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EP lubrication is a "boundary lubrication" phenomenon in which the full fluid film is absent, and in which additives of the film-forming EP additive types chemically combine with the metal surfaces, thereby preventing welding and galling of those surfaces. Sulfur-Phosphorus, the Chlorines, ZDDP, MoTDC, and SbDTC all fall under this category.
EP lubrication for the dispersed additives involves an electrostatic attraction of the dispersed molecules to the metal surface. These materials, such as borates, calcium complexes, or molybdenum disulfide (powder) are suspended in the oil carrier and literally coat the metal surfaces after attraction.
Friction Modifiers, Extreme Pressure additives, and Anti-Wear additives are all friction modifiers to one extent or another, but anti-wear additive films do not provide protection under long-lasting or sustained pressures, due to attrition. However, they will quickly reform providing enough additive reserve was included. So at times the delineation between AW and EP additives is not always clear and somewhat arbitrary. EP films, under extreme pressure, stay intact at higher temperatures and form/reform these films rather quickly after the pressure has passed. Most EP films also form at room temperature, unlike engine oil AW or FM additives. So we can say that the major differences between AW and EP films are that EP films:
a.) Form/reform more quickly
b.) Act or get to the base metal at room temperature, i.e., does not require
elevated pressures/temperatures to start the film reaction (the exception is the phosphate esters)
c.) Act as anti-galling, anti-welding additives
d.) Act/react when the pressures or loads are above what the AW additives can handle.
e.) AW additives form plastic films which "glide," whereas EP additives form films which shear or "slide."
