Preparation And Mechanical Properties Of (Ti_0.2Zr_0.2V_0.2Nb_0.2Ta_0.2)(C_0.6N_0.4)-Based High-entropy Cermets

Ti（C,N）-based cermets have good wear resistance,high-temperature red hardness,thermal stability and oxidation resistance,and are ideal materials for high-speed cutting tools.However,due to the existence of its micro"core-rim"structure,the number of material sub-interfaces increases,which easily leads to the initiation and propagation of cracks,limiting its further application.In order to weaken the influence of"core-rim"structure,(Ti_0.2Zr_0.2v_0.2Nb_0.2Ta_0.2)(C_0.6N_0.4)-Co high-entropy cermets were designed and prepared by two-step method,that is,high-quality single-phase high-entropy ceramic powders was prepared by carbothermic reduction nitridation from metal oxides,and then mixed with Co,and its high-entropy cermets were controlled by pressureless sintering.The synthesis mechanism of the powders and the influence of process parameters on the phase and morphology of the powders were systematically studied,and the relationship between the densification process of the cermet and the structure and properties was explored.The main progress are as follows:（1）High-entropy ceramic powders were prepared by carbothermal reduction nitriding method,and the reaction sequence,phase and morphology evolution during the synthesis process were studied.The specific phase evolution follows the following rules:（Ti O₂,Zr O₂,V₂O₅,Nb₂O₅,Ta₂O₅）→（Zr O₂,Ta O₂,Ta₂O₅,Nb₂O₅,Ti₃O₅）→（Zr O₂,Ta₂（C,N）,Nb C,Ta C）→（Zr N,（Ti Zr VNb Ta）（CN））→（Ti Zr VNb Ta）（CN）,the morphology gradually evolved from fine and loose aggregates to nearly spherical particles with uniform particle size.Finally,high-entropy ceramic powders with good solid solution and uniform distribution of elements were synthesized at temperature above 1500°C.（2）Increasing the sintering temperature and increasing the holding time can promote the solid solution of the system,and the holding time is too long to cause the abnormal growth of the ceramic grains.The addition of nitrogen promotes the reaction process,and controlling the nitrogen flow rate and the amount of carbon in the raw material can effectively regulate the C/N atomic ratio of the product.When the final nitrogen flow rate was 0.6 L/min and the carbon content was 19 wt.%,(Ti_0.2Zr_0.2v_0.2Nb_0.2Ta_0.2)(C_0.6N_0.4)powders with a C/N atomic ratio of 6:4 were prepared at 1600°C for 2 h.,the elements of the prepared powders are uniformly distributed on the nanoscale,the average particle size is～0.8μm.（3）(Ti_0.2Zr_0.2v_0.2Nb_0.2Ta_0.2)(C_0.6N_0.4)-Co cermets were prepared by pressureless sintering,the relationship between the densification process and the structure properties were studied.Studies have shown that the densification process is mainly divided into the bonding stage in the early sintering stage,the sintering neck growth stage,and the closed cell spheroidization and shrinkage stage.Appropriately increasing the sintering temperature or increasing the holding time can effectively increase the densification of the sintered body and promote the uniform distribution of the metal phase.High-entropy cermets with dense structure,uniform grain distribution and best performance was obtained by heat preservation and sintering at1500°C for 2 hours.The porosity was only 0.08%,and the hardness and fracture toughness were 15.72 GPa and 12.9 MPa·m^1/2,respectively.