The Influence of Fibre and Geometry on the Mechanical Characteristics of C/C-SiC Composites

Kurzfassung

The mechanical properties of fibre reinforced ceramic matrix composites (CMC) are predominated by the properties of their individual constituents (fibres, matrix), the arrangement of the fibres (uni-, bi-, multi-directional, random) and the manufacturing parameters (temperature, pressure, atmosphere). In addition the nature of the fibre/matrix interface also has a major effect on the mechanical properties. The C/C-SiC materials under investigation were developed using the Liquid Silicon Infiltration (LSI) process of the DLR in Stuttgart [1] for the thermal protection of reusable space transportational systems and have already been successfully assigned to many missions. Studies towards the design and qualification of future reentry bodies, such as those currently being conducted within the nationally financed DFG-Sonderforschungsbereich 259 „High Temperature Problems of Reusable Space Transportational Systems“, lead to an ever increasing complexity of the thermal protection system (TPS). They consist primarily of load bearing carbon fibres and matrices of carbon, silicon carbide and some free silicon. The constitution of the matrix and the materials microstructure mainly depend on the choice of the matrix precursor and fibre structure, the processing parameters and the interfacial bonding. An attractive aspect of LSI processing lies in the possibility to vary these parameters in order to tailor the microstructure and thus the properties of C/C-SiC materials [2]. The C/C-SiC material can be tailored to the different structural specifications for the material within a TPS by means of parameter variations during the LSI manufacturing process, without a (dramatic) loss in the advantageous material characteristics such as excellent thermal stability, high specific characteristics and thermal shock stability. In this paper, the influence of fibre type and orientation, in relation to testing load, on the mechanical properties of the C/C-SiC materials have been determined. Hereby, is sample geometry a decisive factor. As such, three test methods have been compared with each other. In order to determine the flexural strength of the composites both short span three point bend tests and long beam, four point flexure tests have been employed. The results of the short span bend tests have also been compared to those from shear compression tests to ascertain the interlaminar shear strength of the composites. It has been shown that the type of fibre used has no significant influence on the mechanical properties of materials manufactured via prepregs, but that the fibre architecture has an effect on the shear and bend strength of these materials. In addition, the results from this study have shown that the short beam, three point bend test cannot be used to determine the shear strength of this class of ceramic materials, but may be used for first approximations in the flexural strength.