HIGH-TEMPERATURE PROPERTIES OF MIXED ALPHA' BETA'-SIALON MATERIALS/

The mechanical behavior of mixed alpha'/beta'-SiAlON materials was stu died at elevated temperatures. Two different alpha'/beta'-SiAlON compo sitions along the Si3N4-Y2O3 . 9AlN composition line in the Si3N4-Al2O 3 . AlN-YN . 3AlN plane (alpha'-SiAlON plane) were prepared using thre e different raw-material mixtures. Six different materials were obtain ed with significantly lower values in alpha'-SiAlON than expected. The high-temperature properties of the materials studied were influenced strongly by the chemical composition (alpha' content and grain-boundar y phase) of the SiAlONs, A high content of alpha'-SiAlON was beneficia l in terms of creep behavior of the materials. The same materials, how ever, were characterized by a considerably degradated fracture behavio r at elevated temperatures in air because of a crack-growth process en hanced by the poor oxidation resistance of these materials. It was con cluded that, despite some superior features of the materials studied, a long-term application of mixed alpha'/beta'-SiAlON materials at 1400 degrees C in air is problematical. A combination of all properties re quired for such applications was not observed. For that reason, it is suggested that the real high-temperature potential of these materials in air should be limited to temperatures <1300 degrees C.