Graphite Lubrication

The slipperiness of graphite is not inherent in the crystal structure alone but depends upon adsorption films, especially water, which cover the carbon atoms and provide surfaces of low cohesion. In vacuum graphite rods or brushes seize upon a moving base (copper or graphite) and wear away as a fine dust. This wear and accompanying high friction are instantly stopped by some condensible vapors at low pressures (H2O, C6H6, NH3, etc.) which apparently effect the coverage of the graphite by a reversible adsorption governed by the pressure and surface temperature. Under conditions of optimum coverage (zero wear) the graphite friction is still appreciable but may be accounted for as the heat of cohesion of water monolayers attached to the carbon atoms and undergoing successive makes-and-breaks. From the measured frictional energy and an assigned heat of cohesion based upon surface energy data, the area of molecular contact is estimated to be 4×10−6 cm2 under a real pressure 2×104 kg/cm2. The wear dust of unlubricated graphite shows a unique adsorptive power at room temperature, greater for hydrogen than that of activated charcoal by five orders of magnitude. This and other characteristics may throw new light upon the mechanism of friction in general and upon the structure of stroked graphite surfaces in particular.