First Principle Studies On The Structures And Properties Of TiN And Ti-Si-N Under Pressures

Nano-materials have great significance and wide application for electronics,information, new ceramic, biological, chemical, pharmaceutical, machinery, transportation,energy, national defense and so on. The TiN and Ti-Si-N films, as a class of representativesof hard thin films, have attracted wide attention for their high hardness, wear resistance,excellent thermal and chemical stability. At room temperature and zero pressure, a lot ofexperimental and theoretical studies have been carried out on hardness and lattice dynamicsof TiN and Ti-Si-N films, but the study on the nature under pressures are reported relativelyfew. TiN and Ti-Si-N, as reinforced surface coatings, have been widely used aswear-resistant materials and cutting tools in practical applications. And when working, theymay endure enormous instantaneous stress; therefore, the research promises practicalsignificance on the relative stability and performance under different pressures. The researchof this subject can be divided into two parts: firstly, the studies on TiN under pressures;secondly, the studies on Ti-Si-N under pressures.The present calculations were done using the first-principles method based on densityfunctional theory (DFT) as implemented in the VASP (Vienna Ab initio simulation package),GIBBS and PHONOPY code. Related calculations about energy, elastic properties andphonon spectrum were carried out, and the stability and law of phase transition underdifferent pressures were analyzed. Three configurations were studied, it included the NaClstructure (B1), CsCl structure (B2) and zinc blende structure (B3). By analysing the E-V(energy as a function of volume) relationship, P–H (enthalpy as a function of pressure)relationship, it’s easy to obtain that the phase transition of TiN from NaCl to CsCl structurewould occur when the pressure is about347GPa, and from NaCl to zinc blende structurewould occur when the pressure is about-17.5GPa. And the same conclusions were obtainedby the studies of elastic properties and phonon dispersion relations under different pressures.The quasi-harmonic approximation and quasi-harmonic Debye model were applicated tocalculate the NaCl structure TiN’s Gibbs free energy, vibration energy, enthalpy, constantvolume molar heat capacity, constant pressure molar heat capacity, thermal expansioncoefficient, Debye temperature and so on. The thermodynamic properties of TiN underdifferent pressures were discussed.The same method was used when investigating the structural stability of Ti-Si-N. Andthe stable structure at atmospheric pressure was found out. Related studies were carried out on the stability of energy, elastic properties and phonon spectra. The possible phasetransition pressure range of Ti-Si-N was found out. Its stability under different pressures andthe law of phase transition were analyzed, and the thermodynamic properties under differentpressures were discussed. It’s easy to obtain that the thermal expansion coefficient decreasedwith increasing pressures under higher temperatures; its thermal expansion coefficient wasbigger than the TiN’s, indicating that it’s more important in the preparation of Ti-Si-N whenchosing reasonable substrate, temperature, pressure conditions, etc.