Ordinary solid phase sintering

In the past, ceramics were usually sintered in heavy oil tunnel kiln, while nowadays mainly in electric furnace, including tube furnace, vertical type furnace, box type furnace, etc. And sintering environment is decided by the performance requirement of product, also by economic factor. Shielding gas can be used, such as hydrogen, argon, nitrogen, also can be sintered in vacuum or just atmosphere condition. For some oxide product, especially semi-conductor ceramics, product quality is very sensitive to oxygen concentration of sintering environment.

It’s hard to have pure ceramics sintered , so if it does not affect the performance, some sintering auxiliaries are added to decrease the sintering temperature. For instance, when sinter Al2O3, a spot of Tio2, MgO, MnO is added, and MgO, Y2O3, Al2O3 can be used for sintering Si3N4, temperature can be decreased significantly. Another effective measure is decreasing the powder particle size, since the smaller size, the higher interface energy, which means easier sintering process.

Reaction Sintering, RS, Reaction Banding, RB

Reaction sintering

Reaction sintering of Si3N4 is putting porous silicon pressed blank in N2 environment, heat up to 1400℃ and forming Si3N4. Since this is an exothermic reaction, it’s very important to control the reaction rate. If the reaction is too fast, local temperature will be higher than melting point of silicon, therefore in one hand, further reaction will be blocked, and in the other hand, large sized grain in product will be formed. As the reaction goes on, it will become more and more difficult for nitrogen diffusion, therefore reaction can not be completely finished, so density of product is relatively low, usually reach up to around 90%.

Reaction sintering of Si2ON2 is mixing Si, SiO2 and CaF2(or glass forming agent such as CaO, MgO) and pressing them into blank, under high temperature environment, silicon will react with sintering gas, and CaF2, CaO, MgO will form glass phase with SiO2. Nitrogen will diffuse into molten glass and Si2ON2 crystal will separate out from nitrogen-saturated glass phase, density of reaction sintered silicon oxynitride will be higher than 90%.

Reaction sintering of SiC is dipping porous blank into liquid silicon, and get the final product.

The advantage of reaction sintering is that dimension of blank will basically unchanged during the process, so to keep the production dimension precise. Process is simple and economic, so apply to mass production. However, there is also disadvantage, due to low density, material mechanical property is not high.