Grain growth of β-Si₃N₄ using Y ₂O₃ and Al₂O₃ as sintering aids

The mechanical properties of silicon nitride ceramics are closely linked to the α-β phase transformation. Spark plasma sintering (SPS) technique has been used to density pure α-Si₃N₄ commercial powder, using Y₂O₃ and Al₂O₃ as additives; from 2.5 and 5.0 wt% and 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. The sintering treatment included: 3 to 20 min holding time, 38 MPa axial load, sintering temperature of 1500°C and heating rate of 300°C/min. The maximum relative density developed on studied specimens is nearly 99% of theoretical value, which corresponds to 2M-series specimens and could only be attained once the β-phase nucleated from the α-silicon nitride matrix. X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses confirmed the presence of two mains phases in the resultant microstructures: α- and β-Si₃N₄. Both the β-Si₃N₄ grain growth and the role that different amount of additives can play during SPS-sintering are analyzed.