Micro-décharges en milieu électrolytique aqueux et leur interaction avec les matériaux : le cas du procédé d'oxydation par plasma électrolytique (PEO)

Abstract : Plasma Electrolytic Oxidation (PEO) is a surface treatment of light metallic alloys (e.g Al, Mg, Ti) that makes possible to grow thick and hard oxide protective coating on those materials. To overcome the limitations of anodizing the PEO process takes benefit of anodic micro-discharges resulting from the dielectric breakdown in an aqueous electrolyte under a high applied current density or voltage (typ. 20 A/dm2; 700 V). Therefore this work aims first at studying both the macroscopic parameters (breakdown conditions, surface density, lifetime, size) of such micro-discharges and their behavior, and second to correlate these studies to the growth mechanisms of the oxide coatings within various electrical (applied current waveform) conditions and alkaline electrolyte composition. These coupled studies allowed us to evidence that the transition from arc regime to soft regime (previously determined) corresponds to the growth of a loose spongy silicon-rich phase which is likely amorphous, inside and around cracks of the pancake structures issued from the dielectric breakdown and composed of crystalline alumina. Meanwhile, analyses of combined PVD + PEO coatings lead us to improve our understanding of some breakdown mechanisms occurring during the PEO process, with a particular attention to the phenomena at the coating-substrate interface. Finally, a particular study of cathodic micro-discharges (unusually observed in PEO) allowed us to propose breakdown mechanisms of the dielectric layer during that negative half-period of the current. Besides it has been shown that those cathodic micro-discharges are detrimental to the layer growth though the cathodic half-period of the current is mandatory to improve the coating growth. It is therefore necessary to manage the current waveform to avoid creating such detrimental discharges
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Alexandre Nominé. Micro-décharges en milieu électrolytique aqueux et leur interaction avec les matériaux : le cas du procédé d'oxydation par plasma électrolytique (PEO). Autre [cond-mat.other]. Université de Lorraine, 2014. Français. ⟨NNT : 2014LORR0154⟩. ⟨tel-01751049⟩

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