handbook of carbon, graphite, diamond and fullerenes

Molded Graphite 103
Hardness. As mentioned in Ch. 3, the distance between the basal
planes of the graphite crystal is relatively large (0.334 nm) and the van der
Waals bond between these planes is weak (7.1 kJ mol). The interplanar
distance is further increased by lattice defects, interstitial foreign atoms and
other irregularities. As a result the basal planes can slip easily over each
other without loosing their coherence (as long as the slip is not hindered by
cross-linking boundary conditions), and graphite is able to yield plastically.
This characteristic makes it difficult to interpret hardness measurements
except with the ball-identation method which can be considered
reasonably accurate. The resulting "contact hardness" is defined as the
average pressure required to indent the material to a depth equal to 2/100th.
of the radius of the ball. Other hardness-measurement methods such as the
Scleroscope (which is the measure of the rebound height of a falling
diamond-tipped hammer) are convenient but, as they are not based on the
same principle, cannot readily be correlated with the ball hardness.
Table 5.6 lists typical contact- and Scleroscope-hardness values of
several types of molded graphiteJ 13 !
Table 5.6. Hardness of Molded Carbon and Graphite
Material
Electrographite
Hard carbon
Coke-based carbon
Lampblack-based carbon
Contact
Hardness
kg/mm 2
18
35-50
Scleroscope
Hardness
40-80
70-100
70-90
70-110
Frictional Properties. Molded graphite materials have inherently low
friction due to the ease of basal-plane slippage and the resulting low shear
strength mentioned above.
When graphite is rubbed against a metal or ceramic surface, a thin
transfer film is formed on the rubbed surface. This lowers the coefficient of
friction which can be less than 0.01 after the transfer film is fully developed.