Definition - What does Asperity mean?
An asperity is a rough portion of material. Even when materials are polished very finely and appear to be completely smooth, they still have asperities at the microscopic level. The size of an asperity has a very strong effect on the way the material behaves when placed into contact with another material, and can contribute to thermal or electrical constriction resistance.
Large asperities can also increase the risk of corrosion. Some substances can be trapped in the recesses of the asperities. If the substances trapped happen to be chemically reactive, they can cause corrosion that starts in the asperity and then propagates throughout the material until failure occurs.
Corrosionpedia explains Asperity
Asperities are always present on any material; therefore none are completely free of asperities. If a material were completely free of asperities, then the reduction of energy due to friction would be zero. Reducing the number or size of the asperities decreases the friction. When asperities are large and extremely uneven the friction that occurs when an object slides across another object is large. Components subjected to friction such as rotating shafts are often polished to reduce the amount of wear and energy loss. Lubricants can also help reduce the effect asperities have on friction.
Asperities also cause incomplete contact to occur among materials, which can result in an increase of resistance between the components. The resistance created by the asperities can be electrical resistance, thermal resistance, or both. Care must be taken when designing electrical circuits or heat sinks to reduce the size or number of asperities in order to ensure that energy loss due to resistance is minimal.