Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 77

• Friction varied directly with viscosity; it was proportional to velocity at lower speeds but
  varied inversely with velocity at higher speeds [65].

As the surfaces move closer together, the lubricant is squeezed out from between them. Some addi-
tives, when adsorbed onto the surface, display a molecular orientation perpendicular to the surface
that reduces the level of contact and hence lowers the friction. Such products are known as friction
modifi ers. Those additives effective in reducing wear and (usually) friction in the mixed friction
zone are called antiwear additives, whereas products effective in reducing wear (and increasing
seizure loads) in the boundary lubrication process are known as extreme-pressure additives. How-
ever, due to the importance of temperature in the lubrication process, it has been pointed out in the
past that the latter should, perhaps, be better described as extreme-temperature additives.
The temperature at which an additive reacts physically or chemically with the metal or metal
oxide surface signifi cantly affects its activity. Each AW/EP additive type has a range of temperature
over which it is active (Figure 3.7) [66]. The lowest temperature in the range would normally be
the temperature at which physical adsorption takes place. This can occur at ambient or at higher
temperatures depending on the polarity of the additive and the impact on surface energy. The
greater the reduction in surface energy, the stronger will be the absorption of the surface fi lm and
the greater will be the likelihood that the additive remains in place for a chemical reaction with the
surface. Additives that are only weakly bound to the surface may desorb as the temperature rises
and cease to function further in the wear-reducing process.
As the temperature increases so does the surface reactivity. Fatty acids and esters react at fairly
low temperatures to produce metal soaps followed by chlorine-containing compounds (to form
chlorides), phosphorus (as phosphates, polyphosphates, and/or phosphides), and, fi nally, sulfur,
which reacts at very high temperatures to form metal sulfi des [66].
Chlorine-based additives can be fi lm-forming even at ambient temperatures, but as the tem-
perature rises they become aggressive and, with the release of HCl, can cause signifi cant corrosion.
Although the FeCl 2 fi lm has a fairly well-defi ned melting point at 670°C, the optimum operating
temperature is much lower. Klamann [67] indicates that the effi ciency of metal chlorides starts to

Friction

Viscosity × speed

Boundary

friction

0.15−0.25

0.001−0.002

Minimum fluid

friction

Boundary lubrication

Mixed

Hydrodynamic lubrication

(Fluid film)

Regions of lubrication

load

(ZN/P)

FIGURE 3.6 Relationship between coeffi cient of friction and ZN/P.