Novel processes for near net-shaped fabrication of monolithic and reinforced oxide ceramics

Mg reinforced {dollar}rm Alsb2Osb3{dollar} composites were fabricated by pressureless infiltration of molten Mg into porous {dollar}rm Alsb2Osb3{dollar} preforms. Such pressureless infiltration is thought to be driven by a displacement reaction that was observed to occur at interfaces between liquid Mg and solid {dollar}rm Alsb2Osb3.{dollar} The feasibility of fabricating near net-shaped, monolithic, {dollar}rm MgAlsb2Osb4{dollar} spinel bodies by the oxidation of the solid Mg-{dollar}rm Alsb2Osb3{dollar}-bearing composites was demonstrated. By controlling the preform porosity and the infiltration conditions, Mg-{dollar}rm Alsb2Osb3{dollar}-bearing composite bodies could be produced with the proper overall stoichiometry for spinel. The {dollar}rm Mg/Alsb2Osb3{dollar} composites could be machined into complex shapes. Oxidation of the Mg in the shaped composite was conducted in pure, flowing oxygen at {dollar}430{lcub}-{rcub}700spcirc{dollar}C. Post-oxidation annealing at {dollar}1200spcirc{dollar}C then allowed for complete conversion of MgO-{dollar}rm Alsb2Osb3{dollar} bearing body into {dollar}rm MgAlsb2Osb4{dollar} spinel. A final sintering treatment in flowing Ar at {dollar}1700spcirc{dollar}C yielded spinel specimens with densities {dollar}ge{dollar}92%. The sintered spinel bodies retained the Mg-{dollar}rm Alsb2Osb3{dollar}-bearing precursor shape and dimensions (to within 0.63%). The fabrication of spinel-matrix composites is also discussed.; In addition, a novel approach is presented for the fabrication of dense, shaped ceramic/metal composites by a novel class of displacement reactions. This approach differs from other oxidation-based processes for fabricating near net-shaped oxide/metal composites (e.g. DIMOX, {dollar}rm Csp4){dollar} in that a reaction-induced volume expansion is used to compensate for the porosity within a preform, so as to yield a dense composite with a high ceramic content. In the present case, a displacement reaction between liquid Mg and solid {dollar}rm Alsb2Osb3{dollar} was used to produce composites of MgO and Mg-bearing metal. Porous, shaped {dollar}rm Alsb2Osb3{dollar} preforms were placed in contact with a Mg(l) bath at {dollar}1000spcirc{dollar}C. The liquid Mg completely infiltrated and consumed the {dollar}rm Alsb2Osb3{dollar} preform by the following net reaction:{dollar}{dollar}rm 3Mg(l) + Alsb2Osb3(s) Rightarrow 3MgO(s) + 2Al(l){dollar}{dollar}where Mg(l) and Al(l) refer to Mg and Al in liquid solution. After complete reaction, dense composite bodies comprised of 90 vol% MgO and 10 vol% Mg-bearing metal were produced. The pore volume in the original preform was largely replaced by MgO produced by the displacement reaction above (the volume of 3 moles of MgO is 32% greater than the volume of 1 mole of {dollar}rm Alsb2Osb3),{dollar} so that composites retained the shape and dimensions (to within 2.7%) of the starting {dollar}rm Alsb2Osb3{dollar} preform.