Physical properties of surfaces IV—Polishing, surface flow and the formation of the beilby layer

Abstract

The usual method of polishing surfaces is to rub them together with a fine powder between them. By this process a rough surface, having visible surface irregularities, is changed into one where the irregularities are invisible. If the surface gives specular reflexion the height of these irregularities will be less than half a wave-length of visible light. Beilby has shown that the top layer of the polished solid is “vitreous” in character, it has lost its obvious crystalline properties and has apparently flowed over the surface, bridging and filling up the irregularities in it. The mechanism of the process has been a subject of discussion for many years. Newton (1730, p. 265), Herschel (1830, p. 447) and Rayleigh (1901 a ) considered that polishing was essentially due to abrasion. Beilby (1921, p. 114) considered that the polisher tore off the surface atoms, and the layer below this “retains its mobility for an instant, and before solidification is smoothed over by the action of surface tension”. Adam (1927) has suggested that molecules or small particles are abraded from the surface and subsequently adhere to a different area, so building up an amorphous layer. Hamburger (1932) considered that the abraded units were not molecules but small crystals perhaps 30A in diameter. The view that the temperature is sufficiently high to cause surface melting has been held for some time by Macaulay (1926, 1927, 1931), who was able to detect the products of thermal decomposition of the powder used for polishing glass plates. This evidence has been questioned because many reactions may occur at freshly exposed surfaces due to effects other than temperature. J. W. French (1927) carried out experiments with a thermometer embedded in the polisher, and concluded that the rise in surface temperature was negligibly small. Experiments described in earlier papers (Bowden and Ridler 1935, 1936) provide direct evidence that the surface temperature at the points of contact may, under many conditions of sliding, be sufficiently high to cause a real melting of the metal. The surface temperature was determined by using the rubbing contact of two different metals as a thermo-couple and measuring the e. m. f. developed on sliding. The mass of the metal remained quite cool, the high temperature was localized at the surface of the points of contact. It is just at these points, however, that wear, surface flow and polish occur. The method can, of course, only be applied to metallic conductors, but experiment showed that there was a simple relationship between surface temperature and thermal conductivity. A metal of low thermal conductivity, such as bismuth, showed a much higher surface temperature than copper under the same conditions of sliding. In the case of non-metals such as glass, silk, alumina, etc., which possess a very low thermal conductivity, we should expect the local surface temperature to be very much higher still. If these high local temperatures are reached so easily when one surface is rubbed on another, we should expect it to play an important part in the mechanism of wear, surface flow and polish of solids. The melted or softened solid would be wiped over the surface and would quickly solidify to form the Beilby layer.