Nico Langhof

Synthesis and growth mechanism of SiC nanofibres on carbon fabrics

The SiC nanofibres with a length of few millimetres were synthesized successfully on carbon fabrics using a catalyst-free method. The Si and SiO2 powder mixtures functioned as a source for the generation of gaseous species. Following the chemical vapour reaction route, abundant SiC nanofibres with near stoichiometric composition and high crystallinity were grown on the surfaces of carbon fibres. The phase, morphology and crystal structure were characterized to reveal the nucleation and growth mechanism. SiC nanofibres presented a pagoda structure, which is a consecutive stacking of hexagonal segments with a high density of stacking faults. Combining the fabrication process with an analysis of the morphology and crystal structure, both the vapour–solid mechanism and the screw dislocation-driven mechanism are responsible for the nucleation and the growth of SiC nanofibres in the current study. The significant advantage of the present process is that it can be conveniently implemented and provides an approach to allow the fabrication of SiC nanostructures on a variety of carbonaceous substrates without the assistance of a catalyst, leading to less expensive, long length and high purity production of nanostructures.

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.

The Evaluation of Thermoplastic Precursors for C/C-SiC Manufactured by Liquid Silicon Infiltration (LSI)

C/C-SiC composites, fabricated by the Liquid Silicon Infiltration process (LSI), typically use phenolic resin-based thermosets as carbon precursors. In contrast to this, two different thermoplastics (Polyetheretherketone PEEK and Polyetherimide PEI) were examined for their suitability as carbon precursors for C/C-SiC composites. The carbon fiber surfaces were pretreated between 400 °C and 800 °C in nitrogen atmosphere to modify the fiber/matrix bonding. The microstructures of the materials show an increasing SiC content with increasing fiber pretreatment temperature. The flexural strength of the resulting material was determined by 4-point-bending tests.