Fabrication and characterization of Si3N4 reinforced Al2O3-based ceramic tool materials

Al2O3–Si3N4 composites ceramic tool materials with Y2O3 as sintering additive were fabricated by the hot pressing process at 1450 °C for 30 min under 32 MPa in vacuum. Different mass fractions of Si3N4 as reinforcements were employed. The mechanical properties, microstructure, strengthening and toughening mechanisms of the composites were investigated. The content of Si3N4 had significant effect on the mechanical properties and microstructure of the composites. Al2O3-25 wt% Si3N4-1 wt% Y2O3 had the optimum comprehensive mechanical properties with flexural strength, fracture toughness, Vickers hardness and relative density of 768.7±63.9 MPa, 6.8±0.4 MPa m1/2, 19.6±0.4 GPa and 99.6±0.2% respectively. A core–shell structure with Si3N4 as core and Al2O3 as shell was firstly formed in the composite. The core–shell structure was generated through a rapid dissolution–transport–precipitation process. Elongated β-Si3N4 grains with a diameter of 0.3–0.5 µm and aspect ratio of 5 were in-situ synthesized. The improved mechanical properties were attributed to the strengthening and toughening mechanisms including the mixed fracture mode of intergranular and transgranular fracture, the crack deflection, the high grain boundary strength, the pinning and bridging effects of the core–shell structure and the elongated grains.