Implications of kinetically promoted formation of metastable α-sialon phases

α-Sialon ceramics are interesting materials, because alone or together with β-sialon they can form in-situ reinforced microstructures which offer the best combinations of strength, hardness and toughness. At temperature above 1200°C, the thermal stability of α-sialon phases has been debated since 1992, however, and it has been discussed if any α-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 α-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 α-sialon phase. These observations show that cations with a radius larger than 1 Å may stabilize the α-sialon phase, which contrasts with previous findings. The thermal stability of these α-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.