Page 28 - Tribology in Machine Design

Basic principles of tribology 15

zone is called plastic deformation. Depending on the deformation mode
within the contact, its real area can be estimated from:
the elastic contact

the plastic contact

where C is the proportionality constant.
The introduction of an additional tangential load produces a pheno-
menon called junction growth which is responsible for a significant increase
in the asperity contact areas. The magnitude of the junction growth of
metallic contact can be estimated from the expression

where a % 9 for metals.
In the case of organic polymers, additional factors, such as viscoelastic
and viscoplastic effects and relaxation phenomena, must be taken into
account when analysing contact problems.

2.3. Friction due to One of the most important components of friction originates from the
adhesion formation and rupture of interfacial adhesive bonds. Extensive theoretical
and experimental studies have been undertaken to explain the nature of
adhesive interaction, especially in the case of clean metallic surfaces. The
main emphasis was on the electronic structure of the bodies in frictional
contact. From a theoretical point of view, attractive forces within the
contact zone include all those forces which contribute to the cohesive
strength of a solid, such as the metallic, covalent and ionic short-range
forces as well as the secondary van der Waals bonds which are classified as
long-range forces. An illustration of a short-range force in action provides
two pieces of clean gold in contact and forming metallic bonds over the
regions of intimate contact. The interface will have the strength of a bulk
gold. In contacts formed by organic polymers and elastomers, long-range
van der Waals forces operate. It is justifiable to say that interfacial adhesion
is as natural as the cohesion which determines the bulk strength of
materials.
The adhesion component of friction is usually given as: the ratio of the
interfacial shear strength of the adhesive junctions to the yield strength of
the asperity material