Microstructure and mechanical properties of Al2O3 matrix nanocomposites produced by solid state precipitation

A novel and economical processing route for the production of Al₂O₃-based ceramic nanocomposites via solid solution–precipitation is reported. Dense (>98% ρ_th) and homogeneous solid solutions of 10 wt.% Fe₂O₃ in Al₂O₃ were produced by pressureless sintering at 1450 °C in air. Aging of the solid solutions in a reducing atmosphere at temperatures in the range 1250–1550 °C for different durations (up to 50 h) resulted in the precipitation of FeAl₂O₄ as second phase particles throughout the bulk of the samples. The optimum aging schedule resulted in a final microstructure comprising nano-sized (～100 nm) intragranular FeAl₂O₄ particles, along with coarser micro-sized particles on the matrix grain boundaries and triple point corners. Additionally, surface layers containing metallic Fe and with thicknesses up to ～100 μm were formed due to the further reduction of FeAl₂O₄. After removal of this surface layer, the hybrid nano/microcomposites possessed improved fracture toughness (by ～40%) and flexural strength (by ～50%) with respect to monolithic Al₂O₃.