Preparation Methods of Silicon Nitride Ceramics

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Preparation technologies of silicon nitride ceramics have developed rapidly in the past few years, mainly in the type of reaction sintering, hot pressing sintering, pressureless sintering and gas pressure sintering. Due to different preparation techniques, various types of silicon nitride ceramics have different microstructures (such as porosity, pore morphology, grain morphology, inter-granular morphology and second phase content, etc.), and each property varies significantly. To get the superior Si3N4 ceramic materials, high-quality Si3N4 powder should be prepared firstly. Si3N4 powder prepared by different methods is not with identical quality, which leads to differences in the use of ceramic materials. People tend to attribute failures of ceramic material applications to developers for who do not understand the differences in a variety of ceramic powders and lack of understanding of their natures.

Generally speaking, high-quality silicon nitride powder should have high α-phase content and less impurity with uniform distribution in the ceramics. Besides, its particle size should be small and narrow distribution with good dispersion properties. Superior Si3N4 powder should contain at least 90% of α-phase, because some α-phase turn into β-phase when the Si3N4 is in the sintering process and not enough α phase content will reduce the strength of the ceramic materials.

Reaction Sintering (RS)

The RS adopts the general forming method. Firstly, pressing silicon powder into a green body in desired shape, through pre-nitrided sintering process (part nitride) in the nitriding furnace, the green body thus obtained has a certain strength and can process various mechanical (such as planing, milling and drilling). Finally, at a temperature above the melting point of silicon, the green body would be totally processed by nitride sintering again, getting the products of a small change in the size (the green body’s shrinkage is very small after sintering, with the linear shrinkage <0.1). Generally, the product can be used without grinding. The RS is suitable to manufacture the parts of complex shape and precise size with low cost, but it takes a long time to complete the nitridation.

Hot Pressing Sintering (HPS)

The HPS is to process thermoforming sintering through mixing Si3N4 powder with minor amount of additives (such as MgO, Al2O3, MgF2, Fe2O3, etc.) at the pressure above 1,916MPa and at the temperature above 1,600°C. Some British and American companies use the Si3N4 by the hot pressing sintering with its strength up to 981MPa or more. Additives and phase composition have a great impact on product performance during the sintering. Due to strict control over the composition of the grain boundary phase as well as suitable heat treatment after the sintering of Si3N4 ceramics, the Si3N4 ceramic materials, which their strength (up to 490MPa or more) is not significantly decreased even at the high temperature of 1,300°C, can be obtained with its creep resistance improved three orders of magnitude. If the Si3N4 ceramic material processes the pre-oxidation treatment at high temperatures between 1,400-1,500°C, its surface will form the Si2N2O phase which can significantly improve the oxidation resistance and high temperature strength of Si3N4 ceramics. Mechanical properties of Si3N4 ceramics produced by the HPS are superior to those of Si3N4 ceramics produced by the RS, with high strength and density, but high manufacturing costs and complex sintering equipment. Due to large shrinkage of the sintered body, the dimensional accuracy of products is subject to certain restrictions. So it is difficult to manufacture complex parts, but the parts of simple shape. It is also difficult to machine the workpieces.

Pressureless Sintering (PLS)

In increasing the sintered pressure of a nitrogen atmosphere, as Si3N4 decomposes with temperature increasing (normally at the pressure of N2 = 1atm, the decomposition occurring at the temperature of 1,800°C), it is necessary to conduct the PLS in temperature range of 1,700-1,800°C, and then to conduct the gas pressure sintering in temperature range of 1,800-2,000°C. This method aims to promote tissue densification of Si3N4 ceramics by the gas pressure so as to increase the strength of ceramics. Properties of the product thus obtained are slightly inferior to those of the products produced by the HPS. As for disadvantages, this method is similar to the HPS.