Pressureless sintering SiC, also known as sintered SC (SSiC). It was previously believed that SiC was impossible to sinter without pressure. However, in 1973, Prochazka, a ceramic scientist at GE in the China, first discovered that the submicron β-SiC powder prepared by the gas phase method, with the introduction of a small amount of B and C sintering aids, could achieve high density by pressureless sintering at 2000~2100℃ in an inert atmosphere or vacuum. Subsequently, they used the submicron α-SiC powder prepared by the Acheson method and added B and C aids for pressureless sintering to achieve a density of 98%.
The role of B and C in the process of pressureless sintering SiC is not fully understood, but it is generally believed that B and C participate in the solid-state sintering reaction. According to the ceramic sintering crystal boundary pore model, Prochazka believes that diffusion sintering and pore elimination are related to the ratio of crystal boundary energy to surface energy.
Obviously, to promote sintering and shrinkage and elimination of pores, it is necessary to reduce the grain boundary energy between SiC grains and increase its surface energy. B and C are introduced into SiC, B is located on the SiC grain boundary, and part of B replaces C in SiC to form a solid solution, with a dissolution amount of 0.2% (mol). On the one hand, B increases the volume diffusion of SiC, and at the same time, B segregates on the SiC grain boundary to reduce the grain boundary energy; C can react chemically with the SiO2 layer and impurity Si on the surface of SiC particles, that is, the SiO2 layer is reduced by C to form SiO gas and eliminated, thereby increasing the surface energy of SiC. Since the driving force of sintering is the reduction of surface energy, high initial surface energy will be conducive to sintering.
Mizzrah et al. (1984) further studied the effects of the additional amount of B and C and the particle size and purity of SiC powder on the density of SiC ceramics sintered at normal pressure and found that when B or C is used alone as an additive, SiC ceramics cannot be fully dense. Only when B and C are added at the same time, SiC ceramics can achieve high density, which is consistent with the results of Prochazka (1973). Usually, the amount of B added should be around 0.5% (wt), while the amount of C added depends on the oxygen content in the SiC powder, and generally increases appropriately with the increase of the oxygen content in the SiC powder.
In addition to the above-mentioned simultaneous use of B+C, there are also BC+C, BN+C, BP+C, AlB2+C, and other additives that have a solid-phase sintering effect on SiC. For example, using B1C+C as a sintering aid, using submicron high-purity SiC powder as a raw material, sintering at 2000~2200℃, vacuum or Ar atmosphere, SiC products with a theoretical density of more than 95% can be obtained. Solid-phase sintered SiC ceramics, except for a small amount of residual C, do not have a second phase or a glass phase at the grain boundary, the grain boundary is clean, and the high-temperature performance is very good. It can be used up to 1600℃ without much change in performance. However, solid-phase sintered SiC cannot achieve complete density, and usually, there are a small number of closed pores at the triangular grain boundary of the grain, and high temperature can easily lead to grain growth. The bending strength of this type of solid-phase sintered SiC ceramics is generally 400~500 MPa, and the fracture toughness is 3.5~4.5MPa·m1/2.
Normal pressure sintering to produce a liquid phase is a sintering technology developed after solid-phase sintering of SiC.
It should be pointed out that for the normal pressure sintering of SiC, the initial powder characteristics of SiC and the type and number of sintering aids are two key factors affecting the densification behavior and grain growth of SiC and the final product’s performance.
The initial powder properties of SiC mainly include: 1. Powder particle size and specific surface area. The powder is required to be submicron (regardless of α or β-SiC), with an average particle size of <0.5μm and a particular area of surface of >15m2/g. Because the driving force for sintering is reducing surface energy, the initial high surface energy of ultrafine powder will be conducive to sintering and shorten the diffusion distance; 2. The oxygen content in SiC powder is low (<0.5%), and the free Si content is as low as possible. The oxygen content mainly exists in the form of SiO2, which covers the surface of SiC particles and hinders sintering. The SiC powder raw material can be treated with hydrofluoric acid (HF) or a mixed acid of hydrofluoric acid and nitric acid (HF+HNO2) to improve its sintering properties; the presence of Si impurities causes abnormal grain growth and deteriorates material properties.
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