Fabrication Of Novel Tritium Permeation Barriers Via Hot-dipping Aluminum Combined With Micro-arc Oxidation And The Al₂O₃ Crystalline Structure Adjusting In The Gradient Composite Coating

The conception,principle and developmental states of Al₂O₃ tritium permeation barriers,crystal nucleus to induce a-Al₂O₃ formation,and hybrid techniques combining hot dip alumiinizing?HDA?with micro-arc oxidation?MAO?are reviewed.On the basis of the review,FeAl/Al/Al₂O₃ composite gradient coating was prepared by hot-dipping aluminum and micro-arc oxidation on the surface of 316L stainless steel to increase the thickness Al₂O₃ tritium permeation barriers.Furthermore,the content of ?-Al₂O₃ was improved by doping?-Al₂O₃ and Cr seeds.Firstly,gradient coatings were prepared by hot dip aluminizing on the surface of 316L stainless steel substrate.The surface morphologies and cross-sectional microstructure were investigated with optical microscope?OM?and scanning electron microscopy?SEM?.The energy dispersive spectrometer?EDS?and X-ray diffraction?XRD?was used to analyze the phase composition of the gradient coatings.The evolutions of the gradient structure changjing with HDA temperature and time.The cross-sectional microhardness of the HDA treated specimens were studied.The results indicate that:?1?The gradient structure obtained by HDA consist of four layers from substrate to the outside:316L substrate/intermetallic compound layer/two-phase mixed layer/pure aluminum layer.?2?The intermetallic compound layer consists of a mixture of intermetallic FeAl3+Fe₄Al₁₃ phases.The multi-phases mixed layer consists of Al+FeAl3+Fe₄Al₁₃ phases;the pure aluminum layer contains only Al.?3?The thickness of intermetallic compound layer and pure aluminum layer have maximum values at HD A temperature of 700? and 760?,respectively.?4?In our experiment conditions,the suitable HDA condition for MAO is developed at the HDA temperature of 760? and with the time of 11min.The corresponding maximum thickness of the pure aluminum layer is about 62 ?m.Secondly,then ceramic coatings were obtained on the aluminum coatings by micro-arc oxidation.The surface and cross-sectional morphologies of the composite gradient coatings were observed with OM and SEM.The phase composition of the Al₂O₃ ceramic coatings was analyzed with XRD.The results show that:?1?The composite gradient structure obtained by hybrid techniques of HD A and MAO consist of five layers from substrate to the outside:316L substrate/intermetallic compound layer/two-phase mixed layer/pure aluminum layer/Al₂O₃ layer.?2?The phase composition of the ceramic coatings is mainly composed of ?-Al₂O₃,without ?-Al₂O₃.Finally,alumina coatings were prepared through hybrid techniques of HDA and MAO with doping ?-Al₂O₃ seeds and Cr seeds.The results show that:?1?After doping ?-Al₂O₃ seeds in the MAO electrolyte,the Al₂O₃ layer was composed of both ?-Al₂O₃ and ?-Al₂O₃.The?-Al₂O₃ seeds was effective to induce the formation of ?-Al₂O₃ in MAO coatings.?2?Cr doped gradient coatings were successfully prepared.In the surface of the coatings,Al+Fe₄Al₁₃ + Fe₂Al₅ + FeAl6 + FeAl2 + Cr+ AlCr2 +Al13Cr2 phases was obtained.After doping Cr seeds in the HDA gradient coatings,the MAO Al₂O₃ layer was composed of ?-Al₂O₃+?-Al₂O₃ +?-Al₂O₃ +Cr₂O₃.The inducing effect of Cr seeds was also effective.