Phase Formation Mechanism And Mechanical Properties Of(MgCoNiCuZn)O High Entropy Ceramics Fabricated By Field Assisted Sintering Technology

High-entropy ceramics(HEC),or entropy-stabilized ceramics,are new materials that have only emerged in recent years.Since the system contains a variety of components,the degree of chaos within the entire ceramic system is increased,and the configuration entropy of the system is also increased.Driven by this high entropy in the system,it facilitates the mixing of a variety of metal-nonmetal compounds to form a solid solution phase with a simple crystal structure.Due to its special phase structure and microstructure,high-entropy ceramics have great potential in basic scientific research or development potential and practical application.At present,the research on high-entropy ceramics is still in its infancy,and people are constantly exploring new high-entropy ceramic systems,while constantly exploring the impact of its special structure on performance.In this paper,five kinds of metal oxides,MgO,CoO,NiO,CuO and ZnO,were used as raw materials to prepare high-entropy ceramics.Plasma activated sintering technology was applied to obtain dense bulk(MgCoNiCuZn)O high-entropy ceramic.HEC samples were characterized by XRD,SEM,FESEM,RAMAN and EDS to study phase evolution process and the phase formation mechanism.The bending strength,elastic modulus,and Vickers hardness were also investigated.The dispersion effects of adding dispersion agent and dispersing medium were investigated.The research shows that the PEG/MEK combination used as dispersion agent and dispersing medium,respectively,in the ball milling and drying process has a good dispersion effect on the oxide ceramic powder,which can effectively prevent the coagulation and delamination.The uniformly dispersed ceramic powder helps to obtain a denser structure and reduce the generation of voids.In the paper,the(MgCoNiCuZn)O high entropy ceramics were prepared by field assisted sintering(FAST)under vacuum conditions.The effects of different sintering conditions on the phase composition and microstructure of(MgCoNiCuZn)O were investigated.The results show that the high-entropy ceramics with dense structure and single phase(MgCoNiCuZn)O can be obtained at 800-1000 °C by field-assisted sintering process with axial pressure of 30 MPa and holding time of 3 min.The formation process of(MgCoNiCuZn)O single phase structure and its phase formation mechanism were studied by X-ray diffraction pattern of(MgCoNiCuZn)O at different sintering temperatures.The results show that the process of forming a single phase of(MgCoNiCuZn)O is an orderly process.The(MgCoNiCuZn)O is dissolved into a new lattice structure in a certain order from 650 °C,and finally forms a single-phase solid solution structure.Oxides with the same rock salt phase structure will be first mixed to form a new rock salt phase structure,followed by non-rock salt phase oxides will diffuse into the incoming rock salt phase structure,eventually forming a single rock salt phase structure.The study also shows that the phase transition process from multiphase to single phase is controlled by the self-diffusion rate and inter-diffusion rate of metal elements.The lower the vacancy formation energy of metal,the faster the rate of self-diffusion and inter-diffusion,and the easier to form a single phase.Through TEM analysis,we observed acicular defects and cascade defects caused by Cu segregation.At the same time,by measuring and calculating the lattice spacing and lattice parameters of the single-phase lattice,the single-phase(MgCoNiCuZn)O high-entropy ceramics have severe distortion.These distortions cause large internal stresses in the structure and affect its mechanical properties.In this experiment,the mechanical properties of(MgCoNiCuZn)O high entropy ceramics were characterized,including flexural strength and elastic modulus,and hardness test.According to the results,the flexural strength and the elastic modulus first increase and then decrease with the increase of the density,and the maximum values are 323±19 MPa and 108±5 GPa at the density of 95.6 %.The hardness of ceramic samples decreases first and then increases with the increase of density,and has a negative correlation with the flexural strength.The maximum value can reach 775±15 Hv when the density of the high-entropy ceramic sample is 99.3 % closing to full densification with heating treating 0.5 h.