The influence of amount and type of additives on α→β Si3N4 transformation

During the liquid-phase sintering of silicon nitride-based ceramics, the dissolution of the Si3N4 is inferred from the tendency of the alpha-Si3N4 to transform to beta-Si3N4 In order for this to correlate with densification it is necessary for the dissolved material to diffuse and precipitate, allowing contraction. In the present work the influence of the amount, quantity and type of additives on the alpha --> beta transformation has been analysed. Two systems have been studied: Si3N4-SiO2 and Si3N4-Y2O3-SiO2 All samples were densified through hot isostatic pressing (HIP) of encapsulated compacts. In the Si3N4-SiO2 system the dissolution of alpha-phase follows a zero-order kinetics and the amount of intergranular phase increases uniformly as a function of time, in this way eliminating dissolution as the rate-controlling mechanism in the transformation kinetic. In the Si3N4-Y2O3-SiO2 system, the kinetics of dissolution are first-order and the amount of intergranular phase remains constant with time. This is a sufficient and necessary condition to establish that transformation in this system is a process whose kinetics are controlled by the dissolution of alpha-Si3N4 in the liquid phase. (C) 1999 Kluwer Academic Publishers.