Hexagonal Boron Nitride For Lubrication

The term Tribology is derived by combining two Greek words ‘Tribos’ which refers to the action of rubbing and ‘Logos’ which means word or study. It is a study of managing and controlling friction and wear. One of the widely used dry lubricants is Hexagonal Boron Nitride (hBN).

Hexagonal Boron Nitride has a similar crystal structure to Graphite but is white. It is also called ‘White Graphite’. hBN has excellent lubricating properties. It also has other properties like high thermal conductivity, low thermal expansion, high electrical resistance, good thermal shock resistance, low dielectric constant, chemically inert and microwave transparency.

The hBN monolayer is like Graphene and has a honeycomb lattice structure. In the structure the Boron and Nitrogen atoms are held by strong covalent bonds and the layers are held together by weak Van der Waal forces. This allows the layers to slide against each other when a shear force is applied. This leads to a low coefficient of friction.

hBN is a good lubricant at both low and high temperatures. It can work effectively up to 1000° C in air, 1400° C in a vacuum and 1800° C in an inert gas atmosphere. It has a coefficient of friction as low as 0.15 – 0.7.

Lubrication

Friction and wear have been the main reasons for mechanical failure in many processes. Liquid lubricants have been used to reduce friction and wear. However, the discharge of these lubricants poses a recycling problem and is harmful to the environment. Therefore dry lubricants like hBN powder are used due to their environment-friendliness and lack of emissions.

The three types of lubrication mechanisms are boundary lubrication, mixed lubrication and hydrodynamic lubrication. Friction and wear are very high in boundary and mixed lubrication and it results in high energy loss and wear. Dry lubricants are more effective in boundary and mixed lubrication.

Applications Of hBN

• Dry lubricants are used where the lubricating film between the two surfaces cannot be secured by using liquid lubricants or grease. Liquid lubricants tend to pick up dust particles. Dry lubricants are used where the entry of contaminants is not acceptable.
• hBN can be used in high-temperature and high-pressure applications. It is used when the operating temperature or the load is too high for grease to stay in position. In high-temperature applications, the liquid lubricants may react and evaporate.
• hBN is used in powder form or with binders to make a sticky film on the metal surface. Dry lubricants are available as a dispersion in non-volatile carriers like waxes, soaps, fats and soft metal films.
• Liquid lubricants lead to environmental pollution through spillage, evaporation and total loss lubrication. hBN nanoparticles can be added to water, ethylene glycol or oil and this mixture can be used as a lubricating coating. This mixture will penetrate the friction areas and potentially reduce friction and wear between the interacting components. It will also fill in the surface defects and prevent further wear. It will also result in decreased energy loss and increased lifespan of the components. It will help to protect the environment and give satisfactory results.
• hBN is a preferred lubricant because of its unique properties. It is non-toxic and biodegradable and has a low environmental impact. The production and use of hBN have a lower environmental impact when compared to other lubricants. The manufacturing process involves fewer hazardous chemicals and intensive energy processes.
• hBN can be used in applications that require rapid heat removal as it has high thermal conductivity. It is used in powder form and has good friction-reducing properties. It is chemically inert therefore it is used in a wide range of tribology applications.
• hBN is used as a dry lubricant where it is important to reduce the seizure point of two contacting surfaces. hBn forms a tribological film on the metal surfaces to avoid the collision of asperity contacts and reduces severe wear. There is deposition of the hBN particles in the worn region due to the patching effect. The formation of a protective film on the worn surfaces made them smooth and reduced friction and wear.
• hBN does not require the presence of water vapour in the crystal layer to provide lubrication, unlike Graphite. Therefore hBN is used in vacuum and space applications as a dry lubricant. hBn is a preferred lubricant where the electrical conductivity and the chemical reactivity of Graphite are a problem.

hBN can be sprayed in powder form on hot surfaces to provide lubricating properties. Due to strong thermal resistance hBN is used as an additive for high-temperature applications.