Microstructure and phase morphology of wood derived biomorphous SiSiC-ceramics

Biomorphous SiSiC-ceramics were prepared by spontaneous Si-melt infiltration of beech and pine wood derived biocarbon templates (C-B-templates) at 1550 degreesC for 1 h. The microstructure, phase distribution and interface morphology of the anisotropically structured, microcellular SiSiC-ceramic composite were investigated by X-ray diffraction (XRD) as well as by light scanning and, in detail, by transmission electron microscopy (LM/SEM/TEM). The biomorphous SiSiC-ceramics consist of three different phases: solidified Si in the cell lumina, few residual carbon islands located in the middle of the former wood cell walls and two different, reaction-formed SiC-morphologies. While the majority of the SiC-phase exhibits a grain diameter of a few microns, a second nano-grained SiC-phase was identified at the interface between the coarse-grained SiC-phase and the residual biocarbon. The occurrence of the nano-grained SiC-phase is related to the density and pore size of the initial C-B-template. While in large pores and narrow carbon struts (low density earlywood) of the C-B-template an excess of Si-melt yields the formation of mainly the coarse-grained SiC-phase, in latewood sections (small pores and thick C-struts) of the biocarbon template the nano-grained SiC-phase forms layers of up to several mum in thickness. Due to the uni-directional pore structure of the wood derived C-B-templates, the morphology of the biomorphous SiSiC-ceramics can be analysed in well-defined pore orientations. A microstructural model for the SiC-formation and phase growth will be proposed and discussed with respect to conventional processed SiSiC-materials. (C) 2003 Elsevier Ltd. All rights reserved.