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Dry Lubricants – Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2) & Hexagonal Boron Nitride (hBN)

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dry lubricant sprays

When two surfaces are moving in relative motion with each other they experience a resistance which is called friction. Due to friction a large amount of energy is released in the form of heat. Therefore substances have to be used to reduce friction between the two moving surfaces and such substances are called lubricants.

Lubricants help to reduce the friction and wear between the two moving surfaces. There are liquid, semi-solid, solid and gaseous lubricants. Solid lubricants or dry lubricants are used in applications that require high temperature, high vacuum and high pressure.

Dry lubricants have a layered structure in which the atoms within the layer are bonded with strong covalent bonds and the layers are held together by weak Van der Waal forces. The layers easily slide against each other when a shear force is applied. This gives dry lubricants a low coefficient of friction. 

Some of the dry lubricants that are available in spray forms are Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2) & Hexagonal Boron Nitride (hBN). Dry lubricant sprays are used for lubricating car door hinges, locks, levers, seat belt guides, sunroof rails and other moving parts so that they move easily and smoothly. Many manual tools with moving parts can be lubricated to increase the ease of use and tool life. They can be used to eliminate the squeaks due to friction and protect against rust and corrosion.

Advantages of using dry lubricants 

  • Dry lubricants are used in applications where the lubrication film cannot be secured by using oils and greases due to the operating conditions.
  • Dry lubricants are used where the contamination by dirt and dust of the oil and grease is unacceptable. Liquid lubricants tend to pick up dust and other contaminants which can cause abrasion.
  • When the operating temperature or the load is too high for the grease to stay in position, solid lubricants can be used.
  • Solid lubrication can weigh much less than liquid lubrication. Solid lubricants can be used in weight-limited applications like spacecraft.
  • At high temperatures, liquid lubricants cannot be used because they can decompose and oxidise. In high vacuum applications, the liquid lubricants may evaporate. Dry lubricants can be used for such applications and they can provide relatively low coefficient of friction.
  • Liquid lubricants may flow away from high-pressure areas whereas solid lubricants will stay in place and provide lubricating film for better protection.
  • At low temperatures, liquid lubricant can solidify or become too viscous to provide lubrication. Solid lubricants are suitable for such temperatures.
  • In radiation environments, liquid lubricants can decompose therefore solid lubricants can be used.

Lubrication mechanism

Solid lubricants are used in thin film/boundary lubrication and extreme pressure lubrication.

The boundary lubrication method is used where a thin film of lubricant cannot persist. The clearance space between the moving surfaces is lubricated using dry lubricants. The dry lubricants get adsorbed on the metal surfaces and this adsorbed film helps to keep the metal surfaces apart. Dry lubricants can get adsorbed on the metal surfaces to form a thin film that acts as a lubricant. They can be used for high-temperature applications too.

Extreme pressure lubrication is required where the moving surfaces are under high pressure and have high speed. In such conditions, a high local temperature condition occurs and liquid lubricants cannot be used as they may fail or evaporate. Dry lubricants are used as they can form durable films on metal surfaces that can withstand high temperatures and high pressure.

Types of dry lubricants

Tungsten disulfide (WS2)

It occurs naturally as a mineral ore Tungstenite and is composed of Tungsten and Sulphur. It is used as a dry lubricant for fasteners, bearings and more. It can be easily applied on a metal surface without the need for any binders or curing using high-velocity air impingement. It is mainly used in the military and aerospace industry. check it out 

Molybdenum disulfide (MoS2) 

It occurs naturally as a mineral ore Molybdenite and Jordisite. It is used as a common dry lubricant. It can provide lubricity up to a temperature of 350° C in oxidising environments. It is used as an additive in blends and composites to improve the lubricating properties. It is used in two-stroke engines, universal joints and more.

Hexagonal Boron Nitride (hBN)

It is similar to Graphite and white and is also called ‘White Graphite’. It functions as a good lubricant at both low and high temperatures. It is often used in applications where the electrical conductivity and the chemical reactivity of Graphite can cause problems. It can be used in applications requiring high temperature and pressure. 

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