The preparation of Ce α-sialons and related unstable phases

It has previously been accepted that lanthanum and cerium do not form alpha-sialon structures because their ions are too large to enter the interstices in the structure. Recent work on heat-treatment of rare-earth densified alpha- and mixed alpha-beta sialons has shown that many alpha-sialon compositions prepared by sintering at 1750-1800 degrees C are unstable at lower (1300-1600 degrees C) temperatures. This instability is dependent on alpha-sialon composition, and is more marked for alpha-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 alpha-sialon phase during cooling. The present work demonstrates that cerium stabilised alpha-sialon is in fact stable at sintering temperatures and that this phase has not been observed previously because under normal cooling rates the alpha-phase totally transforms to beta-sialon (plus glass) on cooling. Similar phenomena are discussed in the case of Nd2O3-densified sialons, where a fine-grain size beta-sialon matrix can be observed by decomposition of an alpha-sialon phase initially formed at sintering temperatures. alpha reversible arrow beta transformation provides a useful processing variable for optimising the microstructure and hence the mechanical properties of beta- and mixed alpha-beta sialon ceramics.