Tungsten disulfide (WS2 is composed of Tungsten and Sulphur and occurs naturally as a mineral ore called ‘Tungstenite.’ It is commonly used as a solid lubricant due to its low coefficient of friction. It is a lubricant in automotive, aerospace, manufacturing, and other industries.
Tungsten Disulfide has a layered structure and exhibits tribological properties. WS2 crystals have a hexagonal structure with the W atoms packed between the two layers of S atoms. The W-S bonding is powerful due to covalent bonds, but the S atoms are loosely bound.
It can be used as a lubricant in high-temperature and high-pressure conditions. In a standard atmosphere, it can function in the temperature range of 270° C to 1200° C. In a vacuum, it can work in the temperature range of 188° C to 1316° C. Its load-carrying capacity is as high as 300 000 psi.
Lubrication mechanism
WS2 is one of the most lubricious substances known. It can be used in powder form in various applications. The tungsten disulfide (WS2) powder functions as follows:
- The powder produces a rolling effect on the surface of the moving parts.
- The powder absorbed on the moving surfaces forms a lubricating film. Nanoparticles are very useful because they can penetrate contacts in complex geometries, fill gaps between contacts, and create protective boundary films even under high pressure.
- The powder has a repair effect on the surface of the moving parts.
WS2 as a lubricant
Tungsten disulfide can be applied to lower friction and wear between the two moving surfaces. Dry lubricants can be used in high-temperature environments where other lubricants cannot be used.
WS2 is classified as a transition metal dichalcogenide with a lattice lamer structure. The atoms within the layer have strong covalent bonds and weak Van der Waals forces hold the layers together. Due to this, it can provide low friction as the basal planes in the layer easily slide past one another, thus producing a slipping effect.
The structure is a series of vertically stacked planes supported by weak forces. When the force is applied, the aircraft slips out of place. The strong covalent bonds force the slippage perpendicular to the basal planes.
The lubricant applied should be smoother than the bearing material to reduce the wear. When the WS2 layer is used on a substrate, it becomes a part and does not peel off or crack. WS2 is inert and stays stable in a standard atmosphere. The coating is non-toxic and non-corrosive.
Uses of WS2
When a gun is fired, it leads to high temperatures and erosive wear due to the heat and friction of the shot. This increases the diameter of the firearm barrel and nozzles. This reduces the lifecycle of the gun tubes.
This problem can be solved by applying gunpowder to protect the inner surface. IF-WS2 (fullerene-like tungsten disulfide) is used as a lubricant due to its thermal and mechanical resistance. It protects against wear and helps to prolong life. IF-WS2 is chemically inert and can be used on various substrates.
WS2 nanorods can be added to lubricating oils to improve the anti-wear ability of the oil. When added, WS2 nanomaterials of different morphology help improve the tribological properties of lubricating oils. WS2 nanoparticles, when added to lubricating oils, form a protective layer and cause a patching effect. This makes the interacting surfaces smoother and improves the tribological properties.
It is widely used in boundary lubrication because it reduces friction and wear. But it has properties like high resistance to oxidation and thermal degradation, and it is inert, non-toxic, and non-magnetic. These properties have a positive effect on boundary lubrication under severe environments.
In ships, it is used in moving parts like cylinders and piston rings of diesel engines, tail shafts, bearings, and crankshafts. The cylinder and piston rings work at high temperatures where the lubricating oil can get oxidized and decomposed and may not be able to form a lubricating film. Here, solid lubricants that have higher temperature resistance, like WS2, can be used. These lubricants show good anti-friction properties under dry sliding conditions.
The WS2 nanoparticle will adhere to the moving surfaces and fill in the valleys of the interacting surfaces, thus lowering the surface roughness.
Solid lubricants can replace liquid lubricants when the disposal and recycling of the lubricant is complex and can harm the environment. Solid lubricants can be easily sprayed on the surface.