Hexagonal Boron Nitride Lubricant

Lubricants are essential in controlling friction and wear in various mechanical and tribological systems. Without lubrication, the moving parts in machines would rapidly degrade or fail, making them ineffective. Lubricants come in various forms: solid (dry), semi-solid, liquid, and gaseous.

hBN has a layered structure and within each layer, the Boron and Nitrogen atoms are bonded with strong covalent bonds and the layers are held together by weak Van der Waals forces. When shear force is applied, the layers easily slide against each other. hBN exhibits a low coefficient of friction due to easy shearing of the layers. The atoms bonded with the strong covalent bonds provide good load-carrying capacity.

hBN is the preferred lubricant where cleanliness of working environments is a must. It was evaluated as a ‘Clean’ lubricant in general application as a solid lubricant. hBN is a more effective solid lubricant for high-temperature applications when compared to other lubricants. It is stable up to 900° C in oxidising environments.

hBN has high thermal conduction and electrical insulation properties. It is chemically inert and can be used in harsh conditions. It is not wetted by molten metals or slags and shows excellent lubricating properties. Unlike Graphite, hBN does not need the presence of water vapour between crystal layers to provide lubrication. It is used as a lubricant in vacuum and space applications. hBN is used as a lubricant in applications where the electrical conductivity and chemical reactivity of Graphite are not desirable.

hBN is used as an additive in lubricating oils to reduce friction and wear. It can be added to composite materials to provide lubricating properties.

Synthesis of hBN

HBN can be synthesised using low-temperature growth methods and high-temperature growth methods. For applications like forming composite mixture or as a texture agent for cosmetics, the price is the main factor. In these, the size and the quality of the hBN crystal are not considered. Low-temperature growth methods are preferred. The methods are as follows:

Combustion synthesis

This method is mostly used for producing commercial hBN as it offers large yields and fast reaction. It is based on the nitridation of Boron oxide. Boric acid (B2O3) is mixed with Urea (CH4N2O) and heated between temperatures of 500 °C – 1000 °C. The Hexagonal boron nitride powder produced is called commercial hBN and the particle size ranges between 100 nm to 1 μm.

Solvothermal synthesis

This method requires a lower temperature and offers ease of preparation. It can produce large quantities of micrometer to nanometer-sized hBN particles. Two precursors of BN are mixed in a liquid solvent and heated to moderate temperatures below 500 °C in an autoclave. The quality of the crystals obtained is poor.

Structure and Properties

• hBN has a layered crystal structure:
  • Strong covalent bondshold the Boron (B) and Nitrogen (N) atoms within each layer.
  • Weak Van der Waals forces hold the layers together.
• When shear force is applied, these layers slide easily over one another, resulting in a low coefficient of friction.
• These covalent bonds provide excellent load-bearing capacity.

Advantages of hBN as a Lubricant

• Acts as a clean solid lubricant, ideal for contamination-free environments.
• Stable up to 900°C in oxidizing environments.
• Exhibits high thermal conductivity and electrical insulation.
• Chemically inert and suitable for harsh conditions.
• Not wetted by molten metals or slags.
• Works in vacuum and space applications without needing water vapor (unlike graphite).
• Preferred where electrical conductivity and chemical reactivity of graphite are undesirable.

The high-temperature growth techniques use solvents in the process, which work as a catalyst. Alkali and alkali-earth metals are the best catalysts and the lower the atomic weight, the better. Therefore, alkali and alkali-earth solvents are used in the synthesis of hBN. The different methods are as follows:

High Pressure High Temperature (HPHT) Synthesis

This process required high mechanical pressure provided by a hydraulic press. A closed BN or Mo crucible with BN as a solvent is used. The pressure ranges from 2.5 GPa to 5.5 GPa. A temperature difference of 70 °C is maintained between the top and the bottom of the crucible at 1450 °C. The BN source dissolves at the bottom of the crucible and crystallises at the top end.

• Requires a hydraulic press to apply 2.5–5.5 GPa of pressure.
• BN or Mo crucible is used with BN as the solvent.
• Maintains a temperature of 1450°C with a 70°C gradient.
• BN dissolves at the bottom, crystallizes at the top.

Atmospheric Pressure High Temperature (APHT) Synthesis

This method uses a horizontal tube furnace under gas flow. Ni-Cr is the best and most widely used solvent. Boron and Nitrogen sources are the BN powder or the crucible. The soaking step is 24 hours at 1500 °C. The temperature is then slowly lowered to the temperature of crystallisation and the BN supersaturation increases. The supersaturation drives the crystallisation of hBN.

• Uses a horizontal tube furnace with gas flow.
• Ni-Cr is commonly used as the solvent.
• BN source is added or forms part of the crucible.
• Soaking for 24 hours at 1500°C, then slow cooling initiates crystallization.

Polymer-Derived Ceramics (PDC)

This method uses reactive molecular precursors containing B and N atoms, along with other elements. Precursors used are Ammonia borane (H3NBH3) and Boorn derivatives. The precursors go through a chemical reaction and degrade to form BN at low temperatures. The precursors recrystalline into hBN during the heating phase. This is a fast and versatile method but care must be taken about contamination throughout the process.

• Uses reactive molecular precursors such as Ammonia Borane (H₃NBH₃).
• Precursors degrade into BN at low temperatures, then recrystallize into hBN upon heating.
• Fast and flexible method, but requires strict contamination control.