Lubricant Additives

214 Lubricant Additives: Chemistry and Applications

8.1 INTRODUCTION

To optimize the balance between low wear and low friction, machine designers specify a lubricant
with a viscosity suffi cient to generate hydrodynamic or elastohydrodynamic oil fi lms that sepa-
rate the machine’s interacting surfaces, but not too high to induce excessive viscous drag loss. In
practice, the various contact types in a machine, the incidence of operating conditions beyond the
design range, and the pressure to improve effi ciency by reducing oil viscosity conspire to reduce oil
fi lm thickness below the optimum. The high spots, or asperities, on the interacting surfaces then
start to interact with one another, initially through micro-elastohydrodynamic lubrication (EHL)
fi lms, and at the end through direct surface contact, resulting in increased friction and the likelihood
of surface damage. Antiwear and extreme-pressure (EP) additives are added to lubricating oils to
decrease wear and prevent seizure under such conditions.
A common way to demonstrate the viscosity optimization is shown in Figure 8.1; this is known
as a Stribeck curve. The curve is a composite of a boundary friction curve and a viscous friction
curve that decreases as viscosity and, therefore, fi lm thickness increase and that increases as vis-
cosity and speed increase, respectively. A good operating target is represented slightly to the right of
the minimum in the curve. Improving the surface fi nish of contacting surfaces can move the mini-
mum in the curve to a lower viscosity range, saving energy but increasing the cost of components.
The hardening or coating of surfaces can increase their durability under increased levels of contact
with lower viscosity, but again at an increase in component cost. Notwithstanding these component
manufacturing improvements, the need for antiwear and EP additives will continue, but the nature
of their chemistry is likely to change due to environmental constraints, component material devel-
opments, and the continuing increase in severity of machine operating conditions.
The distinction between antiwear and EP additives is not clear-cut. Some are classed as
antiwear in one application and EP in another, and some have both antiwear and EP proper-
ties. To add to the confusion, EP additives come in mild and strong fl avors, and some are only
effective in low-speed, high-load situations and others only in high-speed, high-temperature
applications. Generally, antiwear additives are designed to deposit surface fi lms under normal
operating conditions and thereby reduce the rate of continuous, moderate wear, whereas EP
additives are expected to react rapidly with a surface under severe distress and prevent more
catastrophic modes of failure such as scuffi ng (scoring), galling, and seizure. Recently, it has
been suggested that EP additives be renamed as antiscuffi ng additives, since there is no pressure

Fluid film

Hydrodynamic

regime

Boundary

friction

0.15

Coefficient

of friction

0.001−0.002

Minimum fluid

friction

ZN

P

Z = viscosity of oil

N = speed of sliding

P = pressure between surfaces (load)

Mixed

lubrication

Boundary

lubrication

FIGURE 8.1 Regions of lubrication—Stribeck curve.