Crystal Field Splitting is Limiting the Stability and Strength of Ultra-incompressible Orthorhombic Transition Metal Tetraborides.

The lattice stability and mechanical strengths of the supposedly superhard transition metal tetraborides (TmB4, Tm = Cr, Mn and Fe) evoked recently much attention from the scientific community due to the potential applications of these materials, as well as because of general scientific interests. In the present study, we show that the surprising stabilization of these compounds from a high symmetry to a low symmetry structure is accomplished by an in-plane rotation of the boron network, which maximizes the in-plane hybridization by crystal field splitting between d orbitals of Tm and p orbitals of B. Studies of mechanical and electronic properties of TmB4 suggest that these tetraborides cannot be intrinsically superhard. The mechanical instability is facilitated by a unique in-plane or out-of-plane weakening of the three-dimensional covalent bond network of boron along different shear deformation paths. These results shed a novel view on the origin of the stability and strength of orthorhombic TmB4, highlighting the importance of combinational analysis of a variety of parameters related to plastic deformation of the crystalline materials when attempting to design new ultra-incompressible, and potentially strong and hard solids.

Stability and strength of transition-metal tetraborides and triborides.

Using density functional theory, we show that the long-believed transition-metal tetraborides (TB(4)) of tungsten and molybdenum are in fact triborides (TB(3)). This finding is supported by thermodynamic, mechanical, and phonon instabilities of TB(4), and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB(3) structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB(3) cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB(3) under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.

Recent attempts to design new super- and ultrahard solids leads to nano-sized and nano-structured materials and coatings.

A brief overview of recent attempts to design new super- and ultrahard materials, which are based on the assumption that materials with high elastic moduli should be super- or ultrahard, is presented in order to show that meeting this condition is not sufficient. Instead, electronic and structural stability upon a finite, relatively large shear strain at atomic level is necessary to avoid structural transformations to softer phases or even a collapse of the structure. We discuss several examples where very high hardness of > 70 GPa has been obtained due to the nano-sized and nano-structured effects. Superhard nano-sized and/or nano-structured ("nanocomposites") materials can be prepared either by limited diffusion or by spinodal phase segregation during their synthesis. The advantages of the latter mentioned approach is the formation of a stable nanostructure with strong interfaces, that avoids grain boundary shear and concomitant softening when the crystallite size decreases below about 10-20 nm. In such a way, hardness enhancement by a factor of 4 to 5 has been achieved. We shall show that nc-TiN/a-Si3N4 nanocomposites can achieve hardness in excess of 100 GPa when properly designed, and prepared with low density of flaws and impurities. The paper finishes with a short overview of industrial applications of the nanocomposites as wear protection coatings on tools for machining.

Friedel oscillations are limiting the strength of superhard nanocomposites and heterostructures.

To obtain a deeper understanding of the mechanism of plastic deformation and failure in superhard nanocomposites and heterostructures we studied, by means of the ab initio density functional theory, the stress-strain response and the change of the electronic structure during tensile and shear deformation of a prototype interfacial systems consisting of 1 monolayer SiN sandwiched between a few nm thick TiN layers. This shows that peak Friedel oscillations of valence charge density weaken the Ti-N interplanar bonds next to that interface, where decohesion in tension and slip in shear occurs. These results provide ways to design new, stronger and harder materials.