Microstructure Evolution Study Of TiCN-based Ceramic Materials During Sintering

TiCN ceramic materials are widely used in making high-speed cutting tool because of their high red hardness,wear resistance and oxidation resistance.However,most of the researches have been focused on Ti(C,N)-based cermets at home and abroad,while the research of TiCN ceramic materials has been ignored significantly so far.The strength,oxidation resistance and wear resistance of cermets are greatly reduced due to the presence of metals such as Ni and Co.Therefore,it is necessary to make a comprehensive study on TiCN ceramic materials.In order to reduce the sintering temperature and improve the densification process,TiCN ceramics and TiCN-based cermets were studied respectively.The microstructural evolutions upon the sintering process were investigated in detail,including solid phase sintering,liquid phase sintering and transient liquid phase sintering.Consequently,the research results can serve to guide the produce of excellent performance TiCN ceramic materials.Firstly,YAG has been chosen as additive to prepare TiCN composite ceramic by hot pressing sintering process.The microstructure evolution of TiCN ceramic materials during solid phase sintering process was studied by means of field scanning microscope,XRD analysis,grains size and density test.The results show that YAG does not react with TiCN and can bond well with TiCN grains.It can inhibit the grain growth of TiCN ceramics,significantly reduce the sintering temperature of TiCN ceramics,and promote the densification of ceramic sintering.During the study of the solid phase sintering mechanism of TiCN ceramic materials,it is also found that the grain growth during the solid phase sintering of TiCN ceramic materials depends on the migration of grain boundaries,the reaction between pores and grain boundaries,and the grain growth process is mainly dominated by surface diffusion and grain boundary diffusion mechanisms.These two sintering mechanisms lead to the formation of submicron particles and pores in the intragranular structure and intergranular structure during grain growth of YAG-TiCN composite ceramic matrix.With the addition of 3 wt%YAG,YAG-TiCN composite ceramics can be fully densified at a lower temperature of 1450°C,with a relative density of 99.81%.Secondly,the Nb2O5 has been chosen as additive to prepare TiCN composite ceramic by hot pressing sintering process because of its low melting point.The same technical method as the solid phase sintering process was used to study the microstructural evolution of TiCN ceramic material in the liquid phase sintering process.The results show that Nb2O5 is an effective sintering aid for TiCN composite ceramics,which can reduce the sintering temperature,inhibit the grain growth,and promote the densification of TiCN ceramic materials.The densification process is divided into two stages.The first stage is the solid phase sintering.The solid solution plays a major role in promoting the densification of the TiCN ceramic material.The second stage is the liquid phase sintering.The TiCN ceramic material particle rearrangement was accelerated after the liquid phase appears.The dissolution-precipitation-reprecipitation densification mechanism plays a major role in the second densification stage.Even the addition of 1 wt%of Nb2O5 has shown a significant promoting effect on the densification and grain refinement of TiCN composite ceramics.When the Nb2O5 addition increases to 2 wt%,a relative density of 99.6%can be obtained at a sintering temperature of 1450? under a hot press sintering process.Thirdly,LiYO2 was also selected as a sintering aid for the study of the microstructure evolution process of TiCN ceramic materials during transient liquid phase sintering in this paper.The results show that LiYO2 is also a good sintering aid.The amount of liquid phase and sintering temperature are two main factors determining the densification behavior of transient liquid phase sintering of TiCN ceramic materials.The TiCN composite ceramic can be densified when the LiYO2 content is 6 wt%and the sintering temperature is 1400?.Finally,the microstructure of Ti(C,N)-based cermets sintering process is studied in three aspects.(1)The results of research on 60%Ti(C0.5,N0.5)-15%Co-13%Mo-12%WC cermet by vacuum sintering method show that the sintering shrinkage behavior of the ceramic green compact of Ti(C,N)-based cermets is as follows:the compact begins to expand between 1000? and 1100?,then shrinkage again,and sharp shrinkage occurs at 1300?.The inner phase phase is formed during solid phase sintering,and the outer rim phase is formed during liquid phase sintering.The main evolution diagram is drawn according to the evolution law of the microstructure.(2)The master sintering curve of Ti(C,N)-based cermet was studied by high temperature thermal dilatometer.The densification process of cermet was studied in master sintering curve.The results show that the apparent activation energy of Ti(C,N)-based cermets is 485 KJ/mol.The master sintering curves can be used to accurately predict densification behavior of ceramic,including relative density and the shrinkage,during sintering process.(3)Preliminary computer simulation has been made to simulate the sintering process.The simulation is visually processed through the selective input of grain orientation number,lattice subdivision number,Hamiltonian energy change and other control processes,output grain growth rate,grain size change and pore and grain boundary evolution.It was found that the grain grows with the increase of the simulation time,tends to be metastable at about 225 MCS,and the grain growth begins to slow down thereafter.The overall number of grains decreases with increasing simulation time,while the average grain size increases significantly,small grains become smaller or are swallowed,and large grains continue to grow.This is in good agreement with the phenomenon of particle diffusion during the sintering process of Ti(C,N)-based cermet,precipitation in the liquid phase and precipitation on the surface of large particles after supersaturation.The simulation results in this study can be a foundation for the sintering simulation of Ti(C,N)-based cermet in the future.