The disc wear surfaces observed in the SEM after the wear tests are shown in Fig. 6. The comparison with the unworn pristine surface (see Fig. 2b), allows us to understand the modifications
introduced by sliding. It is worth starting commenting on the surface with Ra ¼1 μm initial finish. The comparison of Figs. 2b and 6b shows that after sliding, in addition to the white areas
(coating polished asperities) and grey areas (coating unpolished regions), wide black areas can be observed. In Fig. 7a detail of one of these areas at higher magnification is shown. The EDXS analysis taken in correspondence of the black area demonstrates that they are produced by the compaction of fragments originating from the friction material (pin) and then transferred onto the counterface disc. In particular fragments tend to pile-up preferentially into the valleys between the smoothed asperities, as schematized in Fig. 8.If the disc roughness is high (Ra ¼ 5 μm), the amount of such transfer increases (see Fig. 6a). On the contrary, for lower disc surface roughness the amount of the transfer decreases, and it involves only small, limited areas, as shown by the black spots in Fig. 6c and d. The wear of the discs, evaluated by measuring the depth and extension of the wear track by a profilometer, was negligible in all cases.