Abstract: Based on our previous study on formation and densification behavior of AlN-polytypoids(15R, 12H, 21R)^[1],
their mechanical properties and microstructure were studied. The results show that the AlN-polytypoid phases possess relatively poor mechanical
properties, however, their flexural strength can remain constant(21R) or increase as high as 50%(15R and 12H) from room temperature till 1250℃.
The hardness varies with the species of AlN-polytypoids and the additives used in a range of 11.5～14.4GPa(H_v10). The 15R compositions
possess the lowest hardness varying from 11.5 to 12.8GPa. For 12H and 21R compositions, the varying range increases to 12.6～13.8GPa and
12.7～14.4GPa, respectively. The longer stacking sequence in AlN-rich polytypoids is responsible for its higher hardness because of the longer
Burgers vector required for slip dislocations. The fracture toughness and flexural strength has no such close relationships to the type of AlN-
polytypoids. They varied in the range of 3.8～4.8MPa·m^1/2 and 313～366MPa, respectively. The pull out and micro-cracks formed
in the AlN-polytypoids grains are supposedly the main mechanisms for the strengthening effect at high temperature.

Key words: AlN-polytypoids, Sialon, rare-earth oxides, mechanical properties, microstructure