boundary lubrication - a form of lubrication between two rubbing surfaces without development of a full-fluid lubricating film. See full-fluid-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 sometime s 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.