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SiC Ceramic Sintering Technology

Sic is a typical stable compound bonded by covalent bonds. With its low dispersion coefficient, it is difficult to achieve densification by conventional sintering methods. It is necessary to add some sintering aids to reduce the surface energy. Or increase the surface area, and use a special process to obtain dense SiC ceramics.

1. Hot Press Sintering

Hot press sintering is currently the most widely used rapid sintering method for manufacturing silicon carbide and its composite engineering devices. Hot press sintering with additives can strongly promote the densification rate, and can obtain materials close to the theoretical density, which significantly improves product performance. The additives currently used are similar to those used for atmospheric sintering. Liquid phase sintering occupies an important position in the sintering of SiC and its composite materials. The material used in the hot-pressing sintering mold is usually high-purity graphite, the pressure limit is 60MPa, and the pressure is usually about 30MPa. When the pressure is higher than the above value, the Al2O3 mold is used, and the high-frequency induction method is almost used for heating.

2. Hot Isostatic Pressing Sintering

The high-temperature hot isostatic pressing method uses glass or metal foil (low carbon steel, nickel, etc.) as the mold, and H2 or inert gas as the pressurizing medium. Into a non-dense body with a certain shape, and then perform high-temperature isostatic pressing sintering, without the need for metal foil molds, this is the non-wrapped technology. In the hot isostatic pressing sintering process, when Ar or N2 is used as the medium gas, the temperature is about 100C lower than the sintering temperature. There are two kinds of pressurization sequence: one is to pressurize to about 60% of the required final pressure, and then increase the pressure in the cylinder to the final pressure; the other is to increase the temperature first and then pressurize. The key to hot isostatic pressing sintering technology is the selection of sheathing materials.

The basic requirements for the package are:

①Good high temperature performance, does not react with SiC or composite components at the sintering temperature:

②Good deformability, using melting type sheathing material, the degree of tolerance should be large enough to be silent and sintered. The glass encapsulation effect is good, the cost is low, it is easy to form, and it is convenient to directly prepare products with complex shapes, and basically does not require mechanical processing. It is of great significance for the preparation of siC and its composite materials.

Adopting glass sheath, divided into inner and outer layers, the inner layer is made of high melting point glass, and the outer layer is made of low melting point glass. The thicknesses are respectively

It is 0.05mm, 1mm, the high pressure of hot isostatic pressing can reach thousands of atmospheres, so that SiC ceramics or their composite materials that cannot be sintered under normal pressure can be sintered at extremely low temperatures.