First-principles Study Of Electronic Structure And Mechanical Properties Of TiC_1-xN_x

The crystal structure,mechanical properties and electronic structure of TiC_1-xN_x were investigated by the first-principles calculations.TiC_1-xN_x coatings were synthesized by reactive plasma spraying,and the hardness and elastic modulus were measured by nano indentation.Theoretical calculations and experiment anlaysis were combined to analyze the influence of N on TiC_1-xN_x,providing theoretical and technological support for the TiC_1-xN_x structure design and process optimization.TiC_1-xN_x models were established to study the effect of N on the crystal structure and formation energy.The results showed that the equilibrium lattice constants and cell volume of TiC_1-xN_x exhibit a nearly linear increase with the increase of N content.The caculated negative formation energy and cohesive energy indicated that they were all energetically favorable to form and thermodynamically stable.Besides,TiN is the most easy to form,and its thermodynamic stability is also to the best.The elastic modulus of TiC_1-xN_x was calculated to study its mechanical properties.The results showed that the bulk modulus B increased with the increase of N content,whereas the shear modulus G and the modulus of elasticity of E increased first and then decreased.The results of calculated hardness showed that the hardness decreased in the following order:TiC_0.5N_0.5>TiC_0.7N_0.3>TiC_0.2N_0.8>TiC>TiN,and the hardness of TiC_0.5N_0.5is the highest,reaching 31.23 GPa.The analysis of B/G,B/C₄₄,Cauchy pressure and Poisson's ratio?revealed that TiN has good toughness,the brittleness of TiC is the most,and the brittleness of TiCN is between the TiC and TiN.The degree of deviation of Zener's anisotropy parameter A'decreased in the following order:TiN>TiC>TiC_0.5N_0.5>TiC_0.2N_0.8>TiC_0.7N_0.3,and TiN has obvious anisotropy.The density of states?DOS?and charge density of TiC_1-xN_x were calculated to analyze the influence of bond type and bond interaction on its mechanical properties.The results showed that there are three kinds of bonding types in TiC_1-xN_x.The covalent bond is dominated by Ti-C and Ti-N bonds,and there are also a certain amount of C-N bonds,Ti-C and Ti-N ion bonds,and Ti-Ti metal bonds.The doping of N changed the interaction force between metallic d layer rail electrons and non-metallic 2p orbital electrons,and it also changed the force between the d layer electron of the metal bonds which further affected the hardness and toughness of TiC_1-xN_x.With the increase of N content,more electrons were contained in E_F and the increase of the density of states led to the enhancement of the metal bond Tid-Tid,then the toughness was enhanced but the hardness decreased.The transformation of Ti-C into Ti-N bonds led to the reduction of covalently and the enhancement of ionicity,the increase of hardness and the decrease of toughness.The results of XRD showed that the phase structure in the TiCN coatings was related to the carbon source.Based on graphite or carbon black as carbon source,the main phase of coatings were TiC_0.7N_0.3 and TiC_0.3N_0.7,respectively.With the reduction of C,the main phase of coatings transform to TiC_0.2N_0.8.The lattice constant calculated by XRD is similar to the calculated values,and the difference is less than 2%,indicating that the model is reliable.XPS analysis showed that there are Ti-C,Ti-N,C-C and C-N bonds in the coatings,which is in accordance with the theoretical calculation of the electronic structure and bonding analysis.There is a certain deviation between the hardness and the elastic modulus of the coatings and the calculated results according to the presence of crystal defects in real crystal and micro defects in coatings,but the change trend is consistent,indicating that the model and method used in this paper is reliable and the research has guiding significance for preparing TiCN coatings.