Dry Lubricant – Alpha Boron Nitride (ɑ-BN)

One of the polymorphic forms of Boron Nitride (BN) is the hexagonal form. It is also known as alpha-boron nitride (ɑ-BN) and graphitic boron nitride (g-BN). It is the most stable form of BN and is white in colour. It has a structure similar to that of Graphite and is also called ‘White Graphite’.

The most stable crystalline form of BN, Hexagonal Boron Nitride (h-BN), is used as a dry lubricant. A lubricant is used to reduce friction and wear between two surfaces moving in relative motion to each other. When h-BN is placed between two sliding surfaces, it prevents direct contact between the two surfaces, thereby reducing friction and wear.

hBN is used as a lubricant because it has excellent properties. It has excellent thermal stability and chemical resistance, good thermal conductivity and no electrical conductivity. It is non-toxic and is biodegradable, therefore it does not harm the environment. It is a preferred lubricant in applications where the electrical conductivity and chemical reactivity of Graphite are not desired.

h-BN exhibits a low coefficient of friction due to its layered crystal structure. In each layer, the Boron and Nitrogen atoms are arranged alternatively in hexagonal planes and bonded with covalent bonds. The individual layers are connected by weak Van der Waals forces.

The layers easily slide against each other when a shear force is applied, resulting in a low coefficient of friction. The layers align themselves parallel to the sliding direction, while the layers with covalent bonds provide good load-carrying capacity perpendicular to the sliding direction.

h-BN can be used as a lubricant in high-temperature, high-pressure, very low to high sliding speed applications and in vacuum and presence of radiation. Liquid lubricants cannot be used under such conditions. h-BN can be used as an additive to oils and greases to improve the lubricating properties.

Boundary lubrication using h-BN

Boundary lubrication is a condition where the lubricating film becomes too thin to prevent contact between the two sliding surfaces. Here, the friction and the wear between the two sliding surfaces are determined by the properties of the surfaces and the lubricant and not the bulk viscosity.

h-BN powder is used in such conditions as its particles can get adsorbed on the sliding surface due to weak Van der Waals forces. It can provide effective lubrication at the contact points.

Extreme temperature/pressure lubrication using h-BN

In this condition, the sliding surfaces are moving at high speed at high temperature. Here, liquid lubricants cannot be used as they may decompose or evaporate and become ineffective. In such conditions, h-BN can be used as it can withstand high temperature and high load. It has a high melting point and can provide effective lubrication under high temperatures.

Advantages of dry lubricant: h-BN

• hBN has low shear strength and high wear resistance and can provide good lubrication between two sliding surfaces. It can be used as a loose powder, as a coating, or dispersed in oils and greases.
• Dry lubricants like h-BN are less expensive than oils and greases and do not require complex systems. Dry lubricants also reduce the weight of the system, simplify lubrication and improve the materials and processes. In the cost-conscious automotive industry, dry lubricants are replacing oils and greases to provide lubrication. In the weight-conscious aerospace industry, dry lubricants are preferred.
• In high-vacuum environments, liquid lubricants cannot be used as they may vaporise and contaminate the equipment or product. Dry lubricants like h-BN are preferred in such applications.
• At high temperatures, liquid lubricants can decompose or oxidise, and at cryogenic temperatures, liquid lubricants can become highly viscous. They become ineffective and cannot provide effective lubrication.
• h-BN can provide lubrication from cryogenic to high temperatures. In air, it can work up to a temperature range of 800 °C – 1000 °C, in vacuum, it can work in a temperature range of 1200 °C – 1400 °C and in inert gas, it can work in a temperature range of 1400 °C – 1800 °C. 
• Dry lubricants can be bonded to the sliding surface using resins, which can be cured to form strongly adhering coatings. In some plastic bearings, the dry lubricant can be added to the plastic to form a lubricating coating on the mating surface.

Conclusion

h-BN is very similar to Graphite in properties and is used as a dry lubricant in various applications. It is non-toxic, easy to machine, and non-abrasive. It is chemically inert and not wetted by most molten metals. It has the highest temperature-resistance in air compared to any other dry lubricant.