Silicon carbide ceramicsThe material has high temperature strength, high temperature oxidation resistance, good wear resistance, thermal stability, small coefficient of thermal expansion, high thermal conductivity, high hardness, thermal shock resistance and chemical corrosion resistance. It has been widely used in automobile, mechanochemistry, environmental protection, space technology, information electronics, energy and other fields. It has become an irreplaceable structural ceramic with excellent performance in many industrial fields.

The excellent properties of SiC ceramics are closely related to their unique structures.

SiC is a compound with strong covalent bond. The ionic property of Si-C bond in SiC is only about 12%. Therefore, SiC has high strength, large modulus of elasticity and excellent wear resistance. Pure SiC will not be eroded by acid solutions such as HCl, HNO3, H2SO4 and HF, and alkali solutions such as NaOH. It is easy to oxidize when heated in the air, but the SiO2 formed on the surface will inhibit the further diffusion of oxygen, so the oxidation rate is not high. In terms of electrical properties, SiC has semiconductivity, and the introduction of a small amount of impurities will show good conductivity. In addition, SiC has excellent thermal conductivity.

The sintering process of SiC ceramics is described as follows:

Pressureless sintering

Pressureless sintering is considered to be the most promising sintering method for SiC sintering. According to the different sintering mechanism, pressureless sintering can be divided into solid-phase sintering and liquid-phase sintering. S. proehazka sintered SiC with density higher than 98% at 2020 ℃ under atmospheric pressure by adding proper amount of B and C into ultrafine β - SiC powder (oxygen content less than 2%). A. Mulla et al. Sintered 0.5 μ m β - SiC at 1850-1950 ℃ with Al2O3 and Y2O3 as additives (the particle surface contains a small amount of SiO2). The relative density of SiC ceramics obtained is greater than 95% of the theoretical density, and the grains are fine with an average size of 1.5 μ M.

Hot pressing sintering

Without any sintering aids, pure SiC can be sintered compactly only at extremely high temperature, so many people carry out hot pressing sintering process for SiC. There have been many reports on the hot pressing sintering of SiC by adding sintering aids. Alliegro et al. Studied the effect of B, Al, Ni, Fe, Cr and other metal additives on the densification of SiC, and found that Al and Fe are the most effective additives to promote the hot pressing sintering of SiC. F.f.lange studied the effect of different amount of Al2O3 on the properties of hot pressed SiC. It is believed that the densification of hot pressed SiC depends on the mechanism of dissolution and re precipitation. However, the hot pressing sintering process can only produce SiC parts with simple shape, and the number of products prepared in one hot pressing sintering process is very small, so it is not conducive to industrial production.

Hip sintering

In order to overcome the defects of the traditional sintering process, Duna used B and C as additives and hot isostatic pressing sintering process to obtain fine-grained SiC ceramics with a density of more than 98% and room temperature bending strength of about 600MPa at 1900 ℃. Although the dense SiC products with complex shape can be obtained by hot isostatic pressing sintering, and the products have better mechanical properties, but hip sintering must pack the blank, so it is difficult to realize industrial production. 

Reaction sintering

Reactive sintering SiC, also known as self-bonding SiC, is a process in which a porous blank undergoes a chemical reaction with the gas or liquid phase to increase the quality of the blank, reduce the pores, and sinter into a finished product with a certain strength and dimensional accuracy. It is formed by mixing α-SiC powder and graphite into a green body at a certain ratio, and heating it to about 1650 ℃, and simultaneously infiltrating Si or infiltrating the green body through gas-phase Si to react with graphite to generate β-SiC. The existing α-SiC particles are combined. If Si infiltration is complete, a fully dense, reactive sintered body without size shrinkage can be obtained. Compared with other sintering processes, the sintering of reactive sintering has a small dimensional change during the compaction process and can produce precise size products. However, the presence of a considerable amount of SiC in the sintered body makes the reactive sintered SiC ceramics poor in high temperature performance.

SiC ceramics using pressureless sintering, hot press sintering, hot isostatic sintering and reaction sintering have different performance characteristics. For example, in terms of sintered density and flexural strength, hot-pressed sintered and hot-isostatic-pressed sintered SiC ceramics are relatively large, and reactive sintered SiC is relatively low. On the other hand, the mechanical properties of SiC ceramics also vary with the sintering additives. Pressureless sintering, hot-pressing sintering and reaction-sintered SiC ceramics have good resistance to strong acids and alkalis, but reaction-sintered SiC ceramics have poor corrosion resistance to super-strong acids such as HF. In terms of high-temperature resistance comparison, when the temperature is lower than 900 ° C, the strength of almost all SiC ceramics is improved; when the temperature exceeds 1400 ° C, the flexural strength of the reaction-sintered SiC ceramics decreases sharply. (This is because the sintered body contains a

The four sintering methods of SiC ceramics have their own advantages, but with the rapid development of science and technology, it is urgent to improve the performance of SiC ceramics, improve the manufacturing technology, reduce the production cost, and realize the low-temperature sintering of SiC ceramics. In order to reduce energy consumption, reduce production costs and promote the industrialization of SiC ceramic products.