A sharp AFM/FFM tip sliding on a surface simulates just one asperity contact. However, asperities come in all shapes and sizes. The effect of radius of a single asperity (tip) on the friction/adhesion performance can be studied using tips of different radii. AFM/FFM are used to study the various tribological phenomena, which include surface/roughness, adhesion, friction, scratching, wear, indentation, detection of material transfer, and boundary lubrication.
Directionality in the friction is observed on both micro- and macroscales and results from the surface roughness and surface preparation. Microscale friction is generally found to be smaller than the macrofriction, as there is less plowing contribution in microscale measurements. The mechanism of material removal on the microscale is studied. Evolution of wear has also been studied using AFM. Wear is found to be initiated at nanoscratches. For a sliding interface requiring near-zero friction and wear, contact stresses should be below the hardness of the softer material to minimize plastic deformation, and surfaces should be free of nanoscratches. Wear precursors can be detected at early stages of wear by using surface potential measurements. Detection of material transfer on a nanoscale is possible with AFM. In situ surface characterization of local deformation of materials and thin coatings can be carried out using a tensile stage inside an AFM.Boundary lubrication studies can be conducted using AFM. Chemically bonded lubricant films and self-assembled monolayers are superior in friction and wear resistance. For chemically bonded lubricant films, the adsorption of water, the formation of meniscus and its change during sliding, viscosity, and surface properties play an important role on the friction, adhesion, and durability of these films. For SAMs, their friction mechanism is explained by a so-called “molecular spring” model.