Solid lubricants are used in situations such as,

1. When heavy machinery is to be operated at high speed and moderate load or at very high load and low speed, the liquid lubricants cannot be useful. This is because, due to high load and low speed, the film of liquid lubricant cannot remain in between sliding surfaces, due to this the friction increases at selected areas, and so also increases wear and tear.
2. When the machine parts are not easily accessible, that is, if the design is intrinsic, then use of liquid or semisolid lubricant fails.
3. When machine is at high working temperature and pressure, and hence under such conditions combustible lubricants (oils) are unsuitable.

Graphite as a solid lubricant:

• Graphite being an allotrope of carbon, consists of meshwork of hexagonal carbon rings arranged on each other. Graphite, when applied between uneven surfaces, fills into valleys, thereby making surfaces more even.
• When machines is in operation, the graphite particles slide over each other with the motion of machine. Each graphite atom consists of carbon atoms which are arranged in regular hexagons in flat parallel layers.
• Each carbon atom is linked by covalent bonds to three other carbon atoms. The distance from the fourth carbon is almost more than double, due to which this fourth valency atom is flexible and keeps moving about, hence, weakening the bonds between different layers. As a result the graphite is soft and has a lubricating property.
• Graphite is non-flammable, soapy to touch, and not oxidised in the presence of air below 375 degree temperature. It can be used upto much higher temperature in the absence of air. It can be used in powdered form or in the form of its suspension in water or oil.
• The suspension of graphite in water is known as 'aqua-dag'. This is used in the machinery where lubricant free from oil s necessary. eg. Food processing industry.
• The suspension of graphite in oil is known as 'oil-dag'. This is used particularly in I.C. engines.
• Graphite is also mixed with greases to form 'graphite greases'. These are used at still higher temperatures.