Effect of Contamination on the Adhesion of Metallic Couples in Ultra-High Vacuum

Abstract

The present work is concerned with determining the factors controlling the adhesion of metal couples. Most previous studies have been done in the presence of contaminants and, under these conditions, an energy barrier for adhesion has always been observed. This barrier has been ascribed to various factors: the need to disperse the contaminants so that metal-metal contact and, consequently, adhesion, can occur; the energy required to realign the surface metal atoms to form an interfacial bond; and, for lightly loaded conditions, the necessity to overcome the elastic relief stresses which may break any bond formed during the unloading of the couple. A further condition for adhesion which has been postulated, is that the metal couples must be mutually soluble. The present adhesion experiments show that contaminant dispersal is the major barrier to adhesion. Thus, spontaneous adhesion occurred under vacuum conditions for the three systems studied when the surfaces were sufficiently clean, whereas subsequent contamination resulted in nonadhesion. Substantial amounts of contamination could, however, be tolerated. The contaminants may be divided into two classes, stable surface films and mobile gaseous or liquid films. While both are barriers to adhesion, the latter may be removed by application of a vacuum, whereas the former requires a more rigorous treatment. Because adhesion occurred for clean surfaces under vacuum conditions, even where elastic deformation predominated, the postulated energy barrier of adhesion due to the realignment of the surface atoms is considered of minor importance, at least for the softer metals studied here. Further, no evidence for rupture of the bonds by the elastic relief forces on unloading such lightly loaded clean couples has been observed. The latter effect, however, is thought to become more important whenever stable surface films are present, because of the limited metal-metal contact. Since the one immiscible system studied here showed as great a tendency to adhesion as the miscible systems, the condition of bulk miscibility is considered no criterion per se for adhesion.