Adhesion phenomena in bonded joints

Adhesive bonding is a key joining technology in many industrial sectors including the automotive and aerospace industries, biomedical applications, and microelectronics. Adhesive bonding is gaining more and more interest due to the increasing demand for joining similar or dissimilar structural components, mostly within the framework of designing lightweight structures. When two materials are brought in contact, the proper or adequate adhesion between them is of great importance, so it is necessary to device ways to attain the requisite adhesion strength between similar or dissimilar materials including the different combinations of metallic materials, polymers, composite materials and ceramics. To make adhesion possible, it is necessary to generate intrinsic adhesion forces across the interface. The magnitude and the nature of those forces are very important. From a thermodynamic standpoint the true work of adhesion (or intrinsic property) of the interface create free surfaces from the bonded materials. Adhesion mechanisms have been known to be dependant on the surface characteristics of the materials in question. The intrinsic adhesion between the adhesive and substrates arises from the fact that all materials have forces of attraction acting between their atoms and molecules, and a direct measure of these interatomic and intermolecular forces is surface tension. Atomic/molecular understanding of adhesion should be extremely beneficial in selecting or creating the appropriate materials to attain the desired adhesion strength. In the present paper, the following topics are reviewed in detail: (a) the surfaces or interfaces of similar and dissimilar materials, (b) adhesion or bonding mechanisms in the adhesive joints (c) thermodynamic theory of adhesion: surface tension or surface free energy concepts including the wetting, wetting criteria, wettability, and thermodynamic work of adhesion, (d) dispersion and polar components of surface free energies, and finally (e) effect of surface roughness on wettability or adhesion.