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-Si₃N₄commercial powder, having Y₂O ₃ 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: 020 min holding time and 38MPa 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 theα-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 (1588Hv_{(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 Si ₃N₄ particles as occurred upon SPS-sintering is analyzed.