Ceramic-metal composites can be made to near-net-shape by reactivepenetration of dense ceramic preforms by molten metals. Reactivemetal penetration is driven by a strongly negative Gibbs energy forreaction. For Al, the general form of the reaction is (x+2) Al +(3/y) MO_y→Al_2O_3+M_3/yAl_x, where MO_y is an oxide that is wet bymolten Al. In low Po_2 atmospheres and at temperatures above about900 deg.C, molten Al reduces mullite to produce Al_2O_3 and Si. TheAl/mullite reaction has a ΔG_r°(297 deg.) of -338 per mole ofmullite and, for fully dense mullite, the theoretical volume changeon Reaction is less than 1/100.
展开▼
