Molybdenum Disulfide (MoS2) is one of the Transition Metal Dichalcogenides (TMD) and has gained a lot of interest because of its properties. It comprises transition metals like Molybdenum and a chalcogen-like Sulphur and has one atom of Molybdenum and two atoms of Sulphur.
Molybdenum Disulfide is naturally found as a mineral Molybdenite or Jordisite (a low-temperature form of Molybdenite). Molybdenite ore is processed by floatation to get pure Molybdenum Disulfide (MoS2).
MoS2 has a hexagonal crystalline structure and its structure differs from 3D to 2D to 1D to dot structure. Its characteristics also change from one dimension to another and it can be semiconducting, metallic or superconducting.
MoS2 exists in several layers and shapes and its bulk form can be triagonal, hexagonal or rhombohedral. It is relatively inactive and is affected by oxygen and dilute acids. Bulk MoS2 consists of stacked monolayers that are held together by the weak Van der Waal forces.
MoS2 has properties like a large bandgap which changes from an indirect gap to a direct gap in thin structures. The covalent bonds between the Molybdenum and Sulphur and the Van der Waals forces between its layers make it optimal for gas sensing.
MoS2 has contact resistance and high performance and hence it is used to make 1 nm gate transistors. It is used in bio-applications like DNA, coronavirus and cancer detection. It is effective in curing cancer and Alzheimer’s disease. Due to its properties and structure, MoS2 is used in electronics, sensing, microwave and terahertz applications. MoS2 also serves as a lubricant because of its hexagonal layered structure.
MoS2 does not require a humid environment to perform well and its lubricating properties improve in oxygen-deficient environments. It can operate well under a range of temperatures. It is therefore used in aerospace applications.
Why is MoS2 used as a lubricant?
In manufacturing plants, there are extreme operating conditions like high temperature, high pressure, high and low shaft speeds and more. To reduce the friction between the moving parts and to reduce the wear, lubricants are required.
Oil-based fluid lubricants can form an oil film and are used within the temperature range of -20 C to 100 C. At higher temperatures can cause a change in the state of the fluid and the lubricating properties. In such cases, solid lubricants are used for machinery lubrication.
MoS2 is a dry lubricant and can be applied directly to the interacting surfaces to reduce friction. It has a low shear force between its crystalline lattice layers that helps to minimise the resistance between the interacting surfaces. The layers of MoS2 slide over each other allowing them to flow freely over the metal surfaces even under high pressure. It forms a dry lubricating film and works well in dry environments. It is a widely used lubricant and is a stand-alone lubricant.
MoS2 can function well in a hard vacuum because there is no vapour present between the lattice plates. It can also tolerate higher loads and can function as a lubricant up to 700° C. For rough and cut surfaces a larger particle size of MoS2 lubricant should be used than for the higher-finished surfaces. The right particle size should be used.
MoS2 plays an important role in the brake lining composite. It helps to reduce wear that occurs on the brake lining composite which is the contact surface. The coefficient of friction and wear are affected by the use of solid lubricants like MoS2. MoS2 offers good lubricating performance and helps to reduce the coefficient of friction and wear as the temperature rises.
In trains, the braking system works by the process of friction between the brakes and the wheels. Solid lubricants like MoS2 can be used to achieve satisfactory performance on the brake mechanism. MoS2 helps to protect the surface from excessive wear, eliminate the vibrations during the braking process and reduce noise.
MoS2 is found in nature but it can also be synthesised in the laboratory by using processes like Chemical Vapour Transport and Physical Vapour Deposition.
Chemical Vapour Transport
In this process, a two-zone quartz furnace is used for multiple days. The elements Molybdenum and Sulphur are vapourised in the presence of a catalytic transport agent like halogens. This results in the synthesis of bulk MoS2 single crystals.
Physical Vapour Deposition
In this, the MoS2 film is coated on the target surface using spluttering. In spluttering the MoS2 target is bombarded by a noble gas plasma in a vacuum chamber. This is followed by the ejection and deposition of MoS2 particles on the target surface.