Ashless Antiwear and Extreme-Pressure Additives 215
distinction between them and antiwear additives; only an expectation of a performance boost
under severe conditions. EP/antiscuffi ng additives tend to be very reactive, and some can have
adverse effects on oxidative stability of oils, can be corrosive to nonferrous materials, and can
reduce the fatigue life of bearing and gear surfaces. They should only be used when severe dis-
tress is a distinct possibility.
Antiwear additives function in various ways. Some deposit multilayer fi lms thick enough to
supplement marginal hydrodynamic fi lms and prevent asperity contact altogether. Some develop
easily replenishable monolayer fi lms that reduce the local shear stress between contacting asperi-
ties and are preferentially removed in place of surface material. Others bond chemically with the
surface and slowly modify surface asperity geometry by controlled surface material removal until
conditions conducive to hydrodynamic fi lm generation reappear.
EP additives are designed to prevent metal–metal adhesion or welding when the degree of
surface contact is such that the natural protective oxide fi lms are removed and other surface-active
species in the oil are not reactive enough to deposit a protective fi lm. This is most likely to occur
under conditions of high-speed, high-load, or high-temperature operation. EP additives function
by reacting with the metal surface to form a metal compound such as iron sulfi de. They act in a
manner similar to that of antiwear additives, but their rate of reaction with the metal surface and
therefore the rate of EP fi lm formation are higher and the fi lm itself is tougher. Some EP additives
prevent scoring and seizure at high speed and under shock loads; others prevent ridging and rippling
in high-torque, low-speed operations. In both cases, EP additives and surface metal are consumed,
and a smoother surface is created with an improved chance of hydrodynamic action, resulting in
less local distress and lower friction. In the absence of such additives, heavy wear and distress well
beyond the scale of surface asperities would occur, accompanied by very high friction.
A wide variety of antiwear and EP additives are commercially available, and many other chemi-
cals with antiwear and EP functionality have been reported in the literature and in patents. To be
commercially viable, additives must be adequately soluble in lubricant formulations and reasonable
in cost, must neither overly reduce the lubricant’s oxidative stability nor increase the corrosivity of
metals contacted by the lubricant [1–3]. Lead naphthenates were extensively used early in the indus-
try’s history, but environmental concerns have led to their virtual disappearance. Similarly, chlorine-
containing additives are in decline. Zinc dialkyldithiophosphates (ZDDPs or ZnDTPs) are the best
known and most widely used antiwear additives in engine oils, transmission fl uids, and hydraulic
oils. However, the concern for phosphorus poisoning of automotive catalysts and for zinc as an envi-
ronmental contaminant has resulted in a pressure to fi nd metal- and phosphorus-free replacements
for both automotive and industrial applications. This has resulted in a move toward ashless antiwear
and EP additives, and this chapter covers these additives in terms of their chemistry, properties, and
performance characteristics, applications, marketing, sales, and outlook.
8.2 CHEMISTRY, PROPERTIES, AND PERFORMANCE
(CLASSIFIED BY ELEMENTS)8.2.1 SULFUR ADDITIVES
Sulfur-containing additives are used to provide protection against high pressure, metal-to-metal
contacts in boundary lubrication. The magnitude of the EP activity is a function of the sulfur content
of the additive; high-sulfur-content additives are usually more effective EP agents than are low-
sulfur-content additives. The sulfur content of the additive must be balanced against requirements for
thermal stability and noncorrosiveness toward copper-containing alloys. The additive’s composition
and structure represent a chemical compromise between confl icting performance requirements. In
general, any compound that can break down under an energy-input stress, such as heat, and allow
for a free sulfur valence to combine with iron would do well as an antiwear and EP additive. Sulfur
additives are probably the earliest known, widely used EP compounds in lubricants.