ИСТИНА

Si3N4-SiC materials have considerable strength, Si3N4-SiC materials have good corrosion resistance, yet in many industrial applications the decay of N-SiC materials takes place due corrosion or oxidation. The corrosion resistance of refractories in different applications differs, while the priority of oxidation and corrosion resistance of Silicon Carbide above Silicon Nitride and visa versa leaves many questions. The compressive and crushing strength of Si3N4-SiC diminishes with decreasing amount of Silicon Nitride in the compositions, that is obvious, because the Silicon Nitride particles keep the particles of Silicon Carbide, and the porosity decreases as well. However there is no certain optimum of concentrations between Silicon carbide and Silicon Nitride, that gives the optimum properties. In Si3N4-SiC materials the porosity gradients appear due to the fact, that some part of Silicon evaporates due to exothermal effect in the middle of the shapes and moves to the edges the shapes, where reacts with Nitrogen. From 3 to 15-17 % of Silicon in the green mixture may react with Nitrogen in gaseous state. The origin of the porosity gradients in N-SiC materials is in the excessive heat in the middle of the shapes due to the exothermal effect, that give rise to the temperature gradients between the middle of the shapes and the edges. The difference in porosity may be from 0,5-1% to 6-8%. It looks like the decreased concentration of Silicon in the green mixture gives more opportunity for the release of the excessive heat from the center of the shapes, and the porosity gradients diminish as well. The fact, that in the reaction (1) Silicon reacts with nitrogen in solid, liquid and gaseous state, gives the variation of Silicon Nitride gradients along the height of the shapes and the variations of α/β ratios of Silicon Nitride in different parts of the material.