Research Progress and Application Prospects of Sintering Processes for Silicon Carbide Ceramic Materials

1.1 Silicon Carbide Crystal Structure and Sintering Characteristics

Silicon Carbide (SiC) is a compound formed by silicon and carbon bonded covalently. Its crystal structure is primarily composed of silicon-carbon tetrahedra, exhibiting strong covalent bond characteristics (with an ionicity of only 0.2) and extremely low diffusion coefficients. At 2100°C, the self-diffusion coefficients for carbon and silicon are merely 1.5×10⁻¹⁰ cm²/s and 2.5×10⁻¹³ cm²/s, respectively. These structural characteristics make silicon carbide exceedingly difficult to sinter, requiring specialized sintering aids or particular process conditions to achieve densification.

The sintering process of SiC ceramics essentially involves high-temperature treatment to form strong bonds between powder particles, reduce porosity, increase density, and thereby enhance mechanical strength and properties. The typical presence of an oxide layer on the SiC surface further complicates sintering. To address the challenge of sintering SiC, researchers have developed various sintering techniques, including traditional methods such as reaction bonding, pressureless sintering, hot pressing, and hot isostatic pressing, as well as emerging technologies like Spark Plasma Sintering (SPS) and Oscillatory Pressure Sintering (OPS).

1.2 Major Sintering Processes and Progress

1.2.1 Reaction Bonding Process
Reaction-Bonded Silicon Carbide (RB-SiC) involves a chemical reaction between liquid or vapor silicon and carbon within a preform. The in-situ generated β-SiC bonds with the original SiC particles in the preform to form the RB-SiC ceramic material. This process typically first mixes α-SiC powder and graphite powder in a specific ratio to create a porous preform via methods like dry pressing, extrusion, or slip casting. The preform is then exposed to liquid silicon at high temperatures. The carbon within the preform reacts with the infiltrating silicon to generate β-SiC, which bonds with the original α-SiC, while the remaining silicon fills the pores, achieving densification.

RB-SiC usually contains about 8% free silicon. To ensure complete silicon infiltration, the preform must possess sufficient porosity, generally controlled by adjusting factors like the content of α-SiC and carbon in the mixture, the particle size distribution of α-SiC, the shape and size of carbon particles, and the forming pressure. Significant advantages of RB-SiC include low sintering temperature, low sintering cost, and near-net-shape forming. Its strength improves somewhat below 900°C; however, when the temperature exceeds 1400°C, the flexural strength drops sharply due to the presence of free silicon within the sintered body. Additionally, RB-SiC exhibits poor corrosion resistance to superacids like HF.

In terms of application, RB-SiC has become an ideal material for space mirrors. For instance, the large-sized ceramic sealing rings produced by Ningbo Volken Technology Co., Ltd., and the high-performance mechanical sealing systems they contributed to designing, were applied as a core key component in the large radars constituting the deep-space measurement and control network, assisting the Chang'e-5 lunar exploration mission. This marked China's first successful manufacture of high-parameter, 500mm-diameter mechanical sealing products capable of meeting the demands of large-scale radar applications.

1.2.2 Pressureless Sintering Process
Pressureless Sintering is considered the most promising method for sintering SiC. This method is compatible with various forming processes, has relatively low production costs, is not limited by shape or size, and is the most common and easily scalable sintering approach for batch operations. Pressureless sintering involves adding boron and carbon to β-SiC containing trace oxygen. Sintering at around 2000°C in an inert atmosphere can yield SiC sintered bodies with up to 98% of theoretical density.

Pressureless sintering is generally divided into two methods: solid-phase sintering and liquid-phase sintering. Solid-phase sintering typically involves adding boron and carbon. Boron dissolves into silicon carbide, lowering the grain boundary energy at SiC interfaces, while carbon reduces and removes SiO₂ from the SiC surfaces, increasing surface energy. Liquid-phase sintering employs single or multiple low-melting-point oxides as sintering aids. At lower temperatures, the low eutectic point generates a liquid phase that promotes the movement, diffusion, and mass transfer of SiC particles, achieving densification. Commonly used oxide additives include Al₂O₃, Y₂O₃, MgO (Note: corrected from likely typo 'Mg₂O₃'), BeO (Note: corrected from likely typo 'Be₂O₃'), and nearly all rare-earth oxides.

Pressurelessly sintered SiC offers high density, high purity, unique high thermal conductivity, excellent high-temperature strength, and is easily machined into large-sized, complex-shaped ceramic components. Research indicates that by adding 2% carbon and 1% boron as sintering aids, SiC wear-resistant materials with high abrasion resistance can be obtained at 2150°C. Researchers like Wang Shuang and Sun Qinli from the Shenyang Institute of Metal Research, Chinese Academy of Sciences, prepared Al₂O₃/SiC nanocomposite ceramics with a sintering density reaching 98.8%, flexural strength of 89 MPa, and fracture toughness of 6.67 MPa.

Pressurelessly sintered SiC is widely used in wear- and corrosion-resistant sealing rings, sliding bearings, etc. Furthermore, due to its high hardness, low specific gravity, excellent ballistic performance, ability to absorb more energy upon fragmentation, and relatively low cost, it is extensively applied in ballistic armor for vehicle and vessel protection, as well as in civilian applications such as safes and armored cash trucks.

1.2.3 Hot Pressing Sintering Process
Hot Pressing is a process where sintering and forming occur simultaneously under applied pressure and heating. Dry SiC powder is filled into a high-strength graphite mold. During the heating process, a certain pressure (typically 20-50 MPa) is maintained, ultimately achieving both shaping and sintering concurrently.

Since heating and pressurization are simultaneous, the powder is in a thermoplastic state, which aids processes like particle contact diffusion and flow mass transfer. This enables the production of SiC ceramic products with fine grains, high relative density, and good mechanical properties at lower sintering temperatures and shorter times compared to other methods. Hot-pressed SiC can achieve full densification, approaching a purely sintered state.

Sintering aids such as boron, carbon, B₄C, aluminum, Y₂O₃, and Al₂O₃ are often introduced during hot pressing. Researchers like Jiang Dongliang from the Shanghai Institute of Ceramics used B₄C and carbon as sintering aids in a hot pressing process at 2050°C to obtain SiC sintered bodies with strengths up to 500 MPa and densities reaching 3.56 g/cm³.

Hot-pressed SiC was initially used in the 1960s during the Vietnam War as body armor for U.S. helicopter crews. However, with technological advancements, the market for ultra-high-performance armor ceramics based on hot-pressed SiC has largely been surpassed by hot-pressed boron carbide, which has become the top-tier product in the armor market. Besides armor, hot-pressed SiC is also applied in wear-resistant and nuclear industry fields.

1.2.4 Hot Isostatic Pressing Sintering Process
Hot Isostatic Pressing (HIP) refers to sintering conducted within a HIP reaction vessel as the main apparatus by controlling temperature (approximately 1000–2000°C) and vessel pressure (using inert gas as the pressure medium, around 200 MPa) under suitable temperature and pressure conditions.

As the HIP process occurs in a relatively uniform environment, the resulting SiC ceramics have a uniform structure and excellent properties. Dutta et al., using boron and carbon as sintering aids and the HIP process at a temperature of 1900°C with appropriate time and pressure, obtained wear-resistant SiC sintered bodies close to theoretical density (98%) with flexural strengths as high as 600 MPa. Through further research, Dutta also successfully prepared silicon carbide wear-resistant materials without sintering aids via HIP by controlling temperature and pressure at 2000°C and 138 MPa, respectively.

The main disadvantages of HIP are its high production cost, significant influence of temperature and pressure on its properties, and its unsuitability for manufacturing complex-shaped products, which limits its application scope.

1.2.5 Emerging Sintering Processes
Apart from the traditional sintering methods mentioned above, various new sintering technologies have emerged in recent years, including Spark Plasma Sintering (SPS) and Oscillatory Pressure Sintering (OPS). These emerging sintering techniques further improve the sintering activity and final properties of SiC ceramics by introducing additional energy fields or special pressure modes during the sintering process.

Spark Plasma Sintering passes pulsed direct current directly through the mold and powder, generating plasma that activates particle surfaces, significantly increasing the sintering rate. This allows for obtaining high-density SiC ceramics at lower temperatures and in shorter times. Oscillatory Pressure Sintering applies oscillating pressure rather than constant pressure during sintering, helping to break up particle agglomerations, promote particle rearrangement and plastic flow, thereby enhancing material densification and mechanical properties.

These emerging sintering technologies offer new possibilities for preparing SiC ceramics, showing unique advantages, particularly in fabricating nanostructured SiC ceramics and complex-shaped components.

2 Structural Analysis

2.1 Influence of Different Sintering Processes on Material Structure
SiC ceramics prepared by different sintering processes exhibit significant differences in microstructure, crystal morphology, and phase composition. Reaction-bonded SiC consists of original α-SiC particles, reaction-generated β-SiC, and free silicon filling the pores, typically containing about 8% free silicon. This structure shows good mechanical properties at room temperature, but strength drops sharply when the temperature exceeds 1400°C due to the presence of free silicon.

Pressurelessly sintered SiC can be divided into two microstructures: solid-phase sintered and liquid-phase sintered. Solid-phase sintered SiC has clear grain boundaries and uniform grain size. Liquid-phase sintered SiC shows significant structural improvement: the fracture mode shifts from transgranular to intergranular; grains are fine, uniform, and equiaxed; and simultaneously, the material's strength and toughness are notably enhanced.

Hot-pressed SiC, sintered under pressure, exhibits some grain orientation along the pressure direction, with smaller grain size and high density. HIPed SiC, sintered under uniform pressure and temperature fields, has a more uniform structure and less anisotropy.

Emerging SPS and OPS SiC typically possess finer grain sizes and more homogeneous microstructures, as these methods effectively inhibit grain growth while promoting densification.

2.2 Analysis of Material Properties and Application Scenarios
SiC ceramics obtained via different sintering processes possess varied properties, making them suitable for different application scenarios. Experiments show that SiC ceramics produced by pressureless sintering, hot pressing, HIP, and reaction bonding exhibit distinct performance characteristics. In terms of sintered density and flexural strength, hot-pressed and HIPed SiC ceramics are relatively higher, while reaction-bonded SiC is relatively lower.

2.2.1 High-Temperature Application Fields
The high-temperature mechanical properties of SiC ceramics are the best among known ceramic materials. For materials produced by hot pressing, pressureless sintering, and HIP, high-temperature strength can be maintained up to 1600°C, making them the best-performing ceramics at high temperatures. Their oxidation resistance is also the best among all non-oxide ceramics. These characteristics make SiC ceramics perform excellently in high-temperature applications such as kiln furniture, heat exchangers, and burner nozzles. For example, reaction-bonded SiC has become a typical application for high-temperature kiln furniture, radiant tubes, heat exchangers, desulfurization nozzles, etc.

2.2.2 Aerospace and Defense Fields
In the aerospace field, SiC ceramics are highly favored due to their high specific stiffness, low coefficient of thermal expansion, and excellent thermal stability. Dedicated SiC ceramic materials have an extremely low thermal deformation coefficient, possess stiffness far exceeding that of glass materials, and achieve significant weight reduction. They can meet the stringent requirements of maintaining surface shape accuracy to within nanometers under the severe mechanical vibrations during satellite launch.

In the defense field, SiC ceramics are widely used in armor protection systems. Pressurelessly sintered SiC ceramics are extensively applied in ballistic armor for vehicles and vessels due to their high hardness, low specific gravity, excellent ballistic performance, ability to absorb more energy upon fragmentation, and relatively low cost. As a ballistic armor material, it offers good multi-hit resistance, with overall protective performance superior to ordinary SiC ceramics.

2.2.3 Semiconductor and Electronics Industry
In the semiconductor industry, SiC ceramics are widely used due to their high purity, high thermal conductivity, and good insulation properties. With increasing wafer sizes and heat treatment temperatures, reaction-bonded SiC has gradually replaced quartz glass. Using high-purity SiC powder and high-purity silicon, high-purity SiC components containing a silicon phase can be produced and are extensively used as support fixtures in electron tube and semiconductor wafer manufacturing equipment.

In recent years, silicon-infiltrated silicon carbide (SiSiC) ceramics have found increasingly widespread application in the semiconductor field. Due to silicon infiltration, SiSiC ceramics have low porosity, greatly ensuring airtightness; furthermore, silicon doping increases the free carrier concentration in the material, resulting in lower resistivity than SiC, which is beneficial for eliminating static electricity on parts. Their preparation process and characteristics favor the production of large, complex-shaped parts or hollow structures, leading to broader application in semiconductor processing equipment, etc.

2.2.4 Environmental Protection and Energy Fields
In the environmental protection field, SiC ceramic membranes show great potential in water treatment due to their excellent chemical stability and fouling resistance. Recently, the MBR system (Membrane Bio-Reactor) of a 150-ton/day industrial wastewater treatment project in Singapore, utilizing SiC ceramic membranes to replace traditional organic membranes, officially commenced operation, achieving advanced treatment of industrial wastewater within limited space.

In the energy field, Volken has provided key特种 (special) ceramic bearings for the "Hualong One" nuclear power unit and supplied high-performance特种 mechanical sealing products for deep-sea drilling platforms and shale gas development projects. In 2023, the market size of SiC ceramics in China reached approximately 8.99 billion yuan, indicating its broad application prospects in the energy field.

3 Conclusion

1. This paper systematically studied various sintering processes of silicon carbide ceramic materials and their influence on material structure and properties. The research results indicate:

2. There are diverse sintering processes for SiC ceramics, including traditional methods such as reaction bonding, pressureless sintering, hot pressing, and hot isostatic pressing, as well as emerging technologies like spark plasma sintering and oscillatory pressure sintering. Each sintering process has its unique advantages and applicable scenarios. Selecting the appropriate sintering method is crucial for meeting specific application requirements.

3. SiC ceramics obtained through different sintering processes exhibit significant differences in microstructure and properties. Reaction-bonded SiC contains free silicon, limiting its high-temperature performance but offering lower cost; pressurelessly sintered SiC facilitates complex shape forming and is suitable for mass production; hot-pressed and HIPed SiC offer high density and strength but at higher costs; emerging sintering technologies show promise for breakthroughs in preparing nanostructured and complex-shaped components.

4. SiC ceramics have wide applications in traditional industrial fields such as chemical engineering, metallurgy, machinery, energy, and environmental protection, as well as in modern technological fields like semiconductors, optoelectronics, aerospace, and defense. With continuous advancements in preparation technology and cost reduction, the application fields of SiC ceramics are expected to further expand.