R. F. Zhang

Mechanical and electronic properties of hard rhenium diboride of low elastic compressibility studied by first-principles calculation

The mechanical and electronic properties of hcp-ReB2 are calculated by a first-principles approach. The calculated extreme equilibrium mechanical properties are consistent with the available experimental and theoretical data. The calculated elastic moduli suggest that hcp-ReB2 has a low compressibility and is most likely an intrinsically brittle material, but its relatively low ideal shear strength, as compared with c-BN, indicates that it is not intrinsically stronger/harder than c-BN. Based on the calculated electronic density of states and valence charge density distribution, the bonding nature of hcp-ReB2 is examined to obtain a deeper insight into the physical origin of the mechanical properties.

First principles studies of ideal strength and bonding nature of AlN polymorphs in comparison to TiN

The stress-strain relationships under tensile and shear loads and bonding nature of fcc (rocksalt)-, hcp (wurtzite)-AlN, and fcc (rocksalt)-TiN are calculated by first principles method. Compared to fcc-TiN, fcc-AlN shows similar anisotropy of tensile strengths, but lower shear strength is found in both AlN polymorphs. Based on the calculated electronic density of states, bonding nature for both AlN polymorphs is discussed. The hardness enhancement in TiN∕AlN heterostructures and nanocomposites cannot be attributed to the difference of the ideal strength and bonding nature between fcc-AlN and hcp-AlN, but to the formation of semicoherent fcc-TiN/fcc-AlN interface.

Electronic structure, stability, and mechanism of the decohesion and shear of interfaces in superhard nanocomposites and heterostructures

Electronic structure of interfaces, their stability and the mechanism of decohesion in tension as well as of ideal shear have been studied by means of ab initio density-functional theory for heterostructures consisting of a few nanometer thick fccNaCl-TiN slabs with one monolayer of pseudomorphic SiN interface. It is found that the SiN interface sandwiched between fcc001-TiN slabs is unstable in its symmetric fcc structure, but it stabilizes by distortion of the Si-N bonds, which lowers the symmetry. Significant strengthening of the SiN interface occurs due to partial transfer of valence charge to the Si containing interface which induces damped valence charge-density oscillations propagating into the TiN bulk. As a consequence of these oscillations, decohesion, and ideal shear does not occur within the SiN interface, but in the TiN slabs between the Ti-N planes parallel to that interface. We provide a detailed study of this mechanism of decohesion and ideal shear on the atomic scale. The results are discussed in the context of the experimentally found hardness enhancement in heterostructures and superhard nanocomposites.

Origin of the hardness enhancement in superhard nc-TiN/a-Si3N4 and ultrahard nc-TiN/a-Si3N4/TiSi2 nanocomposites

The ideal shear strengths of a variety of TiN/SiN x /TiN interfaces have been calculated with the ab initio density functional theory. Using these data, the high hardness H ≥ 105 GPa, which has been reported for the quasi-ternary nc-TiN/a-Si3N4/TiSi2 nanocomposites, can be explained on the basis of the Sachs average of polycrystal plasticity of randomly oriented equiaxed grains and pressure-enhanced flow stress.

Mechanical strengths of silicon nitrides studied by ab initio calculations

The stress-strain relationships under tensile and shear loads are calculated for hcp(β)-Si3N4 and fcc(NaCl)-SiN by means of ab initio density functional theory. The ideal shear strengths for fcc-SiN are much lower than those for hcp-Si3N4. This is in agreement with experiments which show that the interfacial fcc-SiN can strengthen the TiN∕SiN heterostructures only when its thickness is about 1–2 ML. Based on the calculated electronic density of states, the physical origin of the mechanical strengths is addressed.

Mechanical properties and hardness of boron and boron-rich solids

A brief review of the recent studies of the crystal and electronic structure and its stability under large strain-shear and tensile deformation of selected boron compounds, which have been predicted to be superhard on the basis of their high elastic moduli, is presented. It will be shown that in many cases, the materials undergo electronic instability and transformation to softer phases with a lower shear resistance than the original equilibrium structure. Therefore, high values of elastic moduli (“low compressibility”) do not guarantee high hardness. These results also challenge the recent “models of theoretical hardness of an ideal crystal”, which are based on the equilibrium electronic properties. It is shown that appropriately nanostructured materials open the way to the design of superhard materials.

On the anisotropic shear resistance of hard transition metal nitrides TMN (TM=Ti, Zr, Hf)

The anisotropic shear moduli and strengths of hard fcc-TMN (TM=Ti, Zr, Hf) have been calculated by ab initio density functional theory in order to better understand their shear resistance. TiN shows the largest shear strength among the three nitrides, being consistent with the known facts that TiN is harder than HfN which is harder than ZrN. The electronic origin is further addressed. Based on the smooth shapes of the stress-strain curves and on the variation of electronic structure during shear deformations, the abrupt lattice instability mode reported previously for TiN (110)⟨11¯0⟩ shear deformation is not supported by our data.

Design of ultrahard materials: Go nano!

Intrinsically super- (H ∼ 40–70 GPa) and ultrahard (H ≥ 70 GPa) materials attain high hardness through their large intrinsic strength, whereas extrinsically super- and ultrahard materials reach such hardness through their nanostructure. The recent search for intrinsically super- and ultrahard materials has concentrated on those with high elastic moduli. Several examples are presented of materials with high zero-pressure elastic moduli but relatively low hardness, because, upon imposing a large lattice distortion or substantial plastic shear strain, they undergo an instability of their electronic structure. Recent progress is then elucidated in the understanding of the origin of ultrahardness of nc-TmN/a-Si3N4 nanocomposites (Tm = transition metal that forms hard and stable nitrides), in which 3–4 nm size TmN nanocrystals are joined together by about a monolayer thick SiN x interface material layer. A combined ab initio DFT calculation of the ideal shear strength of the interfaces between the plastically non-deformable randomly oriented TmN grains, followed by Sachs-type averaging of these shear resistances in space to obtain a uniaxial yield strength, Y, employing a proper pressure enhancement of Y and considering the Tabor relation between the hardness H and yield strength Y, shows that these materials can reach hardness significantly larger than diamond, when correctly prepared and essentially free of defects. Such superhard nanocomposites are already available on an industrial scale as protective coatings on tools for machining, such as for drilling, milling, turning, forming, stamping and the like.