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 processes involving physical infiltration of a porous medium, this process 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 that is wet by molten Al, In low P-O2, atmospheres and at temperatures above about 900 degrees C, molten Al reduces mullite to produce Al2O3 and 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 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 enough 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 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 coatings to be fabricated by controlling the penetration time. Thermodynamic calculations indicate that other compositions also are candidates for in situ reaction synthesis, which suggests that reactive metal penetration may be a general route to composite synthesis with the prospect for near net-shape processing.