Why choose pressureless sintering for the preparation of SiC ceramics?

SiCceramics are characterized by high hardness, high strength, high temperature resistance, and corrosion resistance, and are widely used in fields such as aerospace, petrochemicals, and integrated circuits. Given that most Silicon Carbide products are high value-added products with broad market prospects, they have received significant attention from many countries and have continually been a focus of research in the materials science field.

1.Sintering Process: Reactive Sintering vs. Pressureless Sintering

Silicon carbide (SiC) is a compound characterized by strong covalent bonds, which endows the material with properties such as high hardness, high strength, a high melting point, and corrosion resistance. However, these structural characteristics result in a lower diffusion rate during sintering, thus affecting the densification of the material; therefore, methods such as the use of sintering additives and external pressure are necessary to achieve densification. Currently, significant advancements have been made in the research and industrial application of reaction-sintered and pressureless sintered silicon carbide. The reaction-sintering process for silicon carbide is a nearly net shape sintering technique, characterized by minimal shrinkage and dimensional variation during sintering, and it offers advantages such as low sintering temperatures, dense product structures, and low production costs, making it suitable for preparing large and complex-shaped silicon Carbide Ceramic products. However, its shortcomings include a complex pre-processing method for the green body and the presence of contaminants in by-products. Additionally, the usable temperature range of reaction-sintered SiC ceramic materials is affected by the free silicon content in the material; when the temperature exceeds 1400°C, the strength of the material decreases rapidly due to the melting of free silicon.

The pressureless sintering technology of SiC has become very mature, with its advantages lying in the ability to adopt various forming processes, thereby overcoming limitations in product shape and size. With the proper use of additives, higher strength and toughness can be achieved. Furthermore, the pressureless sintering process of SiC is straightforward and suitable for the mass production of ceramic components of different shapes. However, a drawback is that the pressureless sintering SiC powder is more expensive than reaction-sintered SiC, resulting in higher manufacturing costs.

The pressureless sintering process is mainly divided into two types: solid-phase sintering and liquid-phase sintering. Reaction-sintered silicon carbide ceramics exhibit poor high-temperature performance compared to pressureless solid-phase sintered silicon carbide, especially when the temperature exceeds 1400°C, where the bending strength of silicon carbide ceramics sharply declines, and they are not resistant to strong acids and bases. In contrast, silicon carbide ceramics produced by pressureless solid-phase sintering demonstrate significantly better mechanical properties at high temperatures and corrosion resistance against strong acids and bases than reaction-sintered silicon carbide.

Research on the Pressureless Sintering Process

• Solid-state sintering

Solid-phase sintered SiC ceramics operate at high temperatures, but their physical and chemical properties are stable; particularly, their strength does not change at elevated temperatures, providing unique application value.When B and C are added to SiC, B resides at the SiC grain boundaries, with part of B substituting C in SiC to form a solid solution, while C reacts with the SiO2 on the surface of SiC and the impurity Si. These reactions reduce the boundary energy of SiC and increase the surface energy, thereby increasing the sintering driving force and promoting sintering densification. Since the 1990s, the method of preparing pressureless sintered SiC using B and C as additives has been widely applied across various industrial fields.

Advantages: Aside from a small amount of residual C, there is no secondary phase or glass phase at the grain boundaries, the boundaries are clean, and the high-temperature performance is excellent, remaining stable up to 1600°C without significant changes in properties.

Disadvantages: Complete density cannot be achieved; there are typically a few closed pores at the triangular grain boundaries, and grain growth may occur easily at high temperatures.

• Liquid Phase Sintering

The amount of sintering additives added during liquid phase sintering is usually several percent, and a considerable amount of oxides can still be found in the intergranular phase after sintering is complete. Therefore, the fracture mode of liquid phase sintered silicon carbide (SiC) is typically intergranular fracture, demonstrating high strength and fracture toughness. Simultaneously, compared to solid phase sintering, the liquid phase formed during the sintering process effectively reduces the sintering temperature.

The Al₂O₃-Y₂O₃ system is one of the earliest studied and is considered the most attractive system for liquid phase sintering additives in SiC ceramics. This system enables the densification sintering of SiC ceramics at lower temperatures.

1. Samples were subjected to buried burning utilizing a powder bed containing Al₂O₃, Y₂O₃, and MgO. Observations indicated that MgO reacted with the SiO2 on the surface of SiC particles to form a liquid phase, which facilitated sintering densification through particle rearrangement and liquid redeposition processes.
2. Al₂O₃, Y₂O₃, and CaO were used as additives for the pressureless sintering of SiC ceramics. In SiC materials with varying CaO content, the Al₅Y₃O₁₂ phase was formed. As the CaO content increased, the CaY₂O₄ oxide phase also formed within the material. The liquid phases of CaY₂O₄ and Al₅Y₃O₁₂ created rapid permeation pathways at the grain boundaries, improving the sintering characteristics of the material.

Summary

Additives can enhance the density of pressureless sintered SiC ceramics, lower the sintering temperature, alter the microstructure of the material, and improve its mechanical properties. With the continuous in-depth research on additive systems, additives have developed from a single-component system to a multi-component system. In a multi-component additive system, each component has its unique role in enhancing the performance of SiC ceramics; however, the introduction of additives also comes with some noticeable drawbacks. The reactions between additives and SiC can produce gaseous byproducts such as Al2O3 or CO, leading to an increase in the porosity of the material. One of the key research focuses in the future of liquid-phase sintering technology for SiC ceramics will be to reduce the porosity of SiC ceramics and eliminate the impact of additives on the weight loss of the material.