Sliding Behaviour of Friction Material Against Cermet Coatings: Pin-on-Disc Study of the Running-in Stage

Running-in is the initial stage during which a tribological system reaches a steady-state condition. In this study, the running-in behaviour of a commercial brake friction material, pin-on-disc tested under dry sliding conditions, has been investigated to understand the role of different surface finishing of hard coatings. These coatings were deposited onto cast iron counterface discs and then mechanically polished to achieve set values of average roughness, R_a, and roughness skewness, R_sk. The tribological data were modelled using an exponential relation for the wear rate, according to a literature approach. The model parameters were related to the above-mentioned disc surface roughness parameters. The results provided indications on the wear mechanisms acting during the sliding action. Furthermore, these mechanisms were correlated with the formation of the friction layers on the pins and on the disc wear tracks. Of particular relevance is the finding that a negatively skewed surface roughness is fundamental to achieve the best running-in performances. The beneficial effects, coming from this surface treatment, are derived from the reduction in abrasion and from the improved formation dynamics of the friction layer, with particular regard to those parts (secondary plateaus) made of compacted wear debris.