A study of the microtopography of some surfaces by means of a friction technique

Abstract

A range of results has been obtained for a variety of surfaces by means of a friction tech­nique of surface examination in which elastically deforming surface asperities slide against a smooth incompressible plane surface. The effect of the size of the emery grain on the abrasion of sheets of Perspex was investigated in friction experiments using rubber negative replicas of the abraded surfaces. It was found that the roughness index γ/μ 2 where γ is a dimensionless constant and μ is the coefficient of friction was about 40 times as great for surfaces abraded with 40-grade emery cloth (average grain diameter 580 μ m) as for surfaces abraded with 00-grade emery (grain diameter 11 μ m). The variation of γ/μ 2 with load was also entirely different in these two extreme cases. For the smoother replica the asperities were nearly coplanar but for the rougher surface a marked asperity distribution in depth had occurred so that high loads were required to bring all the asperities into contact with the slider. It was found that the negative replicas were consistently rougher than the corresponding positive ones and reasons for this connected with the abrasion process have been suggested. The different results given by abraded steel and Perspex surfaces can be explained as due to the difference in hardness of the two materials. Turned surfaces have also been examined and the variation of γ/μ 2 with load found to be characteristic of the shape of the turning grooves in the surface, giving a result quite different from that obtained with abraded surfaces, where the grooves were roughly cylindrical in shape. It has been concluded that the technique can give a general quantitative index of roughness as defined by γ/μ 2 for any type of surface. If the interpretation of the results is carried further it can give in many cases (where the asperities are roughly coplanar) the average geometry of the asperities.