Abstract
The transformation of Al microparticles into hollow and broken Al2O3 microspheres was investigated by in situ environmental scanning electron microscopy (ESEM) up to 1150°C under different heating rates and 120Pa of gas atmospheres. Slow heating rates (2°Cmin−1) resulted in a better coverage of the particles shell than with fast heating rates (20°Cmin−1) that delayed the threshold temperature at which the particles opened. Above this threshold, the amount of opened spheres increased with heating rate, with the coarser particles opening more than the small ones. It appeared that inert atmospheres (He–4%H2) increased the temperature at which the particles transformed, while air and pure oxygen tended to lower it. In contrast, the temperature interval was larger and was size-dependent when using water vapour. Irrespective of the gas atmosphere, opening of the spheres allowed molten Al to flow out from the core and aluminise the substrate while leaving behind a top coat of hollow alumina spheres.
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