Spark plasma sintering of α-Si 3N 4 ceramics with Al 2O 3 and Y 2O 3 as additives and its morphology transformation

Abstract

The spark plasma sintering SPS technique has been used to densify pure α-Si 3N 4 commercial powder, having Y 2O 3 and Al 2O 3 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 Si 3N 4 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 Si 3N 4 were identified in the resultant microstructures. The morphology evolution of Si 3N 4 particles as occurred upon SPS-sintering is analyzed.