Tungsten Disulfide (WS2) – Properties, Application & Synthesis

Lubricants reduce friction and wear between the two surfaces in relative motion. A film of lubricant needs to be introduced between the two moving surfaces. The type of lubricant to be used depends on the operating conditions and requirements.

When the application requires high or low operating temperatures, a vacuum or a high standard of hygiene solid lubricants are used. Tungsten disulfide (WS2) is a solid lubricant and one of the most lubricious substances.

When solid lubricants are used for lubrication additional oils or additives are not required. Therefore the need for maintenance lubrication is not there. Solid lubricants are useful in applications where maintenance lubrication is expensive or impossible to implement. When solid lubricants are used gaskets are not required as there are no liquids to be kept in place.

Tungsten Disulfide (WS2)

Tungsten disulfide (WS2) is widely used as a dry film lubricant in industrial applications. WS2 occurs as a rare mineral Tungstenite. It is classified as a ‘Transition Metal Dichalcogenide’ (TMD). TMDs have a composition of MX2 where M is a transitional metal like Tungsten (W) and X is a chalcogen-like Sulphur (S).

TMDs have a common structure of a layer of M atoms sandwiched between two layers of X atoms. In Tungsten Disulfide a layer of Tungsten atoms is sandwiched between two layers of Sulphur atoms. Each layer has a hexagonal crystalline lattice structure.

The Tungsten and Sulphur atoms are bonded with strong covalent bonds. The adjacent layers are bonded together by weak Van der Waal forces. When WS2 is subject to shear forces the weak bonds break and the material shears easily. This gives WS2 its low coefficient of friction.

In dynamic applications, WS2 has a coefficient of friction of 0.03 and 0.07 in static applications. WS2 can work in temperature ranges of -270° C to 650° C in normal atmosphere and -188° C to 1316° C in vacuum. The load-bearing capacity of WS2 is 300,000 psi.

When heated in an oxygen-containing atmosphere WS2 converts to Tungsten trioxide. When heated in the absence of oxygen WS2 decomposes into Tungsten and Sulphur at a temperature as high as 1250° C.

How can WS2 be used for lubrication?

WS2 can be used in different ways to provide lubrication. The tungsten disulfide powder can be used to provide dry lubricity or as an additive. When WS2 gets coated on the moving parts it will help to reduce friction and improve load-bearing ability.

• The powder can be coated on a substrate that requires dry lubricity. The powder can be coated by spraying at room temperature. A film of 0.5-micron thickness can be achieved. This film helps to increase durability and wear resistance. It has a long wear life in dry environments.
• Another method is to mix the powder in isopropyl alcohol and the paste can be buffed on the substrate. WS2 has excellent erosion and corrosion resistance
• The powder can be used as an additive in other lubricants like oil, grease and synthetic lubricants. This helps to improve the lubricity of the mixture and also improves the high temperature and extreme pressure properties. The lubricity can last for long cycles.

WS2 is used in the aerospace industry where liquid lubricants cannot be used. It is used as a dry lubricant for fasteners, mould and bearings. It can be easily applied on a metal surface without binders or curing by using high-velocity air impingement.

Synthesis of WS2

The main processes used in the growth of thin layers of WS2 are liquid exfoliation, mechanical exfoliation, exfoliation of bulk and chemical vapour deposition.

Liquid exfoliation

In this, the WS2 powder is mixed with solvents like benzene benzoate, acetone, methanol or isopropanol. This is a time-consuming process and sensitive to the environment.

Mechanical exfoliation

Using this process a single layer of WS2 can be obtained from a bulk crystal on a Si/SiO2 substrate. A thin layer with uniform roughness can be obtained. This method can also be used in vacuum and high temperatures.

Exfoliation of bulk

This process can be used to get WS2 nanosheets. The WS2 powder is sonicated with N-dimethylformamide.

Chemical vapour deposition

This process can be used to obtain WS2 nanosheets. Tungsten Hexacarbonyl is decomposed at temperatures from 420° C to 1000° C. The process is completed in two zones. In the first zone Sulphur vapour is mixed with Carbonyl vapour.  This mixture is transferred to the second zone.

The second zone is heated and the Carbonyl pyrolysis process takes place. The formation of nanopowder is achieved in Sulphur.