Silicon powder compacts were reacted with nitrogen at 1185℃ and 1365℃. Nitridation rate (reaction rate), $\frac{\deltax}{\deltat}$ as the function of reaction time was calculated from the results of weight-gain. Microstructures of nitrided specimens were examined by optical microscope and scanning electron microscope at each stage of nitridation to discuss the effects of nitridation rate on developing the microstructure of reaction-bonded silicon nitride. Reaction products were analysed by X-ray diffraction method.
Calculated nitridation rate at 1365℃ was in the range from 21.4% to 61.7% per hour. The rate was almost 5-10 times larger than that at 1185℃. Alpha to beta ratio at 1365℃ decreased as the amount of reaction increased. On the contrary, at 1185℃, the ratio increased with the amount of reaction. For the specimens formed under the condition with rapid nitridation rate, large pores (20 - 50 ㎛) were produced and a distribution of pores was not homogeneous.
