Formation and densification of 21R AlN-polytypoid

The formation, densification behaviour and mechanical properties of 21R AlN-polytypoid(SiAl6O2N6) were studied. The results show that 12H(SiAl5O2N5) is a transitional phase during the formation of 21R. The additives Sm2O3 and Nd2O3, especially combined with Al2O3 can effectively promote the densification of 21R. The hardness(Hv10) of 21R hot-pressed with 2.5 wt% SmAlO3 at 1750°C/1 h is 1443 with a fracture toughness of 4.6 MPa.m1/2. The X-ray diffraction pattern of 21R was also determined in the present work. The revised unit cell dimensions of 21R are a=3.0461(1)إ, c=56.643(3)إ.