Sintering behavior and oxygen redistribution of fine AlN powders during spark plasma sintering: Effects on microstructure and properties

This study investigates how high-oxygen submicron AlN powders evolve during additive-free spark plasma sintering and how oxygen redistribution governs microstructure and properties. Fine AlN powder exhibited high sintering activity and rapid densification at lower temperatures. During sintering, surface and grain-boundary oxygen was redistributed and partially dissolved into the AlN lattice. At higher temperatures, the decrease in lattice oxygen was accompanied by the formation of crystalline Al-O-N (AlON) secondary phases. Oxygen incorporation and AlON precipitation reduced thermal conductivity and modified high-temperature resistivity. Flexural strength was mainly governed by grain refinement, while hardness was also influenced by AlON precipitation. The ceramics achieved maximum flexural strength (528 ± 32 MPa) and Vickers hardness(1451 ± 17.6 HV) after sintering at 1650 and 1700℃, respectively. These findings highlight the need to balance rapid densification and grain refinement against oxygen-induced thermal and electrical degradation when optimizing additive-free high-oxygen AlN ceramics.