A distinction is drawn generally between static friction (friction at rest) and sliding friction (friction in motion). A state of static friction exists when two bodies resting one on top of the other are required to be set in relative motion (cf. adhesion). After the static resistance is overcome, one of the bodies is made to move, giving way to sliding friction between the bodies. Friction between rotating and stationary bodies can arise on surfaces at an angle between 0 and 90 ° to the axis of rotation. The term face friction is used when the rubbing surfaces are vertical or nearly vertical to the axis of rotation. This is the case with axial seals. Each of the following types of friction can arise in the sealing gap of a mechanical seal, depending on the structural design of the mechanical seal and on the operating conditions:

• full fluid film lubrication exists when the sliding faces are separated by a continuous lubricating film. As there is no contact between solid bodies, the frictional characteristics are affected solely by the dynamic viscosity of the lubricant. The solid bodies (seal face and stationary seat) are not subjected to any wear. Leakage is very high.
• Mixed friction exists when there is a bearing film of lubricant in the sealing gap, but it is interrupted in parts. In addition to the properties of the lubricant, a role now begins to be played also by the sliding characteristics of the face material pairing. Slight wearing occurs. Leakage is high.

• Boundary friction exists when there is only a very thin film of lubricant – interrupted more often than in the mixed friction state – between the sliding faces. The frictional characteristics are conditional in equal measure on the lubricity of the film of lubricant and on the sliding properties of the face material pairing. Wear is usually moderate, with minimum leakage.
• Technically dry friction exists when there is no film of lubricant in the sealing gap. Forces of adhesion and contact between the solid bodies give rise to heavy wear. Thermal stress cracks arise in some face material pairings from dry running.
• Vacuum friction exists when there are very high coefficients of friction as the result of free surface forces due to completely clean sliding faces with no absorbed gases or oxide films. Wear is very high in just a very short time, and metallic material pairings have a tendency to weld.
• Cryogenic friction occurs at very low temperatures. The wear symptoms are similar to those of vacuum friction.

It is possible in principle to operate mechanical seals under all conditions of friction. The high rates of wear from dry, vacuum and cryogenic friction can be reduced considerably by means of suitable impregnations that create films of lubrication and/or by material pairings with very good emergency running properties. Mechanical seal life is decreased appreciably, however. Boundary friction conditions must be aimed at for mechanical seals with sliding velocities up to 20 m/s and pressure loads up to 50 bar, because under these conditions the wear is moderate and the leakage rates are minimal.