Due to advancements in technology, there is an increasing need for oil-free lubrication. Other lubricant types have to be used instead of conventional lubricants. Dry lubricants can be used in powder or granule form to reduce friction and wear.
Dry lubricants can be used in extreme operating conditions, high load, high vacuum, high radiation and high speed conditions where the liquid lubricants cannot be used. Dry lubricants can be used as a solid material between two surfaces moving in relative motion to each other to reduce friction and wear.
Some of the dry lubricants are Hexagonal Boron Nitride (hBN), Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2) and Graphite (C). The materials exhibit a low coefficient of friction due to their lamellar crystal structure.
The materials have a layered structure, and within each layer, the atoms are bonded with covalent bonds. While the layers are connected together by weak Van der Waals forces. When a shear force is applied, the layers easily slide past each other, leading to a low coefficient of friction. The atoms with strong covalent bonds offer a good load-carrying capacity.
When placed in powder form between the interacting surfaces, dry lubricants can provide effective lubrication for long periods. They can be mixed with aqueous media to get an excellent coefficient of friction and best load-carrying capacity. Dry lubricant powders can be used as an additive in oils and greases, and plastics to improve their lubricating and EP properties.
Dry lubricants can be used in different forms: as a coating on the moving surface, as a part of a composite, as a composite coating with particles dispersed in the bulk material and by rubbing the powder on the moving surface.
Properties desired in dry lubricants
- The dry lubricant should shear easily along the moving direction and produce a low coefficient of friction.
- It should have high compressive strength perpendicular to the direction of motion. This allows it to withstand high loads without causing any direct contact between the two moving surfaces.
- The dry lubricant should have good adhesion properties and should adhere to the moving surface even under high-stress conditions.
- It should be chemically stable over the operating temperature range and prevent corrosion. It should have resistance to deterioration in reactive environments.
Applications of dry lubricants
- Conventional lubricants fail in high vacuum conditions as they get vaporised and become ineffective. This can be avoided by using dry lubricants, as they do not vaporise in high vacuum conditions. Dry lubricants can be mixed with binders and bonded to the moving surfaces as a thin solid film.
- At high temperatures, the liquid lubricants can decompose and evaporate and become ineffective. At cryogenic temperatures, liquid lubricants can become too viscous and solidify, thus becoming ineffective. In such conditions, dry lubricants can be used.
- For an oil lubrication recirculating oil system, a pump and a heat exchanger and seals are required. When dry lubricants are used, such systems are not required; therefore, they simplify the design.
- Dry lubricants are preferred where the accumulation of dirt and dust is not desired. Liquid lubricants tend to pick up dirt and dust; therefore, dry lubricants are used widely in such applications.
- Dry lubricants can be applied to parts of assemblies where curing by heat is not possible.
How can dry lubricants be applied?
The different methods used for applying dry lubricants are rubbing, dusting, burnishing, tumbling and aerosol spraying.
Rubbing
In this, the lubricant power is distributed over the moving surface by rubbing with the hands. It produces a very loose coating, but it is effective in the assembly of close-tolerance parts. It is a common method of application.
Dusting
In this, the lubricant powder is not distributed evenly onto the moving surface. It gives an uneven and usually unsatisfactory coating.
Burnishing
This process is the same as rubbing, except that a greater force is applied using a mechanical method. The greater force helps to compact more lubricant powder onto the moving surface. It provides a smooth, glossy and longer-lasting coating. It is a preferred method where the wear life of the parts is not very important
Tumbling
In this method, the tumbling media like spheres, balls, cones and needles are mixed with the lubricant powder and tumbled for a period of time. It does not provide good adhesion but is suitable for coating non-critical small parts.
Aerosol spraying
In this, the lubricant is applied using an aerosol spray by using a volatile carrier like alcohol. The adhesion provided is not good, but it is easy to apply.