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Extreme pressure additives. Applications
under extreme pressure conditions rely on additives.
Lubricants containing additives that protect against extreme
pressure are called EP lubricants, and oils containing
additives to protect against extreme pressure are classified
as EP oils. EP lubrication is provided by a number of
chemical compounds. The most common are compounds of boron,
phosphorus, sulfur, chlorine, or combinations of these. The
compounds are activated by the higher temperature resulting
from extreme pressure, not by the pressure itself. As the
temperature rises, EP molecules become reactive and release
derivatives of phosphorus, chlorine, or sulfur (depending on
which compound is used) to react with only the exposed metal
surfaces to form a new compound such as iron chloride or
iron sulfide. The new compound forms a solid protective
coating that fills the asperities on the exposed metal.
Thus, the protection is deposited at exactly the sites where
it is needed. AW agents in the EP oil continue to provide
antiwear protection at sites where wear and temperature are
not high enough to activate the EP agents.
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