How does fluid-film lubrication keep sliding surfaces apart?

Fluid-film lubrication keeps sliding surfaces apart by interposing a film of fluid—typically a liquid or gas—that completely separates the surfaces. To maintain this separation, the pressure within the lubricating film must effectively balance the load applied to the surfaces.1

The mechanism for generating this pressure determines the type of lubrication:

• Hydrostatic lubrication: The pressure is supplied by an external source.1
• Hydrodynamic lubrication: The pressure is generated by the shape and motion of the sliding surfaces themselves, relying on the viscous properties of the lubricant.1

By increasing the distance between surfaces, this film prevents solid-to-solid contact and reduces damage from surface asperities and abrasive contaminants. Under these conditions, friction is significantly lower than in unlubricated states and is directly proportional to the fluid's viscosity.2