Effect Of TiC_x/VC_x/NbC_x/WC_x/TiB_x/TiN_x On Structure And Properties Of High Entropy Nitride Ceramics

The transitional metal nitride has ionic,covalent and metallic bonds at the same time,with high hardness and strength,high melting point,high conductivity,excellent thermal conductivity and thermal and chemical stability,and is widely used in cutting tools,wear-resistant coatings,electrocatalysis,energy storage,super capacitors and other fields.In recent years,the research on high entropy ceramics has set off an upsurge.The researchers mainly increase the configurational entropy of high entropy ceramics by expanding metal atoms,but the contribution of anions to configurational entropy and valence electron concentration is also indispensable.In this paper,by adding different non-stoichiometric compounds,introducing vacancy defects and polyanion structure to high entropy nitride ceramics,the configurational entropy of high entropy ceramics system is increased in order to achieve toughening effect and reduce sintering temperature.High entropy nitride ceramics with polyanion structure were prepared by mechanical alloying and discharge plasma sintering.The effects of sintering temperature,holding time,vacancy concentration and anion type on the structure and properties of high entropy nitride ceramics were investigated by XRD,SEM,EDS and Vickers hardness tester.The experimental results show that the metal atoms V and W with small atomic radius and the different nonmetallic atoms N,C and B have begun to diffuse with each other through mechanical alloying during the ball milling process of 20h.During the sintering of SPS,under the action of thermal driving and vacancy driving,all kinds of metal atoms diffuse and replace with each other,and the crystal lattice changes.When the concentration of each atom reaches equilibrium state,the diffusion is completed,and the lattice constant is close to the average value of the lattice constant of the raw material compound.The segregation of Nb and Ta elements exists in the sintered ceramics of various series of high entropy,which is related to the interdiffusion ability of various metal atoms,among which Ta element has the weakest interdiffusion ability.In Ti B_1.8-TMN series of high entropy ceramics,Ta will combine with B to form Ta B,and hexagonal boron nituse with laminate structure appears.The series of high entropy nitride ceramics are easy to oxidize and form（Zr,Hf）O_x impurities at high temperature.Therefore,in addition to the characteristic peak of FCC solid solution of high entropy ceramics,there are also diffraction peaks of oxide impurities at low diffraction angles in the XRD pattern.It is observed that the oxides precipitate mainly in the grain boundary of the high entropy nitride ceramics,and the size is small（about 300 nm）,which can play the role of"napping",so as to improve the fracture toughness of the high entropy nitride ceramics.The addition of different non-stoichiometric compounds has different effects on the structure and properties of high entropy nitride ceramics.The addition of compounds with different non-stoichiometric ratios has different effects on the structure and properties of high entropy nitride ceramics.The best toughening effects are VC_0.5 and WC_0.5,and the best hardness enhancement effects are Nb C_0.5 and VC_0.5.In Ti C_0.4-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramics prepared at 1700℃for 20 minutes had good mechanical properties with hardness of 20.8 GPa and fracture toughness of 6.0 M·^1/2.In the VC_0.5-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramics prepared at 1700℃for 20 minutes had good mechanical properties,the hardness is 21.8GPa and the fracture toughness is 6.1 M·^1/2.In the Nb C_0.5-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramics prepared at 1800℃for 20 minutes had good mechanical properties,the hardness is 22.0 GPa and the fracture toughness is 6.1 M·^1/2.In the WC_0.5-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramics prepared at 1700℃for 15 minutes had good mechanical properties,the hardness is 19.7 GPa and the fracture toughness is 7.3 M·^1/2.In Ti B_1.8-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramics prepared at 1800℃for 20 minutes had good mechanical properties with hardness of 20.9 GPa and fracture toughness of M·^1/2.In Ti N_0.3-TMN high entropy ceramic system,the equal molar ratio high entropy nitride ceramic prepared at 1700℃for 15 minutes had good mechanical properties,hardness of 19.5 GPa and fracture toughness of 5.9 M·^1/2.