Cubic Boron Nitride and Its Hardness

Boron nitride is a chemical compound consisting of nitrogen and boron, with BN as the chemical formula. Despite having numerous forms, the most common one is the cubic boron nitride form.

It is a thermal compound that is resistant to heat. It is a lot more identical to the artificial diamond. 

Introduction

This compound is manufactured by a process which is known as the sintering process or the frottage process. The main and basic property that is exhibited by cubic boron nitride is its hardness. It is also because of this property of cubic boron nitride that it is so useful and approachable in the market. Though it has more properties as well which include low melting and boiling point, thickness, and conductivity, but its hardness is one such property that subsides all the rest of the properties. Because of this, it has a wide range of applications as well which makes it more sustainable in the usage and market.

A chemically and thermally resistant refractory compound of nitrogen and boron is known as Boron nitride. Its chemical formula is BN. It exists in numerous forms of crystalline which are isoelectronic to a carbon lattice of similar structure. Among BN polymorphs, the hexagonal form which corresponds to graphite is the softest and most stable, thus being used as an additive to cosmetics and as a lubricant. C-BN is the cubic (sphalerite structure) variety analogous to diamond; even though its chemical and thermal stability is more, it is still soft in comparison to the diamond. In comparison with lonsdaleite, the modification of rare wurtzite BN is the same but a little softer than the cubic form.

Traditionally, due to remarkable chemical and thermal stability, boron nitride ceramics are used as high-temperature equipment parts. In nanotechnology, potential uses are contained by boron nitride. Carbon nanotubes can have a similar structure like BN nanotubes, for instance, despite having very different characteristics, BN (or graphene sheets) rolled on themselves.

Made by mixing nitrogen and boron, cubic Boron nitride is a thermally and chemically heat resistant compound material having BN as the chemical formula.

Cubic boron nitride is made like artificial diamonds. Frottage or sintering is the process that is used by the manufacturer. In sintering, heat is added into the mineral for imparting its integrity and strength. The solid mass is created by boron nitride when its hexagonal form, h-BN doesn’t hit the melting point while facing high pressure and heat. Then catalysts like magnesium or lithium will be added by the producer for lessening the temperatures and pressures that are needed for the conversion. Its structure is crystalline and analogous to diamond. Graphite is more stable than diamond, but at room temperature, the rate of conversion between the hexagonal form and cubic form is negligible, just like a diamond, hexagonal form is more stable than the cubic form. The cubic form has the structure of Sphalerite crystal, just like a diamond, it is also known as c-BN or β-BN.

Properties of Boron Nitride

In h-BN, the BN layer's partly ionic structure lessens electrical conductivity and covalency, meanwhile the interactions of the interlayer increases, leading to more hardness of h-BN as compared to graphite. The absence of large bandgap and color indicates the lessened delocalization of electrons in hexagonal-BN. Bonding is very different, in basal planes, there is strong covalent bonding (Nitrogen and boron atoms are covalently bonded in these planes) and if bonding is weak between them, then high anisotropy of h-BP properties are caused.

For instance, the thermal conductivity, electrical conductivity, and hardness are way more in the planes instead of perpendicular. Also, w-BN and c-BN characteristics are more isotropic and homogeneous.

The hardness of these materials is slightly more than bulk c-BN and the hardness of w-BN is higher than diamond. In comparison to diamond’s hardness, Polycrystalline c-BN with 10nm sizes of grain is higher or comparable. In mechanical applications, diamond is surpassed by c-BN due to greater stability to transition metals and heat, such as in machining steel. Among all-electric insulators, BN's thermal conductivity is the highest.

Boron nitride can be doped n-type with silicon, sulfur, or boron, and be doped p-type with beryllium, or if co-doped with nitrogen and carbon. Both the cubic and hexagonal BN are wide-gap semiconductors with a band-gap energy equivalent to the UV region. UV light is emitted in 215-250 nm range if the voltage is applied to c-BN or h-BN, thus can be used potentially as lasers or light-emitting diodes (LEDs).

On boron nitride’s melting behavior, very less is known. At normal pressure and 2973 C, it sublimates, releasing boron and nitrogen gas. On an increase in the pressure, it melts.

Applications of Boron Nitride

Chemically and mechanically, c-BN is a very famous material as its atoms are very robust and very light. c-BN is usually used as an industrial abrasive. What makes the application really logical is that it is easy to produce and find and it has similar characteristics like a diamond. At high temperature, it will not turn to liquid or dissolve, also, with nickel, iron, and other same metals, it is easy.With these metals, diamond is soluble and in comparison with c-BN, a diamond would liquefy at a lesser temperature, which results in a grinding wheel's breakdown, which is the process's important part. The piece can be finished with poor glazing and burning if the grinding wheel is damaged. Electroplated precision grinding wheels are used specifically by the turbine manufacturers and aviation industry for grinding the components of the engine and the turbine blades. Close tolerance gliding needs the removal of close stock of cobalt alloys, nickel alloys, and other hard materials, electroplated precision grinding wheels are most appropriate for it. In their make-up, a broad range of cutting tools, scratch-resistant surfaces, and drill bits also have c-BN.

Broadly, Cubic boron nitride is used as an abrasive.At high temperature, its insolubility in nickel, iron, and similar alloys makes it useful, because, in the same metals, diamond is rather soluble. Abrasives of the diamond are good for ceramics, stone, and aluminum alloys, whereas for machining steel, abrasives of polycrystalline c-BN (PCBN) are used. Boron oxide's passivation layer is formed by BN at high temperatures when BN is in contact with oxygen. Due to the production of nitrides or interlayers of metal borides, boron nitride binds well with metals. Cubic boron nitride crystal containing materials are often used in cutting tool’s tool bits. For applications of grinding, ceramic binders, and softer binders, for instance, soft metals, porous ceramics, and resin are used. Diamond innovation named commercial products as "Borazon", and Russian Vendors call them as ‘Cubonite’ or "Elbor".

In comparison with diamond, at a lower temperature than that of BN decomposition temperature, the sintering of annealing cubic boron nitride in nitrogen flow produces large pellets of cubic boron nitride. Large BN parts are cheaply produced due to the fusion of the powders of h-BN and c-BN.

Just like a diamond, for heat spreaders, the combination is ideal in c-BN of the highest electrical resistivity and thermal conductivity.         

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For membranes of X-ray, cubic boron nitride is a famous material as c-BN is very robust mechanically and chemically, and comprises of light atoms. Less mass leads to small X-ray absorption, and thin membranes are used because of good mechanical features, therefore lessening the absorption further.

For attaining the films of cubic boron nitride with industrial quality, kinetic energy’ effects and the effect of the substrate’s chemical nature and temperature on the cubic boron nitride film’s synthesis are explored.Thick, adherent, and stable cubic boron nitride films can be deposited with Raman signature by the allowance from Carbon including polycrystalline and nanocrystalline diamond, and amorphous carbon. Higher quality of cubic boron nitride films can be yielded when the deposition is done at high temperatures, even though the temperature is said to be an unimportant parameter.

Through the usage of plasma-enhanced chemical vapor deposition (PECVD), the inserts were cut when cubic boron nitride was being deposited on diamond coated tungsten carbide (WC) at a higher temperature of 800 °C.PECVD grows cubic boron nitride films of a quality which considerably overcomes that which is grown by physical vapor deposition (PVD). The films grown by PVD are affected to a huge extent by the particle’s lower kinetic energies that PECVD uses. The particle's low kinetic energy promotes the mechanism of surface growth which is different from previously proposed growth models.

Cubic boron nitride has many applications, they are in optical instruments as emitters and as UV detectors, for cutting tools they are hard protective coatings, and also in electronic devices of high temperature.Although, the most challenging is synthesizing phase thin films of pure cubic boron nitride.

In this review, the characterization, the synthesis, the optical, electrical, and mechanical characteristics of thin films of cubic boron nitride are discussed in detail. For cubic boron nitride films, the chemical vapour deposition method and the physical methods that are used for cubic boron nitride film’s preparation are also explained briefly. Also, different nucleation and different cubic boron nitride’s growth models formation and growth on different substrates are explained.Theprocess of parameters has influence, for instance, chemical precursors, growth temperature, ion energy, impurities, and bias on the growth and nucleation is studied.We also explained cubic boron nitride’s mechanical features including the stiffness of cubic boron nitride films, their elastic modulus, and their hardness. Also, recent discoveries in the c-BN films’ electrical characteristics based on the heterostructures of metal-insulator-semiconductor (MIS), properties of field emission, conduction mechanism, impurity states, interface states, and negative electron affinity (NEA) are displayed.

Conclusion

Cubic boron nitride is a common and basic form of a chemical compound of boron and nitride widely being used for various purposes but most commonly for the industrial abrasion. This is due to the reason that it is hard or almost impossible for cubic boron nitride to dissolve with iron, zinc, nickel, and other such metals. Diamond is hard to find while cubic boron nitride is easily accessible so better use is being made in the industry of this matter as it is highly impressive in all its forms and functions. Its authentic nature and functionality make it impressive for the consumers to consume it and bring it to more useful terms in both in the fields of technology and industry.

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References

1.https://www.sciencedirect.com/science/article/abs/pii/S092596350500381X

2.https://en.wikipedia.org/wiki/Boron_nitride

3.http://www.docbrown.info/page03/nanochem06.htm

4.https://www.tandfonline.com/doi/abs/10.1179/174328005X67160