Sintered Silicon Carbide Process

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Sintered silicon carbide classification:
(1) Pressure-free sintering
Pressureless sintering is considered as a promising sintering method for SiC sintering. According to different sintering mechanisms, pressureless sintering can be divided into solid phase sintering and liquid phase sintering. S. Proehazka sintered S iC sinter with density higher than 98 at 2200℃by adding appropriate amount of B and C into ultrafine beta-SiC powder (oxygen content less than 2). A. Mulla et al. sintered 0.5 um beta-SiC (with a small amount of SiO 2 on the surface of the particles) at 1850-1950℃ with Al2O3 and Y2O3 as additives. The relative density of SiC ceramics obtained was greater than 95% of the theoretical density, and the grain size was fine with an average size of 1.5 um.

(2) Hot pressing sintering
Without any sintering additives, pure SiC can be sintered densely only at very high temperatures, so many people implement hot pressing sintering process for SiC. There have been many reports about hot pressing sintering of SiC by adding sintering aids. Alliegro et al. studied the effect of metal additives such as B, Al, Ni, Fe and Cr on the densification of SiC. It was found that Al and Fe were effective additives to promote SiC hot pressing sintering. F.F. Lange studied the effect of adding different amounts of Al2O3 on the properties of SiC sintered by hot pressing. It was considered that the densification of SiC sintered by hot pressing depended on the mechanism of dissolution-precipitation. However, the hot pressing sintering process can only produce SiC parts with simple shape, and the number of products produced during the one-time hot pressing sintering process is very small, so it is not conducive to industrial production.

(3) Reactive sintering
Reactive sintering SiC, also known as self-bonding SiC, is formed by mixing a-SiC powder and graphite powder in a certain ratio, then heated to about 1650 C. At the same time, SiC is infiltrated into the green body by infiltration of Si or gas-phase Si, which reacts with graphite to form beta-SiC, which combines the existing a-SiC particles.

Comparison of three common sintering methods:
1. Hot-pressing sintering: SiC parts with simple shape can only be prepared, which has low production efficiency and is not conducive to large-scale commercial production.
2. Pressure-free sintering (atmospheric sintering): SiC parts with complex shapes and large sizes can be produced. It is widely recognized as an advantageous sintering method at present.
3. Reactive sintering: SiC parts with complex shape can be prepared at low sintering temperature, but the high temperature performance of the products is not good.

Characteristic: If Si permeation is allowed completely, the material with zero porosity and no change in geometrical size can be obtained during the whole process.

In actual production, the green billet should have excessive pore to prevent the formation of an impermeable SiC layer on the surface due to the process of Si infiltration, thus preventing the reaction sintering from proceeding. The excess pore in the process of reaction sintering is filled by excess Si, so that the porous compact products can be obtained.

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

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