Reaction Mechanisms and Microstructures of Ceramic-metal Composites Made by Reactive Metal Penetration

Ceramic-metal composites can be made by reactive penetration of molten metals into dense ceramic preforms. The metal penetration is driven by a large negative Gibbs energy for reaction, which is different from the more common physical infiltration of porous media. Reactions involving Al can be written generally as (x+2)Al + (3/y)MOy → Al2O3 + M3/yAlx, where MOy is an oxide, such as mullite, that is wet by molten Al. In low Po2 atmospheres and at temperatures above about 900°C, molten Al reduces mullite to produce Al2O3 and Si. The Al/mullite reaction has a AGr°(1200K) of-1014 kJ/mol and, if the mullite is fully dense, the theoretical volume change on reaction is less than 1%. A microstructure of mutually-interpenetrating metal and ceramic phases generally is obtained. Penetration rate increases with increasing reaction temperature from 900 to 1150°C, and the reaction layer thickness increases linearly with time. Reaction rate is a maximum at 1150°C; above that temperature the reaction slows and stops after a relatively short period of linear growth. At 1300°C and above, no reaction layer is detected by optical microscopy. Observations of the reaction front by analytical transmission electron microscopy show only Al and Al2O3 after reaction at 900°C, but Si is present in increasing amounts as the reaction temperature increases to 1100°C and above. The kinetic and microstructural data suggest that the deviation from linear growth kinetics at higher reaction temperatures and longer times is due to Si build-up and saturation at the reaction front. The activation energy for short reaction times at 900 to 1150°C varies from ∼90 to ∼200 kJ/mole, depending on the type of mullite precursor