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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