Dynamic coefficient of friction

The dynamic coefficient of friction, also called the kinetic or sliding coefficient of friction or friction coefficient (symbol µ or f, dimensionless), is a measure of how large the friction forces are which act between two solids.

Information regarding a coefficient of friction assumes that the type of friction is considered as Coulomb friction, i. e, there is a value for the static friction (if there is no relative movement between the friction surfaces) and a value for the dynamic friction if the surfaces move relative to each other. The dynamic coefficient of friction is independent of the sliding speed and is therefore constant. In practice a corresponding temperature, speed and pressure dependency is identified, which indicates an influence of the surface change and property of the never ideally flat surface (but not the coefficient of friction itself) and therefore apparently affects the material property.

The coefficient of friction is measured on polished surfaces without mechanical keying. The adhesion and cohesion forces between the materials are decisive. Depending on the material, Van der Waals forces form or in polarised materials hydrogen bonds similar forces between the surfaces. The highest is the material adhesion in ionic materials such as e.g. common salt (sodium chloride).