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Excellent properties and sintering methods of silicon carbide ceramic

Silicon carbide ceramic is based on silicon carbide raw materials through a variety of processes made of a special ceramics, the following understanding of the excellent characteristics of silicon carbide ceramics.
Silicon carbide ceramic are mainly oxidation resistance, electrical conductivity, high hardness, high thermal conductivity and so on.
(1) oxidation resistance of silicon carbide ceramic at 800-1140 degrees when the oxidation resistance is poor, at 1140-1600 degrees when the oxidation resistance is very good, higher than 1750 degrees, the oxide film is destroyed, the oxidation resistance decreases sharply.
(2) conductivity Pure silicon carbide is an electrical insulator, but silicon carbide ceramic contain a variety of impurities, so it has some conductive properties.
(3) High hardness The hardness properties of sic ceramic, is determined by the silicon carbide itself, with the increase in temperature, the hardness of silicon carbide ceramics does not decline.
(4) Thermal conductivity At room temperature, the thermal conductivity of sic ceramic is quite high.

The reaction bonded silicon carbide was successfully researched in the United States earlier. The process of reaction sintering is as follows: first mix α-SiC powder and graphite powder in proportion, and then make a porous billet by dry pressing, extrusion or slurry injection. In contact with liquid Si at high temperature, the C in the billet reacts with the infiltrated Si to generate β-SiC, which combines with α-SiC, and the excess Si fills in the pores, thus obtaining a non-porous and dense reaction sintered body. Reaction sintered SiC usually contains 8% free Si, therefore, in order to ensure the complete penetration of Si, the billet should have sufficient porosity. The proper density of the billet is generally obtained by means of adjusting the contents of α-SiC and C in the initial mix, the particle size gradation of α-SiC, the shape and particle size of C, and the molding pressure.

Experiments have shown that SiC ceramic sintered by unpressurized sintering, hot pressurized sintering, hot isostatic pressure sintering and reaction sintering have different performance characteristics. For example, in terms of sintering density and flexural strength, hot-pressure sintered and hot-isostatic sintered SiC ceramics are relatively high, and reaction bonded silicon carbide is relatively low. On the other hand, the mechanical properties of SiC ceramics also vary with the sintering additives. The unpressurized sintered, thermostatically pressurized sintered and reaction sintered SiC ceramics have good resistance to strong acids and bases, but the reaction sintered SiC ceramics have poor resistance to super-strong acids such as HF. As far as the comparison of high-temperature resistance is concerned, when the temperature is lower than 900°C, the strength of almost all SiC ceramics increases; when the temperature exceeds 1400°C, the flexural strength of reaction-sintered SiC ceramics decreases sharply. (This is due to the sintered body contains a certain amount of free Si, when more than a certain temperature bending strength falls sharply due to) for pressureless sintering and hot isostatic pressure sintering of SiC ceramic, its high temperature resistance is mainly affected by the type of additives.

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