Synthesis of Molybdenum Disulfide and its application as a dry lubricant

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 μm, 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.