Ceramic-metal composite formation by reactive metal penetration

Ceramic-metal composites can be made to near-net-shape by reactive penetration of dense ceramic preforms by molten metals. Reactive metal penetration is driven by a strongly negative Gibbs energy for reaction. For Al, the general form of the reaction is (x+2) Al + (3/y) MO(y) --> Al2O3 + M(3)/yAl(x), where MO(y) is an oxide that is wet by molten Al. In low PO2 atmospheres and at temperatures above about 900 degrees C, molten Al reduces mullite to produce Al2O3 and Si. The Al/mullite reaction has a Delta G(r) degrees(927 degrees C) of -338 per mole of mullite and, for fully dense mullite, the theoretical volume change on reaction is less than 1%. Experiments with commercial mullite containing a silicate grain boundary phase average less than 2% volume change on reaction. In the Al/mullite system, reactive metal penetration produces a fine-grained alumina skeleton with an interspersed metal phase, With greater than or equal to 15 vol.% excess aluminum, mutually interpenetrating ceramic-metal composites are produced. Properties measurements show that ceramic-metal composites produced by reactive metal penetration of mullite by Al have a Young's modulus and hardness similar to that of Al2O3, with improved fracture toughness ranging from 4.5 to 10.5 MPa . m(1/2). Other compositions also are candidates for in-situ reaction synthesis, but they exhibit differences in reaction kinetics, most probably due to different wetting behavior. For example, Mg reacts with mullite to form multi-phase composites. These reactions occur at lower temperatures (675 degrees-750 degrees C) than those for Al/mullite (1100 degrees-1500 degrees C). In addition, Mg wets mullite more readily than does Al, and Mg more readily infiltrates porous ceramic preforms. The absence of a passivating oxide layer on Mg can account for this behavior.