Fundamental Properties, Applications, and Chemical Stability of Boron Carbide

Summary:

Boron carbide crystals have a rhombohedral structure, with the lattice belonging to the D3d–R3m space group. Its rhombohedral structure is shown in Figure 7 and can be described as a cubic primitive cell stretched along the body diagonal, forming regularly arranged icosahedra at each corner. Parallel to the body diagonal, this corresponds to the hexagonal c-axis, where linear chains are formed by three boron atoms interconnected with adjacent icosahedra. Consequently, the unit cell contains 12 icosahedral orientations, with three orientations lying along the linear chains. If we consider B atoms as occupying positions defined by the icosahedra and C atoms as residing on the linear chains, then the stoichiometric formula B12C3 is obtained. 1. Basic Properties and Applications of Boron Carbide 1) Low Density Boron carbide exhibits a relatively low density of 2.52 g/cm³. Within the homogeneous phase region, the relationship between density and carbon content can be expressed empirically by Equation (9): ρ = 2.4224 + 0.00489C% (9) Due to its low density, even when achieving high degrees of densification, boron carbide maintains excellent properties such as high strength and hardness. Therefore, it can serve as a lightweight armor material, reducing the weight of tanks and other vehicles while conserving energy. 2) Hardness and Wear Resistance Boron carbide possesses exceptional hardness and outstanding wear resistance. Within the homogeneous phase region, its Vickers hardness increases with increasing carbon content. At a carbon content of 10.6%, the hardness reaches 29.1 GPa; at 20% carbon, it can rise up to 37.7 GPa. Even at elevated temperatures, its hardness remains very high (>30 GPa). The temperature dependence of hardness can be described by empirical Equation (10): H = H0 − exp(−aT) (10) where H0 represents the hardness at room temperature, T denotes temperature, and a is a constant related to carbon content. This equation applies over the range of 20–1700°C. It should be noted that boron carbide ranks among the hardest materials in the world, second only to diamond and cubic boron nitride. The wear resistance of boron carbide improves with rising temperature. Within the range of 20–1400°C, its coefficient of friction decreases as temperature increases, reaching approximately 0.05 around 1400°C, accompanied by a continuous reduction in frictional wear. Owing to its extreme hardness and favorable tribological characteristics, boron carbide has been employed for sandblasting nozzles, diamond‑based nozzles, and water‑jet cutting nozzles—materials renowned for their wear resistance. Militarily, it is widely utilized as an armor material for tanks, aircraft, and other applications [39,40]. With advances in technology and growing demand for high‑precision grinding, boron carbide’s advantages have become increasingly evident, leading to steadily rising consumption in recent years. Additionally, boron carbide can be used to grind hard alloys, ceramics, and gemstones, serving as a free abrasive or ultrasonic machining medium for these ultra‑hard materials. However, compared with Europe and North America, China still lags behind in the application of boron carbide in this field. 3) Coefficient of Thermal Expansion and Specific Heat Capacity Boron carbide melts at 2450°C and boils at 3000°C. Its coefficient of thermal expansion is 5.73 × 10⁻⁶/°C (over the range of 28–1770°C), while its specific heat capacity is given by Equation (11): C = 22.99 + 5.40 × 10⁻³T − 10.72 × 10⁵T⁻² (11) 2. Chemical Stability Boron carbide is one of the most chemically stable compounds. Below 600°C, it resists oxidation; above 600°C, surface oxidation forms a thin B₂O₃ layer that effectively inhibits further oxidation. For this reason, boron carbide is now commonly used as an antioxidant additive in refractory materials. At room temperature, boron carbide generally does not react with chemical reagents; however, above 800°C, it reacts with bromine to form tribromides. At higher temperatures, boron carbide also reacts with metal oxides, producing metal borides and carbon monoxide. The resulting FeB film exhibits exceptionally high microhardness (HV = 24 GPa) and excellent wear resistance, making boron carbide suitable for boriding steel and alloy materials.

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