The True Contact Area Between Solids

Abstract

A new method for observing the points of real contact between two solid surfaces is described. A metallized glass surface is pressed into contact with the specimen, the glass surface being deformed locally at the points of contact. The opposite side of the metallized surface is examined through the glass by phase contrast microscopy, the deformations showing bright on a dark background. Deformations of a few angstroms in depth are detectable if their lateral extension is above the resolving limit of the microscope. Nominally flat, ground, polished, and lapped specimens of silver steel have been examined. With each specimen true contact was made within regions the sizes and distribution of which were determined by the general undulations of the surface and by the magnitude of the applied load. The differences lay in the distribution of the local contact areas within the general regions. With the polished specimen true contact was made over almost the whole of the general region but with the ground specimen it was confined to a few isolated areas along the grinding ridges. True contact with the lapped specimens was made at a very great number of areas each a few microns in linear dimension.