The effect of external grit particle size on friction coefficients and grit embedment of brake friction material

Changes in friction and contact surfaces characteristics of a brake friction material during drag and stop mode test were investigated using a brake model tribo-tester. Scanning Electron Microscopy (SEM) was utilized to reveal the surface topography characteristics and analyze the external particle size effects on friction coefficients and grit embedment. Silica sand with three different particle sizes of 50-180 µm, 180-355 µm and 355-500 µm was used in this work. At higher disc sliding speed, results showed that small grit particles cause higher friction due to greater frequency of particles mixing and modifying the effective contact compared to bigger particles. Good friction stability was attributed to smaller particles size providing more stable contact by actively involved in building up and reducing the rate of changes of the effective contact area. Through SEM analysis, signs of formation and disintegration of contact plateaus correlated well with particle size and hence, suggesting the significant role of particle size as wearing mechanism. Grit embedment (GE) was greatly dependent on presence of compacted wear debris as most particles were found embedded into compacted wear debris. Total GE of 2.7% was observed for silica sand of 50-180 µm, 4.5% for 180-355 µm and 3.0% for 355-500 µm

Effects of external hard particles on brake friction characteristics during hard braking

The effects of external hard particles on the friction coefficients and its oscillation amplitudes during hard braking were investigated. Silica sands of the size between 180 to 355 µm were used during the experiments. The results were compared to the results obtained without the grit particles present in order to determine the change in friction coefficient and the fluctuation of frictional oscillation amplitude. Different sliding speeds were applied and external hard particle of different size is found to significantly affect the friction coefficient and standard deviation of friction oscillation amplitude values. The friction coefficients increase with hard particle due to the rapid changes of the effective contact area and the abrasion mode. Some embedded particles operating in two body abrasion mode help to increase the disc surface roughness and influence the stopping time of the disc. The standard deviation values of friction oscillation amplitude however were stable due to more wear debris produced and get compacted to form friction films assisting friction and they tend to reduce at medium speeds because many contact plateaus and effective contact area started to stabilize