Lubricant Additives

176 Lubricant Additives: Chemistry and Applications

Why is graphite such a good lubricant? The answer lies in the platelet, lamellar structure of the
graphite crystallite (see Figure 6.4). Graphite is structurally composed of planes of polycyclic car-
bon atoms that are hexagonal in orientation. Short bond lengths between each carbon atom within
the plane are the result of strong covalent bonds (see Figure 6.4).
Weaker van der Waals forces hold together a number of planes to create the lattice structure.
The d-spacing bond distance of carbon atoms between planes is longer and, therefore, weaker
than the bond distance between carbon atoms within the planes. As a force is applied perpen-
dicular to the crystallite, a strong resistance is applied against the force. This high yield strength
provides the load-carrying capacity for the lubricant. Concurrent with the force applied perpen-
dicular to the substrate is a sliding force applied parallel to the direction of sliding. The weak
bond between the planes allows for easy shearing of the planes in the direction of the force. This
creates a cleaving of the planes and results in friction reduction. The lamellar motion of graphite
cleavage can be illustrated by the concept of a hand applying a force on a deck of playing cards as
shown in Figure 6.5. Forces applied perpendicular to the deck are resisted by the stack’s thickness

d−Spacing

3.354 Å

C

A

B

1.415 Å

FIGURE 6.4 Structure of graphite.

FIGURE 6.5 Representation of lamellar lubrication.