Asperity (materials science)

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The top image shows asperities under no load. The bottom image depicts the same surface after applying a load.

In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (OED, from the Latin asper — "rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, are not truly smooth on an atomic scale. They are rough, with sharp, rough or rugged projections, termed "asperities".

When two macroscopically smooth surfaces come into contact, initially they only touch at a few of these asperity points. These cover only a very small portion of the surface area. Friction and wear originate at these points, and thus understanding their behavior becomes important when studying materials in contact. When the surfaces are subjected to a compressive load, the asperities plastically deform, increasing the contact area between the two surfaces until the contact area is sufficient to support the load.

Friction is also proportional to the size of the asperities, so larger asperities result in increased friction and vice versa. This has resulted in the development of nearly frictionless materials with minuscule asperities.

The Archard equation provides a simplified model of asperity deformation when materials in contact are subject to a force.