Models of the Interfaces in Superhard TiN-Based Heterostructures and Nanocomposites from First-Principles

Abstract

This chapter reviews structures and properties of various TiN-based heterostructures. The study of such systems is of vital importance in development of new materials with desired optimum properties. After discussion of the current literature, the recently published results of first-principles quantum molecular dynamics (QMD) calculations of heterostructures consisting of one monolayer of interfacial SiNx, SiC, BN and AlN inserted between several monolayers thick slabs of B1(NaCl)-TiN (001) and (111) in the temperature range of 0–1,400 K are reviewed in some detail. The results revealed that SiN(001) exists as pseudomorphic B1-SiN interfacial layer only at 0 K. At finite temperature, this heterostructure transforms into distorted octahedral SiN₆ and tetrahedral SiN₄ units aligned along the {110} directions. At 300 K, the aggregates of the SiN_x units are close to a disordered, essentially amorphous SiN. After heating to 1,400 K and subsequent relaxation at 300 K, the interfacial layer corresponds to a strongly distorted Si₃N₄-like structure. The B1-SiN, Si₃N₄-like SiN and Si₃N₄-like Si₂N₃ interfaces between the TiN(111) slabs are stable in the whole temperature range considered here. The B1-Si₃N₄-like interfaces derived from SiN by the formation of Si-vacancies are unstable at finite temperatures. An estimate of interfacial formation energies showed that the most favorable configurations of the (111) interfaces are silicon atoms tetrahedrally coordinated to nitrogen. The most stable (001) B1-derived heterostructure with Si_0.75 N interface consist of both tetrahedrally and octahedrally coordinated silicon atoms. The TiN(001)/B1-SiC/TiN(001) interface exists as pseudo-morphic B1-SiC layer between 0 K and 600 K. After heating to 900–1,400 K and subsequent static relaxations, the interfacial layer corresponds to a strongly distorted 3C-SiC-like structure oriented in the (111) direction in which the Si and C atoms are located in the same interfacial plane. The Si atoms form fourfold coordinated N-Si ≡ C₃ configurations, whereas the C atoms are located in the Ti₂ = C ≡ Si₃ surrounding. All the (111) interfaces simulated at 0, 300, and 1,400 K have the same atomic configurations. For these interfaces, the Si and C layers correspond to the Si-C network in the (111) direction of 3C-SiC. The Si and C atoms are located in N-Si ≡ C₃ and Ti₃ ≡ C ≡ Si₃ configurations, respectively. The BN(001) interfacial layer forms a disordered h-BN-like structure consisting of BN₃ units in the whole temperature range considered here. Finally, the B1-AlN(001) interface is found to be stable within the whole temperature range.

Phonon calculations show that the observed modifications of the interfaces are due to 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 calculated electronic densities of states (EDOS) of the (001) interfaces suggest that the reconstructed interfaces should be semiconducting.

A comparison with the results obtained by earlier “static” ab initio DFT calculations at 0 K shows the great advantage of the QMD calculations that account for the effects of thermal activation of structural reconstructions. The results, which can be understood also without the knowledge of 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.