The Tribological Significance of Contact and Adhesion

Unfortunately this aspect of tribology has not been studied with the detail and intensity accorded normal contact. Nevertheless it is instructive to examine an analysis such as that of A. P. Green [24] for the junction model of friction. Green pointed out the inadequacy of Bowden and Tabor s simple formula for the coefficient of friction 

In Green s model it is assumed that when the sliding has attained a steady-state condition the number and the size of the junctions remain constant on the average. All motion is tangential there is no normal movement of one surface relative to the other. As the surfaces move past each other, asperities engage to form new junctions as fast as old ones are ruptured, so that the load W is continuously supported and the tan- 

In physical terms, the behavior of a strong junction is governed by the dominant influence of the tensile phase of junction rupture. A weak junction is one in which translational motion of one asperity relative to the other can occur by sliding while the junction is still under compressive stress. In analytical terms, remains at a lower level and the peak of the curve for p. is broader than is found for a strongly adhesive junction. For either a strong or a weak junction, substitution of the results of the stress/strain analysis into Eqns 12-56a and 12-56b gives a steady-state, non-fluctuating value for the coefficient of friction. 

In functioning machinery the contacting parts repeatedly rub one another many times. The interaction of two surfaces on reiterated contact will in part depend on the condition in which the previous iteration left them. Under ordinary circumstances, with the machinery operating satisfactorily, each iteration is much like the one before and an analysis of steady-state wear or friction can be made on the basis of one cycle of surface interaction. Generally in such cases, but not necessarily always, asperity deformation is elastic rather than plastic. Whether an adhesive junction forms depends on the condition of the asperity surface. If the materials f e.n. 4e are easily adhesive but the surfaces are covered by a film which inhibits adhesion, then to initiate adhesion obviously the film must first be removed, broken up or penetrated. The subsequent course of adhesive contact will then be governed by such factors as the size of the contact, the shape of the asperity, the impressed load, the strength of the material, etc., in accordance with the fundamental modes of behavior. 

Timoshenko and J. N. Goodier, Theory of Elasticity, 2nd Ed., McGraw-Hill, New York, 1951, pp. 351 

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