Tribology and surface-related phenomena

From a basic research point of view, the field of tribology involves the contact of surfaces in relative motion, leading to the introduction of surface science concepts, which are coupled with those from other more traditional scientific fields, such as materials science (including metallurgy) and continuum and fluid mechanics. Even if tribology may be considered a multidisciplinary field, 

Hyilrodyrntmic lubrication is based on the formation of a thick lubricant film (typically of thickness from I to 100 pm) which inhibits contact between the moving surfaces. This lubrication mode is governed by the bulk physical properties of the lubricant, mainly the viscosity, and by the speed of the relative motion. 

Elastohydrodynamic lubrication (EHD) occurs when the extent of surface deformation is comparable with the lubricant film thickness, so that a heavy load causes local elastic deformation of the contacting surfaces, but without any significant a.sperity interaction. Taking into account the low lubricant thickness (typically from 0.01 to 10 pm) and the high contact pressures (typically in the GPa range), the lubricant properties differ from tho.se of a traditional bulk liquid. since a strong viscosity increase may occur, when the lubricant behaves more like a solid than a liquid. Moderate temperature rises may occur, thus inducing some thermochemical reactions between the surfaces and the lubricant additives, but no tribochemical phenomena are involved, unlike the next lubrication mode. 

Boundary lubrication is probably one of the most complex phenomena studied by tribologists. in view of the numerous experimental parameters which influence the tribological behavior. Considerable asperity deformation is usually observed, since the contacting surfaces move very close to each other. The role 

Mixed lubrication occurs when the contact behavior is governed by a mixture of EHD and boundary lubrication. Asperity contact and deformation may occur, even when the surfaces are separated by a lubricant film from 0.01 to 1 xm thick. 

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