How does lubrication reduce friction between moving surfaces?

Lubrication reduces friction and wear by interposing a substance between sliding surfaces to prevent direct contact. The effectiveness of this process depends on the type of lubrication:1

• Fluid-film lubrication: A fluid (liquid or gas) creates a film that completely separates sliding surfaces. The pressure within this film balances the load on the surfaces, either through external supply (hydrostatic) or the motion and shape of the surfaces themselves (hydrodynamic). Under these conditions, friction is significantly lower and is directly proportional to the fluid's viscosity.23
• Boundary lubrication: This occurs when surfaces are not fully separated, such as during machine startup or shutdown. Here, friction is determined by the chemical properties of the lubricant and the surfaces rather than viscosity.43

Beyond separating surfaces to prevent solid-to-solid contact and abrasion, lubricants also control corrosion by coating parts with a protective film and help manage heat by carrying it away from the contact area.3