Designing superhard metals: The case of low borides

Abstract

The search for new superhard materials has usually focused on strong covalent solids. It is, however, a huge challenge to design superhard metals because of the low resistance of metallic bonds against the formation and movement of dislocations. Here, we report a microscopic mechanism of enhancing hardness by identifying highly stable thermodynamic phases and strengthening weak slip planes. Using the well-known transition-metal borides as prototypes, we demonstrate that several low borides possess unexpectedly high hardness whereas high borides exhibit an anomalous hardness reduction. Such an unusual phenomenon originates from the peculiar bonding mechanisms in these compounds. Furthermore, the low borides have close compositions, similar structures, and degenerate formation energies. This enables facile synthesis of a multiphase material that includes a large number of interfaces among different borides, and these interfaces form nanoscale interlocks that strongly suppress the glide dislocations within the metal bilayers, thereby drastically enhancing extrinsic hardness and achieving true superhard metals. Therefore, this study not only elucidates the unique mechanism responsible for the anomalous hardening in this class of borides but also offers a valid alchemy to design novel superhard metals with multiple functionalities.

Carbon-Deficient Titanium Carbide With Highly Enhanced Hardness

“…Consequently, the calculated bulk modulus of TiC from first-principles calculation is only 252.4 GPa, which is far lower than that of diamond, Os, and the osmium di-borides. However, according to recent reports of S. H. Jhi and Y. C. Liang, a valence electron number of eight will result in the most reasonable electronic structure, and thus higher hardness [31][32][33][34]. The valence electron of TiC is about eight electrons, and this may be one of the reasons for its exceptionally high hardness.…”

Anisotropies in Elasticity, Sound Velocity, and Minimum Thermal Conductivity of Low Borides VxBy Compounds

“…Superhard materials have long attracted tremendous attention for a wide range of applications, such as high-temperature applications, surface protection, and abrasive materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Diamond is the hardest substance, which maintains records of bulk modulus, shear modulus, and high hardness of 432 GPa, 535 GPa, and 60-150 GPa, respectively.…”

“…The balance of high hardness and excellent metallicity is a central challenge to design superhard metals. However, great success has been achieved with the discovery of transition-metal (TM) light-element (LE) compounds [7][8][9][10][11][12][13][14][15]. It is generally believed that their high hardness stems from a combination of high valence-electron concentration to resist volume compression and strong covalent bonding to counteract shape deformation [7,8].…”

Physical Properties and Electronic Structure of Cr2B Under Pressure

“…Superhard metals show excellent performance, including the high hardness, wear resistance, good thermodynamic stability and metallic behavior, compared with traditional superhard materials. [ 1–9 ] The superhard metals can be synthesized by inserting strong covalent bonds of light element (LE), such as carbon, boron, nitrogen, into transition metal (TM) with a large number of valence electrons. Based on the classical thought, to obtain a high hardness, a straightforward way was to incorporate more light atoms into the transition metal to form strong covalent bonds.…”

“…[ 11 ] Liang et al also found that several low borides showed unexpectedly high hardness, while the hardness reduced for high borides. [ 2 ] Lu et al predicted the hardness of hP4‐WN and hP6‐WN 2 exceeded 40 GPa using the first principles. [ 12 ] The high hardness and low light element content in TM‐LE compound makes the superhard metal possible.…”