Fabrication of silicon nitride with MgO/MgF₂ using pressureless sintering

Because of its superior properties, that is high hardness and strength, excellent creep, oxidation and corrosion resistance as well as low density, silicon nitride is a candidate material for high temperature applications and is frequently used as cutting tools, a typical application for hard materials. In this study, various silicon nitride compositions were prepared using oxide and fluoride additives (e.g. MgO, Y₂O₃, MgF₂). After the green samples were obtained by manual pressing, the samples were cold isostatically pressed under 250 MPa. Cold pressed samples were pressureless sintered under nitrogen atmosphere in the range 1450-1700°C for 0.5-2.5 hour. Silicon nitride sintered with the additions of MgF₂ and Al₂O₃ gave better density and pore free micrographs especially at lower temperatures compared with the samples containing MgO plus Al₂O₃. Densities of samples were measured by using Archimedes principle and analyses of resulting products were carried out using SEM and X-ray diffraction.