High Performance C/C-SiC Brake Pads

Kurzfassung

Melt-infiltrated carbon/carbon composites (C/C-SiC) have already proven their applicability to different light-weight structures in thermal protection systems of spacecraft and rockets. One new application of these composites is their use as brake pads in high performance brake systems of lifts, cranes and other transport systems. In this case, the brake system does not act as a service brake as it is used in passenger cars and high speed trains, but acts as an emergency brake system in case of power failure for example. The main requirements on the brake system are to reduce the velocity constantly, and to keep the load reliably in position. The tribological counterpart is a metallic guide rail whose surface can be contaminated with dust, oil and preservative agents. Conventional braking materials like organic or sintermetallic pads reached their limits with respect to thermal stability, wear resistance and frictional behavior. Novel modifications of C/C-SiC composites have been developed with high thermal and mechanical shock resistance, extremely low wear rates and high coefficients of friction under dynamic as well as stationary conditions. The tests have been conducted with small test samples as well as originally sized brake pads under practically relevant conditions. It could be shown, that the initial state of the metallic guide rail surface influences the friction coefficient essentially. Also, the phase composition of the C/C-SiC composite, namely on the friction surface, determines the friction behavior. High amounts of silicon carbide increase the wear resistance and friction coefficient, but reduce the impact resistance and mechanical strength of the material. As a compromise, graded composites with a symmetrical lay-up of fabric layers have been developed and were tested successfully. They consist of high amounts of carbon fibers in the center, resulting in quasi-ductile cores and surfaces with high SiC contents, resulting in outstanding tribological properties. From an economical point of view, short fiber reinforced C/C-SiC composites are of high interest. Also, modifications of the Liquid Silicon Infiltration process (LSI) with respect to shorter process times have been investigated. Corresponding samples were tested tribologically, showing a high potential for further cost reductions. This paper deals with the correlations between microstructure and mechanical and thermal as well as tribological properties of different C/C-SiC composites.