4.7 Article

Shear anisotropy: Tuning high temperature metal hexaborides from soft to extremely hard

Journal

JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY
Volume 33, Issue 11, Pages 1371-1377

Publisher

JOURNAL MATER SCI TECHNOL
DOI: 10.1016/j.jmst.2017.01.022

Keywords

Microhardness; Electronic structure; Elastic constant; Borides; Structural ceramics

Funding

  1. National Natural Science Foundation of China [51672064, U1435206]
  2. Beijing Municipal Science & Technology Commission [Z151100003315012, D16110000241600]

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Easy machining into sharp lending edge, nose tip and complex shape components plays a pivotal role in the application of ultrahigh temperature ceramics in hypersonic vehicles, wherein low and controllable hardness is a necessary parameter to ensure the easy machinability. However, the mechanism that driving the hardness of metal hexaborides is not clear. Here, using a combination of the empirical hardness model for polycrystalline materials and density functional theory investigation, the hardness dependence on shear anisotropic factors of high temperature metal hexaborides has been established. It has come to light that through controlling the shear anisotropic factors the hardness of polycrystalline metal hexaborides can be tailored from soft and ductile to extremely hard and brittle, which is underpinned by the degree of chemical bonding anisotropy, i.e., the difference of B-B bond within the B-6 octahedron and that connecting the B-6 octahedra. (C) 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

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