Hardness: As mentioned in ch.3, the distance between the basal planes of the graphite crystal is relatively large (0.334nm) and the van der Waals bond between these planes is week (7.1 kJ mol). The interplanar distance is further increased by lattice defects, interstitial foreigh 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.
The 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 graphite.
Table 5.6. Hardness of molded carbon and graphite.
| Material | Contact hardness
Kg/mm2 |
Scleroscope
hardness |
| Electrographite
Hard carbon Coke-based carbon Lampblack-based carbon |
18
35-50 |
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 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.
This hydrodynamic film is maintained only in the presence of absorbable vapors such as water vapor, oxygen or contaminating organic gases. In a chemically pure atmosphere of inert gases or nitrogen, in a high vacuum, or at high temperature, the film is no longer formed or, if formed, is readily broken. As a result, the coefficient of friction becomes high and wear becomes excessive, up to five orders of magnitude greater than in the vapor environment. This phenomenon is known as “dusting”. Under these conditions, graphite is not an effective lubricant and lubricity additives and intercalation compounds are necessary.