Lubricant Additives

188 Lubricant Additives: Chemistry and Applications

lubricant can be used. For example, MoS 2 generally has a higher load-carrying capability than
graphite. Yet, at service temperatures above 400°C, MoS 2 degrades and loses its lubricating capacity.
MoS 2 is, therefore, eliminated from consideration if the service temperature is above 400°C.
The second consideration is environment. Atmospheric restrictions will eliminate the use of
certain solid lubricants. For example, a vacuum environment will eliminate the use of graphite.
As mentioned previously, graphite requires adsorption of water molecules to its surface to function
as an effective lubricant. MoS 2 , on the contrary, as well as PTFE and boron nitride have intrinsic
lubrication properties and do not require water molecules on their surface to provide friction reduc-
tion value.
The third criterion is the nature of the lubricant; either a liquid fortifi ed with solid lubricant
additives or a bonded solid lubricant fi lm. Some pigments are easier to disperse in liquid than
others. For example, graphite and MoS 2 are comparatively easier to disperse in liquids than PTFE
and boron nitride. This is mostly due to particle size-reducing capability, surface energy, and surface
chemistry of the solid lubricant.
The particle size of the pigment has an infl uence on lubrication performance. The size of
the particulate and the size distribution of the particles should be optimized for the application
(see Figure 6.11). For example, larger particles tend to give better performance for applications that
are slow in speed or oscillating in nature.
Large particles also tend to give better performance on substrates where the surface roughness
is relatively coarse.
A fi ner particle size tends to provide superior results for applications with constant motion and
high speeds. Finer particles tend to function better where the surface roughness is relatively fi ne.
Although not always predictable, the infl uence of particle size needs to be considered not only for
dispersion requirements but also for the intended use application.
The fourth criterion involves cost-effectiveness of the lubricant. When the application condi-
tions are met with two or more solid lubricants, cost will dictate the choice. Generally, graphite will
be the least expensive. High-purity graphite is more expensive than lower-purity natural graphite
or secondary synthetic graphite, which are more expensive than low-quality graphite. Molybdenum
disulfi de will be next, followed by PTFE and boron nitride as the more expensive solid lubricants.
Cost-effectiveness for any of the solid lubricants will be infl uenced by the quality of the lubricant

TABLE 6.9
Bench Lubrication Test Results

Four-Ball Lubrication Test Falex Lubrication Test

Wear ASTM

D-4172

Extreme Pressure

ASTM D-2783

Wear ASTM

D-2670

EP ASTM

D-3233

Coeffi cient

of Friction

20 kg

mm

40 kg

mm

Weld

(kg)

Load Wear

Index (kg) Teeth

lb to

Failure Calculated

Base oil 0.678 1.060 126 17.20 Fail 875 0.159
With 1% colloidal
graphite

0.695 0.855 160 18.7 78 1000 0.132

With 1% colloidal
MoS 2

0.680 0.805 200 24.3 8 4375 0.077

With 1% colloidal
PTFE

0.50 0.84 200 29.04 10 4500 + 0.0568

With 1% colloidal BN 0.37 0.72 126 19.9 Fail 500 0.1602

Source: Acheson colloids test data.