How is silicon nitride ceramics sintered and formed?

From the previous two articles, we have learned about some characteristics and application fields of silicon nitride. So how is silicon nitride ceramics sintered and formed? The answer from Kezhong Ceramic Factory to this question is as follows:

Silicon nitride ceramic is a high-temperature, high-strength structural ceramic with broad development prospects. It has high strength, good thermal shock stability, high fatigue toughness, high room temperature bending strength, good wear resistance, oxidation resistance, and corrosion resistance, and has been widely used in various industries.

Preparation method: The preparation technology of silicon nitride ceramics has developed rapidly. Due to Si3N4 being a strong covalent compound, its diffusion coefficient, volume diffusion and grain boundary diffusion speed required for densification, and sintering driving force are very small. This determines that pure silicon nitride cannot achieve densification through conventional solid-state sintering. At present, the main sintering methods for silicon nitride ceramics include: atmospheric pressure sintering, reaction sintering, hot pressing sintering, gas pressure sintering, etc.

1: Normal pressure sintering: Normal pressure sintering is prepared by mixing high-purity, ultra-fine, and high a-phase content silicon nitride powder with a small amount of sintering aid through processes such as forming and sintering. Due to the difficulty in preparing high-density pure silicon nitride materials using atmospheric pressure sintering method, in order to obtain high-performance silicon nitride materials, sintering aids need to be added to react with SiO2 on the surface of Si3N4 powder, forming a liquid phase at high temperature, activating the sintering process, and making it dense through dissolution precipitation mechanism. Therefore, the key to the study of atmospheric pressure sintering of Si3N4 lies in selecting appropriate sintering aids. At present, the commonly used sintering aids mainly include: Mg0, Y203, rare earth oxide, composite sintering aids, etc. These sintering aids can control the liquid viscosity, improve the phase transformation, prevent the formation of solid solution, reduce the content of lattice oxygen, and control the composition and content of glass phase

2: Gas pressure sintering method: In recent years, people have conducted a lot of research on gas pressure sintering and made great progress. Air pressure sintering of silicon nitride is carried out at a pressure of 1-10MPa and a temperature of around 2000C. The high nitrogen pressure inhibits the high-temperature decomposition of silicon nitride. Due to the use of high-temperature sintering, with less addition of sintering additives, it is sufficient to promote the growth of Si3N4 grains and obtain high toughness ceramics with in situ growth of long columnar grains with a density of>99%. Therefore, air pressure sintering has received increasing attention in both laboratory and production. Air pressure sintered silicon nitride ceramics have high toughness, high strength, and good wear resistance, and can directly produce various complex shaped products close to the final shape, This can significantly reduce production costs and processing costs, and its production process is similar to that of hard alloy production, suitable for large-scale production.

3: Reaction sintering method: It adopts a general molding method, which first compresses the silica powder into a green body of the desired shape, and puts it into a nitriding furnace for pre nitriding (partial nitriding) sintering treatment. The pre nitrided green body has a certain strength and can be used for various mechanical processing (such as turning, creating, milling, drilling). Finally, it is above the temperature of the silicon melting point; Completely nitriding and sintering the green body again to obtain a product with minimal size change (i.e., after sintering, the shrinkage rate is very small, and the linear shrinkage rate is<011%). This product generally does not require grinding and processing before use. The reaction sintering method is suitable for manufacturing parts with complex shapes and precise dimensions, and has low cost, but the nitriding time is long.

4: Hot pressing sintering method: It involves combining Si3N4 powder with a small amount of additives (such as MgO, AI2O3, MgF2, Fe203, etc.), Hot pressing sintering is carried out at a pressure above 1916MPa and a temperature above 1600 "℃. Some companies in the UK and the United States use hot pressing sintering of Si3N4 ceramic ceramics, which have a strength of over 981MPa. The additives and phase composition during sintering have a significant impact on product performance. Due to strict control of grain boundary phase composition and appropriate heat treatment after sintering of Si3N4 ceramic ceramics, strength can be obtained even at temperatures up to 1300C The Si3N4 ceramic material (up to 490MPa) will not significantly decrease, and its creep resistance can be increased by three orders of magnitude. If 14001500 is applied to Sj3N4 ceramic material The high-temperature pre oxidation treatment at ℃ forms Si2N2O phase on the surface of ceramic materials, which can significantly improve the oxidation resistance and high-temperature strength of Si3N4 ceramics. The mechanical properties of Si3N4 ceramics produced by hot pressing sintering method are superior to those of Si3N4 produced by reaction sintering, with high strength and density. However, the manufacturing cost is high and the sintering equipment is complex. Due to the large shrinkage of the sintered body, the dimensional accuracy of the product is limited, making it difficult to manufacture complex parts and can only manufacture simple shapes The mechanical processing of parts and products is also more difficult.

5: Other production methods: Resintering involves placing reaction sintered Si3N4 billets in the presence of sintering aids in silicon nitride powder and re sintering at high temperatures to obtain dense Si3N4 products. Resintering encapsulates a mixture of silicon nitride and sintering aids in a metal or glass package, vacuuming and sintering at high temperatures using high-pressure gas to obtain silicon nitride ceramics with theoretical density, but the process is complex and costly. In addition, in recent years, sintering and densification processes such as ultra-high pressure sintering, chemical vapor deposition, and explosive forming have also been developed, achieving good results

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