Effect of Mechanical Milling Treatment and Reaction Temperature on the Fabrication of Silicon Nitride

Silicon nitride has attracted a considerable attention because of its excellent properties such as high-temperature strength، good oxidation resistance، high corrosion resistance، good thermal shock resistance، high creep resistance and good thermal and chemical stability. There are several different fabrication methods for synthesizing Si3N4 particles. Such methods are mostly costly and kinetically slow and require lengthy heat treatment. In this study، Si3N4 compounds were synthesized by means of mechanical milling. In the mechanical milling route،Si powder (≤99. 0%) was milled under nitrogen gas for 25 h and heated at various temperatures 1100-1200-1300 and 1400 C for 1 h at the nitrogen atmosphere at a rate of 200 ml/min. Silicon powder was also annealed under a similar condition in order to evaluate the impact of milling process on the low temperature synthesizing of Si3N4. Phase identification and microstructural characteristics of products were evaluated by X-ray diffraction، scanning electron microscopy and energy dispersive spectroscopy. The Fourier transform infrared spectroscopy and thermal analysis were used for characterization of the formed bands and thermal treatment of the sample، respectively. The obtained results exhibited that Si3N4 powder was fully formed with two kinds of morphologies including globular particles and wire with a width of 100–300 nm and length of several microns at sintering temperature of 1300 C. This was confirmed by the Si–N absorption bonds in the FTIR trace. Based on XRD results، 25 h milling reduced temperature of reaction remarkably in comparison with direct nitridation of Si powders for 1 h. With an increase in the reaction temperature، the Si3N4 samples had a phase transformation α→β، and variation of the morphology followed the vapor–liquid -solid mechanism.