Molybdenum Disulfide (MoS2) is an inorganic compound made up of Molybdenum and Sulphur. It is classified as a Transition Metal Dichalcogenide (TMD). It is a silvery block solid and occurs naturally as a mineral ore Molybdenite and Jordisite.
MoS2 is a type of dry lubricant and is used to reduce friction and wear between two surfaces moving in relative motion to each other. Though it is in solid state it can effectively provide lubrication between the two surfaces without the use of a liquid medium.
Dry lubricants can be used for lubrication at higher temperatures than the liquid or oil lubricants can operate. MoS2 can work as a lubricant effectively up to a temperature of 350° C in oxidising environments and in non-oxidising environments up to 1100° C.
The low friction characteristic of Molybdenum disulfide is due to its unique layered lattice crystal structure. In MoS2 a layer of Molybdenum atoms is sandwiched between the two layers of Sulphur atoms. The bonds within the layer are strong covalent bonds in a sequence of S – Mo – S. The layers are connected together by weak Van der Waals forces.
The layers can easily slide against each other and can shear under very low force which gives MoS2 excellent lubricating properties. The shear force required to overcome the weak bonding between the layers is related to the compressive force perpendicular to the layers. Therefore the layers are very hard to force perpendicular to them. Due to these properties the MoS2 can prevent the moving surfaces from coming in contact with each other.
Dry lubricants are used to lubricate mechanical components which operate under low pressure and high temperatures. They can be used in harsh environments such as ultra-high vacuum, extreme temperatures both cryogenic and high and corrosive environments. They can be used as lubricants where challenging conditions are prevalent.
In such conditions liquid or oil lubricants cannot be used as they may solidify at cryogenic temperatures and decompose and vaporise at high temperatures. Dry lubricants have emerged as an excellent alternative to liquid lubricants for providing lubrication in challenging environments.
Dry lubricants offer advantages like good thermal stability, high melting point and are effective in humid and high load conditions. MoS2 can be used in solid form or powder form to lubricate the moving parts. It can be used as a lubricant in sensitive environments like food processing and textiles.
Properties of MoS2
MoS2 has a load carrying capacity of about 2,50,000 psi and a coefficient of friction of 0.19. MoS2 is insoluble in water and dilute acids. It is chemically inert and unaffected by oxygen. It is non-toxic and does not harm the environment. It can form long-lasting lubricating films which can bear heavy loads and harsh environments.
Use of MoS2 in solid film lubrication
MoS2 is used in solid film lubrication where liquid lubricant cannot be used. The principle used is that when a low shear strength material is introduced between two sliding surfaces in contact the friction force gets reduced. The friction force is related to the lubricant’s shear strength and the contact area.
The different methods used for lubrication are burnished films, spluttered coatings and bonded coatings.
Burnished films
One of the properties of MoS2 is that it can be easily rubbed on the metal substrates to form a lubricating film. This method is used in applications like metal forming dies, sleeve bearings, threaded parts, liquid oxygen valves and more. Methods like Molybdenum disulfide powder dispersion, wire-brushing or sandblasting.can be used to form a thin lubricating film.
Spluttered coatings
In this, very thin films of MoS2 are used for tribological control. To form thin films with a low coefficient of friction it is important to understand how adherence, structure and chemical composition can affect the friction and wear behaviour.
Strong film adherence is important in achieving long endurance lives. Therefore the surface of the substrate should be prepared prior to film deposition. This can affect the degree of adhesion. The most commonly used surface pre-treatment is sputter-etch cleaning. Spluttered MoS2 films are largely used for steel surfaces. Steel surfaces display excellent adhesion.
Most of the MoS2 films are spluttered at ambient or elevated temperatures to get lubricating characteristics.
Bonded coatings
This is the most common method used for achieving dry self-lubricating films on metal surfaces. In this the solid lubricant is dispersed in an organic or inorganic binder and attached to the surface. A typical combination can be 80% of MoS2 and 20% of binder and a carrier liquid.
