Lubricant Tungsten Disulfide (WS2) – Properties And Synthesis

In a machine, the mechanical parts come in contact with each other and rub against each other. This causes friction, and the machine parts wear out. Friction affects the energy dissipated between the two moving surfaces, thereby affecting the energy efficiency. It affects the endurance of the mechanical parts and wear and tear. Repeated and uncontrolled friction can lead to wear and tear of the material of the surfaces. Therefore, it is important to reduce friction when the machine is working.  

One way to reduce friction and wear is to use a lubricant. A lubricant can be placed between the two moving surfaces to reduce friction and wear. It forms a layer between the two moving surfaces and prevents direct contact between them, thus reducing friction and wear. 

One of the dry lubricants is Tungsten Disulfide (WS2), which is an inorganic compound made up of Tungsten (W)  and Sulphur (S). It occurs as a mineral ore called Tungstenite. WS2 is classified as a Transition Metal Dichalcogenide (TMD). TMDs have a structure of type MX2 where M is a transition-metal atom (Tungsten), and X is a chalcogen atom (Sulphur).

Tungsten disulfide exhibits a low coefficient of friction due to its layered lattice crystal structure. Within the layers, the atoms are bonded with covalent bonds and form an S-W-S structure. One plane of the W atoms is sandwiched between the two planes of the S atoms. The individual layers are connected together by weak Van der Waals forces.

When a shear force is applied, the layers easily slide against each other, leading to a low coefficient of friction. The layers align themselves parallel to the moving direction. The layers with covalent bonds provide a good load-carrying capacity perpendicular to the moving direction. Tungsten disulfide powder can be directly applied to the moving surface without using binders.

Properties of WS2

• WS2 can provide effective lubrication over a temperature range of -270 °C to 650 °C in a normal atmosphere. 
• It can be used in high vacuum applications over a temperature range of -188 °C to 1316 °C. 
• WS2-coated films provide a load-carrying capacity as high as 300,000 psi. It is used in high-stress and high-load applications.
• It has a high thermal stability and has a melting point of 1260 °C. Therefore, it provides effective lubrication in high-temperature applications.
• It is chemically inert and non-toxic; therefore, it can be used in reactive environments.
•  It can be used in high-speed applications due to its wear resistance.

 Synthesis of WS2

The most widely used methods to get WS2 monolayers are as follows:

 Chemical Vapour Deposition (CVD)

This method produces high-quality and thin WS2 films. It is a vacuum deposition method where the substrate is exposed to volatile precursors. The precursors react and deposit on the substrate. The volatile by-products produced are eliminated by a gas flow through the reaction chamber. 

A quartz reaction tube with two heated zones is used – one is for the Sulphur powder and one for the substrate. Solid Tungsten precursor W(CO)6 is heated and evaporated. Helium is used as the carrier gas and passed through the quartz tube. Elemental Sulphur is also heated in the tube. The S2 vapour is mixed with the carrier gas and reaches the heated substrate. WS2 monolayer growth is evidenced by the colour (grey/blue) change of the transparent substrate.

 Physical Vapour Deposition (PVD)/ Vapour Phase Transport (PVT) 

It is a vacuum deposition method and can be used to produce thin WS2 films and coatings. In this method, the material changes from a condensed phase to a vapour state and then to a condensed thin film phase. The most common processes used are sputtering and evaporation.

The Pulsed Laser Deposition (PLD) is the most used PVD technique for obtaining WS2 layers. This method allows you to control the uniformity and the thickness of the layers. A SiO2/Si substrate, a third-harmonic Q-switched Nd: YAG, and a laser of 355 nm are used. The shiny laser beam is focused on the target to be ablated and deposited as a thin film on the substrate.

Metal Organic Chemical Vapour Deposition (MOCVD)

It is a chemical vapour deposition method used to produce thin WS2 films. The growth of the crystal is through chemical reaction and not vapour deposition.

WS2 precursors W(CO)6 and (C2H5)2S are used along with H2. They are all diluted using a carrier gas, Ar. The concentration of the reaction can be controlled by the pressure of each reactant. With low partial pressure, the WS2 films grow layer by layer.