Effect Of Coupled Multiple Factors On Formation And Mechanism Of α-Al₂O₃ Coatings As Tritium Permeation Barrier

The preparation of a certain thickness of ceramic material on stainless steels with low diffusivity（so-called penetration barriers） is one of key scientific and technological issues in support of the development of fusion energy. Among the various ceramic materials, α-Al₂O₃ is of special interest owing to its low tritium permeability. However, the high temperature for formation of α-Al₂O₃ phase（usually>1000℃） always causes the degradation of substrate materials’ properties. In this work, FeAl/α-Al₂O₃ coatings were deposited onto the 316 L stainless steel substrate at a lower temperature（580oC） using double-glow plasma surface alloying technique. The formation of Fe-Al transition layer could increase the adherence of the Fe Al/α-Al₂O₃ coatings. The transformation temperature from the metastable phase to α-Al₂O₃ successfully reduced by inducing both α-Al₂O₃ seeds crystals and rare earth element Y.The microstructure, chemical composition and phase components of the prepared coatings were characterized respectively by means of glancing-angle X-ray diffraction（GAXRD）, field emission scanning electron microscope（FESEM）, energy diffraction spectroscopy（EDS）, X-ray photoelectron spectroscopy（XPS） and field emission transmission electron microscope（FETEM）. The growth mechanism of the Al2O3 coatings and the inducing mechanism of α-Al₂O₃ under the action of coupled multiple factors（energetic ion bombardment, α-Al₂O₃ seeds crystals and rare earth element Y） were proposed. Then, the bonding force, hardness, wear resistance and corrosion resistance of the oxide coatings with different factors were investigated individually. Finally, the deuterium permeation properties of the dense coatings were tested and analyzed. The main conclusions were drawn as follows:1) The plasma oxidation mechanism of Al coatings was studied by experimental and theoretical methods. The experimental results indicated that high-energy O2- ions could be injected into the inner part of the coatings, which exhibited a good agreement with that calculated by the SRIM software. Through simulations of the Vienna Ab-initio simulation package, it could be found that the diffusivities of O2- ions were imporved by the vacancies formed in the coatings. Al coatings were deposited directly onto pure iron substrates at different temperatures to form Fe-Al coatings by the double-glow plasma surface alloying technique. Then, the Fe–Al coatings were plasma oxidized at 580 oC. The results indicated that FeAl and Fe3 Al phases were easily formed at the intermetallic/substrate interface owing to the sputtering, re-sputtering, and deposition processes at the beginning of the experiment. The deposition and diffusion occurred simultaneously when the voltage and current of the double cathodes remained the same. The mutual diffusion is more easily performed by thermal diffusion and ion bombardment even at low temperature of 580 oC in this study. The aluminide layer was nanocrystalline with many crystal defects after the aluminizing process. The oxygen diffusivities increased owing to the short-circuit diffusion caused by the existence of large amounts of grain boundaries and crystal defects. After the oxidation, α-Al₂O₃, γ-Al2O3, and Fe2O3 phases were detected. Large amount of voids filled with oxide ions were formed at the outermost aluminide layer owing to the different intrinsic diffusivities of Fe and Al.2) The preparation parameters of double glow plasma technique were optimized to obtain high binding and thick α-Al₂O₃ coatings on the 316 L stainless steel at low temperature of 580 oC. Parameters including working pressure, source electrode voltage, substrate voltage and the parallel distance were designed by L9 orthogonal array. The results indicated that the anode-cathode distance was the factor that mostly affected the thickness of the films. The coatings were very dense, uniform, and compact to the substrate without any cracks and defects with the optimum parameters. The thickness of the coating was about 11.6μm. It was found out that the formation of α-Al₂O₃ was promoted as the increasing of oxygen flow. The FETEM observation indicated that the coatings contain α-Al₂O₃ seeds before oxidation. New-produced Fe-Al phases grew around the α-Al₂O₃ seeds. After plasma oxidation, the surface of the coatings was composed of pure α-Al₂O₃ while the inner layer of the coatings was mostly α-Al₂O₃ with little γ-Al2O3. These intriguing results were attributed to the dual effects of both the α-Al₂O₃ seeds and the high-energy ion bombardment, which provided more nuclei for the growth of α-Al₂O₃ and more energetic species at the surface of the coatings respectively. Furthermore, the coatings were well-bonded to the substrates because a SS/FeAl/（Al2O3+ Fe2O3）/Al2O3 structure was formed at the interface so as to avoid the coefficients of thermal expansion mismatch between the substrate and the deposited coatings.3)On the basis of the doping of α-Al₂O₃ seeds into the sputtering target, the rare earth element Y was investigated to find the influence on the preferred formation of α-Al₂O₃. The FETEM observation indicated that the element Y was discretely distributed in the film, which mainly concentrated in grain boundary of the coating. After plasma oxidation, Y was oxidized to form the Y₂O₃. Under the action of coupled multiple factors（ion bombardment, α-Al₂O₃ seeds and rare earth element Y）, the AlYα coating contained large amounts of α-Al₂O₃.4) The increase of Y content improved the adhesive force of the coatings. The corrosion current of the oxide coatings was reduced by 3 orders of magnitude. It was found that these two oxide coatings at 600 °C showed good retarding effects on the permeation of deuterium. The deuterium permeability was reduced by 4-5 orders of magnitude.