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.

Articles you may be interested inMagnetism, half-metallicity and electrical transport properties of V-and Cr-doped semiconductor SnTe: A theoretical studyThe lattice parameter and electronic structures of CoSi and CoSi 1−x Y x ͑Y = Al and P, x = 0.031 25 and 0.125͒ were calculated using the full-potential linear augmented plane wave approach based on density functional theory. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semimetal and its density of states is very small at the Fermi level. Doping with Y on the Si site can tune the Fermi level and the effective masses as well as the density of the hole carriers or electron ones in the electronic structure of CoSi, which is a valuable way to modulate the transport properties of this compound. Based on the calculated electronic structures and our experimental results on CoSi, the intrinsic relations between the electronic structures and transport properties of doped and undoped CoSi are discussed in detail.

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Determination of atomistic fracture modes under different loading conditions is essential to understanding nanomechanics. Here we report first-principles calculations that unveil intriguing indenter-angle-sensitive fracture modes and stress response at the incipient plasticity of strong covalent solids. We show that indenterangle-dependent distributions of biaxial stresses beneath indenters can produce a variety of bond deformation and breaking patterns that are distinct, at different indenter angles, from each other and from those under a pure shear stress. These results provide key insights for understanding mechanisms of deformation and fracture modes probed by nanoindentation.

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Recently synthesized low-density cubic BC2N exhibits surprisingly high shear strength inferred by nanoindentation in stark contrast to its relatively low elastic moduli. We show by first-principles calculation that this intriguing phenomenon can be ascribed to a novel structural hardening mechanism due to the compressive stress beneath the indenter. It significantly strengthens the weak bonds connecting the shear planes, yielding a colossal enhancement in shear strength. The resulting biaxial stress state produces atomistic fracture modes qualitatively different from those under pure shear stress. These results provide the first consistent explanation for a variety of experiments on the low-density cubic BC2N phase across a large range of strain.