Boundary lubrication is a condition in which a lubricant film becomes too thin to provide total separation. This may be due to excessive loading, speeding or a change in the fluid's characteristics. In such a situation, contact between surface asperities occurs.

Boundary lubrication occurs when the lubricating film is about same thickness as the surface roughness such that the high points (asperities) on the solid surfaces contact. This is generally an undesirable operating regime for a hydrostatic or hydrodynamic bearing, since it leads to increased friction, energy loss, wear and material damage. But, most machines will see boundary lubrication during their operating lives, especially during start-up, shutdown and low speed operation. Special lubricants and additives have been developed to decrease the negative effects of boundary lubrication.

Boundary lubricants generally have long, straight, polar molecules, which will readily attach themselves to the metal surfaces. The lubricant molecules will form a thick protective layer that resembles a molecular shag carpet.

The most common boundary lubricants are probably greases. Greases are so widely used because they have the most desirable properties of a boundary lubricant. They not only shear easily, they flow. They also dissipate heat easily, form a protective barrier for the surfaces, preventing dust, dirt, and corrosive agents from harming the surfaces.