Ashless Phosphorus-Containing Lubricating Oil Additives 75
Table 3.3 [62] offers a generalized classifi cation of the different chemical types of additives used
to improve lubrication performance, but, depending on the structure of the additive, some variation
in the performance can be expected. In reality, the distinction between AW and EP additives is not
clear-cut. AW additives may have mild EP properties, whereas EP additives can have moderate
AW performance, and both produce coatings on the metal surface. In fact, EP additives have been
described as additives that reduce or prevent severe wear [63]. However, as seen from the Table 3.3,
EP additives are unlikely to function satisfactorily as friction modifi ers, and vice versa.
3.4.1 THE BASIC MECHANISM OF LUBRICATION AND WEAR AND THE INFLUENCE OF ADDITIVES
An understanding of the basic mechanism of lubrication is useful to appreciate the way in which
additives behave and their relative performance. The following is therefore a somewhat simplifi ed
explanation of a complex process.
Lubrication can be described as the ability of oil (or another liquid) to minimize the wear and
scuffi ng of surfaces in relative motion. It is a function of the properties of the lubricant (e.g., vis-
cosity), the applied load, the relative movement of the surfaces (e.g., sliding speeds), temperature,
surface roughness, and the nature of the surface fi lm (hardness and reactivity, etc.).
All surfaces are rough. Even those that appear smooth to the naked eye, when examined micro-
scopically, consist of a series of peaks and troughs. The simplest situation arises when the lubricat-
ing fi lm is thick enough to completely separate the two surfaces so that metal-to-metal contact does
not occur (Figure 3.5). Such a situation could arise at low loads or with highly viscous liquids, and
the lubricating characteristics depend on the properties of the lubricant as the load is fully supported
by the lubricant. This condition is known as hydrodynamic or full-fi lm lubrication.
As the load increases, the lubricating fi lm becomes thinner and eventually reaches a condition
where the thickness is similar to the combined height of the asperities on the mating surfaces. At this
stage, metal contact commences, and as the asperities collide, they are thought to weld momentarily
(causing friction) before shearing with loss of metal (wear) (Figure 3.5). The wear particles then
abrade the surface and adversely affect friction, with the resulting damage depending on the hard-
ness of the particle and the surface it contacts. This condition is known as mixed-fi lm lubrication as
it is a mixture of full-fi lm lubrication and boundary lubrication with the trend toward the latter with
increasing load.
As the fi lm thins still further, the load is increasingly supported by the metal surface and fric-
tion rises rapidly. When eventually a fi lm that is only a few molecules thick separates the surfaces,
TABLE 3.3
A General Classifi cation of Chemicals as Friction Modifi ers, AW, and EP Additives
Additive Friction Modifi er AW Additive EP Additive
Natural oils and fats 1 4 5
Long-chain fatty acids, amines, and alcohols 1 4 5
Organo-molybdenum compounds 1 2 4
Synthetic esters 2 3 4
Organo-sulfur compounds 2 2 3
ZDDP 3 1 3
Phosphorus compounds 3 1 3
Sulfur compounds 4 3 1
Chlorine compounds 5 4 1
Note: The lower the number, the better the rating.