Proper functioning of a mechanical seal depends first and foremost on the formation of a film of lubrication between the sliding faces. Needed for cooling as well as for lubricating the sliding faces, this lubrication film is formed by the process medium or by an external medium. Thickness and distribution of the lubrication film over the sliding face vary according to the medium, sealing pressure, temperature, sliding velocity and face material combination. Either boundary friction, mixed friction or fluid friction (see friction types) arises between the sliding faces. The absence or inadequate formation of a lubrication film is a frequent cause of damage (dry running).