|SEM view (photomontage) of a typical C/C-SiC micro-structure. |
The department „ceramic composite structures“ concentrates on the development of SiC based ceramics. This covers fibre reinforced CMC materials (Ceramic matrix Composites) as well as usually not reinforced SiSiC materials. Both material groups are manufactured via the LSI process (liquid silicone infiltration), which has been developed at the institute. Carbon fibre reinforced SiC ceramics, so called C/C SiC materials has been developed since the late 1980s. By tailoring the material properties to the requirements of different application fields, like hot structures for aerospace or friction materials for brake systems, various standard C/C-SiC materials have been obtained, characterized by significantly different material compositions as well as thermal and mechanical properties. C/C SiC materials are multiphase materials consisting of load carrying carbon fibers, respectively C/C bundles, which are embedded in a matrix of crystalline silicon carbide.
The main advantages of C/C-SiC materials are:
- High strength even at high temperatures
- Low density
- Quasi-ductile breaking behavior
- Extreme temperature and thermal shock stability
- High corrosion resistance
- High abrasive resistance
- Very low, adjustable thermal expansion
For long term applications in oxidising atmospheres and temperatures above 450 °C carbon based CMC materials can not be used. Therefore, new CMC materials based on high temperature stable, non oxide CMC are currently in development. Goal applications are hot gas liners for jet engines and stationary gas turbines.
In addition to the non oxide CMC materials, high temperature and oxidation resistant CMC materials are developed on the basis of oxide fibres and matrices. These so called OXIPOL materials (Oxide CMC based on polymers) are manufacture by the PIP process (polymer infiltration and pyrolysis). In the first step, polymer preforms on the basis of 2D fiber fabrics and polysiloxane precursors are manufactured via RTM (resin transfer moulding). During the following pyrolysis the polymer is converted to a SiOC matrix. By repeated cycles of polymer infiltration and pyrolysis, CMC porosities below 10 volume-% volume can be obtained. In contrast to other oxide CMC materials the ductility of the CMC material is not obtained by a highly porous matrix, but by a so called fugitive coating of the fibres. Therefore a thin layer of carbon is applied before the first polymer infiltration and is removed by a controlled oxidation step after the last pyrolysis.
For high temperature stable or extremely stiff structures, biomorphic SiSiC materials are favourable. Due to the possibility of a near net shape manufacture of large sized, complex shaped components via LSI, biomorphic SiC ceramics offer a high potential for the development of novel SiC structures, which can not be manufactured with common technologies up to now. Additionally, the use of low cost raw materials, e.g. commercially available wooden preforms, and relatively low process temperatures for ceramization can open new, cost-sensitive application fields, such as lightweight armour and high temperature heat exchangers. The material development is currently focused on tailoring the material composition and characteristics (e.g. hardness, damage tolerance, ductility) for the special requirements in different applications.
Current development focuses on:
- New, oxidation stable CMC materials on the basis of non oxide (e.g. SiC) and oxide fibres and matrices.
- C/C-SiC gradient and hybrid materials for extreme applications in rocket propulsion systems.
- High strength and highly stiff C/C SiC materials.
- Novel CMC materials by melt infiltration of metallic alloys into porous C/C structures (e.g. C/C-CuTiC composite materials).
- Biomorphic SiSiC as well as C/C-SiC materials on the basis of low cost silicon.
- Further development of biomorphic SiSiC materials for specific applications.