Molybdenum disulfide (Mos2) belongs to the family of Transition Metal Dichalcogenides (TMDs). The lubricating properties of MoS2 are due to its crystal structure. It can reduce friction and wear between the two sliding surfaces.
It has a layered structure with a layer of Molybdenum atoms sandwiched between low layers of Sulphur atoms. Strong covalent bonds exist with the layers, and the layers are connected by weak Van der Waals focus. This allows the layers to sidle easily when a shearing force is applied leading to a low coefficient of friction.
Greases and oils are used as usually used for lubrication of the mechanical system in the industries. However, in extreme conditions sold lubricant like MoS2 is used for lubrication. Solid lubricants can be used in high/low temperature, high pressure, ultra-high vacuum and high load conditions.
In aerospace applications, low temperatures can cause the liquid lubricants to become too viscous and they may become ineffective. In machining applications, solid lubricants are used to reduce friction and wear which otherwise would require large amounts of liquid lubricants. In food and textile industries solid lubricants are used as the contamination of the liquid lubricants can ruin the products.
MoS2 can be used as a dry lubricant and as an additive in greases, oils and composite coatings. It can work effectively in a wide temperature range and at low and high speeds. Molybdenum disulfide powder can be used for lubrication where the liquid lubricant does not meet the requirements of the application. Solid lubricants can simplify lubrication, reduce weight and can be less expensive to use. MoS2 performs best in vacuum environments and is ideal for space applications.
MoS2 has properties like good thermal and chemical stability, high-temperature resistance, high wear resistance, good electrical conductivity and high melting point. MoS2 forms a film between the two sliding surfaces which adheres strongly to both surfaces. It is not lost rapidly from the sliding surfaces. It provides resistance to wear which leads to a useful life of the moving parts.
MoS2 can shear easily parallel to the layers and can support heavy loads at a right angle to the layers. This property is used in the lubrication process.
Synthesis of MoS2
Different approaches used to synthesise MoS2 are top-down techniques and bottom-up techniques. In the top-down technique, different methods used are mechanical exfoliation, liquid phase exfoliation and spluttering. In the bottom-up technique, different methods used are physical vapour deposition, chemical vapour deposition, atomic layer deposition and chemical solutions.
Top-down techniques
In the mechanical exfoliation method, bulk crystals are used and external stress or intercalation is used to separate the adjacent layers. In this sticky adhesive tapes are used to transfer the MoS2 flakes onto the substrate. This method gives a low yield and is good for use in the lab.
In the liquid phase exfoliation method, solvents and mechanical force are used to separate the layers of MoS2. The guest species are intercalated in the Van der Waals gaps of the MoS2 layer and the expanded crystal is separated by an external driving force. This method can be used for producing MoS2 nanomaterials. This method uses toxic chemicals and therefore is not used widely.
The sputtering method involves two steps: depositing MoS2 films and sulfidising the films. A Molybdenum target is sputtered in an Argon atmosphere. The Molybdenum film is sulfidised in
Hydrogen and Hydrogen sulfide atmosphere at high temperature.
Bottom-up techniques
In the physical vapour deposition (PVD) method, the MoS2 powder is evaporated and deposited on the substrate. The MoS2 powder is evaporated as gas and the gas is deposited on the substrate. This method gives uniform films and is easy to operate but the impurities in the raw material can be difficult to remove.
In the chemical vapour deposition (CVD) method, Molybdenum precursors and Sulphur are used. Molybdenum precursor is made to react with Sulphur to produce a film on the SiO2 substrate. Using moderate temperature conditions a large area of MoS2 films with high crystallinity can be obtained.
In the atomic layer deposition (ALD) method, pulsed precursors are used to get high-quality films. It uses a layer-by-layer deposition method. The precursor is used in self-limiting reactions at high temperatures.
In the chemical solutions method, the Mo and S precursor solution is put into a stainless steel autoclave which is tightly sealed and heated at moderate temperature. It is then cooled and the product is rinsed and dried overnight. By controlling factors like temperature, precursor concentration and reaction time you can get MoS2 of different particle sizes and crystallinity.
