Machinery used in giant industries is used in extreme environments and has to be maintained. The maintenance cost is high for mechanical parts and industries face an economic loss. Therefore lubricants are used to reduce friction between the surfaces that come in contact and the wear due to high temperatures.
At high temperatures, liquid lubricants become unstable and tend to lose their lubrication properties. Solid lubricants are used in extreme environments as they can sustain load and high temperatures. Solid lubricants can be used in two forms – Granular lubricants and Powder lubricants.
Lubricants in powder and nanopowder form have shown excellent lubricating properties at extreme temperatures. Powder lubricants can be classified as dry, soft and cohesive particles that radically deform under load. They are used in industries like pharmaceuticals, coal-based gasification, food processing and more.
Lubricants when used in powder form can enhance the lubrication and lower the friction between two surfaces in relative motion. The powder usually adheres and coats the surfaces and reduces the damage to the surfaces. It is used in dry and boundary lubrication.
Powders can withstand some deformation when not pressed too hard, flow under certain circumstances and can be compressed to a certain degree. Adhesion between them enables them to coalesce and provide lubricating properties.
For example, When two component parts are sliding between surfaces it can lead to a higher wear rate and coefficient of friction due to surface properties. A lubricant can be applied to reduce the wear rate and the coefficient of friction. Powder lubricant can be used to protect the sliding surfaces from unexpected damage during the sliding process.
The amount and the character of the lubricant added directly affects the friction that is encountered. When the surfaces are lubricated the friction produced is much less than when the surfaces are not lubricated.
Wear occurs between the two surfaces in motion due to abrasion, corrosion and solid-to-solid contact. Proper lubrication can combat each of the problems. When the lubricant film is used it increases the distance between the sliding surfaces and thus reduces the abrasion and solid-to-solid contact.
The role of lubricant in controlling the corrosion is two-fold. When the machine is idle the lubricant acts as a preservative and when the machine is in use the lubricant controls the corrosion. The ability to reduce corrosion depends on the thickness of the lubricating film and the chemical composition of the lubricant.
Lubricants can also assist in controlling the temperature by reducing friction and carrying off the heat generated.
Type of Powder Lubricants
Some of the lubricant powders are as follows:
- Tungsten Disulfide nanopowder (WS2)
- Hexagonal Boron Nitride (hBN) powder
- Molybdenum Disulfide (MoS2) powder
- Poly Tetra Fluoro Ethylene (PTFE) micronised powder
Tungsten Disulfide nanopowder (WS2)
This is an inorganic compound composed of tungsten and sulphur and a chemical formula WS2. It exhibits the properties of a dry lubricant and is used in fasteners, bearings and moulds. It can be applied to a metal surface without binders through high-velocity air impingement. It is used in industries like military and aerospace.
Hexagonal Boron Nitride powder (hBN)
This is a ceramic material known for its high thermal conductivity, inertness and lubricant. It is a good lubricant at low as well as high temperatures. It is used as a lubricant where the electrical conductivity and the chemical reactivity of graphite can cause problems. As hBN does not require water or gas molecules to be trapped between layers it can be used in a vacuum such as in space applications.
Due to its properties, it is used in cosmetics, paints, dental cements, pencil leads and more.
Molybdenum Disulfide powder (MoS2)
MoS2 is an excellent lubricating material due to its layers structure and low coefficient of friction. The wear resistance of this lubricant can be increased by adding Chromium (Cr). It is mainly used in flexible electronic applications and composites that require low friction like plastics and polymers. It is used in polymers like Nylon, Teflon and Vespel.
It is a common dry lubricant and is used in motorcycle engines, bicycle coaster brakes, automotive CV and universal joints, ski waxes, bullets and more.
Poly Tetra Fluoro Ethylene micronised powder (PTFE)
This is a synthetic fluoropolymer and has numerous applications. It is used as a lubricant to reduce wear, friction and energy consumption in machinery. The commonly known brand of PTFE-based composition is called Teflon.
It maintains self-lubricating properties at temperatures as low as -268.15 C. It is so slippery that insects cannot get a grip when a surface is coated with this material.