DEVELOPMENT OF OXIDATION PROTECTED CARBON CARBON/

Manufacturing of continuous fibre reinforced ceramics is the only alte rnative to realise thin walled, integral light weight structures for h igh temperature applications (T greater than or equal to 1200 degrees C). There exist two main routes to fabricate CMC materials: chemical v apour infiltration (CVI) and liquid phase infiltration. One manufactur ing method of the latter route currently under development at the DLR Institute of Structures and Design is the Liquid Silicon Infiltration (LSI). This manufacturing process is based on conventionally produced carbon fibre-reinforced plastics (CFRP), containing a polymer matrix w ith a high char yield. The second step is the pyrolysis of the CFRP to a porous carbon/carbon (C/C). During the subsequent liquid silicon in filtration, the silicon reacts with the remaining carbon matrix formin g silicon carbide, thus resulting in an internal oxidation protection of the C/C material. It is noteworthy, that all three manufacturing st eps (CFRP, C/C, C/C-SiC) are carried out by an one-shot fabrication me thod without any re-infiltration steps, resulting in a low cost manufa cturing process. At temperatures above 400 degrees C, carbon fibre-con taining composite materials oxidise in air very rapidly. Thus these ma terials need an additional protective coating, mainly on cut surfaces, which effectively prevents oxidation. Among other methods, the polyme r pyrolysis is one interesting possibility for the manufacture of prot ective coatings. In this method soluble and fusible organic silicon po lymers, especially polysilazanes are used as starting materials. These pre-ceramic polymers become cross-linked by thermal, chemical or phot ochemical treatment. Because of this, insoluble and infusible material s are formed. The following thermal decomposition (pyrolysis) up to 11 00 degrees C in a suitable atmosphere leads to amorphous ceramics of t he type Si/C/N, which show very good oxidation resistance. The volume shrinkage during pyrolysis is prevented by adding a certain amount of a filler like silicon powder which reacts with the pyrolysis products and the pyrolysis atmosphere, respectively with volume extension. By a simple dipping-process it is possible to fully coat the sample with a certain pre-ceramic polymer/filler suspension. During the following p yrolysis-step the polymer coating is transformed to a strong adherent, crack-free ceramic layer. This paper describes the development of the manufacturing process of liquid siliconized C/C as well as the additi onal oxidation protective coating.