Transition metal dichalcogenides (MX2) are widely used as solid lubricants where M stands for metals like Tungsten and X stands for Sulphur. Solid lubricants are used where the use of liquid lubricants is adequate or impractical. They can be used in applications like space technology and automotive transport.
Tungsten disulfide (WS2) is silvery-grey-black and occurs naturally as tungstenite ore.
Tungsten disulfide has a layered structure with strong covalent bonds within the structure and weak Van der Waal forces between the layers. The W-S layer is very strong but the layers of the S atom are loosely bound.
This structure is responsible for interlayer mechanical weakness. The weak binding forces allow easy and low-strength shearing. This gives WS2 a lower coefficient of friction.
Tungsten disulfide is the most lubricious material and offers dry lubricity, unmatched by any other substance. It is very stable at high temperatures like 550° C and when heated in the presence of oxygen it converts into tungsten trioxide (WO3). When heated in the absence of oxygen it decomposes into tungsten and sulphur.
Tungsten disulfide can perform well in extreme load, temperature and vacuum conditions. It has excellent thermal stability and resistance to oxidation even at high temperatures.
Synthesis of WS2
Various methods are used to produce WS2 of different sizes and morphology. The different processes are the gas-solid phase reaction, chemical vapour deposition, hydrothermal method, solid-solid phase reaction and the mechanochemical activation method.
- The gas-solid phase reaction follows a simple approach for producing WS2. The tungsten trioxide WO3 is reacted with sulfur-containing compounds at high temperatures for a long time. It can be synthesised in a tubular furnace by using WO3 particles and H2/H2S gas at a temperature of 840° C. However, this process leads to exposure to harmful and toxic H2S gas at high temperatures.
- WS2 flakes can be synthesised using a chemical vapour deposition method. A Sulfurisation of tungsten trioxide WO3 powder at high temperatures is carried out on a Silicon substrate. This method does not release any harmful gas. The process is very demanding but WS2 of high purity can be produced.
- In the hydrothermal method, the WS2 can be synthesised by autoclaving the mixture of WO3 and Sulphur precursor. It can be washed and dried to get the final product. This process leads to low production and additional thermal treatment is required because it produces WS2 with an amorphous structure.
- In the mechanochemical activation method, a ball-milled mixture of tungsten trioxide WO3 and Sulphur is annealed at 600° C in an atmosphere of Argon (Ar) gas. This method is environmentally advantageous but is a complex process. This method can yield the WS2 particles with small dimensions.
- In the solid-solid phase reaction method, the sulfurisation of tungsten trioxide WO3 is carried out in thiourea and N2 atmosphere at 850° C for 1 hour.
Applications of WS2
Tungsten disulfide is more stable and heavy and can be used in various applications. But it is an expensive lubricant. Now it is available at competitive prices therefore this super lubricant can be used in various applications.
- WS2 can be used as an additive to lubricating oils and other synthetic lubricants. This helps to enhance the lubricity and the high temperature and pressure properties. The WS2 in the mixture will get coated with the moving parts and this helps to reduce friction and improve the lubricity and load-carrying ability.
- The tungsten disulfide powder can be coated on a substrate that requires lubricity. The powder can be applied by spraying the substrate. You can make a 0.5-micron thick coating at room temperature.
- The WS2 can also be mixed with isopropyl alcohol and buffed onto a surface. The coating is used in applications like racing car engines, automotive parts, machinery components and more.
- WS2 shows the lowest coefficient of friction in a wide temperature range. It usually does not affect the precision machine tolerances. It is used in stainless steel fastening applications, precision bearings and vacuum applications as a sole lubricant.
- Due to rapid development, the demand for a lubricant with outstanding properties is increasing in aerospace machinery. Solid lubricants are used due to their excellent tribological performances. They can withstand harsh space environments with high vacuum, alternating temperatures and more.
Solid lubricants are used between toe surfaces in relative motion to prevent friction and wear. They are usually used in environments where other lubricants like oil and grease are not effective.
They can be used in very high and low temperatures, extreme pressure and vacuum applications. They are used where lubrication is required for the long term.
Also read: Materials Matter: Selecting the Right Materials for Custom CNC Projects
