pressure structural behaviors of Zr B12 have been performed with angle-dispersive synchrotron X-ray powder diffraction at room temperature. The results show that the structure of Zr B12 crystal is stable under high pressure up to 40 GPa. The high-pressure synchrotron X-ray powder diffraction data of Zr B12 were fitted and the value of bulk modulus, B0=221 GPa was obtained. This value is identical to the results of theoretical simulation reported in the literature slightly lower than Zr B2. This result indicates that the bulk modulus is not directly related to the hardness. The variations of the lattice parameters with increasing pressure were also analyzed. The results show that the variation of a, b, c are consistent, and α、β、γ does not change. And the variation of crystal plane spacing is also consistent. This demonstrates that the mechanical property of Zr B12 crystal is isotropic. |
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