Solid Lubricants as Friction Modifi ers 177
and yield strength. Yet, a far easier force is required to rupture the stack when the force is applied
parallel along the face of the deck, resulting in the shearing of the cards.
The effects of the lamellar structure of graphite can be observed when sliding conditions are
applied onto metal surfaces. Coeffi cient of friction data can be generated by various bench test methods
for measuring the lubricity of sliding conditions. In comparison to unlubricated or oil-lubricated metal
surfaces, graphite provides excellent lubricity [4]. This is summarized in Table 6.1.
6.2.1.1 Sources of Graphite
There are many types and sources of graphite. These sources infl uence the properties of the graphite,
which affects the performance of the end product that uses graphite. Graphite is characterized by
two main groupings: natural and synthetic.
Natural graphite is derived from mining operations worldwide. The ore is processed to recover
the usable graphite. Varying quality of the graphite will be evident from the ore quality and the
postmining processing of the ore. High-purity natural graphite will normally be highly lubricating
and resistant to oxidation. This is due to the high degree of crystal structure and graphitization
usually associated with naturally derived graphite.
Natural graphite of lesser quality is also available. A lower total carbon content and a lower
degree of graphitization characterize the lesser quality. The end product is graphite that is more
amorphous in nature, with a higher content of ash components, which are mostly oxides of silicon
and iron. Lubrication functionality decreases as crystallinity and graphitization decrease. Lubrica-
tion functionality also decreases as total ash content of the graphite increases.
Commercially available natural graphite is provided in a variety of grades. The suitability of
the grades depends on the intended application and economic constraints. Table 6.2 characterizes
examples of commercially available natural graphite.
Selecting the type of natural graphite to use is based on the degree of lubrication required for
the application, the particle size of the graphite necessary for the application, and the economic
constraint. For situations where the lubrication demand is severe, a high-carbon crystalline fl ake
or crystalline vein graphite is desired. The high degree of crystallinity and graphitization provides
superior lubrication. A more economical alternative is the lower-carbon-content fl ake graphite.
TABLE 6.1
Coeffi cients of Friction Provided by Graphite Films
Test Method Graphite Film Unlubricated Metal Mineral Oil on Metal
Three-ball slider 0.09–0.12 0.16–0.18 0.15–0.17
Bowden–Leben machine 0.07–0.10 0.40 0.17–0.22TABLE 6.2
Natural Graphite
Amorphous Crystalline Flake 1 Crystalline Flake 2
% Carbon –85.0 90–95 96–98
% Sulfur –0.30 0.15–0.20 0.10–0.70
% SiO 2 6.0–7.0 0.20–0.30 0.05–0.2
% Ash 10–15 7–10 2.0–3.0
Mesh −325 −325 −325