An investigation of the sliding wear mechanisms of a plasma-nitrided CrMo steel is undertaken using a running-in procedure that eliminates severe wear under mixed and boundary lubricated conditions. In the running-in procedure, using a pin-on-disc wear machine, smooth contact surfaces are obtained rather than the rough contact surfaces generally found in laboratory experiments. This enables the wear mechanisms to be investigated more clearly, particularly mild wear processes. The work shows no measurable nor visible wear under full fluid film lubricated conditions, the existence of polishing under mixed lubricated conditions and micro-pits under boundary lubricated conditions. The results presented in this paper indicate that a mild abrasive wear mechanism predominates under mixed lubricated conditions and a micro-fatigue wear mechanism under boundary lubricated conditions. Examination of the microstructure reveals the formation of white layer regimes on the contact surfaces after tests under boundary lubricated conditions, which suggests severe work-hardening of the contact surfaces.