What is Wurtzite boron nitride?
Wurtzite boron nitride is a metastable superhard material that is a high-pressure polymorph of BN. Clarifying how the metastable high pressure material can be stabilized at atmospheric pressure is a challenging issue of fundamental scientific importance. Here, we fabricate millimeter-size w-BN bulk crystals via the hexagonal-to-wurtzite phase transformation at high pressure and temperatures. By combining transmission electron microscopy and ab initio molecular dynamics simulations, we reveal a stabilization mechanism for w-BN, i.e., the metastable high-pressure phase can be stabilized by 3D networks of planar defects which are constructed by a high density of intersecting (0001) stacking faults and {1010} inversion domain boundaries. The 3D networks of planar defects segment the w-BN bulk crystal into numerous nanometer-size prismatic domains with reverse crystallographic polarities. Our findings unambiguously demonstrate the retarding effect of crystal defects on the phase transformations of metastable materials, which is in contrast to the common knowledge that the crystal defects in materials will facilitate the occurrence of phase transformations.
Nanoindentation measurements of a highly oriented wurtzite-Type boron nitride bulk crystal
We succeeded in synthesizing a bulk crystal of wurtzite-Type boron nitride (w-BN) by the direct conversion method. The synthesized crystal was approximately 2mm wide and 350μm thick and highly oriented to the c-Axis. We performed nanoindentation measurements on the c-plane of the w-BN crystal at room temperature to evaluate the mechanical properties of w-BN. The hardness and Young's modulus of w-BN from the obtained curves were simultaneously determined to be 54 + 2 and 860 + 40GPa, respectively. The underlying physical mechanism that dominates the mechanical properties of group-III nitride semiconductors is also examined.
Elastic constants of cubic and wurtzite boron nitrides
We synthesized pure polycrystalline cubic boron nitride (cBN) and wurtzite boron nitride (wBN) by the direct conversion method from hexagonal boron nitride. We measured their longitudinal-wave elastic constants CL between 20 and 300 K using picosecond ultrasound spectroscopy. Their room-temperature values are 945 ± 3 GPa and 930 ± 18 GPa for cBN and wBN, respectively. The shear modulus G of cBN was also determined by combining resonance ultrasound spectroscopy and micromechanics calculation as G = 410 GPa. We performed ab-initio calculations and confirmed that the generalized gradient approximation potential fails to yield correct elastic constants, which indicated the necessity of a hybrid-functional method.
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