Comparative study of Y₂O₃–MgSiN₂ and Y₂O₃–MgO sintering additives on microstructure and thermomechanical properties of Si₃N₄ ceramics

Si₃N₄ ceramics are promising substrates for high-power electronics, but their thermal and mechanical properties often conflict. To address this, we systematically compared two commonly used sintering aid systems, Y₂O₃-MgSiN₂ (YMSN) and Y₂O₃-MgO (YM), on the phase evolution, microstructure, and thermomechanical properties of Si₃N₄ fabricated by gas pressure sintering. The effects of the Y₂O₃-to-magnesium compound molar ratio and total additive content (3.5 - 14 mol%) were investigated in parallel. The YMSN system formed a Y-Mg-Si-O-N liquid phase with a much higher N/O ratio, creating a nitrogen-rich atmosphere that suppressed Si₃N₄ decomposition, reduced lattice oxygen, and promoted the growth of large and high-aspect-ratio β-Si₃N₄ grains with discontinuously distributed intergranular phases. In contrast, the YM system produced a more oxygen-rich liquid phase, leading to a larger volume fraction of continuous intergranular phases and finer grains, which reduced thermal conductivity. YMSN consistently outperformed YM. The optimal properties including thermal conductivity of 98.14 W m⁻¹ K⁻¹, flexural strength of 941 MPa, and fracture toughness of 7.23 MPa m^1/2 were achieved at a Y₂O₃:MgSiN₂ ratio of 3:7.5 and 10.5 mol% total additives, significantly surpassing YM. This work elucidates the dual role of MgSiN₂ in providing active nitrogen while consuming oxygen, demonstrating that a high-N/O liquid chemistry effectively decouples the trade-off between thermal and mechanical performance.