In the context of this article, adhesion, friction, and wear are defined as follows: Adhesion is the resistance against separation of two bodies in a direction normal to the common interface. Friction is the resistance against motion of the contacting bodies in the direction of the common interface. Wear is the gradual change of geometry and loss of material due to the different contact processes that may occur in moving contacts.

Bowden and Tabor have shown convincingly that adhesion, friction and wear are interrelated at least to some extent. This article will review some of the current arguments that support this notion.

If surfaces are not separated by a viscous film, both friction and wear have their origin in the contact phenomena occurring at the roughness asperities of the sliding bodies.

Within one contact cycle, three primary events can be distinguished:

1. In order to obtain a (normal) load-carrying capacity, the contacting asperities must be compressed or indented (Figure 1), similar to a Brinell hardness test.

2. At the beginning of the contact cycle, the asperities interfere in the direction of sliding (Figure 2). Thus during the first half of the contact cycle, frontal loading of the asperities occurs (Figure 2b).

Particularly if the resulting deformation is plastic (as it will usually be for metals, see Reference 1), the net effect of this contributes to friction, the so-called “ploughing” effect.