Silicon carbide ceramics have excellent properties such as high temperature strength, high temperature oxidation resistance, good wear resistance, good thermal stability, low thermal expansion coefficient, high thermal conductivity, high hardness, thermal shock resistance, and chemical corrosion resistance.

Pressureless sintering

Pressureless sintering is the sintering of a powder compact (sometimes at very high temperatures, depending on the powder) without applied pressure. This avoids density variations in the final component, which occurs with more traditional hot pressing methods.

Pressureless sintering is considered to be the most promising SiC sintering method. According to different sintering mechanisms, pressureless sintering can be divided into solid phase sintering and liquid phase sintering. By adding appropriate amounts of B and C (oxygen content less than 2%) to the ultrafine β-SiC powder, S. Proehazka was sintered to a SiC sintered body with a density higher than 98% at 2020°C. A. Mulla et al. Using Al2O3 and Y2O3 as additives, 0.5μm β-SiC (with a small amount of SiO2 on the surface of the particles) was sintered at 1850 to 1950°C. The relative density of the obtained SiC ceramics is greater than 95% of the theoretical density, and the crystal grain size is small and the average size. It is 1.5 microns.

Hot press sintering

Nadeau pointed out that pure SiC can only be densely sintered at a very high temperature without any sintering additives. Therefore, many people apply hot press sintering to SiC. There have been many reports on the hot press sintering of SiC by adding sintering aids. Alliegro et al. studied the effects of metal additives such as boron, aluminum, nickel, iron, and chromium on the densification of SiC. The results show that aluminum and iron are the most effective additives to promote SiC hot press sintering. F.F.Lange studied the effect of adding different amounts of Al2O3 on the properties of hot-pressed sintered SiC. It is believed that the densification of hot-pressed sintered SiC is related to the mechanism of dissolution and precipitation. However, the hot-pressing sintering process can only produce SiC parts with simple shapes, and the number of products produced by the one-time hot-pressing sintering process is very small, which is not conducive to industrial production.

Hot isostatic pressing sintering

Hot isostatic pressing (HIP) is a manufacturing process, used to reduce the porosity of metals and increase the density of many ceramic materials.

In order to overcome the shortcomings of the traditional sintering process, the scuba adopts type b and type c as additives and adopts hot isostatic pressing sintering technology. Under the condition of 1900°C, a fine crystalline phase ceramic with a density greater than 98 is obtained, and the bending strength at room temperature can reach 600 mpa. Although hot isostatic pressing sintering can obtain dense phase products with complex shapes and good mechanical properties, the hip sintering must seal the gap, which is difficult to achieve industrial production.

Reaction sintering

Reaction sintered silicon carbide, also known as self-bonding silicon carbide, refers to the process of porous steel billet reacting with gas or liquid phase to improve the quality of the billet, reduce pores, and sinter the finished product with a certain strength and dimensional accuracy. The α-SiC powder and graphite are mixed in a certain proportion and heated to about 1650°C to form a square billet. At the same time, it infiltrates or penetrates into the steel billet through vapor phase Si, reacts with graphite to generate β-SiC, and combines with existing α-SiC particles. When Si is completely infiltrated, a reaction sintered body with complete density and no shrinkage in size can be obtained. Compared with other sintering processes, the size change of reaction sintering during the densification process is smaller, and products with precise dimensions can be produced. However, the presence of a large amount of SiC in the sintered body makes the high temperature performance of reaction sintered SiC ceramics worse.

Pressureless sintered SiC ceramics, hot isostatic pressing sintered SiC ceramics and reaction sintered SiC ceramics have different properties. For example, in terms of sintering density and flexural strength, SiC ceramics have relatively more hot pressing sintering and hot isostatic pressing sintering, and reaction sintering SiC is relatively low. On the other hand, the mechanical properties of SiC ceramics change with the change of sintering aids. The pressureless sintering, hot pressing sintering and reaction sintering of SiC ceramics have good acid and alkali resistance, but the reaction sintered SiC ceramics have poor resistance to super acid corrosion such as HF. When the temperature is lower than 900°C, the flexural strength of almost all SiC ceramics is greatly improved than that of high-temperature sintered ceramics, and the flexural strength of reaction sintered SiC ceramics drops sharply when it exceeds 1400°C. (This is due to a certain amount of free Si, which is caused by a sharp decrease in the bending strength above a certain temperature on the sintered body.) For pressureless sintering and hot isostatic pressing sintered SiC ceramics, the high temperature performance is mainly affected by the types of additives.

The four sintering methods of SiC ceramics have their own advantages. However, in today's rapid development of science and technology, there is an urgent need to improve the performance of SiC ceramics, continuously improve the manufacturing process, reduce production costs, and achieve low-temperature sintering of SiC ceramics. In order to reduce energy consumption, reduce production costs, and promote the industrialization of SiC ceramic products.