Preparation And Mechanism Of Ti-Al-C Ternary Layered Ceramic Powders By Combustion Synthesis

Combustion synthesis is a method to synthesize materials by relying on the exotherm of the reaction itself and making the whole combustion process spontaneous and continuous.It has obvious advantages of low cost,environmental friendliness,easy batch production,and high product purity.As a typical representative of MAX phase,Ti-Al-C ternary layered ceramics have a wide range of applications,people focus on the mechanical properties and applications of in situ synthesized materials,but the study of the formation mechanism has been neglected.In this study,Ti-Al-C lamellar ceramics are taken as the research object,the thermodynamic stability of Ti-Al-C lamellar ceramics and intermediate phases were calculated by thermodynamics,and high purity Ti₂AlC powder was prepared by combustion synthesis.The combustion wave quenching was used to study the kinetic process of the system.In view of the difficulty of stopping the reaction of the high exothermic system,a method of doping diluent Cu to reduce the high exothermic reaction is proposed,the effects of doping with different contents of Cu on the adiabatic temperature,phase composition and microstructure of the synthesized products were studied.In this paper,the Φ function method is used for the first time to calculate the thermodynamic parameters of Ti-Al-C system,by calculatingΔG_T^θfunction of different intermediate phases,the trend of reaction occurrence and the stability of the product are analyzed.Then,the regression analysis method is used to convert the data table into the binomial form of the Gibbs free energy of the standard reaction.According to the calculation ofΦ-function method,the predicted intermetallic compounds formed in the Ti-Al-C system are:Al₄C₃,Ti C,Ti Al₃,Ti Al.Based on the values of Gibbs free energy function,it is known that their thermodynamic stability is Ti C,Al₄C₃,Ti Al₃,Ti Alfrom the highest to the lowest.Ti C is the most stable binary phase in the reaction system.In the reaction process,other phases such as Ti-Alintermetallic compounds have the tendency to combine with C to form more stable Ti C,and the Gibbs free energy decreases with the increase of temperature.The Gibbs free energy of the formed compounds in the whole system is from small to large as Ti₃AlC₂<Ti₃AlC<Ti₂AlC<Ti C<Al₄C₃<Ti Al₃<Ti Al.The Ti₃AlC₂ phase is more easily generated at high temperature,and if the generation of Ti₂AlC needs to be controlled,it can be achieved by controlling the temperature and the molar ratio of the reactants.The effects of different combustion atmospheres,Sn doping and different ratios on the phase composition of the synthesized products were investigated.The results showed that the product purity was higher under argon atmosphere compared with vacuum conditions,indicating that argon is more advantageous for the preparation of Ti₂AlC ceramic powders by combustion synthesis.The effect of adding the sintering aid Sn on the composition of the products shows that the addition of the right amount of Sn helps to obtain the single-phase Ti₂AlC,while the addition of excessive Sn makes the Ti₂AlC peak intensity decrease and tends to form Ti₃AlC₂.Ti₂AlC was prepared by selecting different molar ratios of Ti:Al:C.The main crystalline phase of the products obtained from all raw material ratios was Ti₂AlC,and other phases such as Ti C,Ti₃AlC₂and Ti Alwere also included.The effect of increasing Alcontent in the system is more obvious for the synthesized products in the carbon deficiency ratio.The specimens prepared by 2.9:2.1:1 had the least impurities compared to the other two ratios,and the morphological features were regular and complete and more fully developed.The effects of different Cu ratios on the product phase composition were studied,and Ti-Al-C-30 wt%Cu was selected for quenching experiments to analyze the effects of Cu content in the reactants on the T_ad,reaction process,product phase composition and microstructure of Ti₂AlC synthesized by combustion of Ti-Al-C system.The results showed that the Cu content should not exceed 47.25 wt%to make the reaction of Ti₂AlC self-sustainable,with the addition of Cu,the Ti₂AlC in the system decreased significantly,which proved that the addition of Cu would inhibit the growth of Ti₂AlC.In the combustion synthesis process of Ti₂AlC,the reaction process is as follows:at low temperature,Alwith low melting point melts first.With the increase of temperature,Aland Ti react to form intermetallic compounds,after the temperature rises,they will melt again and react with Ti C in the system to form Ti₂AlC.The reaction between Cu,Al,Ti occurs to form Al-Cu,Cu-Al-Ti reaction,which inhibits the direct growth of Ti C,Ti_xAl_y,thus suppressing the growth trend of Ti₂AlC.