Preparation And Properties Of Oxide Tritium Permeation Barrier By A Duplex Treatment Of Double Glow Plasma Surface Alloying And Plasma Oxidation

Tritium permeation loss in the production and use process of fusion reactor is an important issue. The preparation of thin layers of ceramic material on stainless steels with low diffusivity (so-called penetration barriers) seems to be the most practical method to reduce or hinder the permeation of tritium through the substrates. 316L stainless steel is one of the most promising candidates as structural materials for fusion reactor owing to its excellent plasticity, high strength and low tritium permeability. Therefore, it was proposed in this work to prepare oxide tritium permeation barrier on 316L stainless steel by means of a duplex treatment of double glow plasma surface alloying and plasma oxidation. The existence of metal transition layer induced by this method can reduce the mismatch of thermal expansion coefficients between oxide coatings and steel substrates and improve the adhesive strength of the coatings. Hence, it can avoid the failure of the tritium permeation barrier due to excessive residual stresses generated in the coatings.The microstructure of the prepared coatings was characterized respectively by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy diffraction spectroscopy (EDS) and transmission electron microscope (TEM). And the growth mechanism of the coatsing was proposed. Then, the relationships between the microstrucure and the hardness, adhesion strength, thermal shock resistance and corrosion resistance of the oxide coatings were investigated individually. Finally, the tritium permeation properties of the dense coatings were tested and analyzed. The main conclusions were drawn as follows:1) Aluminizing was performed on the surface of 316L stainless steel with double glow plasma technique. A dense and homogeneous aluminizing layer formed at the optimum parameters. It consisted of FeAl3 and Al phases and exhibited a nanocrystal structure. The formation of nanocrystalline was because a large number of nano particles, which escaped from the surface of source materials under Ar ion sputter by glow discharge and subsequently deposited on the steel materials, could transform into an amorphous structure. The subsequent non-homogeneous nucleation made the amorphous structure transform into nano-crystalline. After 600°C plasma oxidation treatment, the oxide coatings mainly consisted ofα-Al₂O₃,γ-Al₂O₃ andθ-Al₂O₃ phases. The main phase of them isα-Al₂O₃.α-Al₂O₃ content in the coatings increased with oxygen flow rate and reached the maximum content of 62%. TEM characterization indicated that the Al₂O₃ coatings exhibited an exceptional structure of outmost surface amorphous and underlying subsurface nanocrystal. It was noticed that the oxide growth of aluminizing layer was controlled by outward diffusion of aluminum cations and inward diffusion of oxygen. Oxygen flow rates had a great effect on the microstructure of the prepared Al₂O₃ coatings. The dense and smooth Al₂O₃ coatings were prepared at the oxygen flow rate of 15 sccm. The coatings exhibited the best mechanical properties with a maximum hardness of 31 GPa and elastic modulus of 321 GPa. The adhesive force was 47 N. After 150 cycles of thermal shock quenched from 550°C to room temperature, no cracking was found in the coatings. The compact structure coatings showed excellent corrosion resistance with a corrosion current density of 3 orders lower than that of the steel substrate in a 3.5% NaCl solution.2) The chromizing layer formed on the steel substrate by means of double glow plasma technique mainly composed of Cr deposition layer, chromium carbide intermediate layer and the solid solution of Cr in Fe diffusion layer. Chromium oxide growth was determined by outward diffusion of chromium ions through the growing oxide layer. Oxygen flow rates had a great effect on the morphology of the prepared Cr₂O₃ coatings. It was found that the dense and smooth coating with a Cr/Cr₂O₃ structure prepared at oxygen flow rate of 10 sccm exhibited the best combined properties. The corresponding hardness and elastic modulus was 31 GPa and 370 GPa, respectively. The adhesive force was 32 N. The prepared coatings endured 150 cycles of thermal shock quenched from 550°C to room temperature without cracking and possessed a lower corrosion current density of about 2 orders of magnitude than that of the steel substrate in a 3.5% NaCl solution.3) During the process of aluminizing and chromizing with double glow plasma technique, the increase of Cr contents reduced the structural defects in the coatings. When the Cr contents reached 50%, a dense and smooth layer was formed. This nanocrystal structure of aluminizing and chromizing layer exhibited a duplex microstructure of an out Al-rich layer and inner Cr-rich layer. After 600°C plasma oxidation, a solid solution ofα-(Al,Cr)₂O₃ coatings formed. The oxide coatings presented an amorphous outmost surface and an underlying subsurface nanocrystalline structure. The analysis revealed that the oxidation of this layer proceeded mainly by inward diffusion of oxygen. Comparing to the single Al₂O₃ and Cr₂O₃ coatings, the (Al,Cr)₂O₃ coatings with denser and more smooth structure possessed better adhesion strength and thermal shock resistance.4) It was found that the oxide coatings on 316L stainless steel showed good retarding effects on the permeation of deuterium. The deuterium permeability at 600°C was reduced by 3 orders of magnitude, and the deuterium resistance of oxide coatings was the result of O?D formed in the coatings. Furthermore, our study found that amorphous/nanocrystalline composite structure could improve the deuterium permeation resistance of the oxide coatings significantly.