Effects Of Combined Addition Of Silicon Nitride And Silicon Carbide On Properties Of Mullite Refractory

Refractory is an indispensable basic material for high temperature technology. It relates to the pillar industries of steel, building materials, non-ferrous metal smelting, chemical machinery, and a number of national economy. As a kind of traditional refractory material, mullite refractories have a high degree of fire resistance, low thermal expansion coefficient, good thermal shock stability, excellent high temperature creep resistance, high wear resistance and peeling resistance, corrosion resistance and a series of good performance. The change of its spatial structure of [SiO4] and [AlO4] caused by the change of the Al/Si ratio can lead to different degrees of periodic oxygen vacancies in the crystal structure, which makes the space between the lattice more, leads to the strength of mullite material is low, affects its application in industrial production. Silicon nitride and silicon carbide are important high temperature resistant hard materials, using them as a reinforcing phase for the preparation of mullite based composites can improve the comprehensive performance of mullite composite materials significantly.In this paper, the composite refractories were prepared by using mullite, andalusite, silicon carbide, silicon nitride as the main raw material, silica powder as binder, PVA as binder, adjusting the addition ratio of silicon nitride and silicon carbide under 1450℃ for 400 minutes. The apparent porosity, bulk density, cold compressive strength, flexural strength, high temperature heating line change rate, load softening temperature of the composite materials were tested.The microstructure and phase composition of the materials were studied by means of X-ray diffraction, scanning electron microscope, X-ray spectrometer. Observe the changes of the materials in the process of thermal shock, test the apparent porosity, bulk density and cold compressive strength of the specimens. The thermal shock resistance of the composite was evaluated with a variety of testing methods.The results showed that the apparent porosity of the materials remained at about 15%, and the volume density was about 2.45 g/cm3, the materials produced with different degrees of expansion, the high temperature flexural strength and softening temperature have reached a high level, can reach 29.8MPa and 1560℃, basically not affected by the addition of silicon nitride and silicon carbide. But the normal temperature compressive strength decreased with the increase of silicon nitride contentThe silicon nitride and silicon carbide in the materials were oxidized in the sintering process, and form a layer of brown oxidation products on the surface of the materials, the oxidation products was mainly glass phase. Dense layer appeared in the cross section of the composite refractory materials.The thickness of the dense layer of the materials decreased with the increase of silicon nitride content; There was a large number of fibers in the composite refractory material. They were formed by the reaction of the SiO with oxygen in the outward diffusion process of the SiO. The SiO was the oxidation of silicon carbide and silicon nitride under high temperature and low oxygen partial pressure.The silicon nitride can increase the thermal shock resistance of the composite refractory materials, and with the increase of silicon nitride content, the thermal shock resistance of the composites was better. When all the silicon carbide in the formula were replaced by silicon nitride, the compressive strength of the composite after ten times of the thermal shock can reach 84%.