Hexagonal Boron Nitride (hBN) has a similar structure as that of Graphite and is known as ‘White Graphite’. hBN has a low coefficient of friction and exhibits good lubricating properties. It exhibits properties like chemical inertness, electrical insulator, good thermal conductor, low thermal expansion and low dielectric constant.
The hBN has good load-bearing properties and high-temperature stability (1000° C in air, 1400° C in vacuum and 1800° C in inert gas). It does not get wetted by most molten metals, molten glasses, and molten salts and thus provides strong resistance to chemical attacks.
hBN has a lamellar structure and is an effective solid lubricant for high-temperature applications. In severe conditions, the oils and grease lubricants cannot meet the application requirements. Solid lubricants have low volatility and high load-carrying capacity therefore they can be used to control friction and reduce wear in high-temperature applications.
hBN can be used as an additive for lubricant for high-temperature applications. hBN powder is used as a lubricant additive and can be dispersed in oil, water, solvents, and grease. When it is mixed with water and binders it can be applied as a paint for lubricity coatings. It can be sprayed on hot surfaces to provide dry lubricity. It can be mixed with alloys, ceramics, resins, plastics, and other materials to provide self-lubricating properties.
Due to high lubricity, hBN is used as a release agent in plastic injection and metal injection molds.
Synthesis of hBN
hBN can be obtained by various methods like low-temperature growth processes and high-temperature growth processes. ]
Low-temperature growth processes
a. Combustion synthesis
This is the most common process used to produce commercial hBN as it uses a fast
reaction and offers high yields. It is based on the nitridation of boron oxide. Boric acid is mixed with urea (urea can be replaced with N2 gas) and heated in an oven between 500° C – 1000° C. The hBN thus obtained has a crystallite size ranging from 100 nm to 1 μm.
b. Solvothermal synthesis
This method requires low temperatures and offers easy preparation to produce large quantities of hBN. It produces micrometer to nanometer-sized hBN particles which are low-priced. In this method, two precursors of Boron Nitride are mixed in a liquid solvent and heated at moderate temperatures below 500° C in an autoclave. The crystals produced are of poor quality.
High-temperature growth processes
Boron Nitride has a very high melting temperature which is lowered by mixing it with a solvent. All the high-temprature growth processes use solvents which are also called catalysts or crystallisation promoters.
It is important to use the right solvent for the processes, usually alkali and alkali-earth solvents were used. These solvents have a low melting point below 1000° C. Boron Nitride is highly soluble in such solvents leading to the crystallisation of hBN in the solvent matrix. After cooling, the dissolution process is carried out using Aqua Regia to free the crystals.
a. High-pressure high-temperature (HPHT) synthesis
This process requires high mechanical pressure and therefore hydraulic press is used. It uses a closed BN crucible with a BN source and a solvent. The solvents that can be used are Lithium Nitride or alkali-earth Barium Nitride. The pressure applied ranges from 2.5 GPa to 5.5 GPa. It is also called the temperature gradient method.
The exact temperature and duration of the process in the HPHT chamber are not quantified. A temperature difference of 70° C is maintained between the top and bottom ends of the crucible at 1450° C.
The BN source dissolves at the bottom end of the crucible and crystallises at the top end as the temperature decreases. The whole HPHT assembly along with the BN-sealed crucible is free from external contaminations.
b. Atmospheric pressure and high-temperature (APHT) synthesis
In this method, a horizontal tube furnace is used. Pure Boron powder can be used as a source of Boron and N2 gas is used as a Nitrogen source. A metal solvent like Ni-Cr is used and it is the best and widely employed solvent.
In this process, the metal solvent requires a high-temperature dwell time of as long as 48 hours to ensure the dissolution of the Boron and Nitrogen atoms. The temperature is lowered to crystallisation temperature. BN supersaturation increases in the liquid metal solvent and drives the crystallisation of hBN.
c. Polymer-derived ceramics (PDC) of hBN
In this reactive precursors of Boron and Nitrogen are used along with N2 gas. At low temperatures, the precursors degrade to form BN. During the heating phase, this will recrystallise into hBN.