Phase characterization of debris generated in brake pad coefficient of friction tests

▶ Magnetite in low speed as opposed to cementite formation in high speed COF tests. ▶ Erratic COF correlates with presence of cementite in debris and pad surface. ▶ Complex phase composition possibly hides presence of relevant phases in XRD. ▶ Relevance of Mössbauer spectroscopy in tribo-couples involving cast iron. Debris from coefficient of friction (COF) tests with semimetallic vehicle brake pads tested against cast iron discs in a full scale inertial bench dynamometer were characterized with Mössbauer spectroscopy, X-ray diffraction and scanning electron microscopy. The debris was collected during 4 brake test runs that consisted of short and long burnishing procedures (100 and 200 cycles) followed by 1000 full stops decelerating from low and high speed (from 46 and 96 km/h). At the low speed tests the COF was high and stable and the debris had increased magnetite content. At the high speed stops COF was lower and more erratic and the debris showed high cementite content. Cementite formation should be prevented in order to get stable COF.