Effect of a silicon carbide 'nano-dispersion' on the mechanical properties of silicon nitride

The hypothesis that a synergistic effect by fine SiC dispersoids operating on the submicrometer scale is capable of enhancing the deformation and fracture properties of Si3N4 ceramics has been examined. In order to single out the effect of the SiC dispersion from other microstructural factors affecting the material properties, experiments were conducted on a highly pure and dense Si3N4 material, suitable for basic investigations. Fracture mechanics and creep characterizations were performed at room temperature and at 1400°C on composites containing 25 vol% submicrometer SiC particles, for which the intragranular fraction was varied by changing the sintering conditions. Despite the obtained difference in composite microstructure, almost no improvement in either the fracture toughness or strength, as compared with the monolithic material, was found. Similarly, the slow crack growth and creep resistance at 1400°C were still dictated by the inherent properties of the matrix. This study emphasizes the need for scientific rather than empirical approaches on simple systems, before deducing general rules for the microstructural design of structural ceramics.