On Model Studies of Metallic Surface Asperities

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Abstract
An apparatus for measuring forces generated at the tips of two cones throughout the sliding‐contact life cycle is described. The cones are used to simulate surface asperities. Test results under atmospheric conditions show the following: (1) For cones with 90° internal angles, the ratio of the integrated value of the horizontal force component over the length of the life cycle to that of the vertical force component, a measure of coefficient of friction after Green, is not sensitive to the presence of lubricants in cases where the cone tips are in line or nearly in line with the direction of relative motion; but the ratio is substantially lowered in the presence of lubricants when the cone tips are far from being in line with the direction of relative motion. The former cases are always accompanied by the formation of macroscopic wear debris. (2) By increasing the internal angle of cones, the ratio defined above is decreased, but its value does not seem to approach zero for the angle approaching 180°, a flat surface. (3) Size effect as discussed by Morrison and Shaw is evidenced in this experiment. The mutual inclusiveness of the classical interlock theory and the popular adhesion theory of friction is discussed. Moreover, the coefficient of friction for a pair of idealized surfaces with asperities in the form of 90° cones and randomly distributed is calculated from considerable amount of test data. While actual surfaces have asperities roughly in the form of cones with internal angles much larger than 90°, right circular cones were used, nevertheless, for simplicity and for amplifying the effects noted.

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