Coating Characteristics - Wettability

In addition to the chemical reactivity of a surface, the surface tension, or surface energy may be of importance. This is frequently measured as the contact angle between the air and the liquid of a droplet on a surface, which defines the wettability of the surface.

This has been recognized as sometimes being an important factor influencing its tribological characteristics. It influences the behaviour of any liquid lubricants present. It can affect thermal transfer, such as in heat exchanger systems, and increasingly it is being studied in terms of the ability of surfaces to ‘self-clean’.

Although usually simply determined by the contact angle of a droplet of liquid in contact with a solid surface, in practice the wetting angle is a thermodynamic variable which is given by the adhesion energy of the solid/liquid interface minus the liquid surface tension. The wettability is influenced by the surface state, e.g. the roughness, its degree of cleanliness and the degree of oxidation, and it can be assumed that both friction and wear can be affected by changes in the wettability or hydrophobicity of a surface.

Especially, since the surface energy of a material depends on the nature of the medium on either side of the material boundary, it is clear that the wettability of a surface is subject to changes in theoperating environment.

When considering the subject of wettability in tribology, there is a tendency to think of liquid/solid contacts at the non-elevated temperatures experienced by lubricants.

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