Ceramic Matrix Composites for High Performance Friction Applications

Ceramic Matrix Composites (CMC) show due to their fiber reinforcement high strength, thermal shock resistance and damage tolerance, a low coefficient of thermal expansion (CTE) and a partially porous and micro-cracks containing matrix. Hence, C-fiber reinforced silicon carbide materials (C/SiC resp. C/C-SiC), manufactured by the LSI (Liquid Silicon Infiltration) process, are very suitable for friction applications, e.g. for brake discs and pads, because of their low wear rates and high coefficients of friction (COF). Fundamental studies and ongoing investigations are the basis for the introduction of fiber reinforced ceramic brake discs for passenger cars since more than 10 years. Today, C/SiC friction materials can be found in emergency brakes for elevators, conveying systems and passenger cars. With respect to the selected friction couple, the tribological performance remains on a high level over a large range of sliding speed and braking pressure. In order to develop C/SiC lifetime brakes for passenger cars, the corresponding brake pads have to be modified appropriately as well. Within a certain friction system, the C/SiC ceramic brake discs withstand a mileage up to 300,000 km, compared with about 70,000 km of grey cast iron rotors. The economic success of these innovative, damage tolerant ceramics depends on the further reduction of the fabrication costs, which are considerable higher, compared to the competing metallic materials. This article describes the technological steps in the development of ceramic friction materials and the current status. It gives an overview about the most important, forthcoming challenges in terms of the material's development and processing.