Stefano Guicciardi

Effect of MoSi2 Particles on the Fracture Toughness of AlN-, SiC-, and Si3N4-based Ceramics

The effect of 30 vol% MoSi2 addition on the fracture toughness of AlN-, SiC-, and Si3N4-based composites is investigated. To isolate the contribution of MoSi2 particulate, the toughness of the composite is compared to that of the reference AlN, SiC, and Si 3N4 matrixes. It turns out that the main toughening mechanisms are residual stress and crack deflection. The residual stress is evaluated through the strain measure in the reinforcing particles by X-ray diffraction, and the crack deflection by analysis of the crack paths. The overall toughening effect is calculated as the sum of these two contributions compared to the experimental values of the toughness of the composites.

SINTERING BEHAVIOR, MICROSTRUCTURE AND PROPERTIES OF FINE SILICON NITRIDE CERAMICS

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

Analysis of nanoindentation tests on SiC-based ceramics

Two silicon carbide-based ceramics, with very different mean grain size, and a standard fused silica sample were characterized by nanoindentation. The values of hardness and Youngs modulus were measured at several peak loads and calculated using the models developed by Oliver and Pharr (O&P) and by Cheng et al. (C&C). For all the materials, the values of hardness and Youngs modulus were strongly dependent on the adopted model. In the silica specimen, the C&C Youngs modulus and hardness were lower than those calculated by the O&P model. In the SiC ceramics, the differences between the two models were both qualitative and quantitative. The C&C Youngs modulus values were lower than those calculated by the O&P model but the hardness values were higher. For most of the peak loads, the O&P model distinguished between the two SiC specimens while the C&C model did not.