Lubricant Additives

78 Lubricant Additives: Chemistry and Applications

drop above 300°C and that the friction coeffi cient at 400°C is already a multiple of the optimum
value. However, the dry friction coeffi cient of the chloride fi lm is substantially lower than that for
iron sulfi de (Table 3.4) [68]. The relatively low friction associated with this fi lm is probably one
reason why chlorinated products are so effective as EP additives. Phosphorus, by comparison, does
not react until at higher temperatures and then at slower rates. However, the upper temperature limit
of ~550°C in an air environment is thought to be a result of the oxidation of the carbon in the fi lm
rather than the degradation of a metal soap (Forster, N.H., Private Communication, July 2007).
The soaps, phosphates/phosphides, chlorides, and sulfi des formed on the metal surface
were originally considered to produce a lower melting and less-shear stable fi lm than that of the
metal/metal oxide. This fi lm would cause a smoothing of the metal surface that was then able to
support a higher unit loading. This is now thought to be an oversimplifi ed explanation as research
has found the EP fi lms to be considerably different to those postulated and without the expected
lower shear stability [69]. What it certainly does not consider are additional “subprocesses” of
removal of the fi lm by mechanical wear and its possible regeneration in situ by further action of the
AW/EP additive (Figure 3.8).
Since surface temperature is largely dependent on load, additives that might be effective at
high loads may be completely ineffective at low loads (and vice versa). Under such circumstances,
therefore, signifi cant wear could occur before the load-carrying properties of the EP additive come
into play. To minimize this effect, additives are often used in combination, resulting in extending
the temperature (and load) range over which they are active.

Mineral base oil

Fatty acids

Cl S

Cl + P + S + fatty acids

P

0 100 200 300 400 500 600 700 800 90 1000 1100

Temperature (°C)

Friction coefficient

0

0.1

0.2

0.3

0.4

0.5

FIGURE 3.7 Effect of temperature on EP additive activity. (From Mandakovic, R., J. Syn. Lubr., 16(1),
13–26, 1999. With permission.)

TABLE 3.4

Corrosion Films Formed on Sliding Iron Surfaces

Lubricant Type Nature

Friction Coeffi cient

(Dry)

Melting Point

(°C)

Dry or hydrocarbon Fe 1.0 1535

FeO 0.3 1420

Fe 3 O 4 0.5 1538

Fe 2 O 3 0.6 1565

Chlorine FeCl 2 0.1 670

Sulfur FeS 0.5 1193

Source: Fundamentals of Wear, Lubrication, 12(6), 61–72, 1957. Permission from Chevron.