Effect of aluminum nitride-scandia content on the microstructural and mechanical properties of sintered silicon carbide ceramics

Two different SiC ceramics, one with 10 vol.% AlN-Sc2O3 in a 2:3 molar ratio, and the other with 20 vol.% of the same additives with the same ratio, were fabricated by hot-pressing at 1900 A degrees C for 1 h and subsequent annealing at 2000 A degrees C for 6 h in nitrogen. The grain boundary structures of both materials were observed by using high-resolution transmission electron microscopy. The results showed that both materials had clean boundaries without any amorphous films. Although both samples exhibited the same boundary structure, the sample with the higher AlN-Sc2O3 content contained more junction phases. The SiC ceramic with 10 vol.% AlN-Sc2O3 maintained its room-temperature strength up to 1400A degrees C, whereas the SiC ceramic with 20 vol.% AlN-Sc2O3 showed a gradual decrease in strength at above 900 A degrees C. The present results suggest that the high-temperature strength is dependent on the amount of junction phase, as well as the characteristics of the intergranular phase.