Effect of the additive content on the sintering of pre-sintered Si3N4 composite.

Sintering of Si3N4 compounds requires additives that promote densification through the dissolution-precipitation mechanism in liquid phase sintering. The sintering of the Si3N4 with the SiO2-Y2O3-Al2O3 system has been extensively studied. Nevertheless, because SiO2 depends on the natural amount on the Si3N4 surface, little has been studied about SiO2 control. A pre-sintering treatment can increase the content of SiO2 through controlled oxidation of Si3N4. In this work the effect of the additive content on the densification of Si3N4 composite ceramics was evaluated. For this purpose, powder mixtures of Si3N4, Y2O3 and Al2O3 were prepared. The Y2O3 and Al2O3 ratio were modified in percentages of 4, 8 and 12% by weight. The mixtures were compacted and pre-sintered at 1300°C for 1 hour. Then, these were sintered at 1450°C for two hours. The samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results showed that during pre-sintering, the Y5(SiO4)3N phase is formed; then, it's subsequently oxidized at the sintering stage. A second phase identified as Y3Al5O12 was found in samples with higher proportions of Al2O3. Composition and quantity of the observed phases depend of Y2O3 and Al2O3 proportion in the added additive.