Low‐Temperature Densification of Si ₃ N ₄ Ceramics With MnO as a Sintering Additive

ABSTRACT Dense Si 3 N 4 ceramics were fabricated by spark plasma sintering (SPS) using a MnO–Al 2 O 3 additive system, and the effects of sintering temperature, pressure and holding time on densification behavior, phase transformation, microstructure development and mechanical properties were systematically investigated. Near‐theoretical densification was achieved at a relatively low temperature of 1450°C, demonstrating the strong fluxing effect of MnO in promoting liquid‐phase‐assisted sintering. The α–β phase transformation was found to be highly sensitive to the sintering conditions, enabling continuous tuning of the α/β phase ratio over a wide range. This controlled phase transformation resulted in pronounced microstructural changes, from fine equiaxed α‐Si 3 N 4 grains to elongated columnar β‐Si 3 N 4 grains, leading to a broad and tunable hardness‐fracture toughness range. The highest hardness of 23.6 ± 0.4 GPa was obtained in the 1550‐100‐5 sample, while the highest fracture toughness of 8.4 ± 0.4 MPa·m 1/2 was achieved in the 1700‐100‐20 sample. These results demonstrate that MnO is an effective non‐rare‐earth sintering additive for low‐temperature densification and microstructure design of Si 3 N 4 ceramics, providing an alternative strategy to regulate phase composition and mechanical properties using transition metal oxides.