Boundary lubrication

Brief description of lubrication

Lubrication is the process of reducing wear of surfaces which are in moving contact with each other, by interposing a lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...

between the surfaces to more evenly spread the load (pressure generated) between the surfaces.

Adequate lubrication is obtained when the relative motion of the parts is smooth and continuous, with only mild wear. If wear is excessive, or if the parts seize up or break, the lubrication has been inadequate or has broken down.

Lubrication regimes

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, which is called Hydrostatic Lubrication, or totally by the hydrodynamic forces acting in the conjunction due to the relative movement of the surfaces and the viscous resistance of the fluid, which is called Hydrodynamic lubrication.

  • Elastohydrodynamic lubrication In this regime the opposing surfaces are separated by the film of lubricant, but some interaction occurs between the raised solid features called asperities, and there is an elastic deformation on the contacting surface enlarging the loadbearing 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 tenacious layer or film on the moving solid surfaces (boundary film) which is capable of supporting the load; thus 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.

The lubricating 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
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