Boron carbide crystals exhibit a rhombohedral structure, with their lattice belonging to the D3d5-R3m space group. As shown in Figure 7, this rhombohedral structure can be described as a cubic unit cell extended along the body diagonal direction, forming highly regular icosahedra at each corner. Parallel to the body diagonal lies the c-axis, which follows a hexagonal symmetry and consists of linear chains composed of three boron atoms interconnected with adjacent icosahedra. Consequently, the unit cell contains 12 icosahedral orientations, with three of these orientations aligned along the linear chains. If we consider the boron atoms as occupying positions dictated by the icosahedral geometry, while the carbon atoms reside within the linear chains, the chemical formula derived from this arrangement is B4C. --- ### 1. Fundamental Properties and Applications of Boron Carbide #### 1) Low Density Boron carbide boasts a relatively low density of 2.52 g/cm³. Within its homogenous phase range, the relationship between density and carbon content can be expressed empirically by Equation (9): \[ \rho = 2.4224 + 0.00489 \times C\% \quad (9) \] Due to its low density, even when achieving high levels of densification, boron carbide retains excellent mechanical properties such as high strength and exceptional hardness. This makes it an ideal material for lightweight armor applications, significantly reducing the weight of vehicles like tanks while also improving fuel efficiency. #### 2) Hardness and Wear Resistance Boron carbide is renowned for its extraordinary hardness and outstanding wear resistance. In its homogenous phase region, the Vickers hardness of B4C increases with rising carbon content. For instance, at a carbon concentration of 10.6%, the hardness reaches 29.1 GPa; further increasing the carbon content to 20% elevates the hardness to as high as 37.7 GPa. Remarkably, even at elevated temperatures, boron carbide maintains impressive hardness levels—exceeding 30 GPa. The temperature dependence of hardness can be modeled using Equation (10): \[ H = H_0 - \exp(-aT) \quad (10) \] Here, \( H_0 \) represents the hardness at room temperature, \( T \) denotes the temperature, and \( a \) is a constant influenced by the carbon content. This equation is applicable across a temperature range of 20°C to 1700°C. It’s worth noting that boron carbide ranks among the hardest materials globally, second only to diamond and cubic boron nitride (c-BN). Moreover, the wear resistance of boron carbide improves as temperature rises. Between 20°C and 1400°C, the coefficient of friction decreases with increasing temperature, dropping sharply to around 0.05 at 1400°C. These exceptional tribological properties have led to its widespread use in high-wear-resistant applications, such as sandblasting nozzles, diamond-coated nozzles for waterjet cutting systems, and other critical components in military equipment like tank and aircraft armor [39, 40]. With advancements in precision machining technologies driving demand for ultra-hard materials, boron carbide continues to gain prominence. In recent years, its usage has steadily increased, particularly in industries requiring advanced grinding techniques. Additionally, boron carbide is increasingly employed for grinding hard alloys, ceramics, and gemstones—serving as both free abrasive particles and ultrasonic machining media for processing these exceptionally tough materials. However, compared to regions like Europe and North America, China currently uses boron carbide sparingly in these applications. #### 3) Thermal Expansion and Specific Heat Capacity Boron carbide exhibits a melting point of 2450°C and a boiling point of 3000°C. Its coefficient of thermal expansion is 5.73 × 10⁻⁶/°C within the temperature range of 28°C to 1770°C. The specific heat capacity can be calculated using Equation (11): \[ C = 22.99 + 5.40 \times 10^{-3}T - 10.72 \times 10^5T^{-2} \quad (11) \] --- ### 2. Chemical Stability Boron carbide is one of the most chemically stable compounds known. Below 600°C, it resists oxidation under normal conditions. However, when exposed to temperatures above 600°C, a thin layer of boron oxide (B₂O₃) forms on its surface, effectively preventing further oxidation of the bulk material. As a result, boron carbide is now widely utilized as an anti-oxidant additive in refractory materials. At room temperature, boron carbide remains inert to most chemical reagents. Yet, at temperatures exceeding 800°C, it reacts with bromine to form tribromides. At even higher temperatures, boron carbide undergoes reactions with metal oxides, yielding metal borides and carbon monoxide. Notably, the resulting FeB films demonstrate remarkable microhardness values, reaching up to 24 GPa, along with exceptional wear resistance. These unique properties make boron carbide a valuable material for boriding steel and alloy surfaces, enhancing their durability and performance.

Boron carbide is a general term for compounds composed of carbon (C) and boron (B), forming two distinct types—B4C and B6C—depending on the specific reaction conditions. When referring to "boron carbide" without further specification, it typically means B4C. ### I. Fundamental Properties of Boron Carbide B4C belongs to the trigonal crystal system, with each unit cell containing 12 boron atoms and 3 carbon atoms. Notably, the carbon atoms within the unit cell are arranged along the body diagonals, leaving them in a highly mobile state that allows them to be readily replaced by boron atoms, leading to the formation of substitutional solid solutions. In some cases, these carbon atoms may even detach from the lattice, resulting in high-boron compounds with inherent structural defects. B4C has a molecular weight of 52.25 g/mol, with carbon accounting for 21.74% and boron making up 78.26% by mass. It typically appears as a grayish-black material, with a density of 2.519 g/cm³ and a Mohs hardness of 9.36. Its microhardness is around 50 GPa, second only to diamond and cubic boron nitride. As a result, B4C powder exhibits exceptional abrasive capabilities, achieving grinding efficiencies comparable to 60%-70% of those attained by diamond, while surpassing SiC by 50% and being 1-2 times more effective than corundum-based abrasives. The melting point of B4C is 2450°C (decomposition occurs at this temperature). Between 1000°C, its coefficient of thermal expansion is approximately 4.5 × 10⁻⁶/°C. At 100°C, its thermal conductivity is 121.4 W/m·K, dropping to 62.79 W/m·K at 700°C. Primarily used as an abrasive material, hot-pressed B4C products are also employed in manufacturing wear-resistant and heat-resistant components. In the refractory industry, B4C serves mainly as an additive—for instance, when incorporated into carbon-bonded refractories, it acts as an antioxidant; when added to unshaped materials, it enhances the green strength and improves resistance to chemical attack. ### II. Composition and Typical Properties of Boron Carbide In industrial applications, the most common method for producing B4C powder involves reducing boron trioxide with excess carbon: \[ 2\text{B}_2\text{O}_3 + 7\text{C} \rightarrow \text{B}_4\text{C} + 6\text{CO} \uparrow \] This synthesis reaction can be carried out either in a resistance furnace or an electric arc furnace. When using a resistance furnace, the mixture of boron trioxide (B₂O₃), carbon (C), and other additives is heated below the decomposition temperature of B4C, yielding a product with minimal free carbon content—though occasionally, it may still contain 1%-2% free boron. This method is considered superior due to its ability to produce higher-quality B4C. In contrast, when synthesizing B4C in an electric arc furnace, the extremely high temperatures cause B4C to decompose into a carbon-rich phase and elemental boron around 2200°C. Additionally, the high heat leads to significant evaporation of boron, resulting in a final product with a much higher free carbon content—often ranging from 20% to 30%. Consequently, the quality of B4C produced via this method tends to be slightly lower compared to the resistance-furnace approach. When producing B4C in an electric arc furnace, typical raw materials include boric acid (containing over 92% boron), artificial graphite (with fixed carbon content exceeding 95%), and petroleum coke (containing at least 85% fixed carbon). Based on stoichiometric calculations derived from the reaction equation, the amount of boric acid added should exceed the theoretical requirement by about 2%, while artificial graphite and petroleum coke each account for roughly 50% of the total carbon input. To ensure optimal results, these materials are then mixed thoroughly in a ball mill before being fed into the electric arc furnace, where the reduction and carburization processes occur at temperatures between 1700°C and 2300°C. Finally, the resulting molten mass undergoes a series of refining steps—including sorting, washing, crushing, grinding, acid leaching, and sedimentation-based grading—to yield B4C powders of various particle sizes.