MoS2 is a silvery black solid and occurs naturally as a mineral ore Molybdenite or Jordisite. It is chemically inert and insoluble in water as well as oil. It is unreactive with most acids but cannot resist an attack of hot concentrated sulphuric and nitric acids. It dissolves in strong oxidising agents like Aqua Regia.
Molybdenum Disulfide (MoS2) is an inorganic compound and has lubricating properties because of its layer-lattice structure. It has a layered structure consisting of a hexagonal plane with a plane of Mo atoms sandwiched by a hexagonal plane of the S ions. The layers have a strong covalent bond between them and the bonding between the layers is of weak Van der Waals forces.
The shearing takes place along the plane of weak bonding and results in low friction. This property is inherent in its structure. It can shear more easily parallel to the layers than across them. Therefore it can support high loads at a right angle to the layers and still slide easily parallel to the layers. This property is used in lubrication.
MoS2 has a high melting point and low vapour pressure. The lubricating effect of Mos2 persists even if there is oil loss as it does not evaporate at low pressures. It can be used as a lubricant in heavy machinery and low-pressure applications. It is used in boundary lubrication where there is a contact between two metal surfaces.
MoS2 as a lubricant
It is used as an additive in grease and used in many applications to achieve better performance. For rough surfaces, a large particle size of MoS2 is preferred to fill in the gaps and produce a smoother finish. For smoother surfaces mild to small particles will offer a better load-carrying ability. The medium particle size of 6 μm is most commonly used in greases. Greases with MoS2 as an additive are used in the construction, transportation, agricultural and mining sectors.
It can be added to fluid lubricants to reduce friction and wear in boundary lubrication, increase the load-carrying capacity and prevent/delay failure. It can be added to grease oil to increase the load-carrying capacity. Such lubricants are used in worm gears, hypoid gears and helical gears.
It can be used in bonded coatings to achieve a dry and self-lubricating film on metal surfaces. Though resins have to be added to MoS2 to enhance the bonding to the base metal. To be effective the MoS2 layer has to be bonded firmly on the surface of the material which requires lubrication.
Uses of MoS2
1. When the stainless steel objects slide against each other under a high load they may fail due to the deformation of the contact points. When two objects touch at very small regions of contact the surface area is much smaller than that of the total surface area. The stress on these contact points is very high when compared to that of the total surface area.
When the two objects slide relative to each other friction and stress can cause the deformation of contact points. To prevent this a substance is required which can maintain the separation of the surfaces under high load. This means to provide a boundary layer between the two surfaces. Molybdenum Disulfide (MoS2) powder can be used because of its high compressive strength and its ability to adhere to sliding surfaces.
Different techniques are used to apply MoS2 to the surface like vacuum spluttering, dropping the powder between the surfaces and mixing the powder with a binder to form a bonded coating. A bonded coating of MoS2 can provide good lubrication performance for temperatures under 500° C.
2. MoS2 is used as an anti-seize thread compound and bolt coating. The anti-seize compound is used to place a dissimilar material between two metal surfaces to reduce the potential of galling or seizure. MoS2 can provide better galling resistance than any other composition.
High percentage MoS2 anti-seizure compounds are available which are popular for their low coefficient of friction over a range of contact pressures. These compounds help to reduce resulting friction.
A solid lubricant like MoS2 is used to protect against the damage caused by the relative motion of moving surfaces. The solid lubricant should have properties like controlling friction between the two surfaces, being chemically stable over a temperature range, not reacting with the metal surface, adhering strongly to the surface, providing resistance to wear, and being non-toxic and economical. MoS2 has all these properties and can be used in applications where liquid lubricants cannot function.