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.
Impurities, in particularly oxygen, degrade the mechanical properties of superhard nc-TiN/a-Si3N4 nanocomposites. In the present paper we show that relatively small oxygen impurities also hinder the ...diffusion and already at a level of about ≥0.8at.% apparently stabilize the Ti–Si–N solid solution to a high temperature of about 1000°C, thus making it impossible to form stable and strong superhard nanocomposites. Therefore, a hardness enhancement to 30–35GPa, which has been reported in many publications on a variety of Ti–Si–N and other transition metal nitrides with silicon, is likely to be due to a simple refinement of the crystallite size towards the so called “strongest size”. Although the latter mechanism of strengthening is more universal than the design of superhard nanocomposites with strengthened interface, superhard nanocomposites can be prepared in this way only with a limited number of intrinsically very hard materials.
► Degradation of superhard nc-TiN/a-Si3N4 nanocomposites by impurities ► Oxygen is hindering Si-diffusion and apparently stabilizes the solid solution. ► Refinement of the crystallite size enables only a limited increase of hardness. ► We discuss the limits to the preparation of superhard nanocomposites.
The original finding of Veprek et al. that in nc-TiN/a-Si
3N
4 and in nc-TiN/a-Si
3N
4/TiSi
2 nanocomposites, deposited under conditions which allow complete phase segregation by spinodal mechanism, ...the maximum hardness of ≥
45 and >
100
GPa, respectively, is achieved when the thickness of the interfacial Si
3N
4 is about 1 monolayer, has been recently confirmed by both experiments and theory. First principle calculations explain why the decohesion and shear strength of a TiN–SiN
x
–TiN sandwich is higher than that of bulk SiN
x
. Combined
ab
initio DFT calculations of shear resistance of the interfaces, their averaging according to Sachs for randomly oriented polycrystalline material to obtain tensile yield strength, Tabor's criterion, Hertzian analysis and pressure-enhanced flow stress explain in a simple way the experimentally achieved high values of hardness of >
100
GPa, in excess of diamond. Friedel oscillations of the valence charge density, originating from negative charge transfer to the strengthened SiN
x
interface, cause decohesion and ideal shear to occur between Ti–N bonds near that interface. The extraordinary mechanical properties of these and related quasi-binary superhard nanocomposites can be understood in terms of nearly flaw-free strong materials with no need to invoke any new mechanism of strengthening. We shall present selected examples of industrial applications of the superhard nanocomposite coatings.
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.
Using high frequency plasma chemical vapor deposition (P CVD) at a total pressure of several mbar with TiCl4, BCl3, N2 and H2 as reactants, superhard nanocomposite nc-TiN/a-BN and nc-TiN/a-BN/a-TiB2 ...coatings with hardness of 40-50 GPa were reproducibly deposited and characterized in terms of their phase composition, nanostructure, thermal stability, oxidation resistance and mechanical properties. By analogy with earlier studied systems nc-MN/a-Si3N4 (M=Ti, W, V) and nc-TiN/a-Si3N4/a- and nc-TiSi2, the maximum hardness of the nc-TiN/a-BN and nc-TiN/a-BN/a-TiB2 coatings was obtained at the percolation threshold when there is about one monolayer of thin continuous tissue of a-BN between the TiN nanocrystals. It was shown that the thermal stability and oxidation resistance of these coatings is fairly high but lower than that of the nc-TiN/a-Si3N4 and nc-TiN/a-Si3N4/a-TiSi2 coatings. (Substrates: stainless steel, single crystal silicon, alpha-Fe, and molybdenum.)
We conducted first-principles molecular dynamics calculations of the stability and possible transformations of heterostructures consisting of face-centered-cubic (NaCl)-TiN(001) slabs with one ...monolayer thick pseudomorphically stabilized interfacial layer of B1-type BN, AlN, SiC and SiN, respectively. The calculations have been done with subsequent static relaxation of the heterostructures at temperatures between 0 and 1400K. It is shown that: i) the BN interfacial layer forms a disordered h-BN-like structure consisting of BN3 units within the whole temperature range considered; ii) the B1-AlN interfacial layer is stable within the whole temperature range; iii) the B1-SiC interfacial layer transforms into a distorted 3C–SiC(111)-like phase above 600K; and iv) the SiN interfacial layer consists of SiN4 and SiN6 units aligned along the 110 direction at room and high temperatures. Phonon calculations show that the observed modifications of the interfaces are due to the dynamical instability of the B1-type (001) and (111) interfacial layers of BN, SiC and SiN driven by soft modes within the given planes. The results, which can be understood also without the knowledge of the theoretical methods, were used to interpret the available experimental results on TiN-based heterostructures and nanocomposite coatings in order to provide guidance to the experimentalists for the preparation of better coatings.
•First-principles quantum molecular dynamics studies were conducted.•TiN-based heterostructures with SiN, BN, AlN and SiC interfacial monolayers•Stability and structural transformation between 0 and 1400K have been calculated.•The results of the calculations have been compared with experiments.•It is concluded which of the systems may form stable superhard nanocomposites.
The total energies and lattice constants of binary hcp- and fcc-TiN, AlN and ternary Ti0.5Al0.5N phases are calculated by ab initio method using the Vienna ab initio simulation package (VASP). The ...values of total energies are then used to calculate the lattice stabilities of binary hcp- and fcc-TiN, AlN and the interaction parameter of ternary Ti1-xAlxN phases on the basis of the semiempirical, thermodynamic sub-lattice model. Based on these data, the Gibbs free energy diagram of the immiscible quasi-binary TiN-AlN system are constructed in order to discuss the relative phase stability of the metastable ternary hcp- and fcc-Ti1-xAlxN phases over the entire range of compositions. The prediction is compared with the published results from PVD and CVD experiments. The calculated lattice energy and the constructed Gibbs free energy diagram show, in agreement with the experiments, that metastable fcc-Ti1-xAlxN coatings can easily undergo spinodal decomposition into coherent fcc-TiN and fcc-AlN, but there is a relatively large barrier for the formation of the stable hcp-AlN. A comparison with the TiN-Si3N4 system shows that, due to the much higher de-mixing energy of this system as compared to the TiN-AlN one, spinodal decomposition may occur in that system also for semicoherent TiN and Si3N4 phases.
The total energies and lattice constants of binary hexagonal close-packed (hcp)- and face-centered cubic (fcc)-CrN, AlN and ternary Cr0.5Al0.5N phases are calculated using the Vienna Ab-initio ...Simulation Program. The calculated total energies of the structures are then used to calculate the lattice stabilities of binary hcp- and fcc-CrN and AlN, and the interaction parameters of the ternary hcp- and fcc-Cr1-xAlxN solution phases. These results are used in the sublattice thermodynamic model to construct the Gibbs free energy diagram of the immiscible quasi-binary CrN-AlN system at different temperatures. Based on these results, we discuss the relative phase stability of the metastable ternary hcp- and fcc-Cr1-xAlxN solid solutions over the entire range of compositions. The predictions are compared with and supported by the published results from physical and chemical vapor deposition experiments. The constructed Gibbs free energy diagrams show that metastable fcc-Cr1-xAlxN coatings may undergo spinodal decomposition into coherent fcc-CrN and fcc-AlN, but there is a relatively large barrier for a direct formation of the stable hcp-AlN. A comparison with the Ti1-xAlxN and TiN-Si3N4 systems shows that the phase segregation will be more difficult and, therefore, the solid solution more stable in the Cr1-xAlxN case.
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