When two surfaces move in relative motion, you must consider friction and wear parameters. Don’t know what is Friction? Well, consider two surfaces are in motion. Friction can be defined as the force that directly opposes the motion between the two surfaces. On the other hand, wear is referred to as the loss of material leading to the destruction of the surface. Friction may lead to wear, and to reduce friction and wear, it is suggested to use lubricants.
Lubricants in different forms are used to reduce friction and wear between two surfaces. Different types of lubricants are semi-solid, liquid, solid/dry, and gaseous. Lubricants must be introduced as a layer between the two moving surfaces for them to work.
Tungsten Disulfide (WS2), a type of Transition Metal Dichalcogenide (TMD), is a solid lubricant. TMD is known for its elemental build blocks and has a composition of MX2, where M is a transitional metal, and X is a Chalcogen. In tungsten disulfide, the transitional metal is Tungsten, and Sulphur is a Chalcogen.
TMDs have a common structure of a layer of M atoms sandwiched between the layers of X atoms. In the case of Tungsten Disulfide, a layer of Tungsten atoms is sandwiched between two layers of Sulphur atoms. Each layer has a hexagonal crystalline lattice structure.
The Tungsten and the Sulphur atoms are bonded by strong covalent bonds while the structure (the layers) are held by weak Van der Waals forces. When a shear force is applied, the layers slide past each other easily. This leads to a low coefficient of friction.
Properties of WS2:
Tungsten Disulfide can offer dry lubricity, which is unmatched by any other substance. Its coefficient of friction is as low as 0.03; therefore, it can be used in many applications. It can be used as a lubricant in high-temperature and high-pressure applications.
WS2 can work effectively as a lubricant in a temperature range of -270° C to 650° C in a normal atmosphere and -188° C to 1316° C in a vacuum. When heated in the absence of oxygen, it decomposes into Tungsten and Sulphur at 1250° C but does not melt. When heated in an atmosphere containing oxygen, it converts to Tungsten Trioxide.
WS2 has an excellent load-bearing capacity of 300,000 psi. It is relatively stable in ambient air. However, it is attacked by a mixture of Nitric and Hydrofluoric acid.
Application as a lubricant:
WS2 is used as a solid lubricant in coatings or as an additive to composites to increase the self-lubricating properties. As it can offer lubrication over a wide temperature range and in high-pressure environments, it is mainly used in the aerospace industry. In such environments, liquid lubricants cannot be used.
When coatings are required, the tungsten disulfide powder can be sprayed on the substrate with dry and pneumatic air. No binders are required, and spraying can be done at room temperature. The thickness of the coating film is around 0.5 microns. The powder can also be mixed with isopropyl alcohol, and the paste can be buffed on the substrate. The particles form a thin lubrication film with good adherence, leading to a low coefficient of friction.
When WS2 is used in composites, its powder can be mixed with oil, grease, and other synthetic lubricants. This helps to improve the lubricity of the mixture and the high temperature and high-pressure properties. During use, the WS2 particles get coated on the moving surfaces, which helps to reduce friction.
Synthesis of WS2:
The various processes used for the large growth of WS2 are Chemical Vapour Deposition (CVD), Metal-Organic Chemical Vapour Deposition (MOCVD), and Atomic Layer Deposition (ALD).
- In the CVD process, the Chalcogen and the metal oxide precursors are co-evaporated. Further, they are made to react in the vapor phase to form a stable layer on the substrate. To get amorphous or polycrystalline films, the substrate is kept at a low temperature.
- The MOCVD process is a subclass of the CVD process and uses organic and metal-organic precursors as the source materials. The required atoms combine with the complex organic molecules and are carried to the substrate. Heat is used to decompose the atoms, and the required atoms form a thin film with high crystallinity. You can engineer the growth film to meet the specific requirements.
- The ALD uses a layer-by-layer gas-phase chemical process for the deposition of the thin films on the substrate. In this process, the precursors are delivered sequentially, which leads to the film’s layer-by-layer growth. You can vary the number of ALD cycles to control the number of layers. This process can be used for large-area substrates.
Nirmal Sarkar is a Biotechnologist from the city of Joy, Kolkata. He is the founder of this blog and covers a wide range of topics from Gadgets to Software to Latest Offers. You can get in touch with him via nirmal@hitricks.com
