Friction is the resisting force generated between two surfaces which move relative to each other.

Friction gives energy loss with temperature increase and usually produces materials wear with dimensional change of the surfaces.

During the starting phase (1) the static friction is followed by boundary friction (high friction/high wear). With increasing speed (2) the sliding surfaces are partially separated by the lubricant, this phase is called mixed friction (medium friction /medium wear).

In these phases the emergency lubrication film formed by the solid lubricants protects the sliding surfaces.

At even higher speeds (3) the sliding surfaces are separated from each other by a hydrodynamic fluid film, in this phase the lowest value of wear is achieved.

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