The powder mixtures in the AlN-Al2O3 system were hot-pressed under 200kg/cm2 at 1600-2100°C for one hour. The effect of Al2O3 addition on the densification rate was remarkable especially in the AlN rich composition range. Several crystalline phases were observed in the hot-pressed specimens. Oxynitride of spinel type was formed at 1700°C in the entire range of the system. The oxynitride reacted on AlN at the temperature higher than 1900°C and the pseudo-polytypes of AlN were formed. 2Hδ-, 27R-, 16H-, 21R- and 12H-polytypes were observed. Some of them developed into an intersected plate like microstructure.
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Tosikazu Sakai (1978) studied this question.