Solid-phase sintering of sintered silicon carbide at atmospheric pressure

         The sintering of silicon carbide ceramic containing C and B4C elements as sintering aids is solid-phase sintering, and the sintering process is mainly controlled by the diffusion mechanism, with an optimum sintering temperature of 2150°C. The sintering process is simple and easy to control. Add the appropriate content of C + B4C sintering additives of sintered silicon carbide sintering process is simple and easy to control, ceramic sintering compared to the billet has about 30% volume shrinkage, you can get a higher density, mechanical properties of silicon carbide special ceramics. At present, the commonly used sintering additives are B4C + C, BN + C, BP (boron phosphide) + C, AI + C, AIN + C and so on. Add the appropriate content of C + B4C SiC pressureless sintering process, the process for this kind sintered sic is simple, easy to control, the material density is higher, the maximum density of 3.169/cm3 (relative density of 98.75%); mechanical properties are better, the maximum compressive strength of 550MPa.
         Silicon carbide raw material is preferably D50 value of 0.5 – 0.8 micron single micropowder. Usually it is chemically treated green silicon carbide microns with a specific surface area of 20 m3/g. And the oxygen content should be as low as possible; furthermore, the amount of B added should be chosen to be around 0.5% – 1.5%, while the amount of C added depends on the level of oxygen content in the SiC powder. Chemical composition SIC>99%, F-C<0.1, Si+SiO2<0.1, Fe2O3<0.08. Particle shape and size composition, the particle shape is nearly spherical in order to achieve the most compact stacking.
        The addition of B4C and C belongs to the category of solid-phase sintering, which requires higher sintering temperatures.SiC sintering driving force is: the difference between the surface energy of the powder particles (Eb) and the wobble surface of the grains of polycrystalline sintered body (Es), which leads to a decrease in the free energy of the system. Doped with an appropriate amount of B4C, B4C is on the SiC grain boundary during sintering, partially forming a solid solution with SiC, thus reducing the grain boundary capacity of SiC. The doping of a moderate amount of free C is beneficial to solid-phase sintering because the SiC surface is usually oxidized, resulting in the generation of a small amount of Si02, and the addition of a moderate amount of C helps to make the reduction of the Si02 film on the SiC surface be removed, thus increasing the surface energy Eb.
         The SiC system undergoes decomposition and sublimation at 1.013x105Pa and a temperature greater than 1880°C. The SiC system contains gas phases such as Si, Si2, Si3, C, C2, C3, C4, C5, SiC, Si2C, SiC2, and so on, and the temperature difference is the fundamental driver of sublimation process during the growth of SiC crystals, and the whole process is dominated by the mass transport. These various gas phases in the SiC system coalesce on the SiC crystal mother by diffusion, leading to the growth of SiC crystal particles. For the samples of the C+B4C sintering aid system, the required sintering temperature is higher due to the predominantly solid-phase sintering, and argon is passed in as a protective atmosphere at about 1300 °C, because argon is favorable for reducing the decomposition of SiC at high temperatures above 1300 °C. Measure the quality of SiC sintered body has two necessary conditions: low porosity as dense as possible; the grain as small as possible.

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