Kinetics of Ceramic-Metal Composite Formation by Reactive Metal Penetration

The rate of composite formation via reactive metal penetration has been determined. The metal penetration depth (i.e., the reaction-layer thickness) was measured from cross sections of partially reacted samples. Samples were fabricated by immersing dense mullite preforms in a bath of molten aluminum at temperatures of 900°–1300°C and reacting the combination for up to 250 min. In general, the reaction-layer thickness increased linearly as the time increased. Penetration rates as high as 6.0 mm/h were measured; however, the aluminum penetration rate varied dramatically with time and temperature. The penetration rate increased when the reaction temperature was increased from 900°C to 1100°C, and the reaction-layer thickness increased linearly as the time increased in this temperature range. At temperatures of 1150°C and above, reaction-layer formation slowed or stopped after a relatively short period of rapid linear growth, because of an increase in silicon concentration near the reaction interface. The duration of the rapid linear growth period decreased from 25 min at 1150°C to <1 min at 1250°C. At temperatures of 1300°C and above, no reaction layer was detected by using optical microscopy. Kinetics data and transmission electron microscopy analysis suggest that the reaction was inhibited at higher reaction temperatures and longer times, because of silicon buildup and saturation at the reaction front. Calculations show that, as the reaction temperature increased, the silicon production increased faster than the silicon transport. The two rates were approximately equal at a temperature of 1100°C.