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 α-Al₂O₃ was improved by dopingα-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 μm.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 η-Al₂O₃,without α-Al₂O₃.Finally,alumina coatings were prepared through hybrid techniques of HDA and MAO with doping α-Al₂O₃ seeds and Cr seeds.The results show that:（1）After doping α-Al₂O₃ seeds in the MAO electrolyte,the Al₂O₃ layer was composed of both η-Al₂O₃ and α-Al₂O₃.Theα-Al₂O₃ seeds was effective to induce the formation of α-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 η-Al₂O₃+α-Al₂O₃ +δ-Al₂O₃ +Cr₂O₃.The inducing effect of Cr seeds was also effective.