in the field of tribology Friction is generally described as a force that opposes the relative motion of two bodies in contact. It is called friction force - in other words, whenever bodies come into contact or move against each other, friction forces occur. This is caused by unevenness in the body surface and cohesive forces that act between the molecules of the bodies that rub against each other.
The friction results in a loss of mechanical energy perpendicular to the normal force. When it comes to friction, a distinction is made between internal and external friction:
Example of internal friction: Viscous friction
Examples of external friction: static, sliding and rolling friction
The friction or the coefficient of friction μ depends on the normal force (diagram 1). With polar friction pairings (e.g.: deva.tex®), the adhesive portion of the coefficient of friction is very high due to the attraction of the friction surface with low normal force. As the normal load on the friction contact increases, this proportion initially drops sharply to a minimum and then remains relatively constant. The background is the actual contact surface of the friction partners. The contact area increases degressively as the normal force increases. The movement-inhibiting friction force caused by the adhesive is proportional to the contact area. The relationship described above between the normal force and the adhesive component of the coefficient of friction results from the ratio μ = FR-adh/FN. The deformative portion of the coefficient of friction increases with increasing normal load on the friction contact.
The sum of the adhesive and deformative friction components results in the coefficient of friction μ.
In friction pairings with at least one non-polar friction partner, the coefficient of friction μ increases continuously from the start due to the lack of adhesion (Diagram 1).
The relationship between the friction force FR and the normal force FN is defined as the coefficient of friction µ.