hBN is used as a lubricant in the industry as it exhibits a low coefficient of friction. It has a low coefficient of friction because of its structure. hBN has a lamellar crystalline structure similar to that of Graphite. The molecules within a layer are bonded with strong covalent bonds while the binding between the layers is through weak Van der Waals forces.
hBN is a solid lubricant and can be used in thin film or as free flowing powder to reduce friction and wear. Hexagonal Boron Nitride (hBN) can be used to form a lubricating film between the two sliding surfaces. When used in powder form they should have the required properties to form a thin lubricating film. hBN will shear more easily under a given load and lead to a low coefficient of friction.
The main purpose of the solid lubricant is to form a consistent adherence to a soft or hard film on the sliding surfaces. The films can be applied using mechanical, chemical or physical processes on the surface.
Solid lubricants can be used for lubrication in extreme conditions such as high temperature, pressure and vacuum. They are preferred in situations where two sliding surfaces have to be effectively separated and it is important that the lubricant stay in place.
Solid lubricants like hBN can be used in dispersions and suspensions in water can be used for coating for cold and hot forming. Dispersion and suspension in oils can be used for lubricating purposes. They can be used with greases in high concentration to enhance the lubricating properties. In application an average particle size between 5 – 15 μm is used.
Properties
hBN is a soft, white slippery powder and also known as ‘White Graphite’. It has properties like good adherence of the solid lubricating film, low abrasivity and thermochemical stability. hBN can surpass other solid lubricants in terms of thermochemical stability and adherence.
hBN can be used in extreme pressure applications and can be used as an additive. Unlike Graphite, hBN does not occur naturally but is synthesised. A variety of hBN powders can be synthesized by changing the process conditions.
hBN can offer lubrication at high temperatures and this is an important property. It can work effectively in air up to a temperature of 800° C, in vacuum up to 1400° C and in inert gas up to 1950° C. hBN has a coefficient of friction between 0.5 – 0.7.
hBN is a good thermal conductor and can dissipate heat easily. It is chemically inert and has a high melting point of 3000° C. hBN does not cause harmful emissions and does not break down to form other hazardous materials therefore it is environment friendly.
Lubrication using hBN
hBN can be applied on the sliding surface as a loose powder, coatings or dispersed in oils or greases. It helps to protect from the damage caused due to the relative motion because of friction and wear. hBN meets the requirements of modern technology and is less expensive than liquid lubricants.
Lubrication is simple when solid lubricants are used and their weight is less when compared to liquid lubricants. Solid lubricants can offer significant weight savings. hBN can be used in space applications where liquid lubricants cannot be used as they may evaporate. hBN can be used in food processing machines as it does not contaminate the products.
hBN is chemically inert and can be used in corrosive environments where the liquid lubricants may just decompose. Under high temperatures, high vacuum, corrosive environments solid lubricating is the only option available.
Synthesis of hBN
hBN can be synthesised using Chemical Vapour Deposition (CVD) method. The method involves the following steps:
- Decomposition of precursors such as Ammonia Borane and Borazine where Boron and Nitrogen decompose into Boron and Nitrogen containing species under high temperature.
- Deposition and nucleation where the Boron and Nitrogen containing species are deposited onto the metal surface and combine to form clusters.
- Continuous growth where these clusters grow into large hBN islands and which are attached to form a continuous film.
Metals such as Copper, Nickel and Iron are used for synthesis due to their catalytic efficiency. They help to decompose the precursors and improve the crystallinity of hBN. The weak interaction between the metal substrate and the synthesised films of hBN is possible by peeling the layers directly from the metal substrate using polyvinyl alcohol (PVA) film. The layers can be laminated on the desired substrate at a temperature of 120° C and PVA film is dissolved in water.
