178 Lubricant Additives: Chemistry and Applications
For most situations, these types of graphite perform adequately in lubricating conditions that do
not require the purity and lubricity of higher-quality crystalline graphite. For occasions where only
minor lubricity is needed and perhaps a more thermally insulating coating is required, amorphous
graphite would be chosen. Amorphous graphite is also the least expensive of the commercially
available natural graphite grades. Combining amorphous and crystalline graphite can also be done
to modify the amount of lubrication to suit the requirements of the application.
Synthetic graphite is an alternative source for lubricating graphite. Synthetic graphite is char-
acterized as primary or secondary grade (see Table 6.3). Primary grade is derived synthetically
from production within an electric furnace, utilizing calcined petroleum coke as well as very high
temperatures and pressures to produce the graphite. The result is usually a product of high purity
and can approach the quality of natural graphite fl ake in terms of percent graphitization and lubrica-
tion capability.
Secondary synthetic graphite is derived from primary graphite that has been used for the
fabrication of electrodes. This type of graphite is usually less lubricating than natural or primary
grades of graphite because of its lesser degree of crystallinity and graphitization and the presence
of binding agents and surface oxides that do not contribute to lubrication. Secondary synthetic
graphite is perfectly capable of lubricating effectively for many applications that can afford a lesser
degree of lubricity. The chief benefi t in using secondary synthetic graphite is the cost, with the
secondary graphite costing signifi cantly less than primary-grade synthetic graphite or high-purity
natural graphite.
6.2.1.2 Lubrication
Appropr iate-quality graphite is able to meet the fi ve criteria for an effective solid lubricant. Graphite
possesses the necessary yield strength for successful lubrication. It is able to adhere suffi ciently to
metal surfaces due to its affi nity to metal and its packing within and above the microstructure of
the surface. Graphite has a burnishing capacity desirable for lubrication mechanisms that require
a “memory” effect. Proper orientation of graphite particles is achieved by the natural tendency
for the graphite crystal to orient itself parallel to the substrate and in the direction of lowest shear.
The anisotropic characteristic of graphite lends itself well to its lubricating capability and friction
reduction property. The planar orientation of the graphite particles on the substrate takes advantage
of the anisotropic property. Proper orientation allows the lamellar functionality of graphite where
easy shear is achieved along the crystal plane when sliding forces are put along the length of the
particles. The high yield strength in graphite is maintained in the direction perpendicular to the
direction of shear force, providing for the load-carrying capability.
Graphite is best suited for lubrication in a regular atmosphere. Water vapor is a necessary com-
ponent for graphite lubrication. The role that adsorbed water vapor plays in the lubricating proper-
ties of graphite has been studied [5]. It is theorized that water vapor helps to reduce the surface
energy of the graphite crystallite. The adsorption of a water monolayer onto the planar surface of
TABLE 6.3
Synthetic Graphite
Typical Values
Primary Secondary
% Carbon 99.9 99.9
% Sulfur Trace 0.01
% SiO 2 0.02 0.05
% Ash 0.1 0.1