PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Perfect ceramic vacuum sintering process

Vacuum sintering is sintering under vacuum conditions. Vacuum sintering has many advantages, such as promoting sintering. Compared with pressureless sintering, vacuum sintering can obtain ceramics with higher density and finer grains at lower temperatures. Another example: because there is no gas in vacuum sintering (even the impurity gas generated during the sintering process will be pumped away), the sintering environment is very clean and the pollution to the material is small. It is naturally advantageous when preparing materials with particularly high requirements for component purity, such as transparent ceramics. It can also be used to sinter metal materials that are easy to react chemically with oxygen, nitrogen or other gases.Perhaps it is precisely because of the existence of many advantages that vacuum sintering is now widely used, and new technologies such as vacuum hot pressing sintering and vacuum reaction sintering have been developed. Let’s talk about why vacuum sintering can promote sintering today?

Regarding this issue, some books explain it this way: When ceramics are sintered to the later stage, isolated pores will be formed. If there is air in the pores, then as the densification proceeds, these pores gradually become smaller, the internal pressure gradually increases, and it is difficult to compress further. Only when the gas is dissolved into the grains and slowly diffuses out can the pores be finally eliminated. Under vacuum sintering conditions, there is no air in the pores, so it is easier to eliminate them naturally – this is the main reason why vacuum sintering can promote sintering.

This explanation is certainly not wrong. At least for transparent ceramics, the above mechanism is very important. However, as long as we think about it a little, we will find that this statement is far from comprehensive. Because many experimental results show that the effect of vacuum sintering on ceramic densification is not limited to the later stage of sintering. In fact, from the perspective of sintering mechanism, the existence of vacuum will have an impact on various mass transfer processes in the green body. In solid-phase sintering, the most important mass transfer mechanism is diffusion, and vacuum will affect the diffusion in the green body. Take the sintering of titanium oxide ceramics under vacuum as an example. Due to lack of oxygen, oxygen vacancies are easily formed in the titanium oxide grains (including grain boundaries and surfaces). The formation of vacancies will cause lattice distortion, increase activity, accelerate the diffusion and migration of substances, and thus promote sintering. Some people have compared the densification and grain growth process of magnesium oxide ceramics under pressureless sintering and vacuum sintering conditions, and found a very interesting phenomenon. At a sintering temperature of 1500℃, pressureless sintering is mainly grain boundary diffusion, while vacuum sintering is mainly volume diffusion. The reason behind this is probably that there are many vacancies inside the grains during vacuum sintering, and the distortion is serious, so it is easy to stimulate volume diffusion. If it is liquid phase sintering, that is, the liquid phase is formed in the green body during the sintering process, then compared with ordinary pressureless sintering, the liquid phase of vacuum sintering has two characteristics: one is that it is easier to form. The melting point of most substances decreases with the decrease of pressure, so in a vacuum state, the temperature of the liquid phase in the green body may be lower than that during pressureless sintering; second, the flow in the pores of the liquid phase green body may be faster than that during pressureless sintering. Because at this time, vacuum sintering is equivalent to forming a negative pressure outside the green body, just like a pump pulling the liquid phase along the pores. The final result is that the liquid phase is easier to fill the pores, and at the same time, it makes it easier to rearrange the grains, thereby achieving the effect of promoting sintering.

If we are more serious, we can even suspect that vacuum will have some effect on the density of the green billet in the early stage of sintering. The reason is simple. The green billet is compacted in the air. After the air is removed, its size will shrink in theory, which means that the initial density will increase. Of course, since the pressure during the green billet molding is generally relatively large, the role of this negative pressure caused by vacuum may be very small and basically negligible.

In short, vacuum sintering can promote densification, which is the result of multiple mechanisms acting simultaneously. However, there are not many literatures related to the densification mechanism of vacuum sintering, so what exactly is going on, or which mechanism is the most important, needs further in-depth research.