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Boron Carbide: A Mystery of The Cemented Carbide

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Update time : 2020-01-15 15:50:12

Boron carbide is widely used in refractories, engineering ceramics, nuclear industry, aerospace, and other fields because of its high melting point, high hardness, low density, and excellent neutron absorption and chemical resistance.
 

1. Crystal structure of boron carbide

Complex crystal structure of boron carbide with typical icosahedral boride. The stoichiometric formula is B4C.
 

There are many isomers in B4C and boron carbide; the carbon content is from 8% to 20%. The most stable boron carbide structure is b13c2, b13c3, B4C with rhombohedral structure, and other phases close to b13c3. There are 12 icosahedral clusters in the hexagonal structure of boron carbide, which are connected by a covalent bond and have a three atom chain on the diagonal of the hexagonal. The dodecahedral structure of the boron is located at the apex of the rhombohedron.

Boron and carbon atoms can replace each other on the icosahedron and atomic chain, which is the main reason why boron carbide has so many isomers. Because of its particular structure, boron carbide has many excellent physical and mechanical properties.


 

2. Characteristics of boron carbide

2.1 Hardness of boron carbide

The most crucial performance of boron carbide lies in its extraordinary hardness (Mohs 9.3, microhardness 55gpa-67gpa), which is an ideal high-temperature wear-resistant material. Therefore, boron carbide, also known as black diamond, is one of the three hardest materials known (the other two are the diamond and the cubic boron nitride), which is used in tank armor, bulletproof clothing and many industrial applications.
 

But, is boron carbide harder than a diamond?
 

The answer is No. Boron carbide is no harder than diamond. As a hard black glossy crystal, boron carbide has a lower hardness than industrial diamond, but higher hardness than silicon carbide. That is to say, and boron carbide is the fifth hardest material known after boron nitride, diamond, fullerene compound, and diamond whole fiber tube.
 

2.2 Density of boron carbide

Boron carbide's theoretical density is 2.52g/cm3,and relative density is (d204) 2.508 ~ 2.512.

It is the lightest ceramic material with minimal density and can be used in the aerospace field.
 

2.3 Chemical properties of boron carbide

Boron carbide has excellent chemical properties. It does not react with acids, bases, and most inorganic compounds at room temperature. It only erodes slowly in the mixture of hydrofluoric acid, sulfuric acid, and hydrofluoric acid, nitric acid. It is one of the compounds with the most stable chemical properties.
 

2.4 Other properties of boron carbide

Compared with the pure elements B and CD, boron carbide has the advantages of low cost, excellent corrosion resistance, and thermal stability, and is widely used in the nuclear industry;
 

Boron carbide also has the advantages of high melting point, high elastic modulus, low expansion coefficient, and functional oxygen absorption capacity;
 

Boron carbide is also a p-type semiconductor material, which can maintain the semiconductor characteristics even at a very high temperature.
 

3. The application of boron carbide

Nozzle material

The super hardness and excellent wear resistance of boron carbide make it a vital nozzle material. The boron carbide nozzle has the advantages of long life, relatively low cost, time-saving, and high efficiency.
 

Neutron absorption and radiation protection materials

Element B has a neutron absorption cross-section of up to 600 barn, which is the primary material for the reduction element control rod or radiation protection part of the nuclear reactor.
 

Composite armor material

It can be used as a light bulletproof vest and armor material by its light, super hard, and high modulus. The bulletproof vest made of boron carbide is more than 50% lighter than that made of the same steel. Boron carbide is also a critical bulletproof armor material for armored land vehicles, armed helicopters, and civil aviation airliners.


 

Semiconductor industrial components and thermoelectric components

Boron carbide ceramics have semiconductor characteristics and better thermal conductivity, which can be used as high-temperature semiconductor components, as well as gas distribution plate, focusing ring, microwave, or infrared window, DC plug, etc. in the semiconductor industry. The combination of B4C and C can be used as a high-temperature thermocouple element, with the operating temperature up to 2300 ℃, and also as a radiation-resistant thermoelectric element;
 

Abrasives

The high hardness of B4C makes it an excellent wear-resistant material or antifriction material. Boron carbide is used instead of diamond abrasives in the polishing, precision grinding or grinding process of cemented carbide and engineering ceramics, which can significantly reduce the cost of grinding process.

The B4C coating can also be coated on the substrate to form a protective film and improve the wear resistance of the substrate. For example, applying a B4C layer on the gear surface of the gearbox can effectively improve the wear resistance of gear and improve the service life of the equipment.

 

Other applications

For example, in the field of refractories, boron carbide is used as an antioxidant, mainly in low-carbon magnesia carbon bricks and castables.
 

Boron carbide plays an essential role in aerospace, national defense, nuclear energy, and wear-resistant technology because of its excellent performance. The preparation methods include the carbothermal reduction method, self-propagating thermal reduction method, mechanochemical method, direct synthesis method, sol-gel method, and so on.

At present, the main problem hindering the application and popularization of boron carbide lies in the high cost of preparation, and the high sintering temperature and weak oxidation resistance of boron carbide itself.

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