Friction and wear of Cu-free and Sb-free environmental friendly automotive brake materials

The purpose of this research was to study the friction and wear performance of Cu-free and Sb-free environmentally friendly automotive brake friction materials. Model brake material samples were manufactured and put to the test in the full scale automotive brake dynamometer (Dyno) using a SAE J2430 test procedure. The SAE recommended a J2430 test procedure provided the necessary data for the Brake Effectiveness Evaluation Procedure (BEEP) by the Brake Manufacturers' Council. The modified Cu-free and Sb-free brake materials were formulated using the Akebono brake formula. The modified formula contains environmentally friendly geopolymer and natural hemp fibers as a fraction replacement of phenolic resin and synthetic kevlar fibers, respectively. The Dyno results indicate that the friction level of the Cu-free samples was higher than when compared to the baseline material (T-baseline). The samples T-baseline and modified Cu-free and Sb-free materials (T303 and T403) exhibit average effectiveness of 0.32, 0.41 and 0.33, respectively. All tested brakes have passed the Brake Effectiveness Evaluation Procedure (BEEP). The Dyno results show that the modified samples had better performance when compared to the T-baseline as the temperature of the brake increased in the fade section of the SAE J2430 test. The average effectiveness in the fade section of T-baseline, T303 and T403 are 0.33, 0.41 and 0.37, respectively. However, the modified samples exhibit higher wear rate than the T-baseline. The T-baseline has a thickness loss of 0.37 mm, but the T303 and T403 samples lost 1.43 mm and 2.36 mm in thickness, respectively. Analyses using scanning electron microscopy and energy dispersive x-ray microanalysis on the tested samples show that the friction surface of sample T-baseline is covered with a fully developed and stable friction layer (third body) consisting mostly of Fe oxides, different formats of carbon and compounds of materials originally present in the bulk brake material. The T303 and T403 samples, formulated without Cu and Sb did not develop the sufficient friction layer. The friction layer seems to be responsible for the detected lower wear rate in T-baseline compared to T303 and T403 samples, and it acts as a solid lubricant on the interface between the rubbing pad and the cast iron disc lowering the adhesive forces. The friction and wear of the T-baseline sample is controlled by adhesive mechanisms. The T403 material, formulated without Cu and Sb and with the highest content of geopolymer replacing phenolic resin matrix, exhibited extensive abrasive wear in addition to adhesive mechanisms. Higher adhesion forces in T403 as well as in T303 samples, when compared to the T-baseline sample, are responsible for higher detected friction values and for the efficient removal of the dominant part of the friction layer. The capacity to form a friction layer on the surface plays a considerable role when lowering the wear of brake materials. The friction layer is formed by the compaction and interaction of friction wear debris particles and its stability and character depend on the chemistry of the bulk materials in contact as well as the temperature, pressure and sliding speed during a friction process.