Mechanisms and kinetics of reaction-bonded aluminium oxide ceramics

Reaction-bonded Al2O3 (RBAO) ceramics were fabricated starting from mechanically alloyed Al2O3/Al, Al2O3/Al/ZrO2, and Al2O3/Al/ZrO2/Zr mixtures. Isopressed compacts were heat-treated in air up to 1,550 C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO2 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 Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al2O3, 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 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO2 additions exhibit nearly the same activation energies as ZrO2-free samples, though ZrO2 has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported