The fundamental properties, applications, and chemical stability of boron carbide

Boron carbide crystals exhibit a rhombohedral structure, with their lattice belonging to the D3d5-R3m space group. The rhombohedral structure, as shown in Figure 7, can be described as a cubic unit cell extended along the body diagonal of the cube, forming highly regular icosahedra at each corner. Along the direction parallel to the body diagonal lies the hexagonal c-axis, where linear chains are formed by alternating sequences of three boron atoms connected to neighboring icosahedra. Consequently, the unit cell contains 12 icosahedral orientations, with three of these orientations situated precisely along the linear chains. If we consider the boron atoms as occupying positions dictated by the icosahedra, while the carbon atoms reside within the linear chains, the resulting chemical formula simplifies to B4C.

1. Basic Properties and Applications of Boron Carbide

1) Low density

B4C has a relatively low density of 2.52 g/cm³. Within the homogeneous phase region, the relationship between density and carbon content can be described by Equation (9):

ρ = 2.4224 + 0.00489C%(9)

Due to boron carbide's low density, it achieves high functional performance—such as exceptional strength and hardness—while maintaining a high level of densification. As a result, it can be used as a lightweight armor material, reducing the overall weight of vehicles like tanks and thereby improving fuel efficiency.

2) Hardness and Wear Resistance

B4C boasts exceptional hardness and outstanding wear resistance. Within its homogeneous phase region, the Vickers hardness of B4C increases as the carbon content rises. At a carbon content of 10.6%, the hardness reaches 29.1 GPa; however, when the carbon level climbs to 20%, the hardness can soar to an impressive 37.7 GPa. Even at elevated temperatures, B4C retains impressive hardness levels—exceeding 30 GPa. The variation in hardness with temperature can be described by Equation (10):

H = H₀ - exp(-aT)(10)

In the formula: H0 represents the hardness at room temperature;

T—represents temperature;

a—represents a constant related to carbon.

This formula is applicable from 20 to 1700°C. It’s worth noting that B4C is one of the hardest materials in the world, ranking second only to diamond and cubic BN.

The wear resistance of B4C increases with rising temperature. Within the range of 20–1400°C, the coefficient of friction decreases as temperature climbs, dropping to around 0.05 at 1400°C—and continues to decline thereafter. Due to its exceptional hardness and outstanding tribological properties, B4C is already widely used as an abrasive material in applications such as sandblasting nozzles, diamond-tipped nozzles for waterjet cutting, and other wear-resistant components. In the military sector, it’s extensively employed as armor material for tanks, aircraft, and other defense systems [39, 40]. With advancements in technology driving the growing demand for high-precision grinding, B4C is increasingly demonstrating its superior performance. As a result, its usage has been steadily rising in recent years. Moreover, B4C can also be utilized effectively for grinding hard alloys, ceramics, and even gemstones—serving as a free abrasive or ultrasonic tool when working with these ultra-hard materials. However, compared to Europe and North America, China still lags significantly in terms of B4C consumption in this area.

3) Coefficient of thermal expansion and specific heat capacity

Boron carbide has a melting point of 2,450°C, a boiling point of 3,000°C, and a thermal expansion coefficient of 5.73 × 10⁻⁶/°C (ranging from 28°C to 1,770°C). Its specific heat capacity is calculated using 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, showing minimal oxidation below 600°C. However, when the temperature exceeds 600°C, its surface oxidizes to form a thin B₂O₃ layer, effectively preventing further oxidation of the bulk B₄C. As a result, B₄C is now widely used as an antioxidant additive in refractory materials. At room temperature, B₄C typically remains unreactive with most chemical reagents. But above 800°C, it reacts with bromine (Br) to form a tribromide compound. Moreover, at elevated temperatures, B₄C interacts with metal oxides, yielding metal borides along with carbon monoxide. Notably, the resulting FeB film exhibits exceptional microhardness (HV = 24 GPa) and outstanding wear resistance. This unique property makes B₄C an ideal material for boriding steel and alloy surfaces.