Adhesive Wear
During frictional contact, peaks or high points on the surface begin to degrade. The removed material becomes abrasive and begins to compound.
Oxides on the surface, which normally protect the surface from various mechanisms of wear, begin to fragment and aid in cutting away yet more high points. As the material begins to gall and plastically deform, heat and surface contact pressure increase. Fragments from one piece transfer to the other piece and cold-weld themselves together. Surface oxidation films and lubricants generally suppress adhesive wear.