Introduction to Tribology

There are several topics that are integrally related to the core areas of friction, wear and lubrication, but that deserve their own description. These are surface roughness, contact mechanics and nanotribology. Each topic will be briefly introduced here.

The behavior of sliding interfaces can be significantly affected by the roughness of the surfaces of the two bodies. Surface roughness is typically measured using profilometry and is often reported as the root-mean-square value of the height of the surface features, or asperities. Other parameters that describe surface roughness are the average of the surface heights and the skewness and kurtosis of the distribution of surface heights. The effect of roughness on friction and wear is dependent on the type of sliding. For example, larger roughness will increase friction and wear in an interface that is dominated by abrasion while it may decrease friction and wear in an adhesive interface. Regardless, roughness is a key property of any sliding interface.

Another property of contacting bodies that can affect sliding is elastic deformation. Many tribological interfaces are subject to very high loads and, more importantly, high pressures. In these cases, the bodies themselves deform elastically, which can facilitate sliding in a lubricated interface. This situation is called elastohydrodynamic lubrication, since the film thickness is due to both the elastic deformation of the bodies and the hydrodynamic flow of the fluid. Elastic deformation is described by classical contact mechanics, typically Hertz contact theory, which enables us to predict the amount of deformation for a given geometry, elasticity and load.

An emerging field within tribology is friction, wear and lubrication at the nanoscale, called nanotribology. Nanotribology is relevant to a variety of novel small-scale devices as well as characterization tools, all of which rely on the nanoscale contact between two materials to function. Nanotribology is also scientifically fascinating because some of the "laws" that we use to describe larger-scale tribological phenomena no longer apply at the nanoscale. A significant amount of research in this area is performed using an atomic force microscope, where an extremely sharp probe interacts with a surface. The contact between the probe and surface is only nanometers in size. Such contacts have been found to exhibit unique and sometimes counter-intuitive behavior, and understanding that behavior is the focus of many exciting research projects currently underway.

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