SYNTHESIS OF AL2O3-AL COMPOSITES BY REACTIVE METAL PENETRATION

Ceramic-metal composites have been made to near net-shape by reactive penetration of dense ceramic preforms by molten Al, In contrast to pro cesses involving physical infiltration of a porous medium, this proces s works with dense ceramic preforms, Ceramic-metal composite formation by 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/y)Al(x), where MO(y) is an oxide tha t is wet by molten Al, In low P-O2, atmospheres and at temperatures ab ove about 900 degrees C, molten Al reduces mullite to produce Al2O3 an d silicon, The Al/mullite reaction has a Delta G(r) degrees(1200 K) of -1014 kJ/mol and, if the mullite is fully dense, the theoretical volu me change on reaction is less than 1%, Experiments with commercial mul lite containing a silicate grain boundary phase average less than 2% v olume change on reaction, In the Al/mullite system, reactive metal pen etration produces a fine-grained alumina skeleton with an interspersed metal phase, With enough excess aluminum, mutually interpenetrating c eramic-metal composites are produced. Properties measurements show tha t ceramic-metal composites produced by reactive metal penetration of m ullite by Al have a Young's modulus and hardness similar to that of Al 2O3, with improved fracture toughness ranging from 5 to 9 MPa . m(1/2) . For penetration times less than 1 h, reaction layer thickness varies as the square root of time, which allows ceramic-metal composite coat ings to be fabricated by controlling the penetration time. Thermodynam ic calculations indicate that other compositions also are candidates f or in situ reaction synthesis, which suggests that reactive metal pene tration may be a general route to composite synthesis with the prospec t for near net-shape processing.