Microstructural development of P-Si3N4 materials

The adaptation of silicon nitride materials to various applications leads to an increase in the variety of microstructures (see Fig. 1). This was the impetus for 

The most important commercial silicon nitride powders have a-Si3N4, i.e. the phase which is metastable at high temperatures, as the main constituent [1]. The consequence of this is that during densiflcation, a phase transformation takes place, having a great influence on the subsequent formation of the microstructure. 

It was shown that high fracture toughness and strength can be achieved when the grains have a needle-like shape. Characterisation of the shape is usually done using two parameters the thickness of the grains and the aspect ratio, i.e. the ratio of needle length to thickness [23 26]. 

A detailed analysis of different microstructures [29-31,33] shows that they can be explained by the number of p nuclei in the starting powder, even at very low P Si3N4 contents. In Fig. 8 the calculated volume fractions of growing nuclei which are necessary in the starting composition to produce two different fine microstructures in the sintered materials are given. This calculated content is the lowest P-Si3N4 content in the starting powder required to explain the microstructure. For crystallite sizes between 50 and 100 nm [20,35], the necessary amount of the p fraction in 

These experimental data suggest that P-SijN4 nucleation has no influence on the microstructure of silicon nitride materials under common sintering conditions and using common powders. 

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