Molybdenum disulfide (MoS2) is a solid lubricant with special value due to its properties. It has a
layered lattice structure, which gives it a low coefficient of friction. It can be used as a solid
lubricant by itself, as a material in composites or as an additive. MoS2 can offer effective
lubrication over a wide temperature range.
MoS2 shows a low coefficient of friction due to its layered crystal structure. Each layer consists
of a plane of Molybdenum atoms placed between two planes of Sulphur atoms. The atoms
within the layers are bonded with covalent bonds. The layers are stacked together and
connected by weak Van der Waals forces.
When two surfaces are moving in relative motion to each other, MoS2 is used to reduce friction
and wear. The stack of layers shears easily when a force is applied, and this easy shearing of
the layers leads to a low coefficient of friction. The layers shear easily parallel to each other than
across. This allows MoS2 to support heavy loads perpendicular to the layers without breaking or
deforming.
The layers reduce friction when the sliding movement occurs parallel to them. MoS2 will be
effective only if its layers are parallel to the direction of sliding. This property is used while
lubricating moving surfaces.
Properties
● MoS2 is a dark blue-grey solid which is slippery to touch. When used as a lubricant, it
offers advantages like no tendency to flow and strong adherence to the surface to be
lubricated. It forms a lubricating film and does not move away from the contacting
surfaces.
● MoS2 does not tend to pick up dirt or dust and therefore does not contaminate the
products. It is used for lubrication in the food and textile industries.
● MoS2 has a very low volatility; therefore, it can be used for lubrication in high-vacuum
applications, like space applications.
● MoS2 is chemically inert and can be used for lubrication in reactive chemical
environments.
● MoS2 is stable in radioactive environments and can be used in nuclear power plants.
● MoS2 has a good load-carrying capacity and is non-toxic. It can be used in applications
where oils and greases do not have sufficient load-carrying capacity.
● It can be used in high-temperature applications. MoS2 begins to oxidise at a
temperature of 350 °C and forms Molybdenum trioxide which itself is a good lubricant. It
can be used for short periods up to a temperature of 450 °C.
Methods of using MoS2
The main purpose of solid lubricant is to form a film on the lubricating surface. There are
different methods used for applying MoS2 in dry form.
Burnished films
This is the simplest way to apply MoS2 on the lubricating surface. Molybdenum disulfide powder can be rubbed onto the surface that requires lubrication. It can produce a very thin film
up to 1 μm thick. It can also be applied using aerosol where a carrier takes the fine powder to
the surface and then evaporates. This method can be used for fine instruments for pivots, fitting
components together to prevent galling and more.
Physical Vapour Deposition
In this method MoS2 is vaporised and deposited onto the surface in a vacuum chamber. To
vapourise MoS2 techniques like thermal evaporation and sputtering are used. The vapours are
condensed on the surface to form a thin film. The files are also called ’Sputtered’ films. Films of
less than 2 μm thick can be produced using this method.
Bonded solid lubricants
There are paints with a high solid content along with water or volatile solvents. It can be made
using single lubricant powder or more than one lubricant powder. It can be applied like paints on
the lubricating surface. Different methods like brushing, dipping or spraying can be used. After
applying the surface can be air cured at room temperature or heated in an oven to a preferred
temperature. Films of 10 -25 μm thickness can be produced using this method.
This method is widely used in the industry. However, the surface to be lubricated should be free
of any contaminants like oil, grease, water and others. The surface should be pretreated using
mechanical or chemical methods before applying the film.
Conclusion
MoS2 is a solid material which can be used for lubrication in dry conditions. It forms a
continuous and adherent film onto the sliding surfaces and reduces friction and wear. The
advantage of using solid lubricant like MoS2 is that you do not need oil pumps or galliers to
lubricate. MoS2 is stable at cryogenic and high temperatures and has good load-carrying
capacity.
