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 behavior 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 the cast iron counterface disc and then mechanically polished to achieve set values of average roughness, Ra, and roughness skewness, Rsk . The tribological data were modeled using an exponential relation for the wear rate, according to a literature approach. The model parameters were related to the above disc surface roughness parameters. The results were interpreted in terms of the main wear mechanisms, that determine the formation of the friction layers on the pins as well as discs 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 derive from the reduction in abrasion and from improved dynamics in the formation of the friction layer, with particular regard to those parts (secondary plateaus) made of compacted wear debris.