Sintering Behavior, Microstructure and Properties of Fine Silicon Nitride Ceramics

Abstract

Potential engineering applications of structural ceramics are greatly limited by their intrinsic brittleness, i.e. their low fracture toughness. With respect to Si3N4-based materials, the improvement of strength and toughness based on the development of “in situ” reinforced microstructures has been recently investigated,1–5 where elongated grains are grown in a fine-grained matrix: the resulting bimodal grain size distribution can improve fracture resistance and toughness. However, microstructural features such as grain size and aspect ratio, type and distribution of secondary phases, flaw population influence properties and performances. Thus the tailoring of microstructures, the optimisation of processing parameters and their correlations to resulting properties are of great interest. In Si3N4SiC composites and nanocomposites, an increase in hardness, strength and toughness can be obtained because of the effects of nanosize particles which pin and pile-up the dislocations creating subgrain boundaries within the matrix.6–10 Recent studies suggested that the incorporation of SiC particles into Si3N4 matrices influence densification, microstructure and properties.7–10 Furthermore Si3N4-SiC composites with microstructural refinement and improved sinterability were obtained with the addition of ultrafine Si/C/N powders to submicronic commercial Si3N4 powders.11–13 These materials, with homogeneous and fine microstructures, exhibited a plastic deformation, thereby predicting a useful behaviour for superplastic forming.9