Influence of Process Parameters on the Thermophysical Properties of C/C-SiC

Kurzfassung

Ceramic matrix composites (CMCs) with low porosity are obtained via the well estab-lished Liquid Silicon Infiltration Process (LSI), which is characterized by short processing times and fairly low manufacturing costs [1]. The key step is the infiltration of liquid silicon in porous car-bon/carbon composites (C/C) whose distinct microstructure is built in the pre-ceding py-rolysis step of carbon fiber reinforced plastics (CFRP). Those were produced by resin trans-fer molding (RTM) in a first step. This process leads to C/C-SiC composites being character-ized by high mass-specific proper-ties in combination with an extreme thermal shock stabil-ity. Besides aerospace applications, such as hot structures for reentry vehicles, more and more applications beyond this classic field of CMCs are of increasing interest, e.g. brake discs, zero expansion materials, high temperature heat exchangers, etc. By applying special process pa-rameters the microstructure as well as the physical properties can be tailor-designed to match specific re-quirements. The thermal conductivity of C-fibers and thus of a C/C-SiC composite is con-siderably increased with pyrolysis and graphitization temperature as known from C/C-com-posites [2,3]. This paper reports upon the influence of various C-fibers, precursors and annealing process parameters of C/C as well as the microstructure, obtained by pre-treating C-fibers, on the thermophysical properties of C/C-SiC. Therefore, C-fabrics were used as received or ther-mally pretreated and submitted to LSI-processing. Another route is to graphitize the C/C in-termediate with subsequent sili-conizing [4,5]. Due to the fact that the thermal conductivity (TC) of carbon fibers after graphitization can be higher than that of SiC the dominating con-tribution to the TC of the composite is expected from the fibers. Because of the pronounced orientation of the fibers in one plane there is a strong anisotropy of the TC parallel and trans-versal to main fiber orien-tation, whereby the values parallel to the fibers are by a factor 2 to 3 higher compared to transversal direction [6]. In this paper, only the results of measurements in transversal direc-tion are presented.