Microstructural characteristics on the surface and subsurface of semimetallic automotive friction materials during braking process

In this study, a series of friction tests on semimetallic automotive friction materials were conducted on a friction test machine by pressing test samples against a rotating cast iron brake disc, thus simulating actual braking. After each friction test, the morphological changes of the wear surface and subsurface were investigated using scanning electron microscopy. Microstructural examinations showed that the major wear mechanisms in operation during braking are comprised of the following: (i) abrasive, (ii) adhesive, (iii) fatigue, (iv) delamination, and (v) thermal. The wear mechanism and wear transition are found to be influenced by the applied loads and braking times. In the study of the subsurface morphology, microcracks generated in the subsurface are thought to be due to the following phenomena; (i) growth of microvoids, (ii) coalescence of microvoids, (iii) coalescence of second phase particles, and (iv) coalescence of microvoids and second phase particles. The microcracks generated in the subsurface grew and propagated parallel to the sliding surface as the braking times as well as applied loads are increased. Finally, the microcracks grew and joined each other producing wear particles on subsequent braking. These mechanical and thermal failures manifested a complex wear mechanism, causing plastic collapse in the local region which subsequently produced wear particles in different shapes depending on the modes of failure.