Molybdenum disulfide (MoS2) powder
Molybdenum disulfide (MoS2) is a class of 2D Transition Metal Dichalcogenides (TMDs) and is an inorganic material. It is a silvery black solid and occurs naturally as a mineral ore Molybdenite and Jordisite.
It is chemically unreactive and is not affected by dilute acids and oxygen. It has a low coefficient of friction and is widely used as a lubricant. MoS2 has a lattice layered structure where the atoms within a layer are bonded by strong covalent bonds and the layers are held together by weak Van der Waals forces.
This allows MoS2 to shear easily parallel to the layers than across them. It also allows them to support relatively high loads at right angles to the layers and still easily slide parallel to the layers. This property is used in the lubrication process. As the low coefficient of friction occurs parallel to the layers, MoS2 will be effective when its layers are parallel to the direction of sliding.
MoS2 can be used in powder form or as a film to provide protection from the damage caused during the relative motion and reduce friction and wear. Dry lubricants are used where extreme operating conditions exist and the liquid lubricant and grease are unsuitable, Dry lubricants are used in the space applications, food industry, textile industry and more.
MoS2 has properties like low coefficient of friction, high wear resistance, excellent thermal stability. It can be used to reduce friction and wear in various applications. As the coefficient of friction increases the shear strength of MoS2 increases,.This property is called superlubricity.
Properties of dry lubricants
- Dry lubricants provide a low and constant coefficient of friction between the two surfaces in relative motion. MoS2 does not require the presence of adsorbed vapour to function as a lubricant. Therefore it can be used in high temperature and vacuum applications.
- They are chemically stable over the operating temperature range. The lubricants do not attack or react with the metal surface. MoS2 begins to oxide in air at a temperature of 350° C though it can be still used for short periods for temperatures up to 450° C. MoS2 is stable to most chemicals but gets attacked by strong oxidising acids and alkalies.
- Dry lubricants adhere strongly to the surfaces and are not rapidly lost.. They do not flow, creep or migrate. MoS2 can be used as a dry powder or with binders to make it stick firmly to the surface,
- Provide resistance to wear and make the parts useful for a longer time. Friction and wear seriously affect the normal operation and the life of components.
- Dry lubricants are non-toxic, environment-friendly and economical to use. When liquid lubricants are used complex systems are required to supply the lubricant. The recycling of liquid lubricant and their disposal can cause harm to the environment. Burning the used liquid lubricants can release harmful gasses.
- Dry lubricants have a minimum tendency to pick up dust and contaminate the products and environment. Liquid lubricants have a tendency to pick up dust, dirt and other contaminants.
- Dry lubricants have very low volatility and can be used in vacuum. Liquid lubricant will vaporise in vacuum and hence cannot be used.
- They have very good load carrying capacity. Dry lubricants are more effective than liquid lubricants in high load and high speed conditions.
How can MoS2 be used for lubrication?
One of the ways to lubricate is to use bonding agents with the Molybdenum disulfide (MoS2) powder. The binder and the powder lubricant can be mixed in the right proportion and can be applied to the surface using dipping, brushing or spraying.
It can be dispersed in oil, water or grease to improve the lubricating properties. If the parts cannot be accessed after assembly then it can be sprayed. The solvent is allowed to evaporate which leads to formation of the MoS2 coating.
Pastes have a high percentage of dry lubricant like MoS2 and can be used for lubricating high-load and low speed lubrication. In high-temperature applications using paste metal powders are also added to prevent the oxidation of the metal parts.
MoS2 is used in anti-friction lubricating paints along with a binder. After applying the coating it is cured to allow the dry lubricant to adhere to the surface and form a solid film. This helps to reduce friction and wear.
MoS2 is used in self-lubricating composites which are used in carbon brushes, sleeve bearings and more. It is added in plastics to make a self-lubricating thermoplastic composite. MoS2 is compounded with Nylon to reduce friction and wear.
