Is Osmium Diboride An Ultra-Hard Material?

Abstract

We report first-principles calculations of ideal tensile and shear strength for the recently synthesized orthorhombic OsB2 that is a primary example of a new class of ultra-hard materials synthesized by combining small, light, and covalent elements with large, electron-rich transition metals. Our calculations show that the shear strength on the (001) plane is highly anisotropic with a low peak stress of 9.1 GPa in the (001)[010] shear direction but a much higher peak stress of 26.9 GPa in the (001)[100] direction. The strong resistance against (001)[100] shear deformation prevents the indenter from making a deep imprint, giving rise to a high Vickers hardness on the (001) plane, despite the weak shear strength in the (001)[010] shear direction. The calculated peak stress of 26.9 GPa in the (001)[100] shear direction agrees well with the 30 GPa Vickers hardness observed experimentally on the (001) plane in OsB2. However, the weak shear strength (9.1 GPa) in the (001)[010] shear direction severely limits its application as abrasives and cutting tools for ferrous metals as well as scratch-resistance coatings. Our results highlight the importance of understanding atomistic deformation modes under various loading conditions in designing new ultra-hard materials.

Intrinsic hardness of crystalline solids

“…From experiments it is found that the mixed alloy of cubic TiC x N 1 -x with a rock-salt structure achieves the maximum hardness when the valence electron concentration is about 8.4 per unit cell. Figure 4 shows the correlation of the measured microhardness in Vickers scale of TiC x N 1 -x with calculated shear modulus (c 44 ) as a function of the valence electron concentration (VEC). The VEC was modified by varying the relative amount of C and N atoms.…”

“…6, a) shows that the fourth band is composed of mainly Ti replacement of C by N increases the number of valence electrons. In a rigid band description, the additional electrons occupy the empty states of the fourth band near Γ, raising the Fermi level and strengthening the Ti-C interactions, which results in a higher c 44 . The complete filling of the shear-resistive bonding states is at a VEC ~ 8.4, which corresponds to the observed maximum in the experimental microhardness.…”

“…Calculations of the ideal tensile and shear strengths of OsB 2 have been reported in [44]. OsB 2 is formed from alternate layers of Os and B atoms.…”

Exploring the high-pressure behavior of superhard tungsten tetraboride

“…9,10 Although this hardness value is relatively high, it does not assign this material to the "superhard" category. 11 One reason for this is that the OsB 2 structure contains double Os layers, alternating with covalent B layers. The weak OsOs metallic bonds within the layers likely reduce the resistance of OsB 2 to large shear deformations in the easy-slip direction, which is parallel to the layers.…”

“…The weak OsOs metallic bonds within the layers likely reduce the resistance of OsB 2 to large shear deformations in the easy-slip direction, which is parallel to the layers. 11 To create potentially harder materials, hexagonal rhenium diboride was synthesized by completely replacing Re for Os. The ReB 2 structure consists of alternating single layers of hexagonally-packed Re and puckered interconnected hexagonal rings of boron.…”