Effect of porous Si3N4 preform on the mechanical properties of Si3N4/Al composites with interpenetrating network structure

Si3N4/Al composites with three-dimensional interpenetrating network structures were fabricated by direct squeeze casting AlSi12 melt into the open porous Si3N4 preforms, which were synthesized by pressureless sintering method using different sintering additives. The effect of the sintering additive (Y2O3 and Re2O3 [Re=La, Gd, Er]) on the microstructure and mechanical properties of the porous Si3N4 preforms and the final MMCs were investigated. The SN–La samples with porosity ~38% showed the highest flexural strength of 263 MPa and the highest fracture toughness of 4.3 MPa·m1/2, due to the bimodal microstructure in which some large elongated β-Si3N4 grains with high aspect ratio were embedded in the fine matrix grains. The results showed that the Si3N4 preform had a significant impact on the mechanical properties of the composites. The strongest Si3N4 preform led to the highest strength of the infiltrated sample SNA–La composite (562 MPa) because the load was transferred from the aluminum matrix to the β-Si3N4 grains. Although the fracture toughness of KIC of Si3N4 preforms was quite different, the KIC of Si3N4/Al composites was increased slightly from 5.2 to 6.0 MPa·m1/2. It was because the interfacial characteristics between elongated Si3N4 grains and infiltrated Al phase of the composites were similar.