In situ formation mechanism of spinel-like Al5O6N and plate-like Al7O3N5 in the two-step sintered Al–Al2O3 composites

A designed two-step sintering was implemented in the resin bonded Al–Al2O3 composites under nitrogen to get active AlN precursor, in which the sample was firstly heated at 580 °C for 8 h then at 1700 °C for 3 h. The introduction of active AlN precursor promoted the synthesis of reinforcements. XRD and SEM were employed to investigate the two-step sintered Al–Al2O3 composites. In addition to the preferentially formed Al2OC-AlN solid solution, plate-like Al7O3N5 and spinel-like Al5O6N were generated. Due to the higher content of active AlN from the nitridation of Al, Al7O3N5 nucleus was synthesized from solid-state reaction. Then, plate-like Al7O3N5 was obtained from liquid phase reaction and gas phase reaction. In the meanwhile, the AlN with lower content from carbothermal nitridation of Al2O3 reacted with Al2O3 to Al5O6N. In addition, gas phase reaction also contributed to the formation of spinel-like Al5O6N. And the reaction models were established in this paper.