Investigations Of Synthesis And Formation Mechanism Of Ti₃SiC₂ Ceramic By Self-Propagation High-Temperature Sintering

Ti₃SiC₂ combines unusual properties of both metals and ceramics. Like metals, it is a good thermal and electrical conductor with relatively low Vickers hardness and high elasticity modulus. Like ceramics, it has high melting point, high bend strength, high thermal stability and good oxidation resistance. It exhibits excellent corrosion resistance. Unlike conventional ceramics, it can be machined using high-speed steel knife tools. Additionally, Ti₃SiC₂ possesses ultralow friction coefficient, which is lower than that of MoS2 and graphite. These excellent properties mentioned above make it possess broad application foreground becoming important object in investigating new materials and attracting increasing interest. To date, many methods have been used for synthesizing Ti₃SiC₂, such as chemical-vapor deposition, self-propagation high temperature reaction synthesis, hot pressing and hot-isostatic pressing. Although high purity Ti₃SiC₂ can be fabricated by these methods, the sintering temperature is high limiting its quantization production. Self-propagation high temperature sintering (SHS) is a method with low cost, simple process and convenience operation, which benefits quantization production of Ti₃SiC₂. Nowadays, reaction thermodynamic and kinetics for synthesizing Ti₃SiC₂ hasn't been extensively investigated. It is difficult to explain many experimental phenomena. Therefore, the reaction mechanism of synthesizing Ti₃SiC₂ by SHS is needed to be researched. In this study, taking 3Ti-Si-2C system as the research object, Ti₃SiC₂ material was synthesized by SHS. Reaction thermodynamic and kinetics for synthesizing Ti₃SiC₂ were investigated. The objective is to control SHS parameters synthesizing Ti₃SiC₂ with high content.Effect of ignition method, raw materials ratio, raw materials'granularity, raw materials species, aids and compact density on the changing rate of Ti₃SiC₂ synthesized by SHS was investigated using X-ray diffraction (XRD) phase analysis and semi-quantitative analysis with Scanning electron microscopy (SEM) morphology observation. The results showed that different SHS products were obtained using different ignition method. Both products obtained by plasma and laser induced SHS have basically the same phase composition and content. The products were consisted of Ti₃SiC₂, Ti5Si3 and TiC. The Ti₃SiC₂ content in the products was about 23%. Two kinds of products obtained by mechanical induced SHS were powder and granule. These products were consisted of TiC, Ti₃SiC₂, TiSi2 and Ti5Si3. Ti₃SiC₂ content of the granule was much higher than that of the powder. Changing Ti and Si powders granularity did not enhance the synthesis of Ti₃SiC₂, but inhibited its synthesis. Graphite granularity had obvious effect on the content synthesis of Ti₃SiC₂. The finer graphite significantly enhanced the content synthesis of Ti₃SiC₂. The raw materials species had an important impact on the content synthesis of Ti₃SiC₂. Ti₃SiC₂ content in the SHS products of Ti/Si/2TiC and 3Ti/SiC/C powders was much higher than that of 3Ti-Si-2C. The aids had important effect on the content synthesis of Ti₃SiC₂. Adding TiC, SiC and Sn could enhance the content synthesis of Ti₃SiC₂ to a certain extent. Al, AlN and Al2O3 had obvious effects on its synthesis. The raw materials ratio and compact density had minor effect.Adiabatic temperature and Gibbs free energy of different phase's formation in Ti-Si-C system as a function of temperature were computed using thermodynamic law. The results showed that stability sequence in the Ti-Si-C system was Ti5Si3, Ti₃SiC₂,β-SiC, TiC, TiSi, TiSi2,α-SiC at different temperatures. The adiabatic temperature of Ti₃SiC₂ during SHS was about 2906 K.One new flame quenching method for investigating SHS kinetics-spark plasma sintering (SPS) method was proposed. Using this method, the products obtained at different temperatures were distinguished. SHS of 3Ti-Si-2C, 3Ti-SiC-C and Ti-Si-2TiC was systematically researched by this method. The finer graphite enhanced the synthesis of Ti₃SiC₂. When TiC was directly induced, for example, Ti-Si-2TiC was used as the raw materials, Ti₃SiC₂ yields were obviously improved. The metallographic specimen was prepared easily and observed by SEM.Reaction kinetics behaviors and microstructures evolution of 3Ti-Si-2C green body was extensively investigated using traditional combustion flame quenching method and SPS quenching method. The formation mechanism of Ti₃SiC₂ was definite. The results showed that the reaction mechanism for synthesizing Ti₃SiC₂ was solution-reaction-precipitation. The size of graphite in the raw materials had a significant effect on the reaction synthesis of Ti₃SiC₂. Coarse graphite was harmful for forming Ti₃SiC₂. The finer the graphite granularity, the higher convert ratio of Ti₃SiC₂.