The requirements of initial powder when producing silicon nitride ceramics and the effect of commonly used additives

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Today the silicon nitride ceramics are regarded as the advanced material of structural ceramics. This kind of material has very high strength and damage resistance at various temperatures and it also has high heat stability, hardness, abrasive resistance, oxidation corrosion resistance and so on. Therefore, it is a kind of structural ceramics which have broad development prospects. At present, the engine parts, cutting tools, bearings, automatic welding work pieces and other products made by the silicon nitride ceramics have already shown excellent usability and resulted in huge economic benefits. However, the potential of silicon nitride ceramics is still far from exploiting. Among the various factors that restricts the structure and performance of silicon nitride ceramics, silicon nitride powders and additives occupy an important position.

The requirements of silicon nitride powders

The high dispersity and good homogeneity of the particles

For most of the processes, the powders with sub-micron size which have the superficial area of 10~25m2/g are all needed. Thus the high density particles structure material can be obtained.

The content of a-phase

Currently, there are no silicon nitride powders of which the quality can fully meet various requirements. As for themselves, the powders compounded with plasma chemical method can be used to produce the best quality of silicon nitride ceramics. The high dispersion and defect level of its particles can ensure the hyperreactivity and good sinterability of the powders. This is because the sintering and structure formation process of silicon nitride ceramics is connected with the phase change of the accompanying a-Si3N4→β-Si3N4 and this kind of change occurs based on the liquid phase recrystallization mechanism. Although such requirements are universally accepted, there is still amorphous formation or crystal phase contained in the successful use of silicon nitride powder.

The control of oxygen content

Oxygen, served as the impurities, participates in silicon nitride powder in the forms of adsorption and in the forms of SiO2 and Si2N2O which cover in the surface of Si3N4 particles. The oxygen content will determine the liquid phase amount when sintered and will influence the phase composition, structure and performance of the materials.

Very few metal impurities and carbon contents

The silicon nitride powders which contain iron, calcium, and magnesium will reduce the liquid viscosity in the sintering process and will increase its final density by 1 percent to 5 percents. But it can promote the grain growth of Si3N4 and the formation of ceramics coarse grain structure, reduce the high-temperature strength under the action of mechanical stress and quicken the strain rate. With carbon participation, it will reduce the amount of liquid phase and change its composition, inhibit the sintering and promote the condensation of its structure. Besides, the   performance of silicon nitride powders depends on its synthetic method and the quality of the initial reaction agent.

The effect of commonly used additives

Through the introduction of special additives (usually oxide additives), we can achieve the aims of adjusting the liquid phase and its formation temperature when the silicon nitride is being sintered. These kinds of additives which can activate the sintering participate in the phase formation and microstructure formation process of silicon nitride powders, which will have an important impact on the usability of the sintered material.

MgO additives

The introduction of MgO additives can ensure the liquid phase formation and make high density hot-pressing silicon nitride material. The liquid phase formation is a result of the interaction between the magnesium oxide and silicon dioxide (always in the formation of Si3N4 surface film to participate in). At first people thought that the formed liquid phase composition approximately conforms to that of the Mg SiO3-SiO2 eutectic, but later it was confirmed that it has the complex composition which includes four components of Mg-Si-O-N. When cooled, this kind of liquid phase forms the glass phase with low softening temperature (1000 degrees celsius) in the grain boundary of Si3N4 and will determine its low temperature strength and creep resistance.

Y2O3 additives

It can form the liquid phase which can promote the sintering by bringing in Y2O3 into silicon nitride in the sintering process or in the relatively low temperature hot pressing. This kind of liquid phase reacts with silicon nitride under the final sintering high temperature and will form hot and strong binding phase. Based on the quantity of Y2O3 and the superficial SiO2 which participates in the silicon nitride powders, it can form four different kinds of nitrogen oxide crystal phase in its system – Y2Si3O3N4, YsiO2N, Y4Si2O7N2 and Y10 (SiO4) 6N2. These kinds of phases configure along with the Si3N4 grain boundary which on one hand ensure the high heat resistance of the materials and on the other hand reduce the nitride resistance of the materials. The latter mentioned is because that all of these phases oxidation are accompanied by a huge volume effect. For example, the volume of Y2Si3O3N4 phase will increase by 30% when it is oxidized. As a result, the ceramic will be damaged in the grain boundary when it is working in the oxidizing medium.

The composite additives of Y2O3 and Al2O3

By using the composite additives of Y2O3 and Al2O3, we can achieve the best sintering of silicon nitride as this kind of additives can ensure the formation of Y-Si-Al-O-N liquid phase and with its participation, the silicon nitride can be sintered fullest. The high strength silicon nitride ceramics can be obtained with the introduction of Y2O3-Al2O3 compound additives.

The earth element oxides (La2O3, Sm2O3, Nd2O3, Yb2O3 and so on) additives

With the introduction of rare earth element oxides additives, it can generate strong heat intergranular phase in the ceramics structures by the formation of complex oxide nitride. Also, the silicon nitride materials which contain this kind of additives will have very high strength at high temperature.

ZrO2 additives

The materials which use ZrO2 (within 20%) additives are usually produced by hot-press approach. Their features are high oxidation resistance and good thermal stability.

The additives of carbides, nitrides and silicides

As there is SiO2 surface film existing in the Si3N4 particles, we can use nitrides additives (such as AiN, BeSiN4, YN and so on) to implement the liquid phase sintering of silicon nitride ceramics. However, the phase composition formed at this time may not be the best for the effective sintering. Non-oxide additives are also available when the silicon nitride is under hot pressed. For example, in order to improve the usability of Si3N4 ceramic tools, we can bring in the additives of carbide (TiC, and WC), nitrides (TiN, and Bn), silicides (WSiO2). For this purpose, we commonly used the titanium carbide which can greatly improve the stability of tools. Titanium Nitride and carbon nitride can also be used.

Among the preparation methods of silicon nitride powders, plasma chemical synthesis has the most development prospective. The ultrafine powder made by this method has the least amount of impurities and the most excellent sinterability and the obtained materials have homogeneous particle structures. By the introduction of rare earth element oxides additives, the silicon nitride materials will have the highest level of thermal strength property.