The anisotropic behavior of graphite is illustrated in its ability to act as a solid film lubricant. Graphene layers, stacked along the “c” crystallographic axis, have high inter-layer strength but low intra-layer cohesion. The weak pi bonding that holds adjacent graphene sheets in alignment, yields with minimal energy allowing graphene layers to peel away from each other and the crystal.
Although individual layers or groups of layers can easily be cleaved from the crystal, the layers themselves are extremely tough. In fact, the individual graphene layer is probably the toughest two-dimensional structure known. This in-plane toughness results from the strong covalent sigma (σ) bonds that hold carbon atoms in the graphene layers together. This high in-plane strength is the basis of all carbon fibers and carbon nanotubes.
Groups of graphene layers cleaved away from a graphite crystal will provide a tough, impervious, inert, highly lubricious, thin film, which will effectively fill and “cap” disparities between rubbing surfaces. The film forming properties of graphite provides a perfect example of the relationship between microscopic form, macroscopic function, and anisotropy.
The anisotropic behavior of graphite is exemplified in virtually all of its physical and chemical properties. For example, thermal and electrical conductivity, which result from various modes of within plane electron transfer, are very high in the direction parallel with graphene layer planes (“a” direction). However, in the “c” axis direction no “ambient” mechanism of electron transfer exists resulting in low thermal and electrical conductivity in this direction. Of course, platelet overlap, crystalline imperfections, and some randomness in platelet location in a thin film results in graphite-containing coatings which typically do show a significant degree of through-plane conduction, however parallel plane conductivity is usually much higher.
