Sintering-Free Technology And Properties Of Mechanical Formed Si₃N₄/SiC Refractories

Si3N4/SiC composite is a promising material for its excellent high-temperature mechanical properties, good thermal stability and corrosion resistance. It has been widely used in metallurgical industry, such as electrolytic aluminum. However, the widespread application of Si3N4/SiC composite has been restricted by the problem of high cost, complex process and higher energy cost. This paper studied sintering-free technology of Si3N4/SiC refractories, which is used in electrolytic aluminum industry. The self-sintering in the aluminum environment at800℃can be achieved when the materials were treated via mechanical molding and low-temperature drying, which thus reduce the production cost.The industrial SiC, Si3N4and clay were used as raw materials. The present work studied particle size distribution and dry-compression molding process, discussed bending strength, thermal shock resistance, oxidation resistance and corrosion resistance after being sintered at800℃(simulating the temperature used in the aluminum tank.) in the oxidation atmosphere, and then analyzed the phase composition, fracture and oxidation mechanism in static air via X-ray diffraction, scanning electron microscopy and thermodynamic analysis. The study is the basis of the sintering-free technology of Si3N4/SiC refractories.The experimental results showed that:(1) during the mechanical forming process, the main factors that affected the density were the particle size distribution, pressure and the amount of added clay. The main phases consisted of SiC, Si3N4, SiO2and Al2O3.(2) when the content of0.3～1.7mm SiC was50%,0.125～0.6mm SiC was22%,60μm SiC was14%,1μm SiC was7%,70μm Si3N4was7%and additional70μm clay was5%, the density of the obtained Si3N4/SiC was2.752g/cm3at400MPa, density after drying was2.733g/cm3at300℃and density after sintering was2.718g/cm3at800℃. Meanwhile, the bending strength of refractories reached24MPa.(3) When the content of0.3～1.7mm SiC was50%,0.125～0.6mm SiC was20%,60μm SiC was10%,1μm SiC was5%,70μm Si3N4was14%and additional 70μm clay was5%, the thermal shock resistance and corrosion resistance of the Si3N4/SiC showed excellent properties. The thermal shock was more than25times, and the growth rate of mass was1.17%after eroding in cryolite and molten aluminum for48h. Meanwhile, the oxidation weight was13.1mg per square centimeter after oxidation at1200℃for8h and the oxidation activated energy was62kJ/mol.