Implications of kinetically promoted formation of metastable α-sialon phases

alpha -Sialon ceramics are interesting materials, because alone or together with beta -sialon they can form in-situ reinforced microstructures which offer the best combinations of strength, hardness and toughness. At temperature above 1200 degreesC, the thermal stability of alpha -sialon phases has been debated since 1992, however, and it has been discussed if any alpha -sialon phase can be formed in Ce-, La-, Eu- and Sr-doped sialon systems. Using a novel rapid densification process named spark plasma sintering (SPS), which allows preparation of fully dense compacts of sialon ceramics within a few minutes, we here show that alpha -sialon phases are initially formed in these systems and that subsequent in situ and ex situ post heat-treatment results in a decomposition of the alpha -sialon phase. These observations show that cations with a radius larger than 1 Angstrom may stabilize the alpha -sialon phase, which contrasts with previous findings. The thermal stability of these alpha -sialon phases is strongly dependent on the kinetics of the reactions occurring when one approaches thermodynamic equilibrium. The findings might also have bearing on other sialon systems than those studied here. (C) 2001 Elsevier Science Ltd. All rights reserved.