Why Can Graphite Be Used As a Lubricant?

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Why Can Graphite Be Used As a Lubricant?
Proper lubrication is necessary for machines to run efficiently. (kugellager image by Sascha F. from Fotolia.com)

Graphite is the mineral form of carbon. Veins of graphite occur in limestone due the presence of organic materials. Graphite is a soft mineral and occurs in either lump or flake form. Graphite is the only non-metal element that is a good conductor of electricity. Crystal structures rarely occur and easily break down. This is the reason for the material's greasy feeling despite being a dry lubricant.

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Basal Cleavage

When a crystal structure occurs in graphite, it is a rough six-sided crystal that easily breaks down into flakes. This process is known as basal cleavage. These flakes readily slide across each other. This is the cause of the greasy characteristic of graphite and results in its application as a lubricant. Since the material is a solid, graphite is considered a dry lubricant.

Carbon-Based Minerals

Both graphite and diamonds are carbon-based. Graphite is an excellent lubricant. Diamonds are the hardest abrasive. Diamonds are a good insulator of electricity. Graphite is a good conductor. Graphite is a stable form of carbon. Diamonds near the surface of the Earth are undergoing the change to graphite; however, the process is slow.

Atomic Structures

Graphite and diamonds both have strong bonds between the atoms of carbon. The difference lies in the structure of the atoms. In diamonds, the atoms are arranged in a three-dimensional structure. This results in stronger bonds above and below. Graphite atoms are in two-dimensional sheets with weak bonding above and below.

Water Vapor

Graphite needs water vapour to lubricate. The bonding energy between the water and the graphite is lower than between the graphite and the surface to be lubricated. This means that graphite functions best in a regular atmosphere. Graphite is not suitable for lubrication in a vacuum.

Carbon Content

Natural graphite is mined. Ore quality and processing will determine the grade of the graphite. High-grade natural graphite contains levels of carbon between 96 per cent and 98 per cent. The higher carbon content and high crystallisation will increase the lubricating quality and resistance to chemically bond through oxidation. Synthetic graphite can be created with similar carbon levels to high-grade natural graphite. Where there are lower requirements for lubrication, amorphous graphite with 80 per cent carbon content can be used.

High Temperature Applications

Graphite can be used in continuous temperatures of 450 degrees Celsius. Graphite can withstand even higher temperature peaks. Graphite conducts little heat itself and will provide some thermal insulation.

Graphite Composites

While graphite is a soft material and an excellent lubricant, it can be rolled into fibres. These fibres can be twisted into threads and held in place with a binder such as epoxy resin. This is the method of creating composites for uses in such items as aeroplanes, automobiles and golf clubs.

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