A brake for automotive application, which is based on energy dissipation by friction, must fulfil several requirements The first one is to provide a medium coefficient of friction (CoF) in the range 0 4-0.5 Secondly, constant friction values in spite of differing operation conditions such as velocity, pressure, humidity and temperature must be achieved with high reliability Furthermore, some comfort requirements have to be reached, first of all Noise-Vibration-Harshness (NVH), which requires stability of local friction forces against fluctuations. Finally the raw materials used lor producing brake pads and the wear particles which are ejected to the environment must not lead to adverse effects to nature and human health. Needless to say availability and price oi the raw materials are important factors as well. Modeling the CoF-evolution at the interface between pad and disc on the nanometer scale yields very reasonable values in the range 0.3 - 0.5, depending on the assumed fine structure of the experimentally approved friction layer. Other contact situations like oxide-on-oxide and metal-on-metal do not provide stable values but rather a stick-slip type of sliding and furthermore unrealistic high mean CoF values. If friction layers comprise of an oxide phase and at least 10 % of a solid lubricant or other soft constituent a mechanically mixed layer (MML), which stabilizes friction forces, may form. Higher solid lubricant concentrations will decrease the CoF. lead to the formation of thicker MMLs and thus more stable friction behaviour. Incorporation of hard nanoparticles into a friction layer increases the CoF-level without destroying the capability of MML-formation. Therefore, increasing stable CoF-levels beyond 0.4 by fine tuning of the microstructure of the friction layer seems to be possible.
Friction control during automotive braking - Experimental observations and simulation at the nanometer scale
2008
7 Seiten, 13 Bilder, 20 Quellen
Conference paper
English
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