Boundary lubrication

Brief description of lubrication

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in relative motion to one another by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. When we talk about (adequate) lubrication smooth continuous equipment operation is assumed, with only mild wear, and without seizure or break of any part of the equipment due to excessive stresses within the lubricated conjunctions. When such a catastrophic event does occurs it means that the lubrication has broken down.

The regimes of lubrication

When progressively increasing the load between the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

*Fluid film lubrication In this regime the lubricant fluid carries the externally applied load entirely either with the help of an external pressure, the case which is called Hydrostatic Lubrication, or totally by the hydrodanamic forces acting in the conjunction due to the relative movement of the surfaces and the viscous resistance of the fluid Hydrodynamic lubrication. [San Andrés. L. "Introduction to pump rotordynamics, Part i. Introduction to hydrodynamic lubrication". ("MEEN626 Lubrication Theory Class:Syllabus FALL2006"). [http://phn.tamu.edu/me262/Notes_pdf/Introduction.pdf] (11 Dec 2007)]

*Elastohydrodynamic lubrication The opposing surfaces are separated but there occurs some interaction between the raised solid features called "asperities", and there is an elastic deformation on the contacting surface enlarging the load bearing area whereby the viscous resistance of the lubricant becomes capable of supporting the load.

*Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenatious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided.

It is also defined as that regime of lubrication in which "the load is carried by the surface asperities rather than by the lubricant" [cite book|author= Bosman R. and Schipper D.J.|title=Microscopic Mild Wear in the Boundary Lubrication regime|edition=|publisher=Laboratory for Surface Technology and Tribology, Faculty of Engineering Technology, University of Twente, P.O. Box 217, NL 7500 AE Enschede, The Netherlands |year=|id=]

In fact the film consists of more than one single layer. Three layers are distinguished:
*a physically adsorbed layer
*a chemically adsorbed layer, and
*a chemical reaction layer

References