Sintering mechanisms of Al2O3-based composite ceramic tools having 25% Si3N4 additions

A type of Al2O3-based composite ceramic tool material simultaneously reinforced with about 25 wt% α-Si3N4 and β-Si3N4 grains was sintered using the vacuum hot-pressing technology by optimizing process parameters which included different contents of yttria additive, sintering temperature and time. Effects of these process parameters on densification, microstructure and mechanical properties were investigated. The mechanical properties of the composites increased and then decreased with an increase in the yttria content and sintering time. Low content of yttria led to a poor relative density and excess yttria caused inhomogeneous microstructure, which decreased the mechanical properties of the composites. The shorter sintering time led to the elongated β-Si3N4 grains stunted and porosity, and the longer sintering time led to β-Si3N4 grains coarsened and some agglomerations. The experimental results showed that the composite Al2O3/Si3N4 containing 1.5 wt% yttria, which was sintered under a pressure of 32 MPa at 1500 °C for 20 min, had the highest density and the optimal comprehensive mechanical properties. Its relative density, flexural strength, fracture toughness and hardness were 99.6 ± 0.2%, 1093 ± 32 MPa, 6.8 ± 0.2 MPa m1/2, and 19.5 ± 0.3 GPa respectively. The correlations between densification, mechanical properties and microstructure were discussed. The improvement of the mechanical properties was ascribed to equiaxed α-Si3N4 particles and elongated β-Si3N4 grains which induced the toughening and strengthening effects such as the crack deflection, interface debonding, grain fracture and grain pullout. The mixed fracture mode, high density, fine and homogenous microstructure, the stronger interface bonding and the pinning or bridging connection of the core-shell promoted the mechanical properties of the composite ceramic tool materials.