Adhesion between clean surfaces at light loads

Abstract
This paper describes a study of the adhesion between clean surfaces, in a vacuum of 13nPa (10 -10 Torr), the influence of hardness, ductility, chemical bonding and adsorbed vapours. The geometry was that of crossed cylinders so that the contact region could be well localized and changed from experiment to experiment. In addition, electrical re­sistance measurements (in the case of conducting solids) could be used to follow the formation and breaking of contact. The joining load was very small (of order 1 mN) with the result that, even for the softest metals the deformation during loading was primarily elastic. The observed adhesion, however, is found to depend markedly on the ductility of the solid. With hard elastic solids the adhesion is very small indeed: much smaller than that predicted by theory and the results suggest that this may be due to fine-scale surface irregularities scarcely larger than atomic dimensions. With metals with a limited number of slip planes the adhesion is higher but appreciably less than would be expected from the area of the junctions formed. With metals such as gold and copper the adhesion is high and equal to the tensile strength of the interfacial junctions. Adhesion of soft metals to a hard solid such as titanium carbide can be high and when the surfaces are separated fragments of metal are found attached to the harder surface. With other hard solids such as sapphire or diamond the adhesion of copper is appreciably less, indicating that the bonding between copper and these covalent materials is intrinsically weak. Experiments at room temperature show that argon and hydrogen have no detectable effect on adhesion, whereas oxygen has. If one of the pair is copper, oxygen reduces the adhesion if the exposure is relatively heavy. With iron a minute exposure of oxygen reduces the adhesion to a very small value. It is suggested that with copper an adsorbed film is formed which is relatively ineffective. By contrast, on iron, minute iro n oxide nuclei are formed which reduce the amount of metal-metal contact and also interpose an interfacial region of very limited ductility.