Preparation And Properties Of （TiZrNbMoW）B₂-SiC High Entropy Ceramic Composites

High-entropy ceramics are a kind of new ceramic materials which are composed of five or more ceramic components with similar near/equal molar ratio structure and in the form of solid solution.Among them,group IV to VI metal boride high entropy ceramics have unique hexagonal crystal structure,showing many advantages such as high strength,high hardness,high melting point,etc.,which has broad application prospects in aerospace and other fields.At present,the research on high entropy boride ceramics is mainly in the initial stage,and researchers have developed a variety of preparation methods suitable for high entropy ceramics.However,due to the slow solid diffusion of ceramic materials,there are some problems such as high preparation temperature and oxide residue,which seriously limit its popularization and application.In this paper,high-entropy boride multiphase ceramic materials are synthesized by sintering pre-alloyed metal solid solution powder and nonmetal powder through solid phase reaction.Firstly,high entropy multiphase ceramics（TiZrNbMoW）B₂-Si C were prepared by alloying Ti,Zr,Nb,Mo and W transition metal powders.The temperature needed to prepare high entropy ceramics was reduced by in-situ reaction synthesis.At the same time,the introduction of Si C second phase into the material product can improve the intrinsic problem of poor oxidation resistance of pure boride ceramics in the middle temperature range（1200°C～1550°C）.The main research contents and results of this paper are as follows:（1）Based on the empirical formula of mixing enthalpy,mixing entropy and valence electron concentration,the possibility of solid solution formation of Ti,Zr,Nb,Mo and W metal powders was predicted.The pre-alloyed metal powders were prepared by mechanical alloying method,and it was found by XRD that the metal solid solution with single BCC structure was synthesized by the five metals after ball milling at 350 r/min for 48 h.The results were consistent with the empirical formula.It was also found that the order of solid solution of metal elements was closely related to the melting point.（2）High-entropy multiphase ceramic materials were prepared by spark plasma sintering at 1600℃,1700℃and 1800℃.The microstructure,phase composition and element distribution of the sintered products were characterized by SEM,XRD and EPMA point composition analysis.The results show that the sintered product contains（TiZrNbMoW）B₂,（W,Mo）B and Si C phases.Through the analysis of density,mechanical properties and microstructure,it was found that with the increase of temperature,the density decreased,and the mechanical properties also decreased,which was due to the phenomenon of tip discharge in discharge plasma sintering,which resulted in local high temperature and rapid evaporation of Si,resulting in the phenomenon of over-burning,which affected the properties of the material.（3）The high-entropy multiphase ceramic materials were successfully prepared by hot pressing sintering at 1650℃.The main phases are high entropy（TiZrNbMoW）B₂,Si C and a small amount of（W,Mo）B₂,which proves the feasibility of the new idea proposed in this paper.The multiphase ceramics with 31%vol and 45%vol were prepared by adjusting the composition ratio of raw materials,and their mechanical properties and oxidation resistance were tested.The test results show that the introduction of appropriate Si C phase can enhance the mechanical properties of high entropy boride ceramics and improve the oxidation resistance of high entropy boride ceramics in the middle temperature region,which is expected to solve the problem of rapid oxidation failure of high entropy boride ceramics in the middle temperature region.