The effect of the grain boundary phase on the fracture toughness of beta-silicon nitride ceramics

The mechanical properties of silicon nitride are controlled by the shape and size of the silicon nitride grains and the properties of the grain boundary phase. Although the effects of the grain size and shape on the mechanical properties have been extensively studied, the influence of the grain boundary phase on the mechanical properties has received little attention.;In this thesis, the effect of the grain boundary phase on the fracture toughness of silicon nitride was determined. To produce a wide range of grain boundary phases, a variety of sintering aids were added to the silicon nitride, including combinations of cordierite (Mg;The sintering aids reacted with the silicon nitride to form a sintering liquid, which produced grain boundary phases when cooled. The fracture toughness of silicon nitride ceramics with different grain boundary phases was measured using Vicker's indentation, and was strongly affected by the properties of the grain boundary phase.;The mechanical properties of the grain boundary phases were studied using model glasses with compositions similar to those of the grain boundary phases. The mechanical properties of the oxynitride glasses, including the Young's moduli, fracture toughness and thermal expansion coefficients were measured.;The thermal expansion coefficient of the grain boundary phase determined the crack propagation path in the silicon nitride ceramic. When the thermal expansion coefficient of the grain boundary phase was large, the fracture path was intergranular and the material was relatively tough. Conversely, when the thermal expansion coefficient of the grain boundary phase was small, the fracture path was intragranular and the material was brittle.;Thermal expansion mismatch between the silicon nitride grains and the grain boundary phases produces residual stresses when the ceramic is cooled. The residual stresses which developed in the silicon nitride grains were measured using x-ray and neutron diffraction.;This thesis demonstrates that the grain boundary phase strongly influences the fracture toughness of silicon nitride ceramics by providing a weak path for crack deflection when the grain boundary is under tensile residual stress.