Formation of phases in the AlN-rich corner of the Si-Al O-N system

The reaction of AIN with Al2O3, SiO2 or Si3N4 and Al2O3 at temperatures of 1820°C and 1950°C and for compositions rich in AlN have been investigated using powder X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. At 1820°C, Al2O3 reacted with AlN to form the non-stoichiometric oxynitride spinel phase ( Al23O27N5) whereas samples on the AlN SiO2 and AlN-'PRMX-phase' tie-lines did not react. At 1950°C, AlN-Al2O3 mixtures reacted to form the 27R aluminium oxynitride polytypoid, AlN-SiO2 mixtures reacted to form 21R and 27R sialon polytypoids and AlN-'X-phase' mixtures reacted to form the 12H, 21R and 27R sialon polytypoids, all of which possessed a needle-like morphology. Ordered and disordered intergrowths were also observed in the polytypoid crystals but the disordered 2Hδ sialon phase was not found.