Recent progress in the engineering of long fibre reinforced ceramic matrix composites (CMC) fabricated by hot pressing has led to the production of materials with high density and good mechanical properties. However, hot pressing limits the components to simple plate shapes, which often require very expensive machining. Pressureless sintering of CMC preforms to near net shape is a preferred alternative, greatly reducing costs and allowing the formation of complex component geometry.
However, there are major technical difficulties. Densification by infiltration of vapour or liquid precursors followed by pyrolysis, leads to unacceptable porosity (poor mechanical properties / no gas tightness) and very high cost. Densification from compacted powders is only possible at high temperatures, risking deleterious chemical interactions between the different constituents of the composite. Also, the achievement of a fully dense matrix in a composite manufactured by pressureless sintering is very difficult. The sintering/densification step requires the movement of partially liquefied ceramic particles to allow matrix shrinkage, while the fibre preforms are rigid, constraining shrinkage. Selection of more glassy, more mobile matrix phases assists densification but detracts from the high temperature mechanical properties of the product composite. Effective densification of a high temperature CMC has been achieved to date, only by the application of an external, mechanical pressure (hot-pressing).
The present work looks to improve the densification processing of a silicon nitride matrix composite reinforced with continuous carbon. Conventional low pressure processing requires extended heat treatment at temperatures up to 1850°C. This study has focussed on matrix phase sintering, principally to explore the ability of novel sinter additives to promote the synthesis of lower melting intergranular phases in-situ, thereby promoting matrix phase sintering at lower temperatures. This is targeted to reduce interphase reactions and to enhance matrix phase mobility at the densification temperature, while retaining the high temperature properties of the final ceramic. The primary goal was to identify sinter aid compositions which allow densification of the candidate matrix phases at lower temperatures by liquid phase sintering to greater than 95% Theoretical Density (TD). SI3N4 based ceramics were prepared containing sintering additive combinations of the form; Y2O3 + AI2O3 , Y2O3 + AI2O3 + AIN, Y2O3 + AI2O3 + CaZrC>3 .
The sintering behaviour, chemical stability, micro structural and phase development, and final properties, including hardness and oxidation of the sintered samples were examined.
Selected compositions were then used in the fabrication of a carbon fibre reinforced ceramic matrix composite by pressureless sintering, gas-pressure sintering and hotpressing. The sinterability and compatibility of the densified composites were analysed and simple mechanical testing / microstructural analysis carried out in order to evaluate the degree of “composite” behaviour of the material subjected to mechanical deformation and fracture.
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