Study And Applications Of The Modified Embedded-atom Method Potentials For The Transition Metal Nitrides And Their Composites

Transition metal nitrides,such as TiN,VN and CrN,have been widely applied in coatings of cutting tools because of their remarkable properties,such as high chemical and thermal stability,superior wear and oxidation resistance and high hardness.Especially,the transition metal nitride nano-multilayered coatings,which are arranged by alternating lamellas of its two mononitrides,exhibit a higher hardness than that of the mononitride coatings and thus have been widely employed as coating materials for surface protection to extend tool life and improve machining performance.In the study of hardening mechanism of nano-multilayered coating,it has been found that there is a close relationship between atomic level behaviors and macroscopic properties of materials.However,the atomic level behaviors of the materials are rather difficlt to observe experimentally.Therefore,molecular dynamics(MD)simulation,which can deal with more than a million atoms,has been used to investigate the nano-multilayered coatings.The second nearest-neighbor modified embedded-atom method(2NN MEAM)potentials,as a semi-empirical interaction potential for molecular dynamics simulation,is believed to be highly applicable for the study of nanomaterials.In this disseration,the parameters and applications of 2NN MEAM potentials for transition metal nitrides and composite ternary systems are investigated systematically.The main work and conclusions are listed as follows:1)The second nearest-neighbor modified embedded-atom method(2NN MEAM)potentials for the transition metal nitrides TMNs(TM=Ti,V,Cr,Fe)binary systems are systematically investigated based on the formalism for a unary one,and a new regulation is proposed to determine the MEAM potential parameters for the binary systems.In this new regulation,the number of parameters to be optimized in the binary system is reduced from thirteen to two,which greatly simplifies the optimization process.To confirm the reliability of the present approach,the structural,elastic,surface and thermodynamic properties of TMNs are investigated using the new potential parameters.The obtained potentials can excellently reproduce the structural and thermodynamic properties of the TMNs.Moreover,the 2NN MEAM potential parameters for Ti-X(X=V,Cr and Fe)binary systems are investigated systematically according to the new regulation.And based on the 2NN MEAM potential parameters of the newly developed binary systems,the 2NN MEAM potential parameters for Ti-X-N(X=V,Cr and Fe)ternary systems are developed.According to the verification,the newly developed 2NN MEAM potential parameters are reliable.2)The MD simulations are performed to investigate the deformation and fracture behaviors of single crystal TMNs at different temperatures.It is found that the fracture strength of TiN,VN and CrN decreased with the increasing of temperature,while the fracture strain increases with the increasing of temperature.But for FeN,both the fracture stress and strain decrease with the increase of temperature.The analysis of the evolution during the uniaxial tensile process shows that the propagation of microcrack hindered by dislocation nucleation is the main mechanism of the plastic increaseing for single crystal TMNs.3)The MD simulations are performed to investigate the mechanical properties of TiN/VN composites.the effects of strain rate and temperature on the mechanical properties of TiN/VN composites are studied first.It is found that the strain rates have almost no effect on the Young's modulus but have a great effect on fracture strength and fracture strain.Whereas both the Young's modulus and the inelastic properties are sensitive to temperature.And we also find that the fracture of TiN/VN composite is a brittle fracture taking place in the VN layers.Moreover,the dislocation slip mechanism of TiN/VN composite is predicted and the generalized stacking fault energy(GSFE)curves of different slip systems are calculated using the newly developed potential parameters.We find that the {110}<110> slip system is the preferred of all possible slip systems for each slip location,indicating that the glide and nucleation of perfect dislocations should be the main dislocation mechanisms of TiN/VN composite.And we also find that the slips are most likely to occur on the VN side in TiN/VN composite.These simulations show that the fracture and dislocation slip of TiN/VN composite only occur in the VN layer and it is difficult to cross the interface into the TiN layer,indicating that dislocation blocking by layer interfaces is one of the main hardening mechanisms.