is a form of lubrication between two-(2) rubbing surfaces without development of a full-fluid lubricating film. See full-fluid-film lubrication, mixed film lubrication, ZN/P curve. Boundary lubrication can be made more effective by including additives in the lubricating oil that provide a stronger oil film, thus preventing excessive friction and possible scoring. There are varying degrees of boundary lubrication, depending on the severity of service. For mild conditions, oiliness agents may be used; these are polar compounds that have an exceptionally high affinity for metal surfaces. By plating out on these surfaces in a thin but durable film, oiliness agents prevent scoring under some conditions that are too severe for a straight mineral oil. Compounded oils, which are formulated with polar fatty oils, are sometimes used for this purpose. Anti-wear additives are commonly used in more severe boundary lubrication applications. High quality motor oils contain anti-wear additives to protect heavily loaded engine components, such as the valve train. The more severe cases of boundary lubrication are defined as extreme pressure conditions; they are met with lubricants containing EP additives that prevent sliding surfaces from fusing together at high local temperatures and pressures.

At a sufficiently large scale, surfaces of even highly finished steel are seen as a pattern of "peaks" and "valleys". When two un-lubricated surfaces, say a ball and a raceway, are pressed together contact occurs between the "peaks" or asperities. The percentage of actual contact area is relatively small compared to the overall "footprint" area of the contact, except under the highest of loadings. The load distribution over these asperity contacts results in extremely high-localized stresses and correspondingly high temperatures as asperity deformation takes place. Asperities are thought to weld together and as one body slides against the other, the weaker asperity breaks, forming a wear particle. This is the adhesive wear process and the primary mode of surface wear in the boundary lubrication regime. Further abrasive wear may occur as a secondary phenomenon due to the plowing action of the adhesive wear debris

Lubrication under boundary conditions is afforded by the metallurgical and chemical nature of the manufactured surfaces in contact. Bearing steel substrates are modified at their surfaces by physical working during the manufacturing process as well as by chemical reactions with the environment. These thin films of molecular scale determine the degree of asperity welding and therefore the adhesive (and subsequent abrasive) wear rates. Practical application of boundary lubricants include chemical or physical surface treatments to form surface films, which act as diffusion barriers to prevent asperity welding

Also see: Film Lubrication and Mixed Film Lubrication.