In this view, the group of Kolarik et al. [140] used successfully the slurry technology on austenitic steel using aluminium microparticles to provide the Al supply as depicted in Figure I-11. After an appropriate heat treatment, they observed two major effects. The first one was the formation of an additive layer enriched in Al, which is well known to provide the oxidation resistant property once the substrate will be used at high temperature. The second effect was that the skeleton of the Al microspheres remained on top of the substrate as hollow spheres since the major part of the aluminium content had diffused into the metallic substrate. Furthermore, due to the thermal treatment, the formation of an ü-alumina hollow sphere topcoat was realized.