Kinetics of phase transformations in SiAlON ceramics: I. effects of cation size, composition and temperature

Kinetics of direct α/β-Si3N4→α-SiAlON transformations, reverse α′-SiAlON→β′-SiAlON transformation, and the formation of intermediate phases were investigated for SiAlON ceramics with rare earth stabilizing cations. It was determined that smaller cations (Yb), and α-Si3N4 starting powders lead to faster Si3N4→α′-SiAlON transformations. Using the knowledge of phase stability of α′-SiAlON, these observations have been correlated to the overall driving force for the transformation, and the correlation is further extended to the reverse α′-SiAlON→β′-SiAlON transformation. Formation of intermediate phases during Si3N4→α′-SiAlON transformation is shown to depend on the α′-SiAlON formation and to be inversely correlated to the stability of α′-SiAlON. A thermodynamic interpretation thus emerges to account for the kinetics of SiAlON transformation that encompass the effects of the size of the stabilizing cations, type of starting powder, overall composition, and the reaction temperature.