Tungsten Disulfide is a 2D material and is used in various applications like gas sensing, solid and dry film lubricants and self-lubricating composites. It exhibits better tribological properties and heat resistance than any other lubricating substance.
It is a material in the Transition Metal dichalcogenides which have a formula MX2 where M is the metal like Tungsten (WS) and X is an element from the chalcogen group like Sulphur (S). Tungsten Disulfide is a layered material with a layer of Tungsten atoms sandwiched between two layers of Sulphur atoms in a trigonal prismatic coordination.
The layers are held together by weak Van der Waal forces which allows the sheets to glide past each other. This allows it to exhibit lubricating properties. It is chemically inert but is affected by nitric and hydrofluoric acids.
When it is heated in an oxygen-containing atmosphere it converts to tungsten trioxide. When it is heated in the absence of oxygen it decomposes into tungsten and Sulphur at a temperature of 1250° C.
Tungsten disulfide occurs naturally as a mineral Tungstenite. It can also be produced by using several methods. Some of the methods involve reacting metal oxides with Sulphur. Others use the metal chlorides with hydrogen sulfide gas or are generated in situ. Other processes involve thermolysis of tungsten disulfide or equivalent.
Tungsten Disulfide films can be produced by depositing it on a hydrophilic substrate like Sapphire and coating it with polystyrene. The sample is then dipped in water for a few minutes. The hydrophobic WS2 film peels off easily.
The Chemical Vapour Deposition (CVD) is the most popular method of producing WS2. In this, elemental Sulphur and metal oxides are used.
The WS2 lacks vapour pressure even at 1000° C and under vacuum it decomposes and does not evaporate. Therefore using the Physical Vapour Deposition (PVD) method is a challenge. In this, the bulk form of the desired material is used as the only precursor. Next, the material is heated under a vacuum which produces a vapour. This vapour can be deposited on a cooler substrate.
What is friction and lubrication?
When two surfaces slide against each other a resistance is encountered called ‘Friction’. It always acts in the opposite direction to that of the relative displacement between the two surfaces. Frictic can cause drastic consequences like wear and energy dissipation.
Friction between the two moving surfaces can be reduced by using lubricants. Lubricants can be solid, liquid, semi-solid or gas. Tungsten Disulfide has been used as a solid lubricant for many years. The strong covalent bonds between the Tungsten and Sulphur atoms within the lamellar and weak van der Waal forces allow easy intern lamellar shearing. This leads to a low coefficient of friction.
Tungsten Disulfide can offer dry lubrication which is unmatched to any other substance. It can be used as a lubricant in high temperature, vacuum and high-pressure conditions.
Application of Tungsten Disulfide
Tungsten Disulfide is extensively used in the automotive, military and aerospace industries. Now WS2 is available at competitive prices though earlier it was too expensive. You can choose a superior lubricant and improve the quality and competitiveness of the final product. WS2 has a low coefficient of friction and works over a wide temperature range. It is the thinnest of the dry lubricants and does not affect the precision machine tolerances.
When a substrate requires dry lubricity it can be coated with Tungsten disulfide powder. It can be easily sprayed on the substrate with dry pneumatic air. The spraying can be done easily at room temperature and a layer of 0.5 micron thickness can be formed. Once applied the film becomes a part of the substrate and does not crack, peel off or wipe off. It is used where a thin layer of lubricant is required and where seizure, wear and galling need to be minimised.
Alternatively, the powder can also be mixed with isopropyl alcohol and the paste can be buffed to the substrate. It can avoid failures due to friction and improve mechanical performance. It can reduce wear and improve the life of the machine parts. It can help you to avoid costly downtimes.
The powder can be added to other lubricants to enhance their properties. The powder can be mixed in 1% – 15% to synthetic lubricants, grease or oil. This will help to enhance the lubricity of the mixture and enhance the high-temperature and high-pressure properties. When added to motor oil the mixture shows better anti-wear, anti-friction and extreme pressure properties than any other motor oil.
