Studies On Preparation And Properties Of Cr₂N Coatings And Composite Coating As Tritium Permeation Barriers

As the main fuel of nuclear fusion energy,hydrogen isotopes deuterium and tritium,are easy to diffuse and permeate in the cladding metal structure material of fusion reactor,causing a series of serious consequences such as the loss of fusion fuel,hydrogen embrittlement of structural material and radioactive contamination of tritium.Therefore,how to prevent the leakage of deuterium and tritium has become one of the key problems to be solved.As its good tritium resistance and heat and corrosion resistance,ceramic materials are regarded as the best tritium resistance materials at present.Therefore,the development of high-performance tritium resistant ceramic coatings is of great significance to solve the problem of tritium permeation in the reactor.With the advantages of high chemical stability,low tritium permeability and thermal expansion coefficient close to that of low activated steel,Cr₂N coating is expected to become a new type of high-performance tritium resistance material.In this paper,Cr/Cr₂N double layer coating was successfully prepared,and the effects of nitriding temperature,nitriding time and other technological parameters on the microstructure,and comprehensive properties of Cr₂N composite coating were systematically studied.The main findings are as follows:Uniform and dense Cr₂N coatings was obtained by electrophoretic deposition combined with nitriding heat treatment.The coating surface was flat and without holes and cracks.After thermal insulation at 700°C for 2 h in a nitrogen-rich atmosphere,the Cr₂N coatings was prepared with small and uniform grain size.The grain size was only50 nm,which was much smaller than the grain size of Cr₂O₃（1μm）.The grains were closely bound together without obvious pores.The coating thickness is about 7μm,presenting a double-layer structure.The outer layer is 2μm thick with Cr₂N coating,while the inner layer is Cr/Fe transition layer.The formation of transition layer is conducive to improving the adhesion between the coating and the matrix.The mechanical properties,corrosion resistance and thermal shock resistance of Cr₂N coating were characterized.Its vickers hardness can reach 670 HV_500g.The coatings has good resistance to chemical corrosion,its self-corrosion potential is higher than the substrate,the self-corrosion current density is more than an order of magnitude lower than the substrate.The thermal shock resistance was tested at 600°C after 200thermal shock tests,the coatings remained intact without falling off,indicating that the coating was tightly bound to the substrate and had good thermal compatibility.The oxidation resistance of Cr₂N coatings was tested.The coatings was oxidized at 500～700°C and cooled naturally.Then the change of morphology was studied.Results showed that the coating was not oxidized below 650°C,and much Cr₂O₃ was generated at 700°C.By adjusting the oxidation time,Cr/Cr₂O₃/Cr₂N layered composite structure can be obtained.The formation of interface in the layered structure is conducive to improving its tritium resistance performance.