First-principles Calculations On The Electronic Structure And Bonding Nature Of TaN(111)/TiN(111)interface

Transition-metal nitrides and carbides have high thermal and chemical stability,high melting points,oxidation resistance,etc.Nano-multilayer films,which are made up of alternating monocarbides and mononitrides,exhibit higher hardness,better wear resisitance,higher oxidation resistance,and better corrosion comparing with nitrides and carbides films.These characteristics make the coatings attract considerable attention.All the excellent mechanical properties of multilayered coatings rely strictly on the electronic structure and the nature of bonding at interface.In practice,the interface is buried in the coatings,which renders the experimental processes in exploring the characteristics of the interface to be complicated and difficult.It is well known that the first-principles calculation could not only construct various interfacial configurations but also provide the information of electronic structures and bonding interactions of these interfaces.Thus the first-principles calculation method plays key role in understanding and improving the properties of multilayered coatings.In the present paper we have calculated electronic structure,and interfacial bonding nature of TaN?111?/TiN?111?interfacial systems employing the first-principles calculations method.Firstly,we had calculated the mechanical properties and the electronic structure of TiN and TaN.The values of the formation enthalpy of TiN and TaN with face-centred cubic structure are–2.46 eV and–3.85 eV,respectiyely.They are negative,which show that they are stable.The results of the elastic constants and elastic modulus indicate that both TiN and TaN are hard materials.The calculated hardness of TiN and TaN is 25.45,19.22 GPa,repectively.Our calculated results are in reasonable agreement with the experimental values.The bonding nature of TiN and TaN is mainly covalent character with certain ionic and metallic character based on the analysis of the charge density,the charge density difference,the density of states,and the Mulliken population.In addition,We had investigated the electronic structure and the bonding nature of TaN?111?/TiN?111?interfaces.To construct a TaN?111?/TiN?111?interface model,it is necessary to make sure that the Ta N?111?and TiN?111?sides of the interface are thick enough to exhibit bulk-like interiors.Therefore,we performed the convergence calculations on the surface energy with respect to the thicknesses of TaN?111?and TiN?111?slabs.The results showed that when the slabs contain more than 11 atom layers,TiN?111?and TaN?111?slabs are thick enough to exhibit bulk-like interiors.Considering two terminations?Ta/N and N/Ti?and three atom stacking sites?OT-,SL-,and TL-sites?,we calculate six possible candidate configurations.The calculated results of the work of adhesion showed that the interfacial systems are more stable when the atom stacking sequence at interface retains the order in the nitride bulks?TL site?.The W_ad values of Ta/N-and N/Ti-terminated TL stacking structures are 5.15and 5.95 J/m²,respectively.Then,we had analyzed the electronic structure and the bonding nature at interface.The results of charge density and charge density difference show that both Ta/N and N/Ti interfaces exhibit a mix of covalent and ionic bonds.It is observed that the covalent?ionic?bonding interaction between the interfacial Ta and N?N and Ti?in Ta/N?N/Ti?interface is evidently stronger than that in inner via analyzing the partial density of states and bond population.The bond length across the interface in N/Ti configuration is remarkably shorter than that of the TiN inner.This result,together with the relatively larger work of adhesion,may be factors contributing to the observed excellent mechanical properties in the TaN/TiN multilayered coatings.