A Study On Mechanism Of Self-propagating High-temperature Synthesis Of Ti₃SiC₂ Machinable Ceramic

Ti₃SiC₂ machinable ceramics has characteristics of both metals and ceramics. Like metals, it is a good thermal and electrical conductor, high modulus and readily machinable. Like ceramics, it has low density, a low thermal expansion coeffcient, high strength, high melting point and high thermal stability. In addition, it has lower attrition coeffient and better performance of self-lubricate than M0S2 and graphite, and its application has wide foreground. Because of its unique advantages, the self-propagating high-temperature synthesis (SHS) has become a novel techlogy of producing materials. The Ti₃SiC₂ ceramic can be prepared by SHS method. However, the investigations of mechanism of Ti₃SiC₂ by SHS are still lack. Therefore, studying the SHS mechanism of Ti₃SiC₂ material is of either scientific significance or engineering applied importance.In this paper, a microstructural evolution and mechanism of self-propagating high-temperature synthesis (SHS) of Ti₃SiC₂ machinable ceramic were studied by means of the combustion front quenching method. A process of microstructure conversion in the quenched sample was observed using a scanning electron microscope (SEM), a compositional change in each micro-region of the microstructure was studied by energy dispersive X-ray (EDX), the temperature of combustion reaction was measured, the phase constituents of the product were inspected by X-ray diffraction (XRD) and chemical reactions during the SHS were investigated by differential thermal analysis (DTA). Accordingly the mechananism of combustion synthesis reaction of Ti-C-Si system was discussed and studied.The results show that the mechananism of combustion synthesis of Ti₃SiC₂ ceramic can be described as dissolution-preciptation. Ti-Si melt is formed through sold diffusion between titanium and silicon powders, then the particles of Ti, C and Si gradually dissolve into the Ti-Si melt, and TiC, SiC particles will prepicitate from the Ti-Si-C melt as the Ti-Si-C melt is saturated. With the increase of reaction temperature, the final production (Ti₃SiC₂) is resulted by the solid-liquid reaction between TiC ,SiC and remanent melted Ti.Both the microstructure and the XRD patterm also show that the combustion synthesis reaction of Ti-C-Si system is incomplete because the residual phase TiC and SiC are in the combustion-synthesized product Ti₃SiC₂. The incompleteness mainly results from coarse Ti and Si powders (135～154μm)because the adiabatic temperature (T_ad) of Ti₃SiC₂ is higher. The coarser raw powders, the lower observed combustion temperature, and the coarser raw powders, the longer diffusion distance of atoms. Both low temperature and long diffusion are not beneficial in completeness of combustion synthesis. So the intermediate phases are remained in the final products. In addition, it was nonideal in the adiabatic course of the practical SHS, and the loss of heat quantity in combustion is one of the reasons which arose incompleteness combustion reaction .