Influence of annealing conditions on the formation of hollow Al2O3 microspheres studied by in situ ESEM

• Transformations of Al microparticles with temperature demonstrated by in-situ SEM
• The gas atmospheres changed the temperature of transformation of the Al microparticles.
• The Al supply from the microparticles aluminized the superalloy substrate.
• A potential thermal barrier coating was thus obtained in a single step process.

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 120 Pa of gas atmospheres. Slow heating rates (2 °C min− 1) resulted in a better coverage of the particles shell than with fast heating rates (20 °C min− 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.

(a) Effect of the increasing of oxidizing power of the gas atmosphere and of the heating rate. The former provokes in a better surface coverage of the Al microparticles while the latter results in thinner alumina shells. (b) The supply of Al to the substrate and the interdiffusion mechanisms result in an aluminised layer underneath the top coat of Al microparticles.Figure optionsDownload as PowerPoint slide