Rearch On The Preparation And The Rule Of Phase Formation Of Ti₃SiC₂

The layered ternary carbide,Ti₃SiC₂,has many excellent characteristics of both ceramic and metal,with the structure of hexagonal crystal.Up to now,many methods of preparation of Ti₃SiC₂ have been researched and used,such as chemical vapor deposition（CVD）,self-combustion synthesis reaction（SHS）,spark plasma sintering（SPS）,atmosphericpressurereactivesintering（RS）,reaction-melting infiltration sintering（RMI）,hot pressed sintering（HPS）,etc.However,high density and excellent mechanical properties Ti₃SiC₂ can only be pepared by RMI and HPS during the above methods.Meanwhile,the rule of phase formation is different for different kinds of method.In this article,atmospheric pressure reactive sintering,reaction-melting infiltration sintering,and hot-pressing sintering were used for preparation of Ti₃SiC₂.In order to provide theoretical guidance for obtaining Ti₃SiC₂materials with higher purity and better performance,the rule of Ti₃SiC₂ formation were studied through the analyses of Ti-Si-C ternary phase diagram and Ti-Si-C-Al quarternary phase diagram by software Pandat.First of all,Ti₃SiC₂ ceramic was synthesized by reaction-melting infiltration sintering from powders of Ti,Si and activated carbon.The results show that high density Ti₃SiC₂ materials can be synthesized with the optimal temperature of melting infiltration and raw material ratio.The optimal melting infiltration process is that preservation 60 minutes at 1500℃with 3Ti/0.2Si/2C in Argon atmosphere.In this way,the porosity is 1.02%.Combining the analyses of thermodynamic phase diagram by software Pandat with X-ray diffraction（XRD）and scanning electron microscope（SEM）results,the rule of phase transition during the process of reaction-melting infiltration was investigated.The results indicate that the formation of liquid phase in samples is beneficial to their densification during the process of sintering.The Ti₃SiC₂ content in samples increases with the Si content in performs,which can be explained as that Gibbs free energy of Ti-Si-C decreases with the increase of Si content during the infiltrating process.Ti-Si（L）comes from co-melting of Ti and Si with the increase of temperature during the process of reaction meling infiltration,which can be reacted with TiC and results in Ti₃SiC₂.Secondly,Ti₃SiC₂ was synthesized at atmospheric pressure with Ti/Si/C,TiC/Si/C/Ti,TiC/Si/Ti,TiC/SiC/Ti,Ti/SiC/C as raw materials,respectively.The influence of different raw materials of products on the rule of Ti₃SiC₂ was analyzed by the results of XRD,SEM and porosity.This research shows that under the joint action of thermodynamic conditions and kinetic reaction diffusion path,the different raw materials ratios affect the initiative reaction temperature and the de gree of the reaction.The longer is the chemical reaction path,the higher is the initiative formation temperature of Ti₃SiC₂ as well as the released reaction heat,which results in the more complete degree of the reaction.The content of Si has obvious influence on the producing of the Ti₃SiC₂.Within limits,the content of liquid phase increase with Si content,which is also propitious to the ternary reaction.The content of Ti₃SiC₂ obtained from Si is more compared to SiC used as the raw material,when S i is in the raw material,Ti and Si co-melting to Ti-Si（L）,nevertheless,when Si is replaced by SiC,content of Ti-Si（L）is limited by replacement reaction between Ti and SiC.Thirdly,influence of sintering pressure on the rule of phase formation was investigated.Ti₃SiC₂ was synthesized with TiC,SiC and Ti under different pressures（14 MPa and 25 MPa）by HPS.The results show that sintering pressure has obvious influence on the rule of phase formation.Within limits,the degree of the reaction and content of Ti₃SiC₂ significantly increases with the enlargement of the pressure,and higher pressure is in favor of densification of the sintered sample.Finally,the influence of aluminum（Al）on the phase formation and densification was studied.One can conclude from the Ti-Si-C-Al quaternary phase diagram that the optimal addition of Al is 0.2 mol,for ensuring that not only the content of Ti₃SiC₂ in the product is not lower than 90%,but also the content of liquid phase will increase and promote the reaction.The experimental results suggest that the content of Ti₃SiC₂ in the products were all increasing significantly after adding 0.2 mol Al into the raw materials.Meanwhile,the porosity of sintered sample obviously decreased.