Adhesion to gold: A review

Abstract

Asulab S.A., Passage Max-meuron 6, CH-2001 Neuchatel, Switzerland The adhesion ofsubstances to gold,, and our understanding ofit, is ofpotentialindustrial importance in a number of diverse fields such as heat transfer surfaces and jewelleiy manufacture.Adhesion is a complex subject and is not fully understood as yet. This explains the many different approaches followed in the field. For instance, in physics we calculate the ideal strength (1) and from a mechanics approach the force of adherence (2), while physical chemists try to understand the molecular interactions which are responsible for the adhesion (3).In the first part of this review the more important results obtained from these approaches will be discussed. These results will relate the force of adherence to the energy of adhesion or the substrate surface tension. The middle section is devoted to surface tension on gold and the controversial matter of whether or not gold is wettable. Recent findings on gold wettability will be discussed and it will be shown that contamination by hydrocarbons is not directly related to the apparent hydrophobicity of the metal. The final part of the article discusses experimental results on the strength of adhesive joints between gold surfaces under various stresses.The review concludes with a presentation of the view that slow superficial reconstruction of gold surfaces may be an important reason for the apparent discrepancies in their behaviourwhich have been recorded.Discussion of the various approaches is of necessity somewhat mathematical. The detail has, however, been kept to a minimum and interested readers should consult the references quoted for a fuller treatment. A glossary of terms used has been compiled to assist those readers less conversant with the subject to follow the arguments presented in this article. The Physics of AdhesionIn considering adhesion from a physical point ofviewwe attempt to discover how the force (or energy) necessary to separate a solid body into two parts can be calculated, from a knowledge of the bulk and surface properties. The solutions derived give the ideal adhesion, as discussed below:The Thermodynamic Solution (Ideal Adhesion)Adamson (1) defines the ideal adhesion as `the adhesion expected under one or another model situation', and some models and theircorresponding solutionswill be considered below. The first could be called the thermodynamic approach as it considers the reversible work of adhesion. In this case the energy required to separate the two parts is equal to the energy necessary to assemble them. Breaking a SolidIn the process of breaking a solid, two new surfaces of energy ys* are created. The energy required is W.• W=2y, 1 2y, is the cohesive energy, w/ , of the solid, and is equal to the work done against the cohesive forces, F^, over the effective range of atomic distance, a, during the breaking process: W, =FF .a.A 2 Thus in this model the energy required to break the solid is equated with the cohesive energy I-Y/, and the corresponding force is 1. This cou...

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