DECOMPOSITION OF SM ALPHA-SIALON PHASES DURING POSTSINTERING HEAT-TREATMENT

The microstructural characteristics and the phase assembly of Sm (alph a+beta)-SiAlON ceramics before and after post-sintering heat treatment at 1450 degrees C have been investigated by scanning electron microsc opy (SEM), transmission electron microscopy (TEM) and X-ray diffractio n (XRD). Grain boundary glass crystallization and the alpha-SiAlON to beta-SiAlON (i.e. alpha' to beta') phase transformation were observed as the major phase transformations occuring during the heat treatment. TEM studies indicated that the alpha' to beta' transformation process itself could produce a nano-sized liquid phase inside the SiAlON grai ns, which could facilitate the transformation. Based on these observat ions, it is proposed that the alpha' to beta' phase transformation in Sm SiAlONs is essentially a self-decomposition process of alpha' phase s and may be described as: alpha' --> beta' + M' + AlN polytypoid phas e(s) Therefore, for an unstable alpha' phase, it would decompose, i.e. proceed alpha' --> beta' transformation without having to involve lar ge amounts of grain boundary liquid. However, the existence of the gra in boundary liquid and beta' grains as nuclei could reduce the activat ion energy barrier and accelerate the alpha' decomposition. (C) 1996 E lsevier Science Limited.