show significant degradation over longer sliding times, to the point where the peaks become highly broadened after 10,000 revs of sliding. This is accompanied by the onset of broad peaks at ~220, 530 and 640 cm -1 after 3,200 revs of sliding, which have tentatively been assigned to the formation of copper(I) oxide on the tracks surface. The intensity of the Cu 2 O peaks increased with longer sliding times.