Tungsten Disulfide (WS2) As A Dry Lubricant

Tungsten disulfide (WS2) is widely used as an industrial lubricant in various applications such as automotive, aerospace, military and marine. WS2 is classified as a Transition Metal Dichalcogenide and has a 2D structure. It occurs naturally as a mineral ore, Tungstenite. 

WS2 has a layered lattice crystal structure. A plane of Tungsten atoms sandwiched between two planes of Sulphur atoms forming the S-W-S molecules. The atoms within the layers are bonded with covalent bonds. The layers are connected using the weak Van der Waals forces.

The bonded atoms are confined to the structure and create a positive charge on the surface. This allows easy shear of the layers when a force is applied. The layers easily glide past each other, resulting in a low coefficient of friction. Tungsten disulfide powder can be used between two sliding surfaces to reduce friction and wear. WS2 exhibits lubricity, which is unmatched by any other substance, and it is the most lubricious substance known.

It can work effectively in normal atmospheres in a temperature range of -270 °C to 650 °C and in vacuum from -188 °C to 1316 °C. WS2 can be used in high-temperature applications and has an excellent load-carrying capacity. It exhibits a state of near-zero friction called superlubricity. WS2 is resistant to oxidation and forms WO3 at higher temperatures, which is also a good dry lubricant.

Application Of WS2

• The space application requires lubricants that can withstand the severe conditions. In space machines, there are harsh and alternating temperatures, high vacuum, strong oxidation and radiation. The lubricant should work effectively and provide a long service life in space.
• WS2 is widely used in space applications due to its properties. It can work effectively at high temperatures encountered in space. It can offer good lubrication in a vacuum without getting degraded or vapourised. It helps to reduce friction and wear in the machinery and increase its useful life.
• WS2 is used as a lubricant in applications where galling, wear, seizure, and fretting need to be minimised. The WS2 coating can be applied to the metal surfaces without binders. It can be used in high-precision assemblies. It can be applied using high-speed air impingement methods directly to the surface. Once applied, the WS2 coating becomes a part of the substrate and does not peel off or crack. This prevents direct contact between two sliding surfaces and reduces friction and wear.
• WS2 is used in high-temperature, high-pressure applications to provide dry lubrication. It can be added to oils and greases to improve the lubricating properties. It can be used with different materials like plastics, ceramics and metals.

Synthesis Of WS2 Using WO3

Different methods are used to produce WS2 of different sizes and morphologies using WO3. The methods are as follows:

Gas Solid Phase Reaction

This is a simple method of producing WS2. In this, WO3 is made to react with sulfur-containing compounds at a high temperature and for a prolonged time. It is synthesised in a tubular furnace by a reaction between WO3 and H2/H2S gases at 840 °C for 30 minutes in an Ar gas flow. However, this method causes exposure to highly toxic H2S gas at high temperatures.

Chemical Vapour Deposition (CVD)

In this method, the WS2 flakes can be synthesised using a Si/SiO2 substrate. WO3 and Sulphur powder are used as precursors under an inert gas flux in a quartz tube under atmospheric and low-pressure conditions. The Sulphur vapour reduces WO3 to provide suboxide products which get adsorbed on the substrate. The Sulphur vapour transforms them to WS2. The advantage of this method is that you can get WS2 of high quality. However, it is a difficult and complex process. 

Hydrothermal Method

In this method, WS2 is synthesised by autoclaving a mixture of WO3 and Sulphur precursors. The resultant output has to be washed and dried. You can use inexpensive precursors, but the yield obtained is low. Additional thermal treatment is required because WS2 is produced with an amorphous structure.

Mechanochemical Activation Method

Using this method, WS2 nanosheets can be synthesised. A ball-milled mixture of WO3 and Sulphur is annealed at 600 °C for 2 hours in an atmosphere of Ar gas. This is a very complex process, but it is environmentally advantageous compared to the traditional methods. This method produces WS2 particles with smaller dimensions.

Solid Solid Phase Reaction

In this, WS2 is synthesised in powder form through the sulphurisation of WO3 powder with thiourea in a horizontal tube furnace. The process is carried out in an N2 atmosphere at a temperature of 850 °C.

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