360 / 2019-02-27 23:34:09

Modulating Hardness in Molybdenum Monoborides by Adjusting an Array of Boron Zigzag Chains

High pressure,Hardness,Transition metal borides

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Modulating Hardness in Molybdenum Monoborides by Adjusting an Array of Boron Zigzag Chains
Qiang Tao, Yanli Chen, Min Lian, Chunhong Xu, Li Li, Xiaokang Feng, Xin Wang, Tian Cui, Weitao Zheng, and Pinwen Zhu*
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
*Email: zhupw@jlu.edu.cn
Transition metal borides (TMBs) are superior materials with high hardness, excellent abradability, and chemical stability [1-3]. In TMBs, although it is generally accepted that high hardness is related to high covalent bonds density, the determinant of hardness is always conflicted. In order to break though the restriction of high density of covalent bonds, the low boron content transition metal borides (TMBs) are chosen to explore new way to enhance hardness. We fix the density of covalent bonds, and modulating the hardness by design perpendicular boron zigzag chain (Bzc) skeleton (pe-Bzcs) and parallel Bzc skeletons (pa-Bzcs) in α-MoB (I41/amd) and β-MoB (Cmcm).
In this work, α-MoB and β-MoB with nano graininess and nano lamella substructure were synthesized by high pressure and high temperature (HPHT). Utilizing pe-Bzcs in α-MoB to enhance shear modulus via less slip directions than pa-Bzcs. Pe-Bzcs generates higher grain boundary density in α-MoB than β-MoB to create nano polycrystal morphology under high pressure and high temperature (HPHT). Hence α-MoB has higher hardness (18.4 GPa) than β-MoB (12.2 GPa) attribute to higher shear modulus and higher density of grain boundary caused by pe-Bzcs. This work suggests a new opinion that modulating boron covalent bonds substructures is an effect way to enhance the hardness even in low-boron content TMBs. It is significant to design new hard or superhard functional materials.

References
[1] B. R. Kaner, J. J. Gilman, H. S. Tolbert. Science 2005, 308, 1268.
[2] Q. F. Gu, G. Krauss, W. Steurer. Adv. Mater. 2008, 20, 3620.
[3] R. Mohammadi, M. Xie, A. T. Lech, et, al. J. Am. Chem. Soc. 2012, 134, 20660.
Keywords: High pressure; Hardness; Transition metal borides