1980 Volume 88 Issue 1024 Pages 718-724
Deformation and fracture behavior of polycrystalline aluminum nitride in three-point bending creep was studied under constant rate of loading. Two kinds of polycrystalline aluminum nitride were fabricated by hot-pressing of aluminum nitride powder under the conditions of 2000°C, 200kg/cm2, 30 minutes (Sample-A), and 180 minutes (Sample-B). Relative density and grain size of these polycrystalline materials were 95-96% and 2-2.5μm for Sample-A, and 99% and 8-10μm for Sample-B, respectively. Under the loading rate of 12.7kg/cm2⋅min, the brittle-ductile transition temperatures were about 1400°C for Sample-A and about 1600°C for Sample-B. The cause of the fracture in the brittle region was discussed from the view point of fracture mechanics, and the effect of the surface flaw size of the specimans on fracture strength was also discussed. On the other hand, the gain boundary separation in the deformation process at the ductile region for Sample-A was observed, while the diffusional deformation must have been operative at the same time. The yield value of Sample-A decreased with increasing temnperature. For Sample-B, the yield value did not change up to temperature of 1800°C, and also the grain boundary separation in the ductile deformation process was not observed even at 2000°C. From above observations, it is concluded that high density is not sufficient condition to obtain the excellent high temperature structurual material of polycrystalline aluminum nitride and the polycrystal must have enough heat treatment to attain the strong grain boundary strength.
