Frictional and Particle Emission Behavior of Different Brake Disk Concepts Correlated with Optical Pin Surface Characterization

Brake wear emissions can be reduced by altering the surface of brake disks. A parametric study using a gray cast iron and a laser-cladded brake disk was performed in a pin-on-disk experiment with integrated optical pin surface characterization and particle emission measurement. Significant differences in the friction, wear and emission behavior are present. The high wear-resistance of the laser-cladded disk led to a reduction of 70% of the particle number emission relative to the gray cast iron disk, but the coefficient of friction was unstable. The surface of the pin used with the gray cast iron showed an initial large debris extension and protruding patches that were removed at high braking energies, exposing white patches and creating holes. These observations correspond to known processes from the plateau theory. The surface of the pin used with the laser-cladded disk showed a topography dominated by holes with almost no protruding patches. The braking condition did not influence the pin surface, implying that the disk and not solely the pin surface might be governing the friction process, and therefore challenging the applicability of the plateau theory to laser-cladded disks. To further study this aspect, a segmentation method was developed for the pin surface images and topographical data to extract and quantify different features on the pin, such as debris, patches, holes and the tribolayer. The correlation of the surface coverage ratios of the feature classes with the braking conditions (speed and applied pressure), the coefficient of friction and the emissions confirmed the differences between the gray cast iron and laser-cladded brake disk.