Effect of Surface Oxidation of Si<SUB>3</SUB>N<SUB>4</SUB> Powder on Mechanical Properties and Microstructure of Sintered Products

Abstract

Si3N4 ceramics have strong covalent bonding, which makes them difficult to sinter. Highly densified Si3N4 ceramics are normally obtained through liquid phase sintering. As the liquid phase consists of sintering additives and surface impurity phase on the raw Si3N4 powder, the quantity of grain boundary phase depends on surface impurity oxygen phase and additives content. The effects of surface impurity oxygen content on mechanical properties, microstructures and sintering behavior are investigated. In this work, the surface oxygen contents is controlled by an oxidation or washing treatment. Assuming that all the oxygen atmos form SiO2, the SiO2 content ranges from 1.07 to 9.20 mass%. The sintering additives used were 3 mol% each of Al2O3 and Y2O3. The sintered bodies are obtained by hot-pressing at 1800°C for 60 min under 30 MPa in N2 atmosphere.The grain boundary crystalline phases were not found from 2.68 mass% to 4.68 mass%SiO2 content by X-ray diffraction analysis. While, sintered bodies from 1.07 mass% and above 6.39 mass%SiO2 powder, the grain boundary phases include crystalline phases. The fracture toughness increases with decreasing surface oxygen content. While, the bending strength decreases below 2.0 mass% and above 6.5 mass%SiO2 contents. The maximum values of fracture toughness and bending strength indicate 5.4 MN/m3⁄2 and 980 MPa, respectively. The grain shape changes from elongated to equiaxed with increasing surface oxygen content. This results in the decrease in fracture toughness and critical flaw size. The number of nuclei depends on the surface oxygen content in the raw Si3N4 powders. When the liquid phase content is low, e.g., using low surface oxygen content powder, the liquid phase content decreases, which leads the decrease in nucleation sites. Therefore, the grain shape changes from equiaxed to elongated grains with decreasing the number of nuclei. If the full dense sintered bodies can be obtained by less surface oxygen content Si3N4 powder, the β-Si3N4 grains should have a high aspect ratio. The high fracture toughness Si3N4 ceramics can be obtained by washing treatment.