Tungsten disulfide (WS2) is a solid lubricant used in various applications to reduce the friction and wear between the two sliding surfaces. It has a lamellar crystal structure which gives it lubricating properties, including a state of zero friction or superlubricity.
In WS2, a layer of Tungsten (W) atoms is sandwiched between the two layers of the Sulphur (S) atoms. Within the layer, there are strong covalent bonds between the atoms and the layers are held together by weak Van der Waals forces. The layers slide against each other easily when a shear force is applied. This gives it a low coefficient of friction.
WS2 has the lowest coefficient of friction among all the lubricants known. WS2 is thermally stable and resistant to oxygen. The slow rate of oxidation of WS2 is due to the formation of Tungsten trioxide (WO3) which also provides a low coefficient of friction. It is used in different industries like aerospace, manufacturing, automotive and more.
WS2 can be used in high temperature, pressure and vacuum applications. It has good thermal stability and can be used in extreme environments. It can be used in space applications where the sliding parts need lubrication. Coating can be fabricated using heat treatment of Tungsten disulfide powder and an additive. The coatings show excellent frictional properties at elevated temperatures up to 400° C.
When WS2 is used in powder form, it can form a lubricating film between the two sliding surfaces. The lubricating film can be harder or softer than the substrate material. The film can be generated using mechanical, electrochemical, physical and chemical methods. In dry environments, WS2 can provide low friction and prolonged wear life. The lubricating films formed on the substrate are smooth and adhere firmly to the substrate.
WS2 can be used as a filler dispersed into a bulk counterpart material to make composites. It can be used as an additive in liquid lubricants.
Lubrication mechanism
The lubrication mechanism depends on the lamellar structure of WS2. The shear behaviour of the layers governs the friction and wear properties of the lubricating films. When WS2 is used between two sliding surfaces, it aligns itself along the sliding direction and slides over one another, acting as a lubricating film. The low shear strength leads to a low coefficient of friction at the interface. The layers with strong covalent bonds provide good load-carrying capacity.
Why are solid lubricants preferred in certain applications?
- Solid lubricants like WS2 are used where the liquid lubricants do not meet the needs of modern technology. Solid lubricants are less expensive to use than liquid or grease systems. Solid lubricants reduce weight and simplify lubrication and improve the processes. In the cost-conscious automotive industry solid lubricants can replace oil and grease in many applications leading to highly efficient automobiles.
- In high-vacuum environments or in food processing machines liquid lubricants can evaporate and contaminate the products. At high temperatures the liquid lubricants decompose or oxidise therefore solid lubricants are used. Solid lubricants can work effectively at high temperatures and maintain a low coefficient of friction.
- At cryogenic temperatures the liquid lubricants become highly viscous and ineffective. In corrosive environments or under radiation the liquid lubricants will decompose and get contaminated.
- In high dust conditions, the liquid lubricants tend to pick up dust therefore solid lubricants are used. The contaminants in the liquid lubricant can form a grinding paste and cause abrasion.
- Under intermittent loading conditions and changes in environmental factors like pressure, loading, time, temperature and radiation can affect the efficiency of the liquid lubricants. When the equipment is stored or is idle for prolonged periods solid lubricants offer good lubrication.
The contact between two sliding surfaces cannot be avoided and it leads to friction and wearing out of the material. Due to friction a large amount d of energy is liberated in the form of heat which reduces the efficiency of machines. To prevent these problems lubricants are used. There are different types of lubricants available such as solid, semi-solid, liquid and gaseous. Solid lubricants are placed between the two moving surfaces to avoid direct contact between them and reduce friction and wear.
WS2 has a low coefficient of friction as it shears easily and reduces friction and prevents wear damage. In extreme environments solid lubricants are the only possible option available. At higher temperatures, pressure and load solid lubricants perform better. WS2 can be used in millitary applications where the moving parts need reliable lubrication. It is used in athletic gear like bicycle chains, roller skate bearings and more.
