Dense biomorphic SiC ceramics derived from wood-based composites

Abstract

A new technology for the manufacture of dense SiC ceramics in a low cost approach by imitating the biological structure of wood is aim of the co-operation between the Technical University of Munich (TUM) and the German Aerospace Center (DLR). In a first material screening eight types of commercial wood-based composites with the size 150 x 150 x 20 mm were examined, which differ in their structure and composition of wood and binding agent (e.g. laminated beech panels, chip boards and medium dense fibre boards). First experiments showed that each wood-based composite has to be dried in three heating ramps up to 110 °C prior to pyrolysis to reduce the water content. A mass loss of 3,7-6,5 % and a shrinkage in thickness of 0,3-2,0 % occur during drying. The subsequent pyrolysis up to 900 °C shows a further mass loss of about 70 % and an additionall shrinkage in thickness of about 34 %. The shrinkage in length and width however is about 21 %. Both, the mass loss and anisotropic shrinkage may be responsible for the formation of cracks during pyrolysis. Within the last step a pressureless infiltration of silicon melt (siliconization) was performed under vacuum up to 1650 °C to fill the pores and to create SiC. As the commercial wood-based composites were not adapted for their conversion to SiC ceramics only a few of them could be pyrolysed reproducible and crackfree. So, the modification of the wood-based composites starts by the development and manufacturing of adapted wooden preforms. This paper deals with the actual status of the project and describes the results in terms of properties and microstructures of these wood-based composites, the intermediate carbon/carbon templates and the final SiC ceramics.