Effect of diamond particle size on carbothermal reduction behavior and properties of Si3N4 ceramics

To improve the comprehensive properties of Si 3 N 4 ceramics, diamond powders with different particle sizes (1 and 2 μm) were added. The effects of diamond particle size on carbothermal reduction behavior, phase evolution, microstructure, and thermomechanical properties of Si 3 N 4 ceramics were systematically investigated. The results show that 1 μm diamond exhibits higher reactivity because of its smaller particle size and larger specific surface area, which promotes the carbothermal reduction of surface silica, decreases oxygen-related impurities, and increases the N/O ratio of the liquid phase. As a consequence, the dissolution–reprecipitation growth of elongated β-Si 3 N 4 grains is enhanced, leading to reduced grain-boundary scattering and improved thermal conductivity. In contrast, 2 μm diamond shows lower reactivity, resulting in insufficient carbothermal reduction, hindered densification, and inferior overall properties. Although the sample containing 1 μm diamond exhibits the highest thermal conductivity, its flexural strength decreases slightly, which is attributed to the combined effects of grain coarsening, reduced intergranular phase, and increased defect sensitivity. These results demonstrate that optimizing diamond particle size is an effective approach for regulating oxygen impurity, microstructure, and overall performance in Si 3 N 4 ceramics.