Mr. Felix Wich, University of Bayreuth, GERMANY, Dr. Nico Langhof, University of Bayreuth, GERMANY, Prof. Dr.-Ing. Walter Krenkel, University of Bayreuth, GERMANY
The formation of a stable 3rd-body, independence of the applied sliding speeds and braking pressures, is considered to be crucial for low wear rates and a desired coefficient of friction that can be measured between the regarding friction materials. Therefore, e.g. LowMet pads and grey cast iron discs are a very suitable friction couple. However, there are some drawbacks of both materials. Their thermomechanical stability is limited, they tend to be corroded very quickly and harmful fine dust emission during braking and acceleration causes enormous challenges for future applications, like in electric cars.
One approach that can help to overcome these challenges are ceramic friction materials. The well established ceramic brake disc for passenger cars, has a temperature stability up to 1300 °C, a low density of about 2.4 g/cm3, shows no corrosion and as a life time disc (> 300.000 km) very low wear rates, i.e. significantly less fine dust particle emission. Some alternative approaches still propagating grey cast iron discs, that are coated with different ceramic materials in order to decrease the fine dust particle emission.
However, on the opposite side, the brake pad is still mainly unchanged, metal containing and prone for corrosion. Due to this behavior, in case of an emergency brake event, the maximum COF is reached much slower, compared to a non corroded brake pad surface.
Therefore, novel corrosion resistant (ceramic) brake pads are required as counterparts for those novel developed brake discs.
Within this work different ceramic pads are studied, based on C-fiber reinforced SiC. Previous tribological tests show the promising frictional behavior in terms of low wear rates and very high COFs. However, the COF is strongly depended on speed and pressure. Due to adhesive friction, especially at low speeds [removed] 0.9, partially vibrations occurs, which make such pads nearly impossible to adapt within braking system of a car.
In order to overcome this drawback different graphite additives were selected as modifying phases within the ceramic pads. The goal was to form a suitable 3rd-body that decreases the COF at low speed < 10 m/s and minimize the COF difference between high (20 m/s) and low speed (< 10 m/s).
The brake pads were manufactured by mixing 3k-C-fibers (6 mm lengths) with phenolic resin and three different types of graphite. All graphites differ in terms of particle shape, particle sizes and size distribution. The mixtures were warm-pressed, cross-linked at about 170 °C and subsequently pyrolyzed > 900 °C under inert conditions. This thermal treatment causes the transformation of the matrix resin into an open porous amorphous carbon. During the final step, liquid silicon was infiltrated (>1420 °C, vacuum) within the open pores in order to enable the reaction of silicon with the amorphous carbon matrix to SiC.
All manufactured brake pads were studied in term of their bulk density, open porosity and microstructure, including some thermomechanical properties, e.g. the compressive strength and thermal conductivity.
Afterwards, for each of the three qualities, a reference without graphite and LowMet, 30x30 mm2 pads with 10 mm thickness were prepared and tested on a fly wheel dynamometer (about 100 kgm2) with a starting speed of 20 m/s and a braking pressure of 3 MPa. As a counterpart the commercial available internal ventilated C/SiC ceramic brake disc from BremoSGL was selected (about 410 mm ∅). The COF, the wear rates and the temperature of the braking parings were determined.
The effect of adding 15 wt.-% graphite is visible for every type of the studied brake pads compared to the pads without additives. Extremely high COFs at low speeds can be prevented and due to the selection of suitable particles sizes and graphite amounts, the COF at low speeds was decreased significantly (< 0.7). In order to understand this favorable behavior the frictional surfaces of the pads were studied after braking. The results show, that due to the addition of graphite the dependence of the COF on speed could be reduced significantly and the wear rate is very low as well. In contrast to the C/SiC pads without graphite, notable areas could be detected in scanning electron microscopy, where a 3rd-body can be confirmed. This 3rd-body is presumably responsible for the significantly improved tribological performance of the pure ceramic friction couple.
The very promising frictional properties of these tailored C/SiC ceramic brakes pads, show the possibility to compete with the current conventional brake pads and to overcome the future challenges accompanying with the electric driven vehicles as well.