Dry Lubricant – Molybdenum Disulfide (MoS2)

Molybdenum Disulfide (MoS2) is a member of the Transition-Metal Dichalcogenides (TMDs). It is an inorganic compound of Molybdenum (Mo) and Sulphur (S). It occurs naturally as a mineral ore Molybdenite or Jordisite. It is widely used as a dry lubricant in tribological applications.

MoS2 has a layered structure with a plane of Molybdenum sandwiched between two planes of Sulfide ions. The layers are held together by weak Van der Waal forces and this allows the planes to easily slide against each other when shear force is applied to the material. This gives it a low friction coefficient. 

As the shear strength of MoS2 increases the coefficient of friction also increases. This property is called superlubricity. The coefficient friction of MoS2 is 0.150 at ambient conditions. It readily adheres to solid surfaces under the conditions of sliding and pressure. This gives the components exceptional performance life when lubricated.

Molybdenum disulfide is a dry lubricant and therefore it does not get squeezed out under extreme pressure like liquid lubricants. It forms a boundary layer to prevent the two lubricated surfaces from coming in contact with each other.

Example

When two stainless steel surfaces slide against each other under a high load they can ‘Gall’ or ‘Seize’ due to the deformation of the contact points. The surfaces may ‘cold weld’ themselves to each other and there may transfer of material from one surface to another. This will rapidly increase the friction to a point where further use of the surfaces will lead to damage.

To prevent this an anti-galling and anti-seizure agent has to be introduced between the surfaces. Molybdenum Disulfide (MoS2) powder can be used to form a boundary film that does not allow the surfaces to come in contact even under high compressive loads.

MoS2 is an ideal anti-galling compound due to its high compressive strength and adherence to sliding surfaces. MoS2 can be applied using high-tech processes like vacuum sputtering or by dropping the MoS2 powder on the sliding surfaces. Another method is mixing MoS2 with a binder and s solvent to form a bonded coating.   

When a good layer of Mos2 is formed it can provide excellent lubrication performance over a temperature range of 500° C under high pressure and corrosive exposure conditions.

Lubrication process

MoS2 can shear more easily parallel to the layers than across them. They can thus support heavy loads at a right angle to the layers and at the same time slide easily parallel to the layers. This property is used in the lubrication process.

Low friction occurs only parallel to the layers and MoS2 will be effective only when their layers are parallel to the direction of sliding. It is important that the MoS2 adhere strongly on moving surfaces otherwise it can get easily rubbed away and reduce the service life of the equipment.

Properties of MoS2

MoS2 has a hexagonal layer-lattice structure and does not have a tendency to flow, migrate and creep. There are minimum chances of contaminating the product or environment therefore it is suitable for textile and food industries. It has very low volatility and therefore it can be used in a vacuum such as in space applications. Due to its chemical inertness, it can be used in chemically reactive environments. It has good load-carrying capacity and is non-toxic.

MoS2 begins to oxidise at 350° C in air though it can still be used for a short time up to a temperature of 450° C. The oxidation produces Molybdic oxide (MoO3) which itself is a lubricant but wears rapidly.

MoS2 has all the desired properties of a good dry lubricant like providing a low and constant coefficient of friction between the two moving surfaces. It can stay chemically stable over a wide temperature change and not attack or damage the surface material. It can adhere strongly to the moving surfaces and is not rapidly lost from the surfaces. It is also economical to use MoS2.

Molybdenum disulfide can be added to liquid lubricants to reduce friction and wear and improve the load-carrying capacity. In most cases, it is added to oil and In case of an oil loss, it can prevent catastrophic seizure.

MoS2 can also be added to greases to improve the lubrication. Usually, for rough metal surfaces, large MoS2 particle size is used because they can fill the gaps between the valleys and build a smoother surface. For smoother surfaces, mid to small MoS2 particle size is used as they can provide a better load-carrying ability for a given MoS2 concentration.

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