Oxidation of Micro-Sized Spherical Aluminium Particles

Micro-sized spherical Al particles have recently attracted interest for the development of a new concept for coatings based on their capability to form hollow alumina spheres and aluminized diffusion zones in the substrate. For understanding better their oxidation behaviour, spherical mu m-Al particles with different sizes were oxidized in air on heating up to 1300 degrees C and under isothermal conditions at 800 degrees C and 850 degrees C. The oxide formation was studied in situ by high temperature X-ray diffraction and the oxidised particles were analysed by scanning electron microscopy. On heating the mu m-Al particles begin to form a gamma-Al2O3 scale before reaching the melting point and the molten Al is kept within the gamma-Al2O3 shell. On further heating theta-Al2O3 is detected, which forms simultaneously with the gamma-Al2O3. The gamma-Al2O3 / theta-Al2O3 scale is stable and protective under isothermal conditions up to 800 degrees C within the investigated times. On further heating the gamma-Al2O3 and theta-Al2O3 transform simultaneously to alpha-Al2O3 in a temperature range of 850 degrees C to 1100 degrees C. Under isothermal conditions the gamma -> alpha-Al2O3 transformation is observed after 160 min at 850 degrees C. During the gamma -> alpha-Al2O3 transformation shrinkage occurs that leads to formation of pores. A model is proposed describing the mechanism that leads to the formation of the observed whiskers morphologies during the gamma -> alpha-Al2O3 transformation.