Molybdenum Disulfide (MoS2) is classified as a Transition Metal Dichalcogenide (TMD) and occurs naturally as a mineral ore Molybdenite and Jordisite. It is widely used as a dry lubricant to reduce friction and wear in the machinery.
MoS2 has a hexagonal layer-lattice structure and a layer consists of a plane of Molybdenum atoms sandwiched between two planes of Sulphur atoms. These three layers form a monolayer of MoS2. The bonds within the layer are strong covalent bonds whereas the layers are held together by weak Van der Waals forces.
When a shear force is applied the layer can easily slide past each other. The material can shear more easily parallel to the layers than across them. They can support relatively high loads at right angles to the layers and still slide past parallel to the layers.. This property is used in the lubrication mechanism.
The low coefficient of friction occurs only parallel to the layers therefore MoS2 will be effective when their layers are parallel to the direction of sliding. MoS2 is a versatile material and can be used where oil and greases cannot be used as they do not have sufficient load-carrying capacity.
Advantages of MoS2
- MoS2 can be applied on a metal surface using burnishing, rubbing, air spraying, spluttering or bonded coatings. It has no tendency to creep, flow or migrate. It is important that the dry lubricant adheres to the moving surface otherwise it will get rubbed away.
- It has a minimum tendency to pick up dirt or dust and does not contaminate the products. It is therefore used in food and textile industries.
- It has very low volatility and therefore it can be used in high vacuum applications such as space applications where challenging conditions are prevalent.
- It is chemically inert and can be used in reactive chemical environments. It is also stable to radioactivity and can be used in nuclear power plants. It is stable to most chemicals but gets attacked by strong oxidising acids and alkalis.
- It is non-toxic and has good load-carrying capacity. Unlike Graphite it does not require the presence of adsorbed vapour to act as an lubricant. It can be used in high temperature applications. MoS2 begins to oxidise at 350° C in air though it can be used till a temperature of 450° C. The oxidation process produces Molybdic oxide (MoO3) which itself is a lubricant.
- It offers advantages like higher melting point, better thermal stability and can be used in humid and high load conditions.
- MoS2 can be used for lubrication in challenging environments without the need for frequent replenishment.
Applications
- In space application dry lubricants are used as they are lighter than the liquid lubricants. MoS2 is a preferred choice for space applications due to its properties. It offers exceptional performance in vacuum with extremely low coefficient of friction. MoS2 can be used as a lubricant in cryogenic and high temperatures making it suitable for space applications.
- MoS2 is used in boundary lubrication where metal to metal contact exists. It helps to prevent the asperity contact between the two surfaces and no wear takes place. It can be used in conditions like high temperature, slow sliding speeds, start-stop motion and shock loading.
- MoS2 is added to greases and used in a variety of applications. It is a very useful additive to Titanium-complex greases to get an even better performance. The particle size commonly used in greases is 6 μm. Greases containing MoS2 are used in transportation, construction, mining, agricultural, millitary and aerospace applications.
How can MoS2 be used in solid film lubrication?
In solid film lubrication dry lubricants are used to provide lubrication between two sliding surfaces. It is important that the Molybdenum disulfie powder adhere to the surface of the substrate. Different methods are used to apply MoS2:
- One of the important properties of MoS2 is that it can be easily rubbed onto the surface of the subatartes to develop a lubricating film of 1 – 5 μm in thickness. This type of burnished coating is used in applications like metal forming dies, seelve bearings, liquid oxygen valves and electrical switches and relays.
- Spluttered coating method can be used to get thin film of MoS2 which is more adherent and shows greater endurance than other types of coatings. MoS2 spluttering can be done on a variety of substrates and in vacuum environments.
- Resin-boandeded coatings can be used to get a dry and self-lubricating film of MoS2. In this MoS2 is mixed with an organic or inorganic binder and applied to the substrate.
