Study On The Synthesis And Properties Of High-entropy Carbide Ceramics

High-entropy materials are the class of single-phase solid solutions that are formed by five or more elements in equal or nearly equal atomic ratios,including high-entropy alloys,high-entropy ceramics,high-entropy intermetallic compounds,etc.which are new type of materials that have gradually developed in recent years.High-entropy ceramics include high-entropy oxides,high-entropy borides,high-entropy carbides,high-entropy silicides and polyanionic high-entropy ceramics.Compared with single-component transition metal carbide ceramics,the mechanical properties,oxidation resistance,high temperature performance,and radiation resistance of high-entropy carbide ceramics have been improved different degrees.Therefore,high-entropy carbide ceramics（HECs）is considered as a candidate material for structural components in extreme environments.At the same time,the designability on structure and property of HECs further determines its wide application potential.Although the current research work on HECs has made certain progress,there are still many problems to be solved in the component design and properties improvement.For example,firstly,whether the relevant laws and effects in high-entropy alloys are applicaple to HECs.Secongly,the unified criterion for the formation of single-phase HECs is not really clear.Thirdly,whether the systhesis methods of transition metal carbide ceramics are suitable for HECs.Fourthly,how to adjust the microstructure and properties of HECs by optimizing the preparation process.Fifthly,the internal relationship between the composition,microstructure and properties of HECs is not yet clear.Sixthly,how do defects（such as lattice carbon vacancies and oxygen impurities）and the added second phase affect the properties of HECs.Aim at above problems,this paper focuses on the following items:relationship between the preparation process,microstructure and properties of HECs;the strengthening mechanism of added second phase on the properties of HECs;and correlation of lattice defects with mechanical properties and thermal conductivity etc.These systematic research results can provide theoretical guidance for the component design,properties and performance improvement of HECs.Relative density is one of the most important factors which will affect the properties of ceramics.Similar to single-component transition metal carbide ceramics,HECs is difficult to sinter and densify,too.The relative density of HECs obtained by blending and sintering of several carbide powders is generally lower than 95%,which seriously affects its mechanical properties and thermal conductivity.Aim at above problems,it is necessary to improve the relative density of HECs by modifying the preparation process.Different raw materials and processes are used to synthesis single-phase(Ti_0.2Zr_0.2Nb_0.2Ta_0.2W_0.2)C ceramics,inclunding element method（HEC-E,via 5 metal powders and graphite powder）,carbide method（HEC-C,via 5 carbide powders）and oxide method（HEC-O,via 5 oxide powders and graphite powder）.Although the obtained three specimens demonstrate a very similar XRD pattern of single-phase HECs,the relative density,microstructures and oxygen content of these 3 samples are much different.Among them,the relative density of the samples of HEC-C,HEC-E and HEC-O is 95.7%,98.2%and 99.9%,respectively.The element distribution of HEC-C is homogeneous.In HEC-E sample,the particle size of the raw metal powder is an important factor which will affect the distribution of metal elements in the sintered ceramics.In HEC-O,the distribution of Zr and O is not uniform due to the high reaction temperature of Zr O₂.In addition,the oxygen content of the HEC-O sample is 1.80 wt.%.The research results show that the reaction sintering processes of single-component carbide ceramics are also suitable for HEC.And the densification behavior,phase composition and microscopic morphology of HEC can be adjusted by adjusting preparation method.This research also provides different process methods and routes for the preparation of HECs.Although the relative density of(Ti_0.2Zr_0.2Nb_0.2Ta_0.2W_0.2)C prepared by the oxide process（HEC-O sample）is as high as 99.9%,the distribution of Zr and O elements is not uniform.Aiming at this inhomogeneous elements distribution,(Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C ceramics was successfully synthesized by oxides and graphite via oxide method.The obtained ceramics has a gradient content of the residual graphite,gradient grain size of the HECs phase,as well as gradient hardness.The results show that the structure of HEC phase is FCC,and the distribution of the five metal elements and the C is uniform.In addition,there is residual graphite in the prepared(Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C ceramics,and the content of residual graphite decreases gradually from the center to the edge area of the sample.The generation mechanism of this kind of gradient material was further explored,and the results show that the existence of axial pressure during SPS sintering caused the internal gas pressure of the sample（gradient decreasing from center to the edge）,which further affected the sintering process and chemical reaction.The chemical reaction ultimately leads to a higher content of residual graphite in the central part,a smaller grain size of HEC phase,and finally the HEC material with gradient microstructure and hardness was prepared.The research results further reveal the microstructure characteristics of the HEC material which obtained by the oxide process,and also show that the graphite second phase can effectively refine the grain size of the HEC phase.There are obvious differences in the element distribution and microstructure of oxide method prepared(Ti_0.2Zr_0.2Nb_0.2Ta_0.2W_0.2)C and(Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C ceramics,indicate that the component and content of elements in HEC will affect its microstructure,but the specific mechanism and relationship are still unclear.Therefore,different（Ti_xZr_xHf_yNb_xTa_xW_x）C ceramics were synthesized by the same oxide method in order to analyse the relationship between the elements content and the phase composition,lattice parameters and microstructure of HEC.The results show that with the increase molar content of Zr and Hf,the lattice parameter of the high-entropy phase gradually increases.The addention of different molar content of Hf into the(Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C system can effectively decrease the oxygen content of the material with homogeneous elements distribution.The research results show that by adjusting the components and contents of the elements of HEC,the mixing entropy of the system can be adjusted,and finally we can adjust the microstructure of HEC.Although it is found that graphite second phase can refine the grains of HEC,the effect of graphite on the material properties is still unclear.Similar to carbide ceramics,the fracture toughness of HEC is very low.In addition,the thermal conductivity of HEC is lower than that of single-component carbide ceramics due to the phonon scattering which was caused by lattice distortion.Aiming at these problems,single-phase FCC structured(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C based ceramic composites with different volume contents of graphite were prepared using the self-prepared HEC powders and graphite via SPS.The results indicate that graphite improves the relative density and increases the lattice parameters of HECs by removing oxide impurities.Graphite phase refines the grain size of HECs phase.The indentation toughness of the composite ceramics increases significantly with the increase of graphite content.The fracture energy absorption caused by crack deflection and new crack initiation at the graphite flakes is considered to be the main toughening mechanism.The thermal conductivity results indicate that graphite decreases the oxygen content and effectively improves the thermal conductivity of high-entropy carbide ceramic.Among them,the room temperature thermal conductivity of HEC2（the content of the added graphite is 2 vol%）is about 19 W·m^-1·K^-1,which is 1.58 times of the pure HEC without graphite addition.The results show that the effect of the added graphite on the properties of HEC is consistent with that of carbide ceramics.The results also provide new ideas for reducing the oxygen content and improving thermal conductivity of non-oxide high-entropy ceramics.Group IV and V transition metal carbides usually have a wide range of non-stoichiometric ratios,and carbon vacancies will significantly affect the densification,mechanical properties and thermal conductivity of ceramic materials.However,the relationship between carbon vacancies,microstructure and properties of HEC is still unclear.Therefore,it is necessary to carry out research on non-stoichiometric high-entropy transition metal carbides.The prepared(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C powders were used as raw materials to prepare different non-stoichiometric high-entropy carbide ceramics(HEC_1-x)with different elements components and contents by adjusting the molar content of Hf.The results indicate that the added Hf powders enter the crystal lattice of the high-entropy carbide phase and single-phase HEC_1-x ceramics were synthesised.The lattice parameter of HEC_1-x ceramics is closely related to the elemental components and contents.The addition of Hf can effectively reduce the sintering temperature of HEC_1-x and increase the relative density of HEC_1-x samples.In all HEC_1-x samples,the distribution of 5 metals and C is uniform.The oxygen content results also show that the content of O in the ceramics increases with the decreases of the C/Me ratio.In addition,the Vickers hardness,elastic modulus,fracture toughness and thermal conductivity of HEC_1-x ceramics decrease with the decrease of the C/Me ratio.This means that the increase content of lattice defects such as carbon vacancies and oxygen impurities have a greater impact on the mechanical properties and thermal conductivity of HEC ceramics.