A thermodynamic model of an amorphous grain boundary phase in liquid-phase sintered βSiAlON ceramic

Thermodynamic modeling has been developed to explain the apparent stability of an intergranular amorphous phase in the liquid-phase sintered ceramic. The basic postulate has been to consider such a grain boundary phase as a separate phase of the system. An analytical form of the Gibbs energy functions recently derived for amorphous silicates has been taken to describe this boundary phase. A derived equilibrium condition conforming with the construction of parallel tangents to Gibbs energy functions has been used to estimate the compositions of the bulk and of the boundary phase. The microstructure of liquid-phase sintered β-SiAlON ceramic has been ascertained in this way. This model supports the existence of amorphous boundary phases as thermodynamically stable and provides a relation between their amount and the composition of the sinter additives in agreement with the experimental evidence.