Carbon brushes

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Carbon brushes

Dry or solid lubricants are materials that, despite being in a solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.

Description

Carbon brushes

The two main dry lubricants are graphite and molybdenum disulfide. They provide lubrication at temperatures higher than liquid and oil lubricants can handle. Dry lubricants are often used in applications such as locks or dry-lubricated bearings. Such materials can operate at temperatures up to 350°C (662°F) in oxidizing environments, and even higher in reducing/non-oxidizing environments (molybdenum disulfide up to 1100°C, 2012°F). The low-friction characteristic of most dry lubricants is attributed to a layered structure at the molecular level with weak bonding between layers. Such layers are able to slide against each other with minimal applied force, giving them low friction properties.

However, a layered crystal structure alone is not necessarily sufficient for lubrication. In fact, there are some solids with a non-lamellar structure that perform well as dry lubricants in certain applications. These include some soft metals (indium, lead, silver, tin), polytetrafluoroethylene, some solid oxides, rare-earth metal fluorides, and even diamond.

Limited interest has been shown in the low-friction properties of dense oxide scale layers that form at several hundred degrees Celsius in metal sliding systems. However, due to their physically unstable nature, practical application is still years away.

The four most commonly used solid lubricants are: 1. Graphite. Used in air compressors, the food industry, railway track joints, brass instrument valves, piano mechanisms, open gears, ball bearings, machine shops, etc. It is also very common for lubricating locks, since a liquid lubricant would let particles get trapped in the lock, worsening the problem. It is often used to lubricate the internal moving parts of firearms in sandy environments.
2. Molybdenum disulfide (MoS2). Used in CV joints and spacecraft. Lubricates in a vacuum.
3. Hexagonal boron nitride. Used in spacecraft. Also called “white graphite”.
4. Tungsten disulfide. Similar use to molybdenum disulfide, but due to its high cost it is found only in some dry-lubricated bearings.

The predominant materials used as dry lubricants are graphite and molybdenum disulfide.

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