Solid lubricants

SOLID LUBRICANTS

Solid lubricants are used the following situations.

(i) where the operating temperature and load is too high.
(ii) where contamination of oils or greases by the entry of dust or grit particles are avoied.
(iii) where combustible lubricants must be avoided.

The most widely used solid lubricants are graphite and molybdenum disulphide

8.7.1. Graphite

Graphite consists of flat layers of hexagonal arrangement of carbon atoms.The carbon atmos in hexagone are bonded together by strong covalent bonds. The adjacents layers are held together by weak Vander Walls forces . Since the distance between the adjacent layers can slide easily one over the other co-efficent of friction .This property makes use of graphite as a lubricant.

Graphite is very soapy to touch and non – inflammable. It can be used upto 375oC above this temperature it gets oxidized. Graphite is used as a dry powder or as a colloidal dispersion . A dispersion of graphite in oil is called oil Aqua dag and a dispersion of graphite in oil is called oil dag.

Used
(i) Oil dag is used in internal combustion engines.
(ii) Aqua dag is used in air compressors and in food processing equipments.s
(iii) Graphite is as a lubricant generally used in lathes, machine shop work railway track joints, open gears chains.

8.7.2. Molybdeenum disulphide

Molydbenum disulphide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atoms in the layer are bonded together by strong covalent bonds but the layers are held together by weak Vander Waals forces of attraction

The molybdenum layers and sulphur layers silde over one another .As a result MoS2 possess very low co-efficient of friction. It is also used either as a dry powder or as a colloidal dispersion. It gets oxdised above 800oC. It is mainly in heavy machineries working under heavy load and high temperatures.

Uses
(i) Pure MoS2 is used in the vacuum of outer space.
(ii) It is used in heavy machinery working at higher temperatures.