Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (eg graphite, MoS₂) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (greases) or exceptionally a gas.

In the most common case the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces.

Lubrication can also describe the phenomenon such reduction of wear occurs without human intervention (aquaplaning on a road).

The science of friction, lubrication and wear is called tribology.

When we talk about (adequate) lubrication smooth continuous equipment operation is assumed, with only mild wear, and without excessive stresses within the lubricated conjunctions to cause seizure at the conjunction, or break of any part of the equipment, and when such a catastrophic event does occur 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 is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid-solid contact is avoided.
• Hydrostatic lubrication is a special case of fluid film lubrication in which an external pressure is applied to keep the lubricant in the conjunction, enabling it to support the external load.
• Hydrodynamic lubrication is also a special case of fluid film lubrication which occurs when the lubricant is able to support the load without external pressure, through hydrodynamic forces alone, which deform the shape of the interposing lubricant film into a wedge shape and drags the lubricant into the film, so that the externally applied load can be supported.
• 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.