Studies On Intergranular Phase In Si₃N₄-MgO-CeO₂ Ceramics

Si3N4 ceramics have been prepared by spark plasma sintering(SPS) with 5wt% MgO+5wt%CeO2 additives and SiO2-MgO-CeO2 ternary system has been sintered with hot press sintering in this paper. The SPS process and the microstructure, composition of intergranular phase in Si3N4 ceramics have been studied by means of XRD and SEM. The crystallization of MgO during SPS process has been observed and the mechanism of crystallization has also been discussed.The results show that the densification process was controlled by the mechanism of liquid phase sintering, the densification process began with the appearance of liquid phase in sintering body at 1220, and ended at 1500. When temperature was below 1450 , the densification of sintering body was achieved by grains rearrangement, the relative density can reach 95.74%. The dissolution-deposition of 813 occurred between 1450 and 1500, but its effect on densification was little. When temperature was above 1550, the liquid phase in sintering body would separated into two phases, one was rich-Si liquid phase, which consisted of Ce, N, Si, O etc, another was rich-Mg liquid phase. The phase-separation increased the viscosity of rich-Si liquid phase and reduced that of rich-Mg liquid phase. Finally, the former became high-refractory amorphous cerium silicate, which existed in intergranular phase of Si3N4 ceramics; the latter existed as MgO crystal, which reduced the content of amorphous intergranular phase, it was of great benefit to improve the high-temperature mechanical behavior of Si3N4 ceramics.Besides, the effects of surface grinding and polishing on mechanical behavior of Si3N4 ceramics have also been studied in this paper. The results show that the bending strengths of the specimens ground respectively by surface grinder and tool grinder were reduced, since both might induce the tensileresidual stresses in surface of SiaN4 ceramics. Polishing after grinding could diminish the tensile residual stresses and smooth the surface defects, it could enhance bending strengths obviously.