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 → Al ₂ O ₃ + M _3/y Al _x , where MO _y is an oxide that is wet by molten Al. In low PO ₂ atmospheres and at temperatures above about 900⁰C, molten Al reduces mullite to produce Al ₂ O ₃ and Si. The Al/mullite reaction has a ΔG _r ⁰(927⁰C) 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 ≥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 Al ₂ O ₃ , 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 (675⁰-750⁰C) than those for Al/mullite (1100⁰-1500⁰C). In addition, Mg wets mullite more readily than does Al, and Mg more readily subiltrates porous ceramic preforms. The absence of a passivating oxide layer on Mg can account for this behavior.