The effects of silicon additive content on thermal conductivity and mechanical properties of Si ₃ N ₄ ceramics

Abstract In order to improve the thermal conductivity of silicon nitride (Si 3 N 4 ) ceramics and break through the bottleneck of the inverse relationship between thermal conductivity and flexural strength, the combination of silicothermic reduction treatment (SRT) and gas pressure sintering (GPS) was employed in this study. The “Nitrogen‐rich, Oxygen‐deficient” liquid phase was constructed after SRT, enabling phase transformation to precede densification during the dissolution–precipitation process. β‐Si 3 N 4 grains were developed with less steric hindrance in a porous β‐Si 3 N 4 matrix. A “unimodal to bimodal” transition in the microstructure of Si 3 N 4 ceramics was attributed to the addition of Si. The purification and abnormal growth of large β‐Si 3 N 4 grains and enhanced devitrification of the intergranular phase were achieved via the “Nitrogen‐rich, Oxygen‐deficient” liquid phase, thus improving thermal conductivity. The trend of an initial increase followed by a decrease in flexural strength with the increase in Si content was attributed to the proportion of large grains in the bimodal microstructure. Fracture toughness was enhanced via self‐toughening resulting from the abnormal growth of Si 3 N 4 grains. Thermal conductivity and mechanical properties of Si 3 N 4 ceramics were synergistically optimized, which provide a high‐performance ceramic substrate for the package of high‐power electronic devices.