FIBER REINFORCED CERAMIC FRICTION PADS FOR AUTOMOTIVE APPLICATIONS - THE DEPENDENCE OF FRICTION AND WEAR ON SPEED AND BRAKING PRESSURE

摘要

The frictional properties of two different C-fiber reinforced ceramic composites were investigated. These composites, e.g. C/SiC were manufactured by applying the liquid silicon infiltration process (LSI). C/SiC means C-fiber reinforcement within a SiCmatrix and residual silicon. To study the frictional properties of C/SiC brake pads (material A) and Si-free C/SiC brake pads (material B) decent tests with defined conditions were realized. On a dynamometer test rig braking pressures between 0.54 N/mm~2 and 2.44 N/mm~2 and rotating speeds of a commercial C/SiC ceramic brake disc between 5.5 m/s (300 rpm) and 42.6 m/s (2262 rpm) were applied. Due to the observation of the microstructure and the measurements of the coefficient of friction (COF = μ), the wear rates, the surface roughness and finally the determining friction mechanisms could be detected. Adhesive, abrasive, fatigue wear and oxidative wear occur, partially simultaneously. In general, the COF decreases with increasing speed and pressure. At low braking pressure (0.54 N/mm~2) and moderate speeds (17.4 m/s) no adhesive wear due to free silicon is detectable. Examinations of the surfaces of brake pads (C/SiC material) and brake disc after increasing the pressure and/or the speed show, that the residual silicon causes extraordinary high wear rates at 42.6 m/s by combining adhesion and abrasion of free-Si and SiC. Wide distributed roughness peaks on the surface of the brake disc were observed. Each of them consists of a silicon matrix with embedded hard SiC particles. As a result, significantly deep grooves on the pads surface can be detected at moderate speed (17.4 m/s) and finally at higher speeds C/SiC brake pads (material A) were rapidly abraded. By removing of the residual silicon, the porosity increases and the adhesive wear can be avoided. Therefore, no asperities were formed, the real contact area for friction increases compared to the C/SiC pads and the wear rate decreases with increasing speed and increases with increasing the pressure. Nevertheless, no stable sliding friction independent on the conditions is observable, due to the lack of a friction layer.