When two surfaces move in relative motion to each other, it leads to friction and wear. A lubricant can be used to protect against the damage caused by friction and wear due to relative movement. Lubricant is placed between the two moving surfaces to reduce friction and wear.
There are different types of lubricants, such as solid/dry, liquid, gaseous and semi-solid. Dry lubricants are used in dry operating conditions in powder, thin film or composite material form to reduce friction and wear. Dry lubricants provide a constant and controlled friction between the two moving surfaces.
Dry lubricants have a high melting point and remain chemically stable during the operating temperature range. They do not attack or damage the moving surfaces. They provide resistance to wear and tear, which leads to a longer and useful life of the machine.
Dry lubricants are preferred where using liquid lubricants is not appropriate. Dry lubricants have high thermal stability and can be used in high-temperature applications. They can be used in high-load and high-vacuum applications. They do not tend to pick up dirt and dust like liquid lubricant and are therefore used in applications where cleanliness is a must. Dry lubricants can provide long-lasting lubrication without the need for frequent replenishment.
The purpose of the dry lubricants is to adhere firmly to the moving surfaces. Dry lubricants can be applied using physical, mechanical and chemical methods. A carrier medium or a bonding agent can be used for applying. In some applications, pretreatment of the moving surface is done before applying the dry lubricant.
Types of dry lubricants
There are several types of dry lubricant powders available, like Hexagonal Boron Nitride (hBN), Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2) and Graphite (C). All these substances exhibit a low coefficient of friction due to their layered lattice structure.
Hexagonal Boron Nitride has a hexagonal arrangement of Boron and Nitrogen atoms within each layer. The atoms in the layers are bonded with strong covalent bonds, while the layers are connected by weak Van der Waals forces.
Tungsten Disulfide belongs to the family of Transition Metal Dichalcogenides (TMDs). It consists of stacked layers of atoms. Each layer consists of a layer of Tungsten atoms sandwiched between two layers of Sulphur atoms. The atoms in the layers are bonded with covalent bonds, while the layers are connected by weak Van der Waals forces.
Molybdenum Disulfide is classified as a two-dimensional Transition Metal Dichalcogenide (TMD). Each layer consists of a layer of Molybdenum atoms sandwiched between two layers of Sulphur atoms. The S-Mo-S forms the layered structure. The atoms in the layers are bonded by covalent bonds, while the layers are held by weak Van der Waals forces.
Graphite consists of layers of carbon atoms arranged in a hexagonal form. Within each layer, a Carbon atom is covalently bonded to three other Carbon atoms. While the layers are connected by weak Van der Waals forces.
Lubrication mechanism of dry lubricants
In dry lubricants, the layers are held by weak forces; therefore, the layers easily slide against each other when a shear force is applied. This easy shearing of the layers leads to a low coefficient of friction. The weakly held layers slide parallel to the direction of the moving surface.
Dry lubricants can withstand high compression forces perpendicular to the moving direction. This is because the atoms within the layers are bonded with strong covalent bonds. Therefore, dry lubricants can carry heavy loads.
Applications of dry lubricants
Dry lubricants are used where the liquid lubricants do not meet the requirements of modern technology. In many applications, dry lubricants are less expensive to use than liquid or grease lubrication systems. Dry lubricants simplify the lubrication process.
- Dry lubricants are used in the automotive industry to reduce the cost and are replacing oils and greases. They help to make the automobile as effective as possible.
- Liquid lubricants are not preferred in many applications due to weight, sealing problems and environmental conditions. Dry lubricants are used in the aerospace industry as they can function effectively in extreme conditions.
- In high-vacuum environments like space applications, liquid lubricants evaporate and can be disposed of. Dry lubricants are used in space applications where liquid lubricants do not survive.
- In high-temperature applications, the liquid lubricants decompose, evaporate or oxidise, thus becoming ineffective. Dry lubricants can be used in high-temperature applications and maintain a low coefficient of friction.
Dry lubricants are widely used in applications where the use of liquid lubricants is inadequate or impossible.. They can be used as powders, dispersions, suspensions and pastes.
