Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

Abstract

The fabrication of composites formed by alumina grains (95vol%) in the micrometer size range and aluminium titanate nanoparticles (5vol%) by reaction sintering of alumina (Al2O3) and titania (TiO2) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400°C were established from dynamic sintering experiments. Specimens cooled at 5°C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400°C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5°C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.