Lubrication Mechanisms and Lubricants

Abstract

Sliding between clean solid surfaces is generally characterized by a high coefficient of friction and severe wear due to the specific properties of the surfaces, such as low hardness, high surface energy, reactivity, and mutual solubility. Clean surfaces readily adsorb traces of foreign substances, such as organic compounds, from the environment. The newly formed surfaces generally have a much lower coefficient of friction and wear than the clean surface. The presence of a layer of foreign material at an interface cannot be guaranteed during a sliding process; therefore, lubricants are deliberately applied to produce low friction and wear. The term lubrication is applied to two different situations: solid lubrication and fluid (liquid or gaseous) lubrication. Solid LubricationA solid lubricant is any material used as a powder or a thin, solid film on a surface to provide protection from damage during relative movement to reduce friction and wear. Solid lubricants are used for applications in which any sliding contact occurs, for example, a bearing operative at high loads and low speeds and a hydrodynamically lubricated bearing requiring start/stop operations. The term solid lubricants embraces a wide range of materials that provide low friction and wear (Bhushan, 1987a(Bhushan, , 1987bBhushan and Gupta, 1991). Hard materials are also used for low wear under extreme operating conditions. Fluid LubricationA regime of lubrication in which a thick fluid film is maintained, between two sliding surfaces by an external pumping agency, is called hydrostatic lubrication. B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices