Different types of powder lubricants
By TOI Staff
November 7, 2023
Update on : November 7, 2023
To reduce the wear and friction between two sliding surfaces it is essential to lubricate the machinery. There are different types of lubricants: liquid, gaseous, solid and semi-solid. Solid lubricants are also used in dry powder form and they can provide lubrication even at high temperatures.
The common dry lubricants are Tungsten Disulfide (WS2), Hexagonal Boron Nitride (hBN), Molybdenum Disulfide (MoS2) and Polytetrafluoroethylene (PTFE).
These have a layered molecular structure with strong bonds within the layers and weak bonds between the layers. The formed layers can slide within each other when force is applied. This gives them low friction properties. They can operate at temperatures up to 350° C in oxidising environments.
Types of lubricant powders
Tungsten Disulfide
This is an organic chemical compound and is a part of transition metal dichalcogenide. It is composed of Tungsten and Sulphur and has a layered structure. It occurs naturally as a mineral called Tungstenite. Its chemical formula is WS2 and consists of one layer of Tungsten sandwiched between two layers of Sulphur. This structure is responsible for inter-layer mechanical weakness with low shear strength. This gives it lubricating properties.
Bulk WS2 forms hexagonal crystals within the layered structure and shows lubricating properties. It is used as a lubricant in moulds, fasteners and bearings. It exhibits properties like being chemically stable in a wide temperature range, inert, corrosion resistant, non-toxic and non-magnetic. It has a lamellar structure and has low shear strength and high oxidation and thermal degradation resistance.
Hexagonal Boron Nitride
This is a soft lubricious material and is also called ‘White Graphite’. It is used in a wide range of applications from lubricants to cosmetics. It is suitable for hot and cold processes due to its properties like low thermal expansion, high thermal conductivity and a low dielectric constant. HBN is a cheap lubricant which is used in industries.
It has a lamellar crystalline structure in which the molecules within a layer are bonded strongly and the binding between the layers is through weak Van der Waal forces. Its lubricating property is due to the easy shearing along the basal plane of the hexagonal crystalline structure.
Moving surfaces and separated from direct contact by using hBN to form a protective film. Due to the hBN powder, it leads to reduced wear and friction between the surfaces.
Molybdenum Disulfide
This has excellent lubricating properties due to its layered structure and a low coefficient of friction. It is classified as a transition metal dichalcogenide and is composed of Molybdenum and Sulphur. It occurs naturally as a mineral called Molybdenite.
It is relatively unreactive and is not affected by dilute acids and oxygen. It has a layered structure with a plane of Molybdenum atoms sandwiched between the two layers of Sulphur. When the shear stress is applied to MoS2 interlaying, sliding takes place and dissipates energy. The shear strength of MoS2 increases as the coefficient of friction increases. This property of MoS2 is called superlubricity.
The wear resistance of MoS2 can be increased by doping it with Chromium. MoS2 is used in blends and composites that require low-friction. It is used in applications like two-stroke engines, ski waxes, universal joints, bicycle coaster brakes and more.
Polytetrafluoroethylene
This is a fluoropolymer and is commonly known by the trade name Teflon. It has properties like non-reactivity, hydrophobicity and low coefficient of friction. It is used as a non-stick coating for cookware. PTFE is one of the most non-toxic, chemically inert and non-flammable substances.
This is one of the most stable polymers due to the high strength of the bonds between the Carbon and Fluorine atoms. It is used as a lubricant to reduce the friction, wear and energy consumption of the machinery.
It is a highly crystalline unique plastic with a high melting point. It is used in applications that require frictionless surfaces and resistance to load like bearings. It has an extremely water-repellent surface and excellent non-stick properties.
Lubrication is a common approach to reduce friction and wear. When the lubricant powder is put between two sliding surfaces it fills the gap between the two surfaces. It allows smoother movement of the two surfaces. This reduces the frictional force than when the surfaces are not lubricated.
Lubrication also helps to reduce the wear that can be caused by the two surfaces rubbing together. Lubricants when added as powders form a soft thin layer between the two surfaces. If the surfaces are not lubricated then it can lead to the two surfaces rubbing together and causing damage and finally the failure of the system.
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