Formation of Al₂O₃ grains with different sizes and morphologies during the pulse electric current sintering process

Commercial micrometer Al₂O₃ powder was sintered at 1550 °C under a mechanical pressure of 30 MPa by pulse electric current sintering (PECS). Microstructure observation was performed on polished, thermal-etched cross sections parallel to the direction of mechanical pressure. Platelike Al₂O₃ grains formed when the powder was heated at a heating rate of 5 °C/min, while a heating rate of 200 °C/min resulted in equiaxed grains. These results indicated that PECS is an effective approach to hinder grain growth by application of a higher heating rate. However, Al₂O₃ grains at the upper edge were larger than those at the side edge of the samples in both cases. It implied that there were different temperatures at the upper edge and the side edge of the Al₂O₃ powder compacts during the PECS process.