Microstructure and properties of biomorphic SiSiC ceramics derived from pyrolysed wooden templates

Kurzfassung

Biomorphic SiSiC ceramics are intermediate materials between damage tolerance fiber rein-forced ceramics (CMC) and monolithic SiC ceramics. The structure of this new class of mate-rials is characterized by a pseudomorphic fiber structure. The intention of the materials devel-opment is to obtain ceramics with high strength and fracture toughness by using a state of the art manufacturing. Commercial SiC-ceramic parts are made by powder metallurgy whereas fiber ceramics are manufactured through pressureless silicon infiltration in carbon preforms. This LSI process is advantageous for the manufacturing of biomorphic SiC ceramics. The processing of ceramics derived from cellulose containing preforms offer a potential to obtain a new class of SiC ceramics with appropriate properties by imitating the biological structure. First investigations on the conversion of wood into ceramics were conducted with infiltration of gases, melts, liquids or suspensions into porous carbonized bodies [1]. The bio-organic structure anatomy with its cellular morphology is the premise for the infiltration proc-ess. The open pore channels can be used as a transport system for the precursors which react with the carbon walls. However, natural products have a high scatter due to the non-uniform growing conditions which are transformed in the ceramics. To overcome the anisotropy of natural products technical, wooden preforms can be used as raw materials to achieve porous or dense SiC ceramics. Another advantage is the highly de-veloped industry on this field which is already able to produce parts in any size and shape for packagings, furnitures and constructions. SiSiC ceramics in near net shape can be derived from veneer panels by machining the pyrolysed carbon structure into shape. A high mass loss (69 %) and linear shrinkage (28 %) during pyrolysis occur. The high porosity and low density in the carbonized stage lead to high silicon contents and limits the strength [2]. Especially porous structures derived from paper and pulp are suitable for gas infiltration via CVI-process or other conversion reactions with precursors, so that the biological microstructure with the cell walls can be converted directly. The material might be of interest for applications as high temperature gas filters [3,4]. Another approach to obtain dense biomorphic ceramics is with natural fibers like flax etc. which are pyrolysed, pressed into shape, and finally sintered. The result are ceramic parts with a strength of about 300 MPa in the ceramic state [5]. In this paper wooden preforms derived from raw materials and binding agents and their processing have been investigated. In order to achieve dense biomorphic SiSiC ceramics the pressing is followed by pyrolysis and final infiltration with pure silicon melt. In these studies commercial wooden preforms composed of veneers, chips and fibers were used as raw mate-rials. Uniaxial warm pressing experiments were conducted with different fiber and chip sizes as well as microcrystalline cellulose as a binding agent. The samples were converted into car-bon preforms and siliconized through a modified LSI-process [6]. The manufacturing was done in order to produce homogeneous and dense biomorphic SiC ceramics with a high strength.