Some Further Aspects of Friction

An interesting aspect of friction is the manner in which the area of contact changes as sliding occurs. This change may be measured either by conductivity, proportional to if, as in the case of metals, it is limited primarily by a number of small metal-to-metal junctions, or by the normal adhesion, that is, the force to separate the two substances. As an illustration of the latter, a steel ball pressed briefly against indium with a load of IS g required about the same IS g for its subsequent detachment [37]. If relative motion was set in, a value of S was observed and, on stopping, the normal force for separation had risen to 100 g. The ratio of 100 IS g may thus be taken as the ratio of junction areas in the two cases. 

Even if no perceptible motion occurs (see later, however), application of a force leads to microdisplacements of one surface relative to the other and, again, often a large increase in area of contact. The ratio F/W in such an experiment will be called since it does not correspond to either the usual ns or can be related semiempirically to the area change, as follows [38]. We assume that for two solids pressed against each other at rest the area of contact Aq is given by Eq. XII-1, A W/P. However, if shear as well as normal stress is present, then a more general relation for threshold plastic flow is 

The constant a can be estimated by applying a load so as to establish an Aq and then removing the load and measuring the force Fq to cause motion. Under 

For steel on indium, ot was about 3, while for platinum against platinum, it was about 12 [4]. 

For very clean metal surfaces, m should approach unity, and /t becomes very large, as observed with even a small decrease in m, y, falls to about unity, or to the type of value found for practically clean surfaces. And if a boundary film is present, making m 0.2, Eq. XII-11 reduces to 

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