Studies on AlN-Polytypoids II. The Mechanical Properties and microstructuves of AlN-Polytypoids

Based on our previous study on formation and densification behavior of AIN-polytypoids (15R, 12H, 21R)[1], their mechanical properties and microstructure were studied. The results show that the AIN-polytypoid phases possess relatively poor mechanical properties, however, their fie-cural 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 AIN-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 AIN-rich polytypoids is responsible for its higher hardness because of the longer Burgers vector required for slip dislocations. The fracture toughness and fie-cural strength has no such close relationships to the type of AIN- 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 AIN-polytypoids grains are supposedly the main mechanisms for the strengthening effect at high temperature.