Molybdenum Disulfide (MoS2) is an inorganic compound and is classified as a transition metal dichalcogenide (TMD). It is made up of one atom of Molybdenum and two atoms of Sulphur. MoS2 is silvery black and occurs naturally as Molybdenite or Jordisite and the ore can be processed to get pure MoS2.
MoS2 is not affected by dilute acids and oxygen and is relatively unreactive. All the forms of MoS2 have a monolayered structure in which a layer of Molybdenum is sandwiched between the two layers of Sulphur. Crystalline MoS2 exists in two phases such as hexagonal (H) and rhombohedral (R) symmetry. The 2H and 3R forms are used as dry lubricants. The Bulk MoS2 consists of stacked monolayers.
MoS2 as a lubricant
Lubrication is adding substances between the two sliding surfaces to reduce friction and wear. Lubrication is done to allow smooth operation of the moving parts. When used in dry powder forms they are made to stick firmly to metal surfaces.
As Molybdenum Disulfide (Moly) has a layered structure and a lower coefficient of friction it is commonly used as a lubricant. The MoS2 has a crystal lattice structure arranged in layers. Strong bonds exist between the atoms within a layer and weak bonds exist between the atoms of different layers. This allows the lamina to slide over one another.
The interlayer sliding releases energy when shear is applied to it. The shear strength of MoS2 increases as the coefficient of friction increases. This property is called superlubricity. MoS2 can offer high lubricity and stability up to 350° C in an oxidising environment and can be used in continuous operating situations. It is chemically inert, has high stability and can also be used in high vacuum.
It is used in boundary lubrication where the lubricating film becomes too thin to provide total separation. In such cases, dry moly lubricant is used as the weak Van der Waals forces allow it to get absorbed into the surface. It can be used in low-speed and high-load conditions.
In lubrication applications, the wear resistance of MoS2 can be improved by adding Chromium (Cr). In the particle size of 1 – 100 ?m, it acts as a dry lubricant. It is added in various composites and blends where low friction is required. MoS2 is added to composite coating in high-temperature applications. When added to plastic MoS2 forms a composite with high strength and reduced friction.
MoS2-based lubricants are used in two-stroke engines in motorcycles, bicycle coaster brakes, universal joints, ski wax and more. In bulk form, it is used as a dry lubricant in various industries.
Synthesis of MoS2
The different processes used for the synthesis of MoS2 are the top-down approach and the bottom-up approach.
Top Down Approach
Exfoliation of MoS2
This method can be easily used to get pure MoS2 due to the weak Van der Waal interaction and its layered structure.
In the mechanical exfoliation process, the MoS2 flakes can be peeled off from a bulk crystal using adhesive tapes. It is then shifted onto a specific substrate. When the tape is detached some part of MoS2 remains on the substrate. This way MoS2 nanosheets of one or more layers can be obtained in random sizes and shapes. However, the thickness and the size of the crystals cannot be controlled. Gold is used as a substrate due to its affinity for Sulphur.
Bottom-Up Approach
The different processes used for the synthesis of MoS2 are as follows:
Physical vapour deposition
In this process, advanced technologies like molecular beam epitaxy are used to prepare single-crystal thin films. This process can be used to grow continuous MoS2 films at a temperature of 350° C.
Chemical vapour deposition
In this process, the solid sources of Molybdenum and Sulphur are used like Mo or MoO3 and H2S gas or S powder. The solid MoO3 and vapourised S are made to react in a low-pressure chamber. This leads to the formation of MoS2 nuclei. MoS2 grows slowly and enlarges in size on the substrates at temperatures between 700° C to 1000° C. This process can be used to produce large-size, high-quality and thin sheets.
Solution-based process
This process is commonly used to synthesise MoS2 nanosheets. The Mo source used is usually a Molybdate and the S source used is thiourea, thioacetamide and more. The molybdate reacts with the S in a stainless steel vessel. The reaction takes place at a temperature between 160° C – 200° C for a few hours. The MoS2 powder obtained from this process has different shapes and sizes.
