**Abstract**

This chapter describes two characteristic phenomena of metal wear that are usually not often considered but are related to the basic aspects of wear. The first is a mild-to-severe wear transition caused by the wear itself. Convex sliding pairs are usually accompanied by rolling sliding motion, but rolling sliding motion sometimes produces a peculiar wear profile, leading to high contact pressure. When the contact pressure exceeds a certain value that depends on the material, the wear mode changes to severe wear. This is a common wear transition for convex sliding pairs, but it can also occur for other pairs. The second is the similar appearance of wear tracks on various friction pairs. Rubbing metal under relatively severe conditions creates streaked wear tracks. We found the width and depth of these streaks, that is, wear track profiles are similar regardless of the sliding conditions and the material, which leads to similar appearance of the wear tracks. This suggests the existence of a general mechanism for producing wear tracks.

**Keywords:** wear track, roughness, profile, appearance, wavelet analysis, power spectrum density
