**1. Introduction**

Two characteristic phenomena of metal wear are described in this chapter. The first is mild-to-severe wear transition [1, 2]. Severe-to-mild wear transition is usually observed in the running-in process. The transition is believed to be due to a smoothing of roughness [3], an increase in morphological conformity of sliding pairs due to wear [4], and/or oxide formation in the wear scars [5–8]. Mild-to-severe wear transition is sometimes observed in friction parts used for a long time. There are various causes of this transition. One of the simple causes is deterioration or depletion of lubricant [9]. Fatigue of the sliding surface or temperature increase [10] is also a possible reason.

We have found that convex sliding pairs, such as those used in some latch mechanisms or sliding electrical contacts, often cause the mild-to-severe wear transitions. This wear transition was found to be caused by an increase in contact pressure between the sliding pairs due to an increase in the wear shape inconformity of the sliding pairs. This wear shape inconformity was caused by the rolling-sliding motion of the convex sliding pair.

The second one is about wear track profiles [11, 12]. When metal sliding pairs are rubbed under relatively severe conditions, streaked wear tracks are generated in many cases. By observing the wear tracks of wear specimens which we have obtained for various test purposes and those of several published papers, we noticed that the appearance of many streaked wear tracks is similar. In other words, the width of the streaks does not change much regardless of the sliding material and wear conditions. We thought this suggests the existence of a general mechanism for producing wear tracks that can suggest the way to prevent severe damage from wear.

Though some researchers seemed to have been aware of this feature, few studies were found that focused on the wear track profile characterization [13], though many studies have been conducted on the wear scar or wear track morphology (e.g., [14]).

These two issues are often overlooked but we think they can be one of the key aspects of wear.
