Specific Friction Force in a Graphite Brush Contact as a Function of the Temperature in the Contact Spots

Abstract

Measurements by different workers have indicated that the friction coefficient μ of a graphite brush contact decreases with rising contact temperature T. If P = mechanical load, Ab = load bearing area and F = friction force, the average pressure is p̄ = P/Ab and the specific friction force ψ = F/Ab. Hence μ = F/P = ψ/p̄. It will be shown in this paper that the measured μ(T) effect during smooth sliding is caused by the decrease of ψ with rising T. The specific friction force is a function of the character of the sliding surface alone, and μ is, in addition, a function of the actual pressure in Ab. It is shown that sliding, after a relatively short time, leads to a stage during which Ab, thus also p̄, remains practically constant for more than 10 hr. The (μ,T) test begins when such an ``initial stage,'' defined by μ=μ0, T = T0 and constant contact voltage U, has been reached. It involves T increasing above T0 and then decreasing to T0 again, whereby the time of the test is short enough for constancy of p̄. During a perfect (μ,T) test, the forward and reversed curves agree with each other indicating that Ab has remained constant. For all sliding stages with constant p̄, the relationship mentioned above can be written: μ(T) = constψ(T), which makes it possible to study ψ as a function of T by studying μ as a function of T. The decrease of ψ with rising T is explained by assuming that adherence bonds in Ab are broken or loosened by T according to a probability exp(−φ/kT) = exp(−11600 φ/T) with φ in ev. The measured, strong dependence on T results from the activation energy φ=0.07 to 0.1 ev between the graphite ``platelets'' being of the order of kT. Hence it is expected that μ and ψ are inversely proportional to the probability factor. It was found that at constant Ab up to about 500°K (the upper limit measured): ψ(T)ψ(T0)=μ(T)μ(T0)=exp[11600[open pi](1T−1T0)]gives a very good agreement in the case of graphite members when the binding energy is assumed to be φ=0.09 ev. For these contacts, ψ decreases from 0.12 to about 0.065 ton/cm2; simultaneously μ decreases from 0.13 at 115°C to 0.07 at about 200°C. This has been found at atmospheric air pressure with dew point about 279°K and also in vacuum of 10−4 mm Hg with dew point about 75°K. With a copper and gold ring at atmospheric air pressure, the (μ,T) curves have a somewhat steeper slope and agreement with theory requires ψ=0.12 or 0.10 ev, respectively. At temperatures of about 500°K, μ and ψ decrease very slowly and may reach a lower limit at temperatures when the chemisorbed gases on the graphite platelets begin to evaporate.