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Why choose pressureless sintering to prepare SiC ceramics

SiC ceramics have the characteristics of high hardness, high strength, high temperature resistance, corrosion resistance, etc., and are widely used in aerospace, petrochemicals, integrated circuits and other fields. Since most silicon carbide products are high value-added products, the market prospects are broad.

SiC ceramics’ ultra-high synthesis temperature and difficulty in sintering densely limit its development. The sintering process is very important for SiC ceramics.

Sintering process

SiC is a strong covalent bond compound. This structural feature gives the material high hardness, high strength, high melting point and corrosion resistance. At the same time, it also causes the material to have a low diffusion rate during sintering, which affects the sintering densification of the material. Therefore, it is necessary to use Densification can be achieved by methods such as sintering additives and external pressure. At present, the research and industrial application of silicon carbide reaction sintering and pressureless sintering have achieved great development.

Pressureless sintering processes are mainly divided into solid phase sintering and liquid phase sintering. Compared with pressureless solid phase sintering silicon carbide, reaction sintered silicon carbide ceramics have poorer high temperature resistance, especially the flexural strength of silicon carbide ceramics when the temperature exceeds 1400°C. dropped sharply, and it is not resistant to strong acids and alkalis. The mechanical properties of pressureless solid-phase sintering silicon carbide ceramics at high temperatures and corrosion resistance under strong acids and alkali are much better than those of reaction-sintered silicon carbide.

Research on pressureless sintering process

1.Solid phase sintering

The temperature of solid-phase sintered SiC ceramics is relatively high, but its physical and chemical properties are stable, especially the strength will not change at high temperatures, so it has special application value.

After adding B and C to SiC, B is located on the SiC grain boundary. Part of B replaces C in SiC to form a solid solution. C reacts with SiO₂ and impurity Si on the surface of SiC. The above reaction reduces the grain boundary energy of SiC and increases the surface energy. Increase, thereby increasing the sintering driving force and promoting sintering densification.

Advantages: Except for a small amount of residual C, there is no second phase or no glass phase at the grain boundary. The grain boundaries are clean and the high temperature performance is good. It can be used up to 1600°C with basically unchanged performance.

Disadvantages: It cannot be completely dense. There are usually a small number of closed pores at the triangular grain boundaries of the grains, and high temperatures can easily cause the grains to grow.

2.Liquid phase sintering

The amount of sintering aid added in liquid phase sintering is usually a few percent, and a large amount of oxide will still remain in the intergranular phase after sintering is completed. Therefore, the fracture mode of liquid phase sintered silicon carbide is usually intergranular fracture, which has high strength and fracture toughness. At the same time, compared with solid-phase sintering, the liquid phase formed during the sintering process effectively reduces the sintering temperature.

The Al₂O₃-Y₂O₃ system was the first to be studied and is considered the most attractive SiC ceramic liquid phase sintering additive system. This system can achieve densified sintering of SiC ceramics at lower temperatures.

(1) The samples were buried and burned using a powder bed containing Al₂O₃, Y₂O₃ and MgO. It was observed that MgO reacted with SiO₂ on the surface of SiC particles to form a liquid phase, which promoted sintering densification through particle rearrangement and melt redeposition process.

(2) Al₂O₃, Y₂O₃ and CaO are used as additives for pressureless sintering of SiC ceramics. The Al5Y3O12 phase is formed in SiC materials with different CaO contents. As the CaO content continues to increase, the CaY₂O₄ oxide phase is formed in the material. CaY₂O₄ and The liquid phase of Al₅Y₃O₁₂ forms a rapid penetration path in the grain boundaries, improving the sinterability of the material.