SiC production process for mechanical seals

SiC ceramics production process is briefly described as follows:

A, SiC powder synthesis:

SiC almost does not exist on earth, only found in the meteorite, therefore, the industrial application of SiC powder are synthetic. At present, the main methods of synthesizing SiC powder are:

1, Acheson method:

This is the most widely used synthesis method in the industry, i.e., the mixture of quartz sand and coke heated to about 2500 ℃ high temperature reaction with electricity to produce. Because quartz sand and coke usually contain impurities such as Al and Fe, a small amount of impurities are solidly dissolved in the made SiC. Among them, the less impurities are green, and more impurities are black.

2, chemical method:

In a certain temperature, so that high-purity silicon and carbon black directly react. From this high purity β-SiC powder can be synthesized.

3, thermal decomposition method:

Silicon polymers such as polycarbosilane or trichloromethylsilane undergo decomposition reaction at a temperature range of 1200 to 1500 degrees Celsius, from which submicron β-SiC powder is produced.

4, gas opposite phase method:

SiCl4 and SiH4 and other gases containing silicon and CH4, C3H8, C7H8 and (Cl4 and other gases containing carbon, or CH3SiCl3, (CH3)2 SiCl2 and Si (CH3)4 and other gases containing both silicon and carbon at high temperatures in the reaction, so as to prepare nanoscale β -SiC ultrafine powder.

Silicon carbide ceramics sintering

1, pressureless sintering

In 1974, the U.S. company GE by adding a small amount of B and C in the high-purity β-SiC fine powder at the same time, the use of pressureless sintering process in 2020 ° C to successfully obtain high-density SiC ceramics. At present, this process has become the main method of preparing SiC ceramics. U.S. GE researchers believe that: the ratio of grain boundary energy and surface energy is less than 1.732 is the thermodynamic conditions of densification, when the simultaneous addition of B and C, B solid solution to the SiC, so that the grain boundary energy is reduced, the C to the surface of SiC particles to reduce the removal of SiO2, to improve the surface energy, and therefore the addition of B and C for the densification of SiC to create thermodynamically favorable conditions. However, Japanese researchers have argued that there is no thermodynamic limitation to the densification of SiC. It has also been suggested that the densification mechanism of SiC may be liquid phase sintering, and they found that: in β-SiC sintered bodies with both B and C added, there is a B-rich liquid phase present at the grain boundaries. Regarding the mechanism of pressureless sintering, it is still inconclusive.

Dense sintering of SiC can be similarly achieved by using α-SiC as a raw material and adding B and C simultaneously.

Studies have shown that the use of B and C as additives alone does not contribute to the full densification of SiC ceramics. Only with the addition of both B and C can the densification of SiC ceramics be realized. In order to SiC dense sintering, SiC powder specific surface area should be more than 10m2 / g, and the oxygen content as low as possible. B additions in the 0.5% or so, C additions depend on the SiC raw materials in the oxygen content of the high and low, usually the amount of C additive and the oxygen content in the SiC powder is proportional to.

Recently, some researchers have added Al2O3 and Y2O3 to sub-micron SiC powders to realize dense sintering of SiC at temperatures from 1850°C to 2000°C. The sintering temperature is low and has a significant effect on the sintering temperature. The strength and toughness were greatly improved due to a significantly refined microstructure at a low sintering temperature.

2, hot press sintering

In the mid-50s, the U.S. Norton began to study the effect of B, Ni, Cr, Fe, Al and other metal additives on the SiC hot press sintering. Experiments show that Al and Fe are the most effective additives to promote SiC hot pressing densification.

Some researchers have also achieved densification of SiC by hot-press sintering process with Al2O3 as an additive and concluded that the mechanism is liquid-phase sintering. In addition, other researchers have also obtained dense SiC ceramics by hot-press sintering with B4C, B or B and C, Al2O3 and C, Al2O3 and Y2O3, Be, and B4C and C as additives, respectively.

Studies have shown that the microstructure as well as mechanical and thermal properties of the sintered body vary depending on the type of additives. For example, when B or compounds of B are used as additives, the grain size of hot-pressed SiC is small, but the strength is high. Be is selected as an additive, hot pressing SiC ceramics have a high thermal conductivity.

3, hot isostatic pressing sintering:

In recent years, in order to further improve the mechanical properties of SiC ceramics, researchers have carried out research work on hot isostatic pressing process of SiC ceramics. Using B and C as additives, the researchers used the hot isostatic pressing sintering process to obtain a high-density SiC sintered body at 1900°C. The process was used for the sintering of the SiC ceramics. Further, by this process, dense sintering of additive-free SiC ceramics was successfully realized at 2000 °C and 138 MPa pressure.

Research has shown that when the particle size of SiC powder is less than 0.6 μm, it can be densified by hot isostatic pressure sintering at 1950 ℃ even without introducing any additives. Such as the use of specific surface area of 24m2 / g of SiC ultrafine powder, hot isostatic sintering process, at 1850 ℃ can be obtained in the high density of SiC ceramics without additives.

In addition, Al2O3 is an effective additive for hot isostatic sintering of SiC ceramics. The addition of C does not play a role in the hot isostatic sintering densification of SiC ceramics, and excessive C will even inhibit the sintering of SiC ceramics.

4, reaction sintering:

SiC reaction sintering method was first successfully studied in the United States. Reaction sintering process is: first α-SiC powder and graphite powder mixed in proportion, by dry pressing, extrusion or slurry injection method to make a porous blank. 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, to ensure complete Si penetration, the billet should have sufficient porosity. Appropriate 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 ceramics 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 more, and reaction-sintered SiC is relatively lower. 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 sharply reduced due to) for pressureless sintering and hot isostatic pressure sintering of SiC ceramics, its high temperature performance is mainly affected by the type of additives.

In short, the performance of SiC ceramics due to different sintering methods. Generally speaking, the comprehensive performance of unpressurized sintered SiC ceramics is better than that of reaction-sintered SiC ceramics, but inferior to that of hot-pressure sintered and hot-isostatic pressure sintered SiC ceramics.