Molybdenum Disulfide (MoS2) is an inorganic compound and is classified as a Transition Metal Dichalcogenides (TMDs). TMDs have a general formula of MX2 where M is a metal and X is a chalcogen. TMDs are known to have a layered lattice structure and are suitable for lubrication.
In case of Molybdenum disulfide the metal is ‘Mo’ (Molybdenum) and the chalcogen is ‘S’ (Sulphur). MoS2 has a layered structure with a layer of Mo atoms sandwiched between two layers of S atoms. Within the layer exist strong covalent bonds whereas the layers are connected together by weak Van der Waals forces.
Due to low shear strength the layers easily slide past each other and can provide good lubrication. MoS2 is a dry lubricant and can be used for lubrication in dry environments and vacuum. It can be used for lubrication in air up to a temperature of 350° C – 400° C after which it gets oxidised and turns into MoO3.
MoS2 is widely used as a lubricant in various applications due to its properties like high wear resistance, high melting point, low coefficient of friction and excellent thermal stability. It can work effectively as a lubricant in harsh environments such as extreme temperatures (high and cryogenic), ultra-high vacuum, corrosive environment, radiation applications and preventing material wear.
MoS2 can be used as a lubricant in space applications where challenging conditions exist. It can withstand high-vacuum conditions where the liquid lubricant becomes volatile. MoS2 is used in high-temperature conditions where the liquid lubricants may oxidise and vaporise. MoS2 can be used at cryogenic temperatures where the liquid lubricants will become high viscous or solidify and thus become ineffective. MoS2 weighs less than liquid lubricant and is ideal for space applications.
MoS2 can be used for lubrication in challenging environments without the need of frequent replenishment. It can work effectively in humid and high load conditions. It has a low surface energy which further helps to reduce the sliding motion resistance.
How can MoS2 be applied on the substrates?
There are other methods which can be used to apply MoS2. MoS2 can be dispersed in a volatile liquid in powder form and applied to the substrate by dipping, brushing or spraying. The liquid is then allowed to evaporate by additional heating or at room temperature. Thin films of MoS2 are produced which are soft and randomly oriented.
MoS2 in powder form can be mixed with organic or inorganic binding agents in appropriate proportions and applied to the substrate using dipping, brushing or spraying. The surface is then cured at high temperatures for reaction to take place and the binding agent to get hardened. This leads to a good adherence between the substrate and the MoS2 mixture.
Synthesis of MoS2
The processes used to synthesise MoS2 can be divided into top-down and bottom-up approaches.
Top-down approaches
1. Micromechanical exfoliation
Using this method flakes of MoS2 can be produced on the substrates. Stick tapes are used on the bulk material to peel off and press the MoS2 onto the substrate. As the tape is released some part remains on the substrate due to Van der Waals forces. The process is repeated to produce flakes of MoS2 with random shapes, sizes and thickness. You can get a MoS2 layer of the highest quality.
2. Liquid phase exfoliation
In this atomic intercalation via solution chemistry is used. Mainly Lithium is used for intercalation between the MoS2 layers and enlarge the interlaying spacings. This makes the sonication process easier. This method can be used to produce dry Molybdenum disulfide (MoS2) powder, flakes and also thin films.
Bottom-up approaches
1. Physical vapour deposition (PVD)
In this vacuum sputtering is done using amorphous MoS2 and Titanium (Ti) as targets.
2. Solution chemical process
In this you can use the hydrothermal synthesis and the solvothermal synthesis. The difference between the two processes is that in the latter the precursor used is not aqueous. Usually the precursor used is Ammonium Tetrathiomolybdate and it is made to react with Sulphur in a stainless steel autoclave. Various reactions take place at high temperature and high pressure for several hours. You can get powders of MoS2 in different shapes. The powders are high-temperature post-annealed to improve their quality and purity..
- Chemical vapour deposition (CVD)
In this chemical reactions are used for the growth mechanism and you can get films of MoS2 on the substrate. A Molybdenum precursor in solid state and S precursor in gaseous state and used and vapourised. The growth temperature is maintained between 700° C – 1000° C.
