TOPOGRAPHIC APPROACH TO ADHESION.

There are two basic forces which contribute to adhesion. The first is chemical (primary force); the second is physical (secondary force) and is sometimes referred to as the Van Der Waal's force. A third type, mechanical interlocking, does play some part in adhesion. Of these forces, the chemical forces are by far the strongest. There are three types of chemical forces: ionic, covalent, and metallic. These types of bonding forces operate over very short distances and are basically the interaction of electrons and their nuclei. The second group of forces, referred to as physical forces, are electrostatic in nature. These are: the dipole-dipole, the dipole-molecule, and the molecule-molecule. It is these types of forces that contribute most to the bonding between metals and nonmetals. They operate over larger distances and are of a smaller value. Theoretical considerations are illustrated by photomicrographs.