Lubricant Additives

76 Lubricant Additives: Chemistry and Applications

the roughness, composition, and melting point of the surfaces strongly infl uence the resulting fric-
tion. At this stage, viscosity plays little or no part in the frictional behavior. This stage is known
as boundary lubrication and is characterized by high frictional values that now change little with
further increases in load or sliding speed. The wear process that takes place under boundary condi-
tions is perhaps the most complicated of those involved in lubrication in that it involves four differ-
ent types of wear: corrosive, fatigue, ploughing, and adhesive.
Corrosive wear occurs when the metal surfaces react with their environment to form a bound-
ary fi lm, whereas fatigue wear is the process of the fracture of asperities from repeated high stress.
Micropitting is an example of this form of wear, which is the subject of considerable investigation
today. Micropitting is the result of plastic deformation of the surface that eventually causes the frac-
ture of the asperity, leaving a small pit in the surface. Ploughing wear arises when a sharp particle
is forced along the surface, leaving a groove behind, whereas adhesive wear is the tendency of very
clean surfaces to adhere to each other. However, this action requires the generation of fresh surfaces
during the wear process, perhaps by plastic deformation. It is now thought that this mechanism is
much less prevalent than was earlier believed [64].
The relationship between friction, viscosity, load, and sliding speeds can be represented graphically
for a bearing by what is known as a Stribeck curve. This is shown in Figure 3.6 [65], where the frictional
coeffi cient is plotted against the dimensionless expression ZN/P, where Z represents the fl uid viscosity,
N the sliding speed, and P the load. Friction is reduced as the value of ZN/P is lowered until a minimum
is reached. For a bearing, this minimum value is ∼0.002 for an ideal hydrodynamic condition. At this
point, metal contact begins, and friction rises and continues to do so with increasing contact. In the
mixed friction zone, the friction value lies in the region of 0.02–0.10. Eventually, when the fi lm is very
thin, friction becomes independent of viscosity, speed, and load and can reach a value of 0.25.

By experiment it was established that

• Continually increasing the load reduced the ZN/P value, assuming that speed and the
  viscosity remained constant. The same results can be obtained by reducing either the speed
  or viscosity, or both, provided the unit load remains constant or is increased.

Metal

Metal

Metal

Metal

Oil

Oil

Full-film (hydrodynamic)

lubrication

Mixed-film lubrication

FIGURE 3.5 A diagrammatic representation of full-fi lm (hydrodynamic) and mixed-fi lm lubrication.