DENSIFICATION KINETICS AND MODELING OF GLASS-FILLED ALUMINA COMPOSITE

Kinetics and mechanism of sintering in a model borosilicate glass (BSG ) + alumina composite system have been investigated isothermally at 60 0 to 1000-degrees-C. A faster and greater densification is observed wi th higher sintering temperature, greater BSG content, and larger alumi na particle size. Using measurable densification kinetics, it is found that the activation energy of densification (Q) increases continuousl y with increasing BSG content, from predominant < 110 kJ/mol for BSG < 40 vol. % to 245 kJ/mol for BSG > 90 vol. %. The rate-limiting step d uring densification is identified to be diffusion of alkali ions in BS G when BSG < 40 vol. % (Q = 110 kJ/mol), diffusion of both alkali and aluminum ions when BSG = 40-60 vol.% (Q = 110-170 kJ/mol), diffusion o f aluminum ion in BSG when BSG = 60-80 vol. % (Q = 170 kJ/mol), and vi scous flow of BSG when BSG > 90 vol. % (Q = 245 kJ/mol). These observa tions are attributed to a chemical reaction taking place at the interf ace of alumina/BSG, resulting in a reaction layer adjacent to alumina. Since the composition of the reaction layer is known to be rich in al uminum and alkali ions and poor in silicon, the alkali ions content in BSG is continuously decreased during sintering. Accordingly, when the BSG content in the mixture is low, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the dens ification kinetics and changing the densification mechanism. As the am ount of BSG is increased to > 90 vol. % at the expense of alumina, the reaction as described above becomes less dominant and the sintering p roceeds by viscous flow of glass.