Dense SiSiC Ceramics Derived from Different Wood-Based Composites: Processing, Microstructure and Properties

Kurzfassung

The manufacture of dense SiSiC ceramics from commercial wood-based composites (WBC) via liquid silicon infiltration has been developed in the last years. The process is based on the silicon infiltration of manufactured carbon preforms. An advantage of the carbonization of WBC for the manufacture of SiSiC ceramics is the simple axial pressing process. It is possible to create homogeneous, isotropic and reproducible WBC on the basis of fine grain powders. Fine microstructures and phase compositions of the SiSiC ceramics can be achieved by using particle sizes less than 50 mu-m and additives or by varying the type of wood. So, in a first step different wood powders were homogeneously mixed with phenolic resin, carbon powder and carbon fibers as additives and uni-axially thermal pressed into WBC with the dimensions of 140 x 140 x 10 mm3. After drying up to 110oC the WBC were carbonized up to 1650°C (pyrolysis) to obtain porous carbon preforms for the final siliconization which occurs above the melting point of silicon (greater than 1420°C). The use of C-additives in the WBC results in a reduction of the shrinkage behavior during pyrolysis. Furthermore, it is possible to improve the damage tolerance and the ductility of the SiSiC ceramics. The aim of this study is to create damage tolerant SiC ceramics which can be used for lightweight structures for the application within optical apparatus with high stiffness and low thermal expansion. In this paper the process itself as well as the mechanical, some thermo physical properties of the dense SiSiC ceramics derived from WBC are reported. The microstructure was examined with light microscopy and SEM. The bending strength and fracture behavior, carried out by the three-point short beam flexural test, was correlated with the structure as well as the composition and distribution of the phases.