Mechanical properties of AlN-polytypoids – 15R, 12H and 21R

Mechanical properties of three types of AlN-polytypoids with Ln2O3(Ln=Nd,Sm) and Ln2O3:Al2O3=1:1, respectively, as sintering additives were studied. The results showed that the hardness increased with the AlN-polytypoid compositions shifting towards AlN-rich side, that was 21R having the highest hardness (12.7–14.4 GPa) and 15R the lowest values(11.5–12.8 GPa). The fracture toughness and flexural strength had no such close relationships to the type of AlN-polytypoids, which varied in a range of 3.8–4.8 MPa·m1/2 and 313–366 MPa (at room temperature), respectively. However, for the compositions with suitable amount of additives, the flexural strength greatly increased with increasing temperatures, especially for 15R and 12H. This characteristic was discussed on the basis of the fracture morphology observed by SEM.