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Abstract
The dependence of the measured thermoelectric voltage, generated during sliding between two dissimilar contacting metal surfaces, on the individual flash temperature rises occurring at the load-bearing junctions has been investigated from a theoretical viewpoint. Emphasis has been placed on analysis of the four limiting cases of sliding contact (plastically and elastically-deformed contacts at high and low-speed sliding). In all cases, when there are numerous contacts, it is shown that the measured thermoelectric voltage is effectively a measure of the arithmetic mean flash temperature rise, although the distribution of the load amongst the various load-bearing junctions must be considered. For practical sliding arrangements, where the radii of the load-bearing junctions may be assumed to have a Gaussian distribution, in the two limiting cases of low-speed sliding contact, the measured thermoelectric voltage, V Tn, may be expressed exactly in terms of the thermoelectric power of the metal pair, S, the mean junction flash temperature rise, Tf(mean), the mean junction radius, a, and the standard deviation of the junction radius or asperity peak-height distribution, a. For the low-speed, plastic-deformation sliding mode,
VTR = ST f(mean)[1+σ2/(ā)2]
while, for the low-speed, elastic-deformation sliding mode,
VTR = ST f(mean)[(ā)2 + 3σ2/(ā)2 + σ2]
It is shown that thermoelectric voltage measurements may also be employed to evaluate the number of load-bearing contacts. The results of some practical experiments are presented and discussed. In particular, for reciprocating, sliding conditions under a load of 1.5 kg, amplitude of 2.5 mm and speed of 500 double traversals per minute, contact involves approximately 50 asperity-asperity junctions, giving a mean surface flash temperature rise of 5 K.