| At present silicon nitride ceramics are widely used in a variety of industrial fields for their superior mechanical properties, such as high strength, good thermal shock resistance, and high oxidation and corrosion resistance. However, it has recently been shown that porous beta-Si3N4 can be engineered to provide similar mechanical properties to dense beta-Si 3N4, with the additional benefit of lower mass. Flexural strengths exceeding 1 GPa can be achieved by tailoring the microstructure of the inter-locking beta-Si3N4 grains to have moderately high aspect ratios after sintering (up to 10:1), while maintaining porosity levels in excess of 10%. Porous beta-Si3N4 ceramics have many potentially useful engineering applications, including: high temperature gas filters, catalyst supports and membranes, bioreactor substrates, as well as light weight structural applications. While fully dense silicon nitride has a density between 3.2 and 3.4 g/cm3, depending on the sintering aid composition and content, porous materials have densities typically ranging from 1.5 to 2.8 g/cm3.;In the present work, porous Si3N4 has been produced using Y2O3 as a sintering additive, with graphite powder as a fugitive pore forming agent. Y2O3 promotes minimal densification, but enhances the alpha- to (beta-Si3N4 transformation. While graphite filler was used to produce micro-scale porosity in the green-body, it was subsequently removed by low temperature oxidation. Samples were then sintered between 1500 and 1750°C. Characterization of materials has been explored to analyze the microstructural development and grain morphology. Finally, biaxial strength of the material was determined as a function of porosity. In the present study, for up to 5 wt. % graphite content, the samples have high biaxial strength (up to 180 MPa). With increasing further graphite content, up to wt. 10 %, the strength drops significantly. This was observed to arise due to agglomeration of the graphite particles, creating large pores which are responsible for lowering the strength.;In order to increase the toughness and damage tolerance of the ceramic materials, there is a huge potential from making a fiber reinforced porous matrix ceramic composite. In the present study, electrophoretic deposition has been used to make porous silicon nitride ceramic matrix composites, reinforced by woven fine carbon fiber. Severe reaction occurs between the carbon fiber and the Si3N4 during sintering. This reaction has been reduced by lowering the sintering temperature from 1750°C to 1650°C, although samples are still brittle. Although the sample has not achieved the desirable strength for mechanical testing, the potential benefits of such a strong, low-mass material for engineering applications can be easily appreciated. |
