According to the production method, silicon nitride products can be divided into reaction sintering products, hot pressing products, atmospheric sintering products, isostatic pressing products, and reaction re-firing products. Among them, reaction sintering is a widely used method for the production of refractory silicon nitride products.
The method of reaction sintering makes it possible to obtain products from silicon nitride by pressing or isostatic pressing of finely ground silicon powder (the particle size usually does not exceed 80 μm), drying the green body, heating it to 1350-1400°C in nitrogen and nitriding in the process of roasting. In this production method, the raw material conditions, roasting process, and atmosphere have a great influence on the performance of the product.
Silicon powder contains many impurities, such as Fe, Ca, Al, Ti, etc. Fe is considered a catalyst in the reaction process. It can promote the diffusion of silicon, but at the same time, it can cause defects such as pores. The main role of Fe as an additive: it can act as a catalyst in the reaction process, promoting the formation of a SiO2 oxide film on the surface of the product; form an iron-silicon melt system, and nitrogen dissolves in liquid FeSi2, contributing to the formation of β-Si3N4. However, if the iron particles are too large or too high, defects such as pores appear in the product, which reduces its performance. Generally, the amount of iron added is 0-5%. Impurities such as Al, Ca and Ti easily form low eutectics with silicon. An appropriate amount of additives can promote sintering and improve product performance.
The finer the particle size of the silicon powder, the larger the specific surface area and the lower the firing temperature. Compared with larger silicon powder, finer silicon powder contains more α-Si3N4 in the product. Reducing the particle size of silicon powder can reduce the microscopic pore size in the product. An appropriate particle size ratio can increase the density of the product.
Temperature has a great influence on the nitriding rate. The nitriding reaction starts at a temperature of 970-1000°C, and the reaction rate is accelerated at a temperature of about 1250°C. In the high temperature stage, because it is an exothermic reaction, if the temperature quickly exceeds the melting point of silicon (1420°C), silicon flow may occur, and strong silicon powder blanks will melt and collapse.
