Silicon nitride ceramic is an inorganic material ceramic that does not shrink during sintering. Silicon nitride is very strong, especially hot-pressed silicon nitride, which is one of the hardest substances in the world. It has the properties of high strength, low density and high temperature resistance. Si3N4 ceramic is a covalent bond compound, the basic structural unit is [SiN4] tetrahedron, the silicon atom is located in the center of the tetrahedron, and there are four nitrogen atoms around it. There are several methods for the preparation of silicon nitride.
The reaction sintering method adopts a general molding method. First, the silicon powder is pressed into a green body of the desired shape, and placed in a nitriding furnace for pre-nitriding (partial nitriding) sintering. The green body after pre-nitriding has a certain strength and can be processed by various mechanical processes (such as turning, planing, milling, drilling). Finally, at the temperature above the melting point of silicon, the green body is again fully nitrided and sintered to obtain a product with little dimensional change (that is, after the green body is sintered, the shrinkage rate is very small, and the linear shrinkage rate is < 011%). This product can generally be used without grinding. The reaction sintering method is suitable for the manufacture of parts with complex shapes and precise dimensions, and the cost is also low, but the nitriding time is very long.
The hot pressing sintering method is to press Si3N4 powder and a small amount of additives (such as MgO, Al2O3, MgF2, Fe2O3, etc.) under pressure above 1916 MPa and temperature above 1600 ℃ for hot pressing and sintering. Additives and phase composition during sintering have a great influence on product properties. If the composition of the grain boundary phase is strictly controlled, and proper heat treatment is performed after the sintering of Si3N4 ceramics, Si3N4-based ceramic materials that do not significantly decrease in strength (up to 490MPa or more) can be obtained even when the temperature is as high as 1300 °C. Additives and phase composition during sintering have a great influence on product properties. If the composition of the grain boundary phase is strictly controlled, and proper heat treatment is performed after the sintering of Si3N4 ceramics, Si3N4-based ceramic materials that do not significantly decrease in strength (up to 490MPa or more) can be obtained even when the temperature is as high as 1300 °C. The mechanical properties of Si3N4 ceramics produced by pressure sintering are superior to those of reaction sintered Si3N4, and have high strength and density. However, the manufacturing cost is high, the sintering equipment is complex, and the dimensional accuracy of the product is limited to a certain extent. It is difficult to manufacture complex parts, and only parts with simple shapes can be manufactured.
In the aspect of increasing the pressure of the sintering nitrogen atmosphere, this method utilizes the property of increasing the decomposition temperature of Si3N4, and conducts atmospheric pressure sintering in the temperature range of 1700-1800 °C, and then carries out air pressure sintering in the temperature range of 1800-2000 °C. The purpose of this method is to use air pressure to promote the densification of Si3N4 ceramics, thereby improving the strength of the ceramics. The properties of the obtained products are slightly lower than those of hot pressing sintering. The disadvantages of this method are similar to hot pressing sintering.
Gas pressure sintering of silicon nitride is carried out at a temperature of about 2000 ℃ under the pressure of 1 ~ 10MPa, and the high nitrogen pressure inhibits the high temperature decomposition of silicon nitride. Due to the use of high temperature sintering, the addition of less sintering aids 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 > 99%. Therefore, air pressure sintering has been paid more and more attention both in laboratory and in production. Gas pressure sintered silicon nitride ceramics have high toughness, high strength and good wear resistance, and can directly produce various complex shape products close to the final shape, which can greatly reduce production costs and processing costs. And its production process is close to carbide production process, suitable for mass production.
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