Synergistic effects of Y2O3 and MgB2 sintering additives on the microstructure and properties of Si3N4 ceramics

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Abstract
Sintering additives play a crucial role in the phase transformation, microstructure evolution, and properties of Si3N4 ceramics. In this work, Si₃N₄ ceramics containing a fixed total content of Y2O3 and MgB2 sintering additives with different Y2O3/MgB2 ratios were fabricated by spark plasma sintering and post-annealing. The influence of the additive ratio on phase evolution, microstructure, mechanical properties, and thermal conductivity was systematically investigated. Decreasing the Y2O3/MgB2 ratio markedly suppressed the α→β phase transformation during spark plasma sintering, leading to pronounced variations in grain morphology and mechanical properties. After post-annealing, all α-Si3N4 transformed completely into β-Si3N4, whereas the final grain morphology remained strongly dependent on the additive ratio. The highest flexural strength (1170 ± 26 MPa), fracture toughness (9.17 ± 0.42 MPa·m1/2), and Vickers hardness (19.7 ± 0.6 GPa) were obtained at 2, 3, and 6 mol% MgB2, respectively. Despite significant differences in oxygen content and microstructure, the thermal conductivity remained within 53–58 W/(m·K). Localized boron enrichment was observed in individual Si3N4 grains, indicating that the distribution and chemical state of boron deserve further investigation. These results provide insights into the role of Y2O3/MgB2 additive ratio in controlling phase evolution, microstructure, and the resulting properties of Si3N4 ceramics.

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