Solid lubricants can be used in powder form to reduce the friction and wear between rubbing parts by eliminating direct contact between them. Using solid lubricant powder is the only way to control friction and wear in applications requiring extreme operating conditions.
Liquid lubricants are unsuitable for lubrication in high temperatures and vacuum conditions. Because it can lead to the breakage of chains in the molecules and polymerisation may occur, leading to the formation of sludge. This can affect the lubricating properties and reduce the load-carrying capacity. Dry lubricant powders can be used in such conditions. These powders can work efficiently as lubricants in high temperatures and extreme operating conditions.
Some of the dry lubricants are Molybdenum disulfide (MoS2), Tungsten disulfide (WS2), Hexagonal boron nitride (hBN), Graphite (C) and PTFE (Polytetrafluoroethylene). Dry lubricants are used where the use of other lubricants like oils and greases is not possible or undesirable. They can ensure reliable and maintenance-free lubrication for moving components.
Solid lubricants have a layered lattice structure and the materials can easily shear parallel to the layers then across them. They can support relatively high loads at a right angle to the layers and still easily slide parallel to the layers. This property is used in the lubrication of moving parts. For solid lubricants to be effective their layers must be parallel to the direction of sliding.
Types of Solid lubricants
Molybdenum disulfide (MoS2)
It is an inorganic compound and consists of Molybdenum (Mo) and Sulphur (S). It has a layered structure with a layer of S atoms on either side of the layer of Mo atoms. A strong covalent bond exists between the S and Mo atoms but the layers are held together by weak Van der Waal forces.
Its shear strength increases as the coefficient of friction increases, this property is called superlubricity. The coefficient of friction of MoS2 at ambient conditions is 0.150. MoS2 can function well up to temperatures reaching 350° C in oxidising environments. It is often used in blends and composites that require low friction.
Tungsten disulfide (WS2)
It is an inorganic chemical compound made up of Tungsten (W) and Sulphur (S). It is a 2D material consisting of a layer of hexagonally arranged W atoms sandwiched between two layers of hexagonally arranged S atoms. Strong covalent bonds exist between the W and S atoms within the layer and the layers are held together by weak Van der Waal forces.
When WS2 is heated in an oxygen-containing atmosphere it converts to tungsten trioxide. When heated in the absence of oxygen it decomposes into Tungsten and Sulphur at a temperature of 1250𝆩 C. It is used as a lubricant in the aerospace and military industries.
Hexagonal boron nitride (hBN)
Boron Nitride is a compound consisting of Boron (B) and Nitrogen (N) and Its hexagonal form is the most stable. hBN has a layered structure with the B and N atoms arranged in a honeycomb configuration. Within the layers the B and N are bonded by strong covalent bonds and the layers are held together by weak Van der Waal forces.
hBN can work effectively up to a temperature of 1000° C in air, 1400° in a vacuum and 2800° C in an inert atmosphere. It is used as a lubricant at both low and high temperatures.
Graphite (C)
It is a crystalline form of elemental Carbon (C). It consists of a layered structure consisting of six C atoms arranged in a honeycomb lattice. The individual layers are called Graphene. Within the layer, the C atoms are bonded by strong covalent bonds and the layers are held together by weak Van der Waal forces.
It is used in various industrial applicators because it offers a relatively better lubrication performance. However, it oxidises to carbon monoxide at a temperature of 400° C and carbon dioxide at a temperature of 500° C. Therefore it is used for applications requiring medium temperature ranges.
PTFE (Polytetrafluoroethylene)
It is a synthetic fluoropolymer of tetrafluoroethylene. It consists of a Carbon (C) backbone chain with two Fluorine (F) atoms attached to each Carbon atom. The Fluorine atoms surround the Carbon chain like a protective sheath making it non-reactive.
It cannot be made wet by water and water-containing substances. It is used for non-stick coating for pans and cookware. It functions effectively as a lubricant at temperatures as low as -268.15° C and has good flexibility at temperatures above -79° C. The only chemicals that can affect PTFE are alkali metals at high temperatures.