Ceramics-Silikáty 35, (2) 127 - 145 (1991)

α-silicon nitride containing 35-37 wt. % nitrogen, 3-4 wt. % oxygen and 1 wt. % carbon, with a mean particle size of about 1 μm, was prepared by heating the mixture of finely dispersed silica and carbon at 1 500 °C for 7-8 hours in nitrogen flowing at a rate of 40-50 cm³/min (the flow rate related to 1 g of SiO₂ in the initial mixture). α-silicon nitride is formed by precipitation from gaseous SiO, N₂ and CO. The positive effect of seeding crystals of α-silicon nitride on the reaction rate, the particle size of the product and on its morphology increases with the increasing surface area of the seeding powder. A ceramic material exhibiting a fracture toughness of about 6 MPa m^1/2, which can be further increased by an addition of β-silicon nitride, was prepared by hot pressing of the silicon nitride powder with additions of yttrium oxide and aluminium oxide. Gutting tools8 made by grinding the resultant material were tested by intensive machining of cast iron and showed wearing comparable to that of cutting tools manufactured commercially abroad.