Preparation And Properties Of α-Al₂O₃Thin Coatings By Plasma Discharge Techniques At Lower Temperature

α-Al₂O₃coatings are candidates as tritium permeation barriers for structural materials of nuclearfusion reactors. However, the high temperature for formation of α-Al₂O₃phase （usually＞1000℃）always causes the degradation of substrate materials’ properties. In this work, α-Al₂O₃wassuccessfully obtained at a lower temperature （580℃） by introducing both energetic ionbombardment and the addition of α-Al₂O₃seed crystals. In detail, a duplex treatment of double glowplasma discharge surface alloying and plasma oxidation was carried out respectively to preparealuminide layer with different proportions of α-Al₂O₃seed crystals, and alumina coatings on thesurface of316L stainless steel. Besides, another process of arc added glow Al plating plus plasmaoxidation was also adopted.The microstructure, chemical composition and morphology of the coatings were characterizedrespectively by means of glancing-angle （1°） X-ray diffractometry （GAXRD）, scanning electronmicroscopy （SEM） and X-ray photoelectron spectroscopy （XPS）. Not only the processing parameterswere optimized, but also the bonding force between the coatings and substrates, wear resistance andcorrosion resistance of the coatings were evaluated individually in this study. Moreover, the influenceof seed crystals on the aluminide coatings, plasma oxidation process and properties of aluminacoatings were discussed. The main conclusions were drawn as follows:Firstly, α-Al₂O₃reinforced iron aluminide coatings were successfully prepared. The coatingswere dense with a FeAl₃/FeAl/Fe₃Al construction at the interface, obtaining metallurgical bonding. Inthe surface of the coatings, α-Al₂O₃seed crystals were well-distributed, the relative content of whichin the coatings increased with that in the targets. When the target possessed20%α-Al₂O₃, theas-prepared coating had the maximum bonding force of63.15N with the substrate. The compositecoatings exhibited a good wear resistance because the coatings were reinforced by well-dispersedAl₂O₃particles. And the corrosion resistance depended directly upon the content of α-Al₂O₃in thecoatings.Secondly, alumina coatings were prepared through plasma oxidation of the aluminide coatingswith α-Al₂O₃seeds. Except γ-Al₂O₃,θ-Al₂O₃and α-Fe2O3phases, α-Al₂O₃phase was obtainedsuccessfully at580℃. α-Al₂O₃with the content of65.54wt.%which was10.34%more than thatobtained by the oxidation of pure Al at the same condition, was obtained after plasma oxidation byusing the target dopted with10%α-Al₂O₃. However, the inducing effect became weak with the further increment of seed crystals’ content. When the target possessed20%α-Al₂O₃, the as-prepared coatingshad the maximum bonding force of71.4N with the substrates, and metallurgical bonding wasobtained at the interface. However, the Al₂O₃coatings were measured to be better wear and corrosionresistants when the target with10%α-Al₂O₃was used.Finally, in order to increase the deposition rate of Al layer and remove the impurity on thesurface such as Fe, arc added glow aluminium plating technique was adopted to preparenanocrystalline Al coatings without any other elements on the surface. α-Al₂O₃phase with the relativecontent of60.24%was obtained after heat treatment and plasma oxidation at600℃. The oxidizedcoating performed better wear and corrosion resistance than that obtained by double glow cathodesdischarge technique at the same condition with a lower fiction coefficient （0.55）, less volumetric wearloss （5.28×10⁶μm³ï¼‰ and corrosion current (0.197μA cm^-2).