Solid Surfaces

Abstract

SURFACE ROUGHNESS §98.It was pointed out in §65 that liquid surfaces are in a state of permanent agitation and, consequently, are not absolutely smooth. Analogous vibra tions, although with much smaller amplitudes, take place on solid surfaces also. Reflextion of low-energy electrons (of, for instance, 100 electron volts) from metals shows 1 (see also Reference 2) that the amplitude of vibrations, normal to the surface, of the exterior atoms of palladium is about 0.14 A while the corresponding amplitude in the bulk is near 0.07 A. The results for platinum, lead and so on are similar.However, the roughness caused by molecular motion is negligibly small compared with the mechanical rugosity of all solids, which does not vanish when a time-average is taken. Because of the high viscosity or the high yield stress of solids, their profile usually remains almost unaltered for months or years and, as a rule, demonstrates no tendency to become a straight line. Roughness is perhaps the most striking property of solid surfaces. Several books 3 " 6 have ably reviewed it. In this section, the main geometrical para meters of rugosity are listed; the following seven sections deal with the main methods of measuring these and related parameters, and some typical numeri cal data are presented in §106.Let A A in Fig. 1 be the profile of a solid surface magnified perhaps, 1000 times. If all hills on it were rased and employed to fill in the valleys, the new, perfectly smooth boundary would have been BB. This is known as the main plane or the mean surface of the sample. If it really is a plane, the actual surface (that is AA) has rugosity but no waviness; and if BB itself has (macroscopically) concave and convex segments, as a file has, both waviness and roughness are present. Both can be measured together, but only the latter is considered in the following. METHODS FOR DETERMINING SURFACE ROUGHNESS 171Bearing-area curves 8 can be computed from surface profiles, if the mag nification is much greater than in Fig. 1; see Fig. 2. The line y = 0, identical y FIG. 2. Determination of the bearing area curve from the surface profile (shown by the irregular curve passing through a and b). Abscissa: distance from a point along the plane which is parallel to the mean surface and includes the bottom of the deepest indentation. Ordinate: height above this plane. Only p% of the dotted line (and its continuation) is within the solid. Values of ρ plotted against y form the bearing area curve. After Bikerman. 78with the abscissa, lies wholly within the solid. All other straight lines, parallel to it and situated above it, pass partly within the solid and partly outside it; thus, only ρ % of the line determined by y = y t is in the solid body. When y is plotted as a function of /?, a bearing-area curve results, see Fig. 3. FIG. 3. Bearing area curves. 1: Johansson block. 2: lapped metal. 3: ground metal. After Abbott and Firestone. 8METHODS FOR DETERMINING SURFACE ROUGHNESS §99. As the effects of surface tension are so pervading in liquids, the metho...