Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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in such a way that friction would increase, and attributed the behaviour to adhesion
acting in true contact areas. This statement initiated long lasting contradiction
between adherents of mechanical and molecular theories of friction. In 1781
Coulomb published his “Theory of simple machines” where he acknowledged the
influence of adhesion on friction. However he pointed out to the work that had to be
done during relative sliding of rough surfaces as the main source of friction.
Coulomb expressed the low of friction as follows:
T = N + C, (9.1)
where C was a constant dependent on the molecular interaction of surfaces in
friction (cohesion).
Coulomb postulated that the value of C is constant for flat surfaces, and
independent of the normal load. Leslie criticized Coulomb’s theory in 1804 and
indicated that it should contain the incorporate deformation of surface asperities
as the necessary condition for energy losses to take place. This remark was
supported by work of Bowden [21] who claimed that the character of interaction
between bodies depended on the relation between their hardness, as well as on
temperatures of melting of the substances and the temperature of the contact area.
According to Bowden, friction force is composed of the force necessary to shear
bonds between asperities and the force necessary to draw a groove in the weaker
material. This author did not consider molecular interaction of surfaces nor the
influence of surface roughness, and assumed purely mechanical interaction of bodies
in friction. Progress in technologies of surface treatment did not result in the
elimination of friction, a fact that supported the point of view of followers of
molecular theories of friction. In 1929 Tomlison (following Hebda and Wachal, [62])
proposed that friction was a result of adhesion of sliding surfaces. Dispersion of
energy was a result of continual changes of pairs of interacting molecules and of the
creation of new molecular bonds. Based on laboratory testing, Tomlison formulated
an empirical relationship for coefficient of friction in the form of:
µ = 0.18⋅10 8 (A k + A p ) 2/3 , (9.2)
where A k and A p – material parameters.
Another molecular theory of friction was proposed by Deriagin (following
Kragelsky et al., [86]). This author proposed that friction depended on molecular
roughness of the material that was interrelated with material structure. Deriagin’s
concept was valid in the case of ideal sliding, but did not consider frictional wear of
sliding materials. In 1939 Kargelsky [86] published the principles of a molecular-