Spark plasma sintering of α-Si3N4 ceramics with Al2O3 and Y2O3 as additives and its morphology transformation

The spark plasma sintering SPS technique has been used to densify pure α-Si3N4 commercial powder, having Y2O3 and Al2O3 additions; from 0, 2.5 and 5.0 wt% to 0, 1.5 and 3 wt%, respectively. Such powder admixtures were previously spray-dried at 160 °C in such a way that powder was thoroughly homogenized. Set sintering treatment included: 0–20 min holding time and 38 MPa axial load, sintering temperature of 1500 °C and heating rate of 300 °C/min. The maximum relative density developed on studied specimens ranged from 99.4 to 99.8% and could only be attained once the β-phase nucleated from the α-silicon nitride matrix. Obtained Si3N4 composites combine both α- and β-phases. The later phase becomes evident trough the rod-like geometry, which forms throughout the presence of a liquid face. The largest hardness value developed (1588 Hv(20 kgf)) on studied ceramics (3M-series – 3 min) matched close to the corresponding counterpart found in literature (1600 Hv), the former developed in much shorter sintering times. Using X-ray diffraction XRD and scanning electron microscope SEM analyses, the two major phases of Si3N4 were identified in the resultant microstructures. The morphology evolution of Si3N4 particles as occurred upon SPS-sintering is analyzed.