Sinterización de cerámica de carburo de silicio
Silicon Carbide (SiC) Ceramics
Silicon carbide (SiC) ceramics hold a significant position among high-temperature structural ceramics due to their low thermal expansion coefficient, high thermal conductivity, high hardness, and excellent thermal and chemical stability. They are widely used in fields such as aerospace, nuclear energy, military applications, and semiconductors.
Due to the strong covalent bonding and low diffusion coefficient of SiC, achieving full densification of SiC ceramics is challenging. To address this, various sintering techniques for SiC ceramics have been developed, including reaction bonding, pressureless sintering, hot pressing, and recrystallization sintering. These techniques each have unique advantages, resulting in SiC ceramics with differences in microstructure, properties, and applications. But can you distinguish between these different types of silicon carbide ceramics?
Pressureless Sintered SiC Ceramics (SSiC)
Pressureless sintering of silicon carbide involves densification at high temperatures without applying external pressure, often under inert gas, by adding appropriate sintering aids. This technique has matured, offering low production costs and no limitations on the shape or size of the products. Notably, pressureless sintered SiC ceramics produced through solid-state sintering boast high density, uniform microstructure, and excellent comprehensive properties.
Pressureless sintering is further categorized into solid-state sintering and liquid-phase sintering:
1、Solid-state sintered SiC offers strong density, high purity, unique high thermal conductivity, and excellent high-temperature strength. It is especially suitable for manufacturing large, complex-shaped ceramic components.
2、Liquid-phase sintering, developed by U.S. scientist Muua M.A. in the early 1990s, uses Y₂O₃-Al₂O₃ as the primary sintering additives. This method allows for lower sintering temperatures and produces ceramics with finer grains.
In applications, pressureless sintering is straightforward, cost-effective, and ideal for mass production of ceramic components of various shapes. It is widely used in wear-resistant and corrosion-resistant sealing rings, sliding bearings, and more. Additionally, due to its high hardness, low density, excellent ballistic performance, and ability to absorb energy upon fracture, pressureless sintered SiC ceramics are extensively applied in ballistic armor, including vehicle and ship protection, as well as in civilian safes and armored cash transport vehicles.
Reaction Bonded SiC Ceramics (SiSiC)
Reaction bonded silicon carbide is an attractive structural ceramic with excellent mechanical properties such as high strength, corrosion resistance, and oxidation resistance. It also offers the advantages of low sintering temperature, low cost, and near-net-shape forming.
The reaction bonding process involves mixing carbon sources with SiC powders to prepare a green body. At high temperatures, molten silicon infiltrates the porous body under capillary action, reacting with the carbon source inside to form β-SiC, which bonds closely with the original α-SiC. Residual pores are filled with liquid silicon, achieving densification. This process allows near-net-shape forming, enabling the fabrication of complex-shaped products.
Applications of reaction bonded SiC ceramics include high-temperature kiln components, radiant tubes, heat exchangers, and desulfurization nozzles. Furthermore, due to its low thermal expansion coefficient, high elastic modulus, and near-net-shape forming capability, reaction bonded SiC is an ideal material for space mirrors. With increasing wafer sizes and higher thermal treatment temperatures, reaction bonded SiC is gradually replacing quartz glass.
Hot-Pressed SiC Ceramics (HP-SiC)
Hot-pressing is a sintering process that simultaneously applies high temperature and pressure. Dried SiC powders are filled into high-strength graphite molds, and under heating and pressure, densification and shaping occur simultaneously.
The simultaneous heating and pressing process promotes particle contact, diffusion, and mass transfer in the material, enabling the production of fine-grained, highly dense SiC ceramics with excellent mechanical properties at lower temperatures and shorter sintering times. Hot-pressed SiC ceramics can achieve full densification, approaching pure sintering conditions. Applications primarily include wear-resistant and nuclear industry components.
Recrystallized SiC Ceramics (R-SiC)
Recrystallized SiC ceramics are produced by mixing coarse and fine SiC powders in specific proportions, followed by sintering green bodies at high temperatures under an inert atmosphere. During this process, fine particles evaporate into a gas phase and condense at the contact points of coarse particles, forming R-SiC ceramics. This method, requiring no sintering additives, is widely used for manufacturing ultra-high purity, large SiC components.
R-SiC, formed at high temperatures, has hardness second only to diamond and retains many superior properties of SiC, such as high strength at elevated temperatures, excellent corrosion resistance, oxidation resistance, and thermal shock resistance. As a result, it is ideal for high-temperature kiln components, heat exchangers, and burner nozzles. In the aerospace and military sectors, recrystallized SiC is used to manufacture structural components of aircraft, such as engines, tail fins, and fuselages. Its superior mechanical properties, corrosion resistance, and impact resistance significantly enhance the performance and lifespan of aerospace vehicles.
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