asperity

Asperity is a small projection on a solid surface that can contact another surface. In everyday contact between rough surfaces, actual contact occurs only at a collection of asperities, while the rest remains separated by microscopic gaps. The term comes from Latin asperitas, roughness.

In tribology, asperities govern friction, wear, and heat generation. When a load is applied, asperities deform elastically or plastically, altering contact area and stress concentration. Because many micro-contact points share the load, the real contact area is only a fraction of the apparent macroscopic area. Models such as the Greenwood–Williamson framework describe the distribution and behavior of asperities, while surface engineering aims to reduce or control asperity interactions through polishing, coating, or texturing.

In geology and seismology, asperities on fault surfaces are patches that resist sliding during tectonic loading. The failure of asperities can initiate slip along the fault and generate earthquakes; the size, strength, and spatial distribution of asperities influence rupture propagation and recurrence intervals.

Roughness and asperity characteristics are measured with profilometry, scanning electron microscopy, or atomic force microscopy, and described statistically by parameters such as average roughness (Ra) and asperity height distributions. Understanding asperities is essential for predicting friction, wear, seals, and seismic hazards.