Preparation of Ce and Nd β-sialon ceramics by α → β sialon transformation

It has previously been accepted that lanthanum and cerium do not form α-sialon structures because their ions are too large to enter the interstices in the structure. Recent work on heat treatment of rare earth densified α- and mixed α-β sialons has shown that many α-sialon compositions prepared by sintering at 1750-1800°C are unstable at lower (1300-1600°C) temperatures. This instability is dependent on the α-sialon composition, and is more marked for α-sialons prepared using low, rather than high, atomic number rare earths. An important requirement of such studies is rapid quenching of the initial sintered sample in order to prevent decomposition of the α-sialon phase during cooling. The present work demonstrates that cerium stabilised α-sialon is in fact stable at sintering temperatures, and that this phase has not been observed previously because, under normal cooling rates, the α-phase totally transforms to β-sialon (plus glass) on cooling. Similar phenomena are discussed in the case of Nd ₂O ₃ densified sialons, where a fine grain size β-sialon matrix can be observed by decomposition of an α-sialon phase initially formed at sintering temperatures. The α ⇌ β transformation provides a useful processing variable for optimising the microstructure, and hence the mechanical properties of β- and mixed α-β sialon ceramics.