Study On The Structural Stabilities And Thermophysical Properties Of MAX Phases Nb₂AN（A=Si、Ge、Sn） By First Principles

MAX phase material is a new type of ceramic material with metal properties,which has great application prospects in the field of thermal barrier coatings.However,the MAX phase materials prepared at present have the disadvantages of high thermal conductivity,large coefficient of thermal expansion and low melting point.Therefore,how to design a new type of MAX phase material with excellent high temperature performance and stable structure has become the main problem in the development of new high temperature coatings.In this paper,the structural stabilities,mechanical and thermophysical properties of Nb₂AN（A=Si,Ge,Sn）compounds are systematically studied by using the first principle calculation method.Through the calculation of their crystal structure,mechanical properties and thermophysical properties,the optimal chemical components with low thermal conductivity,matching coefficient of thermal expansion and high melting point in Nb₂AN（A=Si,Ge,Sn）compounds are discussed.The effects of vacancy,antisite and interstitial defects on the crystal structure and thermophysical properties of Nb₂Sn N compounds were further explored,and the effects of the occupation distribution of Ti atom at M site on the properties of(Nb_1-xTi_x)₂Sn N MAX phase solid solution were analyzed.The results show that:（1）The results of binding energy,formation enthalpy,elastic constants and lattice dynamics show that Nb₂AN（A=Si,Ge,Sn）has good structural stability,mechanical stability and intrinsic stability.In particular,Nb₂Sn N performs the lowest anisotropy and the highest ductility among the three compounds,and the low Young’s modulus（E=142 GPa）means that it may have higher strain tolerance and better thermal shock resistance.Although the melting point（2793 K）of Nb₂Sn N is not the largest,it has the best thermal expansion coefficient matching performance and the lowest thermal conductivity with Ni-base alloy in 300～1452 K,which forces it to be an application prospect TBC material.（2）The positive defect formation energy of Nb₂Sn N compound shows that the intrinsic defects are not easy to produce independently,and are easier to form between MX layer and A atom layer,among which the defect formation energy of N atom entering the gaps is the lowest.The formation of vacancy defects will increase the elastic modulus and thermal conductivity of Nb₂Sn N compounds,while the formation of antisite defects will reduce the mechanical properties.Especially,the Young’s modulus is reduced by 14%when Nb _Nantisite defects are formed,but it may have better thermal shock resistance.Moreover,the generation of Nb _Snand Nb _Nantisite defects enhances the scattering effect of phonons and makes them have lower lattice thermal conductivity(3.87 W·m^-1·K^-1and 3.44 W·m^-1·K^-1,respectively).Nb₂Sn N compound can form stable self interstitial defects when N atoms enter the gaps between MX layer and A atomic layer,but it will significantly reduce its mechanical properties.Especially when N atoms occupy the gap of Sn atomic layer,the bulk modulus and Young’s modulus are reduced by 17%and 24%respectively,but it also possesses the lowest lattice thermal conductivity and minimum thermal conductivity(2.3 W·m^-1·K^-1and 0.54 W·m^-1·K^-1).（3）(Nb_1-xTi_x)₂Sn N（x=0,0.2,0.4,0.6,0.8,1）MAX phase solid solution has good structural stabilities.The mechanical properties also improve with the increase of x,and Ti₂Sn N performs the highest Young’s modulus（202 GPa）and the highest hardness（11.23 GPa）.However,the increase of elastic modulus means the decline of thermal shock resistance,Nb₂Sn N and(Nb_0.8Ti_0.2)₂Sn N may have better thermal shock resistance due to their low Young’s modulus（142 GPa and 153 GPa）.The thermal conductivity of(Nb_1-xTi_x)₂Sn N also increases with the increase of x.Among them,Nb₂Sn N and(Nb_0.8Ti_0.2)₂Sn N have lower lattice thermal conductivities in 100～1500 K.The change of Ti content will obviously affect its thermal expansion coefficient,and when x=0.2,it has a very similar thermal expansion coefficient(13.4×10^-6/K)to that of Ni-based superalloy.Therefore,(Nb_0.8Ti_0.2)₂Sn N is expected to become an excellent protective layer material of Ni-based alloy thermal barrier coating.