Solid and dry lubricants differ significantly from liquid lubricants. Liquid lubricants reduce friction due to their fluidity and viscosity. However, while solid lubricants have neither of these properties, they are still capable of reducing friction and wear.

Use of Solid Lubricants

Solid and dry lubricants work in situations where liquid lubricators have no practical value. For instance, a solid lubricant may be sprayed on the outer hull of a boat to improve water resistance. Similarly, some of your kitchen pans also feature solid lubricants, namely in the form of Teflon, to minimize interaction between the food you are cooking and the pan itself.

Solid and dry lubricants are also used in temperatures where liquid lubricants would boil away. A Teflon coated pan, for instance, provides lubrication at high temperatures, allowing you to adequately heat food on a non-stick surface.

When two objects roll across each other or when the two objects must move against each other under extreme pressure, solid lubricants can also be more effective than other types of lubricants. In such cases, a liquid lubricant would be squeezed out from between the two objects, but a dry lubricant will continue to operate.

The best solid and dry lubricants work due to forces at the molecular level. The molecular construction of the solid lubricant allows molecules to easily slide over each other.

The four most commonly used solid lubricants are:

1. Graphite: Used in air compressors, food industry, railway track joints, open gear, ball bearings, machine-shop works etc. It is also very common for lubricating locks, since a liquid lubricant allows particles to get stuck in the lock worsening the problem.
2. Molybdenum disulfide: Used in CV joints and space vehicles. Does also lubricate in vacuum.
3. Hexagonal boron nitride: Used in space vehicles. Also called "white graphite".

4. Tungsten disulfide: Similar usage as molybdenum disulfide, but due to the high cost only found in some dry lubricated bearings.

    

Graphite

Graphite is a soft, crystalline form of carbon. It is gray to black, opaque, has a metallic luster, and is flexible but not elastic. Graphite occurs naturally in metamorphic rocks such as marble, schist and gneiss. It exhibits the properties of a metal and a nonmetal, which makes it suitable for many industrial applications. The metallic properties include thermal and electrical conductivity. The nonmetallic properties include inertness, high thermal resistance and lubricity. Some of the major end uses of graphite are in high-temperature lubricants, brushes for electrical motors, friction materials, and battery and fuel cells. 

Graphite is a layer lattice lamella crystal structure where the bonds between the carbon atoms in the crystal structure of the layer are stronger than the carbon bonds between layers. Graphite is comprised of carbon and water vapor. Each carbon atom is bonded to three other surrounding carbon atoms. The flat rings of carbon atoms are bonded into hexagonal structures, as shown in the figure. The hexagonal carbon structure is referred to as a benzene ring. These plates exist in layers, which are not covalently connected to the surrounding layers.

Properties:

Graphite has a layered lattice structure with weak bonding between layers, providing excellent lubricity as long as moisture is available. Graphite solids provide:

• High-temperature stability
• Good lubrication in high humidity
• Low coefficient of friction under high loads

• Protection against fretting corrosion

   

Uses:

Graphite blends and pure graphite dry film lubricant systems are commonly used in applications such as hot and cold forming, wire drawing and billet coatings; on high-speed cutting tools; as a mold release for die cast, plastic and rubber mold applications; cylinder head and exhaust bolts; ammunition and armament applications; automotive engine and many common industrial applications.