We obtained densified chemically fixed silicon nitride (DCFSN) by pressureless sintering of the Si3N~-Y203-MgO system maintaining the sintering temperature up to 1800~The specimens having a density of 2.4-2.5 g/cm 3 were obtained in advance by synthesizing compact silicon powders containing certain sintering-activators in nitrogen atmosphere and were subjected to densification. In order to avoid dissociation of silicon nitride, the specimens were densified according to the technique of Giachello and Popper [I]: the specimens were placed in a particulate charge consisting of silicon nitride, boron nitride, and the oxides similar to the additives present in the material requiring densification.The KPS-type silicon of semiconductor-purity having an average dispersion of 5 ~m was used as the raw material. In this case, during nitriding, the degree of transformation of silicon into nitride was found to be 100%.The densification processes begin to occur even at 1450~ and their maximum rate is observed in the 1600-1700~ range. Initially, densification of the material occurs according to the liquid-phase diffusion mechanism. As the temperature increases, additional mechanisms begin to operate (dissolution and recondensation of silicon nitride in the glass phase to form elongated B-Si3N 4 grains).The microstructure of the synthesized ceramic consists of silicon nitride crystals having a diameter of 0.1-0.5 ~m in the transverse section (Fig. i). The phase composition of the ceramic is represented by the following crystalline phases: ~-and ~-Si3N~, yttrium silicates and Si3N4.Y203. Magnesium compounds are present in the form of a small quantity of MgSiOs. An analysis of the regions of coherent scattering showed the presence of elongated grains of the crystalline ~-Si3N~ phase [at (hks = (I01), the size is 0.5 ~m and at (hks = (202), it is 15.7 ~m].* After densification, the microstructure is represented by the ~-SisN~ grains having a hexagonal cross section; they are in the form of columnar crystals having a diameter of 0.7 ~m and different degrees of elongation (Fig. 2). Grains measuring 15-17 ~m in length are encountered.An analysis of the microstructure shows that in spite of the sufficiently high content of the activating agents in the original powder, glass phase is virtually absent at the grain boundaries and the fracture of the ceramic is transcrystalline in nature.At a heat treatment temperature of 1750-1800~ the density of the ceramic approaches the theoretical density and amounts to 3.2-3.3 g/cm s and its linear shrinkage attains a value of 6.0-6.5% (Fig. 3a). The increase of the density in the 1750-1800~ range (that is not related to the densification processes and the linear dimensional changes) owes to the diffusion of magnesium oxide from the particulate charge into the specimen. During the densification process, the particulate charge containing the oxide additives creates a specific gaseous atmosphere that hinders not only the dissociation of Si3N~, but also the diffusion of MgO or other magnesium compounds ...
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