The fundamental properties, uses, and chemical stability of boron carbide.

Boron carbide crystals have a rhombohedral structure, and the lattice belongs to the D3d5-R3m space group. Its rhombohedral structure is shown in Figure 7 and can be described as a cubic unit cell lattice extending in the spatial diagonal direction, forming appropriately regular dodecahedra at each corner. Parallel to the spatial diagonal, it becomes the hexagonal c-axis, composed of linear chains formed by three boron atoms connecting with adjacent dodecahedra. Therefore, the unit cell contains 12 dodecahedral orientations, with three orientations located on the linear chain. If the B atoms are considered as orientations caused by the dodecahedra, and the C atoms are seen as being on the linear chain, then the chemical formula for B12C3 is B4C.

1. Basic properties and uses of boron carbide

1) Low density

The density of B4C is relatively low, at 2.52g/cm3. In the homogeneous phase, the relationship between density and carbon content can be expressed by the formula (9):

ρ=2.4224+0.00489C%

Due to the low density of boron carbide, it can achieve high density while maintaining excellent properties such as high strength and hardness, making it suitable as lightweight armor to reduce the weight of tanks and other vehicles, thus saving energy consumption.

2) Hardness and wear resistance

B4C has super hardness and extremely high wear resistance. In the homogeneous phase, the Vickers hardness of B4C increases with the addition of carbon content. When the carbon content is 10.6%, the hardness is 29.1GPa; when the carbon content is 20%, the hardness can reach 37.7GPa. Its hardness remains very high (>30GPa) at high temperatures. The variation of hardness with temperature can be expressed by the formula (10):

H=H0-exp(-aT)

Where: H0 is the hardness at room temperature;

T is the temperature;

a is a constant related to carbon content.

This formula is applicable from 20 to 1700℃. It should be noted that B4C is one of the hardest materials in the world, second only to diamond and cubic BN.

The wear resistance of B4C increases with temperature. In the range of 20 to 1400℃, the friction coefficient decreases with increasing temperature, dropping to 0.05 around 1400℃, and the friction rate also continuously decreases. B4C, with its super hardness and friction characteristics, has been used as a sandblasting nozzle. Diamond nozzles and nozzles for hydraulic jet cutters, among other wear-resistant materials; it is widely used in military applications for armor materials in tanks, aircraft, etc. With advancements in technology and increasing demand for high-precision grinding, B4C has increasingly shown its superiority, and its usage has been continuously increasing in recent years. Additionally, B4C can also be used to grind hard alloys, ceramics, and hard gemstones, as well as for abrasive materials used in free abrasives or ultrasonic processing of these super hard materials. However, compared to Europe and the United States, the usage in China is still relatively low.

3) Coefficient of thermal expansion and specific heat capacity

The melting point of boron carbide is 2450℃, the boiling point is 3000℃, and the coefficient of thermal expansion is 5.73×10-6/℃ (28~1770℃). The specific heat capacity calculation formula is:

C=22.99+5.40×10-3T-10.72×105T-2

2. Chemical stability

Boron carbide is one of the most stable compounds, not easily undergoing oxidation reactions below 600℃; above 600℃, due to the formation of a B2O3 film on the surface, further oxidation of B4C is prevented. Therefore, B4C is now used as an antioxidant in refractory materials. At room temperature, B4C generally does not react with chemical reagents; above 800℃, B4C reacts with Br to form tribromide compounds; at high temperatures, B4C reacts with metal oxides to generate metal borides and carbon monoxide, with the resulting FeB film having very high microhardness (HV=24GPa) and wear resistance. Thus, B4C can be used for boronizing steel and alloys.