Graphite is made up of a giant covalent structure consisting of layers of carbon atoms. Carbon atoms have four valence electrons that are available for bonding. In graphite, each carbon atom is covalently bonded to 3 other carbon atoms. The carbon atoms form layers with a hexagonal arrangement of atoms; each carbon atom has one non-bonded outer electron, which becomes delocalized. While there are strong covalent bonds between carbon atoms in each layer, there are only weak forces between layers.
This structure allows layers of carbon to slide over each other - and because one layer can easily slip over another layer, makes graphite is an excellent lubricant.
One can visualize graphite as a pack of cards - each card is durable and stable,, but the cards can easily slide over each other.
The distance between the graphite layers is about 2.5 times the distance between the atoms within each layer. The layers extend over vast numbers of atoms.
So what holds the layers together?
Graphite is the ultimate example of van der Waals dispersion forces. As the delocalized electrons move around in the layers, very large temporary dipoles can be set up, which will induce opposite dipoles in the sheets above and below - and so on throughout the whole graphite crystal.
Graphite has a high melting point, similar to that of a diamond. In order to melt Graphite, you have to break the covalent bonding throughout the whole structure.
How can graphite and diamond be so different if they are both composed of pure carbon?
Both diamonds and graphite consist entirely of carbon. The differing properties of carbon and diamond arise from their distinct crystal structures.
In a diamond, the carbon atoms are arranged tetrahedrally. Each carbon atom is attached to four other carbon atoms 1.544 x 10-10 meters away with a C-C-C bond angle of 109.5 degrees. It is a strong, rigid three-dimensional structure that results in an infinite network of atoms.
The carbon atoms in graphite are also arranged in an infinite array, but they are layered. These atoms have two types of interactions with one another. In the first, each carbon atom is bonded to three other carbon atoms and arranged at the corners of a network of regular hexagons with a 120-degree C-C-C bond angle. These planar arrangements extend in two dimensions to form a horizontal, hexagonal "chicken-wire" array. In addition, these planar arrays are held together by weaker forces.
The bottom line is graphite is carbon that forms only two bonds with other carbon atoms. This means it has free electrons and the graphite material exists in layers. If the air supply shuts off, air bearings will act as a low friction lubricant bearing and it won't damage the platform or the air bearing system.
CEO, OAV Air Bearings | Chairman of the Board | OAVCO
Murat Erturk
June / 2020