Is Rhenium Diboride a Superhard Material?

Abstract

The average measured Hv for polycrystalline ReB2 and Ti–B composites under different loads shows that the tendency of Hv to decrease becomes weak for large loads, and the measured hardness of ReB2 is always lower than that of Ti–B composites. For comparison, the results from Chung et al. were inserted into the figure.

Indentation strength of ultraincompressible rhenium boride, carbide, and nitride from first-principles calculations

“…6), we estimate that the (asymptotic) Vickers hardness of ReB 2 and Re 2 C are about 20-30 GPa and 15-25 GPa, respectively, which are consistent with the experimental results. 4,8,9,17 This explains the differences between the measured (asymptotic) Vickers hardness (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) of ReB 2 and the previous theoretical hardness predications (35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) in which the normal compressive pressure beneath the indenter are neglected in the calculations. 58 The higher pure shear ideal strength of ReB 2 (35.3 GPa) and Re 2 C (27.2 GPa) suggests that they are suitable for applications in abrasive tools and wear-resistant coatings where compressive pressures normal to their scratching surfaces are not high.…”

Predicting new superhard phases

“…Very recently, a new family of materials formed by heavy transition metals and light elements are proposed to be potential superhard since heavy transition metals can basically introduce high valence electron density into the compounds to resist both elastic and plastic deformation. Many these compounds have then been successfully synthesized, such as, transition metal nitrides [17][18][19][20][21], carbides [22], and borides [23][24][25][26][27][28]. These compounds possess ultrahigh bulk moduli (428 GPa for IrN 2 [20]) comparable with those of the traditional superhard materials.…”

“…These compounds possess ultrahigh bulk moduli (428 GPa for IrN 2 [20]) comparable with those of the traditional superhard materials. Though there exists extensive debates [26,27], ReB 2 and WB 4 have been claimed to be superhard [24,25,29].…”

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

“…Since 2007, inspired by frontier research on ReB 2 by Chung et al [7], there has been a new surge of research interest in searching for ultra-incompressible and superhard transition-metal (TM) borides, nitrides, and carbides. These efforts have led to a series of new system including diborides (ReB 2 [8][9][10] and RuB 2 [10]), tetraborides (WB 4 [11,12], FeB 4 [13], MnB 4 [14,15], and CrB 4 [16][17][18]), nitrides (Re x N [19] and WN x [20]), and carbides (Re 2 C [21]). …”