Decomposition of Sm alpha-SiAlON phases during post-sintering heat treatment

The microstructural characteristics and the phase assembly of Sm (alpha+beta)-SiAlON ceramics before and after post-sintering heat treatment at 1450 degrees C have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (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 grains, which could facilitate the transformation. Based on these observations, it is proposed that the alpha' to beta' phase transformation in Sm SiAlONs is essentially a self-decomposition process of alpha' phases and may be described as: alpha' --> beta' + M' + AlN polytypoid phase(s) Therefore, for an unstable alpha' phase, it would decompose, i.e. proceed alpha' --> beta' transformation without having to involve large amounts of grain boundary liquid. However, the existence of the grain boundary liquid and beta' grains as nuclei could reduce the activation energy barrier and accelerate the alpha' decomposition. (C) 1996 Elsevier Science Limited.