Fundamental Properties, Use and Chemical Stability of Boron Carbide

Boron carbide crystal is rhombohedral structure, the lattice belongs to the D3d5-R3m space lattice. Its rhombohedral structure is shown in Figure 7, which can be described as a cubic proto-cell lattice extending in the diagonal direction of space, forming an appropriately regular icosahedron at each corner. Parallel to the diagonal of space, it becomes the c-axis of the hexagonal symbol, which is composed of three boron atoms and adjacent icosahedron connected to each other to form a linear chain. Thus, the unit cell contains 12 icosahedral orientations, three of which lie on linear chains. If the B atom is considered to be in the orientation caused by the icosahedron and the C atom is considered to be in a linear chain, then the chemical accounting of B12C3 is B4C.

1. Basic properties of boron carbide and its use

1) Low density

B4C has a small density of 2.52g/cm3. In the homogeneous phase region, the relationship between density and carbon content can be shown by the following formula (9):

Ρ=2.4224+0.00489C%(9)

Because of the low density of boron carbide, in the case of obtaining a higher density, its function can reach the standard high strength, high hardness and other excellent functions of boron carbide, so it can be used as light armor to reduce the weight of tanks and other car bodies, saving energy consumption.

2) Hardness and wear resistance

B4C has super hardness and high wear resistance, in the homogeneous phase region, the Vickers hardness of B4C is added with the addition of C content. When the carbon content is 10.6, the hardness is 29.1GPa; when the carbon content is 20%, the hardness of 21 up to 37.7GPa. Its hardness is still very high (>30GPa) at high temperatures. The change of hardness with temperature can be indicated by the experience formula (10):

H=H0-exp(-aT)(10)

In the formula: H0-hardness at normal temperature;

T-temperature;

a-is a constant related to carbon content.

This formula is suitable for 20~1700 ℃. The demand points out 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 the increase of temperature. In the range of 20~1400 ℃, the conflict coefficient decreases with the increase of temperature, and decreases to 0.05 at about 1400 ℃, and the conflict rate also decreases. B4C has been used as a sand spray tip for its ultra-hardness and conflict characteristics. Diamond nozzles, nozzles of water conservancy jet cutters, and other wear-resistant materials; Armor materials widely used in military affairs for tanks, airplanes, etc. [39,40]; With the progress of skills and the demand for high-precision grinding, B4C is increasingly showing its advantages. In recent years, its dosage has been continuously added. Together, B4C can also be used to grind hard alloy, ceramics and gem hard materials, conflict with free abrasive or ultrasonic processing of these super-hard materials with abrasive, but compared with Europe and the United States in this area in China is still very small.

3) Thermal expansion coefficient and specific heat capacity

The melting point of boron carbide is 2450 ℃, the boiling point is 3000 ℃, the thermal expansion coefficient is 5.73 × 10-6/℃(28~1770 ℃), and the specific heat capacity calculation formula (11):

C = 22.99 5.40 × 10-3t-10.72 × 105t-2 (11)

2. Chemical stability

Boron carbide is one of the most stable compounds, below 600 degrees Celsius is not easy to produce oxidation reaction; and the temperature above 600 degrees Celsius, because the appearance of oxidation into B2O3 film, to prevent the further oxidation of B4C. So B4C is now used as an antioxidant in refractory materials. Under normal temperature B4C generally does not react with chemical reagents; above 800 DEG C B4C and Br constitute three bromine compounds; high temperature B4C and metal oxide reaction to generate metal borides and carbon monoxide, the generated FeB thin films with high microhardness (HV = 24GPa) and wear resistance. Therefore, B4C can be used for the boronization of steel and alloys.