This paper is devoted to the study of alumina splats formation and the adhesion/cohesion of alumina coatings on oxidized 1040 steel substrates. When preheating with the plasma torch, a duplex oxide layer is formed with an upper hematite and an under magnetite. Measurements have been performed on oxide layers with 470 nm average thickness corresponding to preheating times of about 10 min. In these conditions, the relative thickness of each sub-layer can be controlled by monitoring the heating rate and the preheating temperature. When the hematite upper layer is thick, it is mostly broken by the droplet impact and correspondingly the resulting coating has a poor adhesion (? 34 MPa) the rupture occurring within the oxide layer. When the hematite upper layer is thin, the droplet impact breaks only partly the oxide layers and the rupture of the coating occurs both within the oxide layer and the substrate surface (? 40 MPa).

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