The Phase Stability And Mechanical Properties Of Transition Metal Nitrides:first-principles Study

The transition metal nitrides(MoN,NbN,TaN)have been the focus of the researchers because of their excellent properties such as high hardness,high melting point,high chemical stability,wear resistance and corrosion resistance.In the industrial field,transition metal nitrides are mostly used as cutting tools,super wear-resistant materials,thermal barrier coating,but its poor oxidation resistance prevents its wide range of applications.It was found that Al doping could improve the oxidation resistance of transition metal nitrides,and the content of Al had a great influence on its crystal structure and phase stability.Therefore,it is very important to study the effect of Al doping on the stability and mechanical properties of transition metal nitrides.In this thesis,the first principles method based on density functional theory is employed to optimize the structure parameters of Mo1-xAlxN,Nb1-xAlxN and Ta1-xAlxN(x = 0,0.25,0.50,0.75,1.0)in the four structures(B1,B4,WC,NiAs).By calculating the formation enthalpy,the stability of Mo1-xAlxN,Nb1-xAlxN and Ta1-xAlxN is determined according to the corresponding criterion.The elastic modulus and hardness are calculated to analyze the change trend of various physical quantities in the whole interval.The G/B value and Poisson's ratio are also calculated to determine the brittleness of the systems.In addition,the correlation between hardness and brittleness is illustrated.Finally,the electronic structure of MoAlN,NbAlN and TaAlN are investigated to analyze the nature of the trend of changes in the hardness.The results show that Mo1-xAlxN,Nb1-xAlxN and Ta1-xAlxN compounds undergo the phase changes with the addition of Al.When the concentration of Al reaches 0.436,the most stable structure of Mo1-xAlxN changes from WC phase to Ni As phase and when the concentration of Al exceeds 0.577,there will be a phase transition from NiAs phase to B4 phase.The most stable structures of Nb1-xAlxN and Ta1-xAlxN are transformed from WC phase to mixed phases when Al element reaches 0.469 and 0.5,respectively.When Al content exceeds 0.6,their most stable structure changes to B4 structure.In the aspects of mechanical properties,the elastic modulus of the material is calculated to obtain the hardness and bonding information of the systems.With the addition of Al element,the hardness of Mo1-xAlxN material first increases to the maximum and then decreases,which has a great relationship with the structure of the material.The hardness of Nb1-xAlxN and Ta1-xAlxN first decrease and then increase with the increase of Al content.The calculations of G/B and Poisson's ratio indicate that the change trend of material hardness has a certain relationship with the directionality of covalent bond and the strength of metal bond.Finally,through the calculation of the density of states(DOS)and partial density of states(PDOS),the orbital hybridization of each atom and the electronic filling condition near the Fermi level are discussed.Combined with the trend of phase stability and hardness to analyze,the micro-mechanism of phase stability and hardness is obtained.The calculation of the charge density difference is used to further analyze the bond formation of the material which is consistent with the result of the DOS.