Reaction-bonded A1,0, (RBAO) ceramics were fabricated starting from mechanically alloyed AI,O,/AI, AI,O,/ AI/ZrO,, and Al2O,/AI/ZrO,/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reactionbonding mechanisms, kinetics, and the influence of ZrO, and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of AI. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% A1 and 55 vol% AI,O,, it is 112 and 152 kJ/mol at -64% and -74% TD, respectively, while above the melting temperature, it lies in the range -26-33 M/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO, additions exhibit nearly the same activation energies as Zr0,-free samples, though ZrO, has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.

Important parameters controlling the reaction-bonding of AI,O, (RBAO) process are Al content, particle size, and green density. Successful fabrication of high-strength bodies requires fine and homogeneous powders. Low milling intensity does not lead to the required particle fineness, whereas overmilling causes extensive oxidation and hydrolyzation of Al. Oxidation of A1 during milling is enhanced by ZrO, additions. As much of the A1 should be oxidized by solid/gas reaction as possible. Milled RBAO precursor powder contains physically-adsorbed and chemically-bonded (aluminum hydroxide) water. Decomposition of chemicallybonded water promotes cracking in the temperature range 450-550°C. Bloating of RBAO occurs when the surface region becomes dense before complete decomposition of aluminum hydroxide in the interior of the body.

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A new technology to fabricate low-to-zero shrinkage Al2O3-based composites, Reaction Bonding of Alumi num Oxide (RBAO), is presented: Al and Al2O3 precur sor powder mixtures are intensively milled, compact ed, and heat-treated in air, such that Al fully oxidizes to Al2O3. The volume expansion associated with the oxi dation partially compensates for the sintering shrink age. The process can be modified by incorporating other metals or ceramic phases to change the final com position, to enhance the mechanical properites, and to further compensate for the sintering shrinkage.In teresting characteristics of this technology are easy green machining without binders, superior mechanical properties of reaction-bonded bodies without final machining (also when green machined), superplastic formability, near-net-shape capability, and broad microstructural and compositional versatility.

The technology of reaction bonding Al,O, (RBAO)