Dry/Solid lubricants are used to reduce the friction and wear between sliding surfaces. Solid lubricant consists of a film of a solid material that is introduced between the two sliding surfaces. Some of the dry lubricants are Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2), Hexagonal Boron Nitride (hBN) and Graphite(C).
Solid lubricants like WS2, MoS2 and hBN are classified as Transition Metal Dichalcogenides (TMD). They consist of one layer of metal which is present between the two layers of Chalcogen atomic layers. Their structures are layered where there exist strong covalent bonds within the layer and the layers are held together by weak Van der Waal forces. This allows the layers to easily slide past each other resulting in a low coefficient of friction.
Graphite consists of hexagonal planes of polycyclic Carbon atoms where the bonds within the layers are strong and the layers are held together by weak Van der Waal forces. This gives it a low coefficient of friction. Depending on your requirements you can buy dry lubricants online.
Properties of dry lubricants
Dry/solid lubricants are used to prevent direct contact between the two sliding surfaces. They can be used in the form of dispersed particles or surface films. A coating of the solid lubricant can be applied on the required surface or a composite (solid lubricant dispersed in a matrix) coating can be used. It can be used in powder form to lubricate the rubbing surfaces. It can also be used as an additive to lubricating oils and greases.
Solid lubricants have a low shear strength in the sliding direction. The easy shear movement of the solid lubricant leads to a low coefficient of friction.
Solid lubricants possess high compressive strength perpendicular to the sliding direction and in the direction of the load. They can withstand heavy loads and avoid direct contact between sliding surfaces.
The solid lubricants have good adhesion on the substrate surfaces even if high shear stresses exist.
The advantages of using solid lubricants are the ability to function under high loads, high thermal stability and diversity in application forms.
Dry lubricants
Tungsten Disulfide (WS2)
It has a hexagonal lattice structure in which a layer of W atoms is sandwiched between two layers of S atoms. It forms a S-W-S structure. It occurs naturally as a mineral Tungstenite and is one of the most lubricious materials known. It has a coefficient of friction of 0.03 which is unmatched to any other substance.
It can be used in high-pressure and high-temperature conditions. It can provide lubricity for temperatures ranging from -270° C to 650° C in a normal atmosphere and from temperatures ranging from -188° C to 1316° C in a vacuum.
Molybdenum Disulfide (MoS2)
It has a hexagonal crystal structure in which a layer of Mo atoms is sandwiched between two layers of S atoms. It forms a S-Mo-S structure. It occurs naturally as a mineral ore Molybdenite and Jordisite. It has a coefficient of friction of < 0.1. The shear strength of MoS2 increases when the coefficient of friction increases. This property is called superlubricity.
It can be used for lubrication up to a temperature of 350° C in an oxidising environment. It is often used as an additive to blends and composites which require low friction. The particle size of MoS2 must be matched with the surface roughness of the substrate to avoid abrasion.
Hexagonal Boron Nitride (hBN)
It has a honeycomb lattice structure in which a layer of B atoms is sandwiched between two layers of N atoms. It forms an N-B-N structure. hBN has a structure similar to Graphite and is also known as ‘White Graphite’. The coefficient of friction of hBN is 0.5 - 0.7. It is chemically inert and an electrical insulator.
It can be used as a lubricant up to a temperature of 900° C in an oxidising atmosphere. It is used for lubrication where the chemical reactivity and the electrical conductivity of Graphite are a problem.
Graphite (C)
It has a hexagonal crystal structure with rings of 6 Carbon atoms arranged in a layer. The individual layers are called Graphene. It forms a C-C-C structure. It occurs naturally in sedimentary rocks as a result of the reduction of sedimentary Carbon compounds. It has a coefficient of friction of 0.1.
Graphite is suited for lubrication in air. Water vapour is an important part of Graphite lubrication and therefore it cannot be used in a vacuum. It can work effectively up to temperatures around 450° C continuously.
