In situ reaction mechanism of MgAlON in Al–Al2O3–MgO composites at 1700°C under flowing N2

The Al–Al2O3–MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M1, M2, and M3, respectively, were prepared at 1700°C for 5 h under a flowing N2 atmosphere using the reaction sintering method. After sintering, the Al–Al2O3–MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M1 was composed of MgO and MgAl2O4. Compared with specimen M1, specimens M2 and M3 possessed MgAlON, and its production increased with increasing aluminum addition. Under an N2 atmosphere, MgO, Al2O3, and Al in the matrix of specimens M2 and M3 reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al–Al2O3–MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N2 atmosphere, the partial pressure of oxygen is quite low; thus, when the Al–Al2O3–MgO composites were soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al2O3 diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al2O3 to form MgAl2O4. When the temperature was greater than（1640 ± 10）°C, AlN diffused into Al2O3 and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and Mg Al2O4 at high temperatures because of their similar spinel structures.