Xuanzhi Chen,Yehua Li,Youwu Liu,Pengjin Huang,Haidong Wu,Shanghua Wu,Xin Deng
Si 3 N 4 ceramics were sintered at 1850 °C and 2 MPa in a nitrogen atmosphere, using Gd 3 Si 2 C 2 -MgO as a sintering aid. The impact of Gd 3 Si 2 C 2 content and sintering dwell time at 1850 °C on the relative density, microstructure, and thermal conductivity of the Si 3 N 4 ceramics was investigated. The results indicate that under 4 h of sintering, increasing Gd 3 Si 2 C 2 content significantly enlarges Si 3 N 4 grain size while reducing microstructural uniformity; however, after 8 h, grain size refines and uniformity markedly improves. This phenomenon is mainly attributed to the increase in Gd 3 Si 2 C 2 content, which raises the number of β-Si 3 N 4 nucleation sites, thereby facilitating grain growth during short-term sintering; whereas during long-term sintering, the further enlarged grains generate a spatial hindrance effect, promoting the homogenization of the microstructure. Furthermore, under the same Gd 3 Si 2 C 2 content, extending the sintering time is beneficial to the growth of silicon nitride grains and the reduction of the glass phase, enhancing the continuity of Si 3 N 4 /Si 3 N 4 interfaces and thereby improving the thermal conductivity of Si 3 N 4 ceramics. The maximum thermal conductivity of 107.25 W·m −1 ·K −1 was achieved with a Gd 3 Si 2 C 2 content of 1 mol% and a sintering period of 8 h at 1850 °C.
In this study, self-synthesized Gd 3 Si 2 C 2 and MgO were used as sintering aids to fabricate Si 3 N 4 ceramics via gas-pressure sintering. The results indicated that Gd 3 Si 2 C 2 reacted with inherent SiO 2 on the Si 3 N 4 surface to form Gd 2 O 3 and β-Si 3 N 4 in situ, thereby increasing the N/O ratio of the liquid phase. In addition, in situ β-Si 3 N 4 induced grain growth, enhancing the thermal conductivity of Si 3 N 4 ceramics while maintaining excellent mechanical properties. Compared to the samples with Gd 2 O 3 and MgO as sintering aids, those with Gd 3 Si 2 C 2 and MgO as sintering aids demonstrated superior overall performance with the thermal conductivity, bending strength, fracture toughness, and Vickers hardness of 101.96 W⋅m −1 ⋅K −1 , 848.26 ± 4.66 MPa, 6.23 ± 0.15 MPa⋅m 1/2 , and 16.56 ± 0.43 GPa, respectively.
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