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The Mechanisms of Friction
 he mechanisms of friction. When two bodies are in contact and relative motion, a finite force is required
to maintain this motion, the friction force . A coefficient of friction, µ, is calculated by dividing the friction force
by the normal force, µ=Ff/FN. To date, despite considerable efforts at understanding
the origins of friction, there is not a model capable of predicting friction coefficients from first principles. Thus, careful
and proven experimental techniques represent the most sophisticated and reliable technique for investigating,
designing, and assessing the tribological worthiness of new materials.
The tribological system must be defined well in
advance of an experimental study in friction. The three basic points that are considered fundamental to studies of
friction are the real area of contact, the surface adhesion and shear strength, and the nature of deformation and energy
dissipation occurring at the asperity junctions. When two surfaces are in contact, the area over which they appear to be
in contact has little to do with the area in which they are actually in contact. Surfaces are actually contacting over
only a very small fraction of the apparent area of contact, the real area of contact is generally less than 1% of the
apparent area of contact. The real area of contact is determined by the load being carried, the roughness of the surfaces,
and the mechanical properties of the materials (elastic modulus, hardness, and Poisson's ratio). Over the real area of
contact, adhesive forces act to resist motion. These forces are the result of various intermolecular forces such as
metallic, ionic, and covalent bonding as well as the generally weaker van der Waals forces. Because of the complexity
and uncertainty about the actual bonding between the asperity junctions, the total adhesive contribution is modeled
as a surface shear strength; thus, the resisting force is the product of the surface shear strength and the real area of
contact.
Resisting forces and energy dissipation during sliding also occur through a variety of material deformation.
In metal and ceramic contacts, these deformations are generally permanent (plastic deformation, plowing, cutting,
tearing, and cracking). With polymers, in addition to the permanent material deformation dissipating energy, there
are also viscoelastic losses that act to dissipate energy and increase the friction coefficient. |
