Applications Of Silicon Carbide And Manufacturing Processes Of Silicon Carbide Products

Silicon carbide is synthesized through a heating reaction in an electric resistance furnace, using natural silica, carbon, wood chips, and industrial salt as the basic raw materials. Wood chips are added to make the bulk mixture porous at high temperatures, which facilitates the discharge of a large amount of gases and volatiles generated during the reaction, preventing explosions. Approximately 1.4 tons of carbon monoxide (CO) are produced for every 1 ton of silicon carbide synthesized. Industrial salt (NaCl) serves to remove impurities, such as alumina and iron oxide, present in the raw materials.

Synthesis and Applications of Silicon Carbide

The synthesis of silicon carbide is carried out in a special electric resistance furnace, which consists essentially of a graphite resistance heating element. This element is formed by stacking graphite or carbon particles into a columnar shape.

The heating element is placed in the center, and the aforementioned raw materials-mixed uniformly in the proportions of 52%–54% silica, 35% coke, 11% wood chips, and 1.5%–4% industrial salt-are tightly packed around the graphite heating element. When heated by electrification, the mixture undergoes a chemical reaction to produce silicon carbide. The reaction formula is as follows:

SiO₂+3C→SiC+2CO↑

The reaction initiates at approximately 1400°C, yielding low-temperature β-SiC with extremely fine crystals. This β-SiC remains stable up to 2100°C, beyond which it gradually transforms into high-temperature α-SiC.

α-SiC can maintain stability up to 2400°C without significant decomposition; however, when the temperature exceeds 2600°C, it sublimes and decomposes, releasing silicon vapor and leaving behind graphite. Therefore, the final reaction temperature is generally set within the range of 1900–2200°C.

The product of the synthesis reaction is a blocky crystalline aggregate, which needs to be crushed into particles or powders of varying sizes while removing any impurities contained within.

To obtain high-purity silicon carbide, the chemical vapor deposition (CVD) method can be used. Specifically, when a mixed vapor of silicon tetrachloride, benzene, and hydrogen passes over a glowing hot graphite rod, a gas-phase reaction occurs, and the resulting silicon carbide deposits on the graphite surface. The reaction formula is as follows:

6SiCl₄+C₆H₆+12H₂→6SiC+24HCl

Pure silicon carbide is colorless and transparent, but industrial-grade silicon carbide exhibits colors such as yellow, black, dark green, and light green due to impurities like free carbon, iron, and silicon. Light green and black are the most common colors.

Silicon carbide has a relative molecular mass of 40.09, with silicon accounting for 70.04% and carbon for 29.96%. Its true density is 3.21, and its melting point (sublimation temperature) is 2600°C.

It has multiple crystal forms: low-temperature β-SiC with a cubic structure, high-temperature α-SiC with a hexagonal structure, and a series of other polymorphs-around one hundred in total-caused by different atomic arrangements in the crystal lattice, generally referred to as polymorphism.

In addition, due to differences in electron affinity within the crystal structure, apart from the dominant covalent bonds, a portion of ionic bonds also exists.

Silicon carbide is a hard material with a Mohs hardness of 9.2. At low temperatures, it features stable chemical properties and excellent corrosion resistance, remaining unaffected even by boiling hydrochloric acid, sulfuric acid, and hydrofluoric acid.

However, it can react with certain metals, salts, and gases at high temperatures, and the reaction details are listed in Table 10-4-16.

Silicon carbide remains stable up to 2600°C in a reducing atmosphere, while oxidation occurs in a high-temperature oxidizing atmosphere:

SiC+2O₂→SiO₂+CO₂

However, its oxidation resistance between 800°C and 1140°C is inferior to that in the range of 1300°C to 1500°C. This is because the silicon dioxide (SiO₂) oxide film formed during oxidation at 800°C–1140°C has a loose structure, which fails to provide adequate protection to the substrate. In contrast, when the temperature exceeds 1140°C-especially within the 1300°C–1500°C range-oxidation becomes more pronounced. The thin oxide layer generated at this stage covers the surface of the silicon carbide matrix, preventing oxygen from further contacting the silicon carbide and thereby enhancing its oxidation resistance. Nevertheless, at higher temperatures, the oxide protective layer is damaged, subjecting the silicon carbide to severe oxidation, decomposition, and failure.

Thanks to its excellent physical and chemical properties, silicon carbide is widely used as a critical industrial raw material. Its primary applications fall into three categories: first, manufacturing abrasive grains and abrasive tools; second, producing resistance heating elements such as silicon carbide rods and silicon carbide tubes; and third, fabricating refractory products. As a special refractory material, it is used to make components for blast furnaces, cupolas, and other equipment that suffer from severe scouring, corrosion, and wear in ferrous metal smelting. In non-ferrous metal (zinc, aluminum, copper) smelting, it serves as furnace linings, molten metal delivery pipelines, filters, and crucibles. In aerospace technology, it is applied in rocket engine tail nozzles and high-temperature gas turbine blades. In the silicate industry, it is extensively used as shelf boards for various kilns, muffle furnace linings, and saggers. In the chemical industry, it is adopted as linings for oil-gas generators, petroleum gasifiers, and desulfurization furnaces.

Manufacturing Process of Products

Producing products using pure α-SiC is quite challenging. Due to its high hardness, grinding it into micron-sized fine powder is difficult. Moreover, its particles are plate-like or needle-like. Green bodies pressed from these particles show no significant shrinkage, even when heated close to the decomposition temperature of α-SiC, resulting in poor sintering performance, low densification of the final products, and inferior oxidation resistance.

Therefore, in industrial production, a small amount of spherical β-SiC fine powder and additives are incorporated into α-SiC to obtain dense products. Additives used as binders for the products can be classified into various categories, such as oxides, nitrides, and graphite, including clay, alumina, zircon, mullite, lime, glass, silicon nitride, silicon oxynitride, and graphite.

Binder solutions for forming can consist of one or a mixture of several materials selected from carboxymethyl cellulose, polyvinyl alcohol, lignin, starch, alumina sol, and silica sol. The firing temperature of the green bodies varies depending on the type and dosage of additives, ranging from 1400°C to 2300°C.

For example, a mixture consisting of 70% α-SiC with particle size larger than 44 μm, 20% β-SiC with particle size smaller than 10 μm, and 10% clay, plus an additional 8% of 4.5% lignin aqueous solution, is uniformly blended and then pressed into shape under a pressure of 50 MPa. The pressed green body is fired in air at 1400°C for 4 hours. The resulting product has a bulk density of 2.53 g/cm³, an apparent porosity of 12.3%, and a flexural strength of 30–33 MPa.

The sintering properties of products with different additives are listed in Table 1.

In general, silicon carbide refractories exhibit excellent performance in multiple aspects, such as high strength over a relatively wide temperature range, high thermal shock resistance, outstanding wear resistance, high thermal conductivity, and good chemical corrosion resistance.

Nevertheless, it should be noted that their key drawback lies in poor oxidation resistance, which causes volume expansion and deformation at high temperatures, thereby reducing their service life. To enhance the oxidation resistance of silicon carbide refractories, extensive research has been conducted on the selection of binders.

Clay (including oxides) was initially used as the binder, but it failed to provide effective protection, and the silicon carbide particles were still subject to oxidation and corrosion. In the late 1950s, silicon nitride (Si₃N₄)-bonded silicon carbide refractories emerged as an improved product. These refractories demonstrated excellent oxidation resistance without significant expansion. However, this type of refractory has drawbacks: it is relatively expensive, may suffer sudden damage during repeated heating and cooling cycles, and the network structure of silicon nitride itself is permeable, making it impossible to fundamentally prevent silicon carbide from oxidizing.

In the early 1960s, silicon oxynitride (Si₂ON₂)-bonded silicon carbide refractories were developed. They offer better oxidation resistance than their silicon nitride-bonded counterparts. This is because silicon oxynitride adheres to the silica film on the surface of silicon carbide and reacts with it to form a continuous protective film that is firmly bonded to the silicon carbide matrix. Moreover, this material is reasonably priced, comparable to oxide-bonded silicon carbide refractories.

To produce dense ceramic products of high-purity silicon carbide and maximize the utilization of silicon carbide's inherent properties, self-bonding reaction sintering and hot pressing processes have been developed.

Self-bonded silicon carbide is fabricated by mixing α-SiC with carbon powder, forming the mixture into green bodies through various shaping methods, and then heating the green bodies in a silicon vapor atmosphere. This process silicidizes the carbon powder in the green bodies into β-SiC, which tightly bonds the α-SiC particles together to form dense products. Therefore, self-bonded silicon carbide is essentially a type of α-SiC bonded by β-SiC. This manufacturing process is also known as reaction sintering. A specific example of the process is as follows:

α-SiC powder with various particle size distributions is uniformly mixed with colloidal graphite in a porcelain ball mill drum for 20 hours. Then, an aqueous solution of carboxymethyl cellulose or an alcoholic solution of polyvinyl alcohol is added as a binder. The mixture is molded in a steel die under a pressure of 50–70 MPa. The amount of graphite added significantly impacts the density of the green body. To ensure that the final density of the silicidized silicon carbide product is close to the theoretical value, the green body must achieve the desired density during die pressing. Based on the green body density, the required amount of graphite can be calculated using the following formula:

Where: x - mass fraction of graphite in the silicon carbide batch, unit: %.

The formed green bodies are first slowly dried at 40°C, then dried at 100°C, followed by silicidation reaction sintering. A schematic diagram of the silicidation device is shown in Figure 1.

1-Furnace tube; 2-Crucible cover; 3-Green body; 4-Crucible; 5-Silicon particles; 6-Graphite base

Silicidation can be carried out in a carbon tube furnace under normal atmospheric pressure, with the silicidation temperature needing to be above 2000°C. If conducted in a vacuum furnace at a pressure of 66.65 MPa, the silicidation temperature can be reduced to 1500–1600°C. The particle size of the silicon powder used for generating silicon vapor ranges from 0.991 mm to 4.699 mm. For silicidation under atmospheric pressure, the silicon powder can be loaded into a graphite crucible. However, for silicidation under vacuum conditions, the silicon powder should instead be contained in a boron nitride (BN) crucible. This is because silicon will penetrate graphite and react to form silicon carbide under vacuum conditions, causing the graphite crucible to crack, whereas boron nitride is non-wetting with silicon.

The duration required for silicidation varies depending on the silicidation temperature and the amount of silicon volatilized at that specific temperature. Upon completion of silicidation, there should generally be no residual silicon left in the crucible, as all silicon should have evaporated. Any silicon adhering to the surface of the product due to evaporation can be removed by treatment with hot sodium hydroxide. Self-bonded silicon carbide products exhibit 7 to 10 times the strength of conventional silicon carbide products, along with enhanced oxidation resistance.

In addition to manufacturing silicon carbide products via sintering, the invention of hot-pressing sintering technology has enabled the production of silicon carbide products through hot pressing, which yields superior sintering performance. The hot-pressing process integrates shaping and firing of the green body into a single step: the green body is shaped and sintered simultaneously under high temperature and pressure. This method has been used in powder metallurgy within the metallurgical industry for decades and has gradually been promoted for the production of special refractory materials.

Adopting hot-pressing shaping and sintering can shorten production time, lower sintering temperature, improve the microstructure of products, increase densification, and enhance material performance. Optimal hot-pressing results can be achieved by selecting appropriate process parameters, such as temperature, pressure, and green body particle size. Hot-pressing is particularly useful for manufacturing refractory compounds.

The molds used for hot pressing must withstand temperatures above 1000°C and pressures of several kilonewtons at high temperatures. Therefore, high-strength graphite is generally used as the mold material for manufacturing refractory products. Molds can be heated by radiation heating, high-frequency induction heating, or resistive heating of the mold itself. The pressure applied to the green body can be provided by an oil press or a conventional hydraulic jack.

The primary disadvantages of hot pressing are the limitations it imposes on product shapes and its relatively low production efficiency, which is why this method is not as widely used as reaction sintering. However, hot-pressed products offer significantly better performance. For example, hot pressing at 1350°C under a pressure of 70–90 MPa: if high-temperature α-SiC is used as the raw material, the density will not exceed 96% of the theoretical value. If low-temperature β-SiC is adopted, the hot-pressed density can reach 3.20 g/cm³, which is close to the theoretical value, and the β-SiC will transform into high-temperature α-SiC during the sintering process. The flexural strength of such hot-pressed sintered compacts is 380 MPa at room temperature and 500 MPa at 1370°C. They also demonstrate excellent thermal shock resistance and good oxidation resistance at high temperatures in air.