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Dependence of the static coefficient of friction on the time of stationary contact Davis, Harold Robert

Abstract

The dependence of the static coefficient of friction on the time of stationary contact has been determined using stick-slip vibration to provide a periodic time of stationary contact between two metallic bodies. To describe this time dependence a theory based on the creep by diffusion for metals in contact has been developed. This approach is similar to the adhesion theory presented by many authors as representative of junction strength in friction. Eleven friction-couples were studied; ten couples being very pure metals run against an annealed steel disk. The eleventh couple consisted of a hardened steel slider against the annealed steel disk. The results indicated that, fundamentally, very little difference in static friction values was apparent between the friction-couples. Because of the difficulty in obtaining material properties it was not possible to compare the experimental results with purely theoretical predictions. However, within reasonable experimental accuracy, the shape of the friction-time curves agreed with those predicted by theory. By varying the system parameters it was found that, in general, the static coefficient of friction was load and area independent but seemed to be very dependent on surface finish and the micro—structure of the friction-couples. Notable exceptions to this rule were indium and silver which showed excessive creep under load thus directly affecting the static coefficients of friction. It is important to stress that quantitative results for the friction-growth curves are not applicable since the system parameters affect the results greatly. However, a qualitative idea of the fundamental problems that exist allows a reasonable prediction of friction values to be made.

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