Applications and Sintering Methods of Silicon Carbide Ceramic

Silicon carbide(SiC) is high temperature resistant and unresponsive to strong acid and strong alkali, and it has great conductive thermal conductivity and strong ability to resist radiation.

Its excellent physical and chemical properties make it widely used in petroleum, chemical industry, microelectronics, automotive, aerospace, laser, atomic energy, machinery, and metallurgical industry, such as grinding wheel, nozzle, bearing, seal, dynamic and static gas turbine blades and reflective screen substrate, engine parts, refractory materials, etc.

Tanks and armored vehicles composite panels made of silicon carbide composite materials are lighter than ordinary steel tank about 30%-50%, and its impact resistance can be increased 1-3 times, which is an excellent composite materials. High-tech ceramics with special functions (electrical, magnetic, acoustic, optical, thermal, chemical, mechanical, biology, etc.) are new materials in the rapid development of nearly 20 years, which is also called the third largest materials after metal materials and polymer materials.

Silicon carbide (SiC) is the compound of strong covalent bond combination, whose sintering diffusion rate is quite low. According to research results from J. D. Hon and others, even under the high temperature of 2100 ℃, the self diffusion coefficient of C and Si is also small, so the SiC is hard to sinter, and it must use additives or external pressure or siliconizing reaction to help realize densification.

At present, the main methods of producing high density SiC ceramics include hot pressing sintering, pressureless sintering and reaction sintering, etc.

A. The hot pressing sintering

If pure silicon carbide powder goes through hot pressing sintering, it can be close to the theoretical density, but need high temperature (> 2000 ℃) and high pressure (350 MPa). The additive can strongly promote the densification rate, and get silicon carbide materials that close to heoretical density. The common used additives: Al203, AIN, BN, B, etc.; The biggest addition amount of B is 0.36%.

Mechanism: the existence of free carbon and B can generate B4C, again with SiC forming solid solution, and it plays an important role for material migration in the liquid phase sintering process.

B. The pressureless sintering (atmospheric pressure sintering)

The sintering mechanism: Diffusion sintering

Diffusion sintering is related with the ratio between grain boundary and surface energy. The pure SiC can not do the sintering; When boron is added, the boron is on the SiC grain boundary, and part of them can form solid solution with SiC, which can reduce the SiC grain boundary; In addition, adding C can help Si02 membrane on the surface of SiC for reduction, thus increasing the surface energy, which  can make the rg/rs superfine powder provide required mechanical force for densification, shortening the diffusion distance.

C. The reaction sintering

Reaction sintering SiC is also called self-bonding SiC. a - SiC powder and graphite powder mix in a certain ratio and are pressed into green body, to heat up 1650 ℃ or so, and at the same time, infiltrate the body Si or by gas phase Si, reacting with graphite generating β- SiC, combining original a - SiC particles together.

Features: if it is allowed to infiltrate completely, the porosity can be obtained is zero in the whole process, and there are no the materials with changing geometry size.

In practical production, green body need have excess air hole, to prevent the formation of a airtight SiC layer, so as to prevent the reaction sintering, and if the redundant stoma is filled by excess Si in the reaction sintering process, no hole density products will be obtained.

The contrast of three common sintering methods:

A. The hot pressing sintering only can produce simple shaped silicon carbide parts, and the production efficiency is low, so it is not conducive to large-scale commercial production.

B. Pressureless sintering (atmospheric pressure sintering) can produce complex shapes and large sizes of silicon carbide parts, which is now widely recognized the greatest sintering method.

C. The reaction sintering can help produce silicon carbide parts with complex shapes and low sintering temperature, but it can not help get good high temperature performance of products.