Moderate adhesive wear is the 'normal' wear that takes place mostly at the Top Dead Center where the oil film is not sufficiently thick to completely separate the piston ring from the liner wall surface. When the temperature, the sliding speed or the load exceeds a critical value severe adhesive wear or scuffing takes place.
Severe mechanical wear begins at a very small area of the contact surface but quickly spreads due to the significant deterioration of the surface. When the friction is intense the surface melts and forms 'white layers', which are very hard and brittle. If they crack, small, hard particles flake off and plow the surface, producing the typical scuffing appearance.
