Superhard CubicBC2NCompared to Diamond

Abstract

Recent experiments claimed successful synthesis of cubic boron-carbonitride compounds BC2N with an extreme hardness second only to diamond. In the present Letter, we examine the ideal strength of cubic BC2N using first-principles calculations. Our results reveal that, despite the large elastic parameters, compositional anisotropy and strain dependent bonding character impose limitation on their strength. Consequently, the hardness of the optimal BC2N structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of BC2N nanocomposites is most likely due to the nanocrystalline size effect and the bonding to the surrounding amorphous carbon matrix. This may prove to be a general rule useful in the quest for new superhard covalent materials.

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

“…The lowest peak stress in all the indentation shear directions determines the ideal indentation strength of the structure, at which the crystal structure starts to destabilize. If we set Φ = 0 (so σ zz = 0) in the calculation, it is equivalent to require that all the five stress components (except σ xz ) become negligible during the structural relaxation, which is the relaxation procedure used in the previous calculations of pure ideal shear stresses [28][29][30][31][32][33][34][35][36][37][38][41][42][43][44][45][46][47][48] that neglect the effects of the normal compressive pressure and geometry (indenter angles) of the indenter. Our test calculations for the lattice vectors of the equilibrium structure of Re, Re 3 N, Re 2 N, Re 2 C, Re 2 B and ReB 2 , their bulk moduli and Poisson's ratios are given in Table I, which agree well with the previous calculation results 19,[21][22][23][24]54,55 .…”

“…The ideal shear strength calculations can provide an estimation of the asymptotic (i.e., load independent) hardness by comparing the calculated ideal shear strength with those of benchmark materials, such as diamond and c-BN whose hardness are well established. However, most previous ideal shear strength calculations did not consider normal compressive pressure beneath the indenter, [28][29][30][31][32][33][34][35][36][37][38][41][42][43][44][45][46][47][48] which makes them appropriate primarily to describe the scratching hardness of materials where normal pressures on scratching surfaces are not high.…”

“…26,27 The limit of structural stability of the specimen in these hardness tests is closely related to its maximum shear strength, which precedes the initiation of cracks and dislocations that lead to plastic deformation. Recent advances in computation physics have made it possible to calculate directly the ideal shear strength of a perfect crystal, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] i.e., the lowest shear stress peak at which a perfect crystal becomes mechanically unstable, that can be compared to the shear strength derived from nano-indentation measurements. 42 These ideal strength calculations, using accurate first-principles methods, also provide atomistic deformation patterns and full range stress-strain relations which offer key insights into the mechanisms responsible for the fracture modes at incipient plasticity.…”

Size dependence of nanostructures: Impact of bond order deficiency

“…Recent tight-binding potential MD calculations of tetrahedron carbon (t-C) graphitization by Zheng et al [688] reveal that the graphitization of t-C cluster with hundreds of atoms happens at a temperature that is 10% higher than that measured (1100e 1200 K). A first-principle calculation [689] predicted that the hardness of the optimal BC 2 N structure is lower than the measured extreme hardness of BC 2 N nanocomposites. It is suggested that the effects of the nanocrystalline size and the bonding with the amorphous carbon matrix in BC 2 N nanocomposites likely play a crucial role in producing the extreme hardness measured in experiments.…”

Ideal strength and structural instability of aluminum at finite temperatures

“…Recent developments in computational physics have enabled firstprinciples calculations of peak stresses (i.e., ideal strengths) in the stress-strain relations of a crystal lattice along specific deformation paths and the structural deformation modes leading to elastic instabilities [1][2][3][4][5][6][7][8][9][10][11] . Meanwhile, dynamic instability of a crystal lattice has been studied via separate calculations of the phonon spectra of the crystal lattice at each step along every deformation pathway.…”