Dense Reaction Infiltrated Silicon/Silicon Carbide Ceramics Derived from Wood based Composites

Abstract

The results of the joint project of the German Aerospace Center and the Technical University of Munich showed, that variation of resin content and bulk density of the wood based composite influences directly the microstructure and properties of the carbon preforms and resulting SiSiC ceramic materials and allow following conclusions: •The mass loss during carbonization can be effectively reduced by increasing the content of phenolic resin •Higher resin contents and higher bulk densities of wood based composites cause a lower bulk density reduction during carbonization •Higher resin contents of wood based composites lead to larger carbon aggregates and thus to more coarsely structured carbon preforms. The consequence is an increased residual carbon content of the resulting ceramic •The higher compression of the structure of wood based composites by increasing the bulk density is still obvious in the resulting carbon preforms and leads to smaller pore diameters. Excessive compression causes a deterioration of the pore interconnectivity and infiltration behaviour of the carbon preform •Higher bulk densities of the carbon preforms lead to an increased silicon carbide content as long as (1) the maximum theoretical density for the complete conversion of 0.963 g/cm³ is not exceeded, (2) a fine grained structure is retained and (3) a complete and even infiltration is possible. •The mechanical strength of the resulting ceramic depends on its structure and phase composition. The strength rises with increasing silicon carbide content and decreases with increasing residual silicon and carbon content as well as with higher porosity •The mechanical properties of SiSiC ceramics derived from wood based composites are promising and show a potential for industrial use. Based on the promising mechanical properties of TUM A1 and TUM A2 further investigations will focus on dense reaction infiltrated SiSiC ceramics with silicon carbide contents up to 90 Vol-% without exceeding a residual carbon content of 1 Vol-%. This means increasing successively the bulk density of the carbon preforms and simultaneously retaining a homogeneous and fine grained structure with a sufficiently interconnected pore system. This aim could be reached by the use of further optimized wood fibres in combination with moderate resin contents.