Solid lubricants generally have a laminar structure
Structure of graphite
Graphite has a layered lattice structure as shown in fig.3.5. The sp2 hybridized carbon atoms are arranged in flat parallel layers. Each carbon atom is linked to four other carbons atoms; to three atoms by covalent bonds (bond length =1.42A) in the same layer and fourth carbon atom is more than double (3.70A) and is due to weak Van der Waal’s force of attraction. So the two neighbouring layers are 3.70apart. The fourth bond is not fixed and hence, breaks and deforms easily.
Graphite forms a film having low frictional resistance, by filling up the flaws; this makes the surface evener. Further, the particles slide over each other as the surfaces of machinery are in motion. The range of coefficient of friction achieved by using solid lubricants is 0.05 to 0.1.
Properties of graphite
- It is soft and soapy to touch.
- It is non-inflammable and does not get oxidized in air below3750C.
- In absence of air, it can be used at a much higher temperature.
Uses of graphite
It is used either in the form of dry powder or mixed with water or oil so that they may stick firmly to the surfaces. The suspension is stabilized by using emulsifying agents like tannin the suspension of graphite in water is known as aqua-dag and that in oil is known as oil dag.
Aqua – dag is used where oil-free lubrication is required
Oil– dag is used lubricant in internal combustion (IC) engines because it forms a film between the cylinder and piston giving a tight fit to increase the compression of the air-fuel mixture.
It is used as a lubricant in air compressors, railway track joints, spacecraft, nuclear reactors etc where oil layer cannot maintain due to various reasons.
Lubricating property of greases is increased by using graphite as an additive.