Nanograined Si3N4 ceramics with Y2O3–Al2O3–MgO as sintering additive exhibited superplastic elongation of >300% at 1923 K with an initial strain rate of 5 × 10−4 s−1. Flow stress was less than 4 MPa up to elongation of 130%. In the later stage of deformation, the flow stress increased with strain due to the grain growth and the alignment of elongated grains. The volume fraction of glassy phase significantly reduced due to vaporization of glassy phase. The chemical analysis by TEM/EDS revealed that the chemical composition of the near Si3N4 grain surface was different from that of the interior of the glass pocket at multigrain junctions due to absorption of additive cations on the Si3N4 grain. The chemistry of intergranular glassy phase changed significantly in the later stage of deformation accompanying the microstructural evolution. It is suggested that the flow stress, which depends on the viscosity of intergranular glassy phase, varies with the change in glass chemistry during deformation.

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additive) was studied by compression tests between 1500 and 1700°C. We studied the effect of CaO additive on flow stress, microstructural evolutions, and thermal stability of the intergranular glass phase during deformation. While the addition of CaO did not affect grain size, the flow stress decreased with the amount of CaO. This result suggested that the addition of CaO reduced the viscosity of intergranular glass phase. The addition of CaO further improved the thermal stability of the glass phase by suppressing the evaporation at elevated temperatures.

Silicon nitride ceramic is a potential material for clinical indications due to its high fracture toughness, strength, and non-cytotoxicity. For this reason, Si3N4 ceramic is interested to apply for dental core. The superiority of Si3N4 ceramic is the low coefficient of thermal expansion (CTE) which is lower than that of zirconia and alumina ceramics that are popular in this field. In this study, borosilicate glass powder with 5 wt% of zirconia addition was prepared by melting at 1450 °C for 1 h. The glass melt was quenched and was then ground to be a powder and mixed with polyvinyl alcohol solution to be a paste. The Si3N4 specimens coated with the veneer were fired in electrical tubular furnace at 1100 °C for 15 min. The appearance of these specimens shows smooth, glossy without defect and crazing. The veneer has thermal expansion coefficient as 3.05x10-6 °C-1 and the Vickers hardness as 4.0 GPa which is close to the human teeth. The specimens were tested by human gingival and periodontal ligament fibroblasts (HGF and HPDLF) and cytotoxicity by MTT assay. The results indicated that Si3N4 ceramic and borosilicate glass can be used as dental materials.