Solid lubricants are used for certain system where oil and grease are either undesirable or inadequate.

Solid lubricants are generally used in the following cases-

• When heavy machinery is to be operated at high speed and moderate load or at very high load and low speed.

• When the machine parts are not easily accessible.

• When machines are at high working temperature and pressure.

They are used in the form of powder or a suspension in water or oil.

The co-efficient of friction lies on range of 0.005 to 0.01. They separate two moving surfaces under boundary conditions.

They are economical, non-toxic, easy to apply, chemically inert, thermally stable and are resistant to radiations.

The substances most commonly used as solid lubricants are graphite, molybdenum-di-sulphide, mica, Teflon, chalk, soap stone etc.