Hexagonal Boron Nitride (hBN) has an analogous construction as that of Graphite and is named ‘White Graphite’. hBN has a low coefficient of friction and displays good lubricating properties. It displays properties like chemical inertness, electrical insulator, good thermal conductor, low thermal growth and low dielectric fixed.
The hBN has good load-bearing properties and high-temperature stability (1000° C in air, 1400° C in vacuum and 1800° C in inert fuel). It doesn’t get wetted by most molten metals, molten glasses, and molten salts and thus offers sturdy resistance to chemical assaults.
hBN has a lamellar construction and is an efficient stable lubricant for high-temperature purposes. In extreme circumstances, the oils and grease lubricants can’t meet the appliance necessities. Stable lubricants have low volatility and excessive load-carrying capability subsequently they can be utilized to regulate friction and scale back put on in high-temperature purposes.
hBN can be utilized as an additive for lubricant for high-temperature purposes. hBN powder is used as a lubricant additive and will be dispersed in oil, water, solvents, and grease. When it’s blended with water and binders it may be utilized as a paint for lubricity coatings. It may be sprayed on sizzling surfaces to supply dry lubricity. It may be blended with alloys, ceramics, resins, plastics, and different supplies to supply self-lubricating properties.
As a result of excessive lubricity, hBN is used as a launch agent in plastic injection and steel injection molds.
Synthesis of hBN
hBN will be obtained by numerous strategies like low-temperature progress processes and high-temperature progress processes. ]
Low-temperature progress processes
a. Combustion synthesis
That is the commonest course of used to supply industrial hBN because it makes use of a quick
response and gives excessive yields. It’s based mostly on the nitridation of boron oxide. Boric acid is blended with urea (urea will be changed with N2 fuel) and heated in an oven between 500° C – 1000° C. The hBN thus obtained has a crystallite measurement starting from 100 nm to 1 μm.
b. Solvothermal synthesis
This methodology requires low temperatures and gives simple preparation to supply giant portions of hBN. It produces micrometer to nanometer-sized hBN particles that are low-priced. On this methodology, two precursors of Boron Nitride are blended in a liquid solvent and heated at reasonable temperatures under 500° C in an autoclave. The crystals produced are of poor high quality.
Excessive-temperature progress processes
Boron Nitride has a really excessive melting temperature which is lowered by mixing it with a solvent. All of the high-temprature progress processes use solvents that are additionally known as catalysts or crystallisation promoters.
You will need to use the best solvent for the processes, normally alkali and alkali-earth solvents had been used. These solvents have a low melting level under 1000° C. Boron Nitride is very soluble in such solvents resulting in the crystallisation of hBN within the solvent matrix. After cooling, the dissolution course of is carried out utilizing Aqua Regia to free the crystals.
a. Excessive-pressure high-temperature (HPHT) synthesis
This course of requires excessive mechanical stress and subsequently hydraulic press is used. It makes use of a closed BN crucible with a BN supply and a solvent. The solvents that can be utilized are Lithium Nitride or alkali-earth Barium Nitride. The stress utilized ranges from 2.5 GPa to five.5 GPa. Additionally it is known as the temperature gradient methodology.
The precise temperature and period of the method within the HPHT chamber usually are not quantified. A temperature distinction of 70° C is maintained between the highest and backside ends of the crucible at 1450° C.
The BN supply dissolves on the backside finish of the crucible and crystallises on the prime finish because the temperature decreases. The entire HPHT meeting together with the BN-sealed crucible is free from exterior contaminations.
b. Atmospheric stress and high-temperature (APHT) synthesis
On this methodology, a horizontal tube furnace is used. Pure Boron powder can be utilized as a supply of Boron and N2 fuel is used as a Nitrogen supply. A steel solvent like Ni-Cr is used and it’s the finest and broadly employed solvent.
On this course of, the steel solvent requires a high-temperature dwell time of so long as 48 hours to make sure the dissolution of the Boron and Nitrogen atoms. The temperature is lowered to crystallisation temperature. BN supersaturation will increase within the liquid steel solvent and drives the crystallisation of hBN.
c. Polymer-derived ceramics (PDC) of hBN
On this reactive precursors of Boron and Nitrogen are used together with N2 fuel. At low temperatures, the precursors degrade to type BN. Through the heating part, this may recrystallise into hBN.