Process-driven role of oxide sintering additives on microstructure evolution and mechanical performance of Si 3 N 4 –Mo–Al cermets

Pressureless sintering of silicon nitride–based cermets remains a critical processing challenge due to the limited self-diffusivity of Si₃N₄ and the resulting poor densification. In this study, the individual and combined effects of oxide sintering additives on phase evolution, densification mechanisms, and mechanical performance of Si₃N₄–Mo–Al cermets are systematically investigated. In contrast to some previous studies that mostly consider individual oxide additives or pressure-assisted sintering paths, the current research studies the synergistic impact of Y₂O₃ and Al₂O₃ additives in Si₃N₄–Mo–Al cermets produced through conventional pressureless sintering. Cermets with a base composition of 75 wt.% Si₃N₄, 15 wt.% Mo, and 10 wt.% Al were fabricated by conventional pressureless sintering at 1500°C using Y₂O₃ and Al₂O₃ additives (2.5–10 wt.%). Unlike earlier studies that focus on single-additive systems, the present work demonstrates that the combined addition of Y₂O₃ and Al₂O₃ promotes liquid-phase-assisted densification and controlled phase evolution. X-ray diffraction and electron microscopy reveal partial α-to-β Si₃N₄ transformation accompanied by the formation of molybdenum silicides and mullite, which collectively govern microstructural refinement. The cermet containing 5 wt.% Y₂O₃ and 5 wt.% Al₂O₃ exhibits the most favourable balance of properties, achieving a relative density of 83.04%, Vickers micro-hardness of 5.73 ± 0.38 GPa, and indentation fracture toughness of 9.40 ± 0.49 MPa·m^1/2. The improvement in mechanical performance is attributed to enhanced particle rearrangement, silicide-induced crack deflection, and β-Si₃N₄-based self-toughening. These findings establish a clear process–microstructure–property relationship and demonstrate that tailored oxide additive combinations provide an effective and scalable route for improving pressureless-sintered Si₃N₄–Mo–Al cermets for high-temperature engineering applications.