Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite

Abstract

Recent indentation experiments indicate that wurtzite BN (w-BN) exhibits surprisingly high hardness that rivals that of diamond. Here we unveil a novel two-stage shear deformation mechanism responsible for this unexpected result. We show by first-principles calculations that large normal compressive pressures under indenters can compel w-BN into a stronger structure through a volume-conserving bond-flipping structural phase transformation during indentation which produces significant enhancement in its strength, propelling it above diamond's. We further demonstrate that the same mechanism also works in lonsdaleite (hexagonal diamond) and produces superior indentation strength that is 58% higher than the corresponding value of diamond, setting a new record.

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

“…The energy convergence of the calculation is on the order of 1 meV per atom, with the residual stresses in the fully relaxed structures less than 0.1 GPa. The quasistatic ideal indentation strength and relaxed loading path were determined using a method described previously, 39,40 in which the lattice vectors were incrementally deformed in the direction of applied shear strains, say ǫ xz . At each step, the applied shear strain is fixed which determines the calculated shear stress σ xz , while the other five independent components of the strain tensors and all the atoms inside the unit cell were simultaneously relaxed until (i) the compressive pressure (σ zz ) beneath the indentor normal to the chosen shear deformation plane reaches a specified value (i.e.…”

“…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.…”

“…39,40 The high-density valence electrons in these materials behave like a low-compressibility liquid. It is difficult to compress their volumes by hydrostatic pressure, which results in their high bulk moduli.…”

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.…”

Simulation of the formation of polymorphic varieties of nanodiamonds

“…However, according to the theoretical research, performing in [1], hardness of hexagonal polytype of diamond (as called as lonsdaleite) exceeds hardness of cubic diamond twice. How lonsdaleite can be synthesized till now remains insufficiently clear.…”