Preparation Of Cr-based Protective Coatings On The Surface Of Zr Alloys And Their Resistance To Radiation Damage

With the use of nuclear reactors and the emergence of nuclear accidents,nuclear safety issues have attracted widespread attention.The key problem of nuclear safety is that although the fuel cladding material(Zr alloy)of the reactor can perform good corrosion resistance and mechanical stability under normal working conditions,including supercritical water corrosive environment and irradiation environment.However,under accident conditions——high temperature steam environment——it is easy to react with steam to produce hydrogen and cause an explosion.To solve this problem,a more promising method is to modify the surface of Zr alloy(that is,deposit a protective coating on the surface of Zr-based materials).This method does not need to change the existing nuclear system,so it has the advantages of economy,simplicity,short development cycle,and easy application.The Cr-based protective coating has many advantages such as low thermal neutron absorption cross section,good mechanical properties,corrosion resistance,and oxidation resistance,and is suitable for surface protection of Zr alloy.Previous studies have shown that Cr-based coatings do have excellent oxidation resistance behaviors and corrosion resistance in supercritical water,but few studies on the damage resistance behavior in the radiation environment under conventional working.The purpose of this paper is to explore the radiation damage resistance behavior of Cr-based protective coatings and provide corresponding experimental and theoretical supports for the application of such coatings in various,complex,and harsh service environments.For this reason,three typical Cr-based protective coatings,pure Cr,Cr-AlSi,and Cr-Al-Si-N,were prepared by magnetron sputtering technology commonly used in the industry.The damage resistance behavior of these coatings under different irradiation energy or dose was systematically studied,and their reliability as Zr-based protective materials was evaluated.Specifically,pure Cr coatings(10 μm-thickness),Cr62.8Al27.9Si9.coatings(5 μmthickness),and Cr37.6Al18.8Si6.3N38.3 coatings(5 μm-thickness)were prepared on Zr substrates by magnetron sputtering technology.These coatings were exposed to different irradiation environments.On this basis,a series of structural characterization methods,such as x-ray diffraction,scanning electron microscopy,atomic force microscopy,focused ion beam-field emission scanning electron microscopy,and transmission electron microscopy,were used to explore the effect of irradiation on the coating structure.In addition,the effect of irradiation on the mechanical properties of the coating was studied by nanoindentation test.The main results and conclusions are as follows:(1)Under the irradiation condition of high energy(5 MeV)and low dose(1× 1014 ions/cm2),the pure Cr coating exhibits the most excellent resistance to radiation damage.Structurally,the crystallinity of the coating deteriorates,the column boundaries are blurred,the porosity decreases,the packing density at the column boundaries increases,the coating surface is relatively flat,and the surface particles are smaller.Meanwhile,the coating has excellent mechanical properties,with high hardness(5.8±0.5 GPa)and high indentation modulus(214.3±7.8 GPa).The internal structure of the coating is compact,the indentation surface morphology is complete,no cracks,and the deformation resistance of the coating is strong.In addition,the wettability of the pure Cr coating surface is reduced after irradiation.(2)The Cr62.8Al27.9Si9.3 coating shows certain radiation damage under the condition of low energy(200 KeV)and high dose(5×1016 ions/cm2).Structurally,the columnar crystal structure inside the irradiated coating is relatively complete,and large particles aggregate on the coating surface.After the nanoindentation test,extended cracks appear at the corner of the indentation,and a large number of microcracks also appear in the range of irradiation depth.In terms of hardness,compared to the as-deposited coating(12.3± 1.7 GPa),the hardness of the irradiated coating(7.2±0.4 GPa)decreases by about 5 GPa,although its hardness is still much higher than that of the bare Zr substrate(3.4±0.3 GPa).The appearance of cracks and the decrease in hardness indicate that the Cr62.8Al27.9Si9.3 coating is difficult to continue to provide protection.(3)Cr37.6Al18.8Si6.3N38.3 coating also shows some radiation damage under the condition of low energy(200 KeV)and high dose(5×1016 ions/cm2).Structurally,the cross-section of the Cr37.6Al18.8Si6.3N38.3 coating after irradiation demonstrates uniform and featureless amorphous morphology.In terms of hardness,compared to the asdeposited coating(10.2±0.5 GPa),the hardness of the irradiated coating(8.0±0.4 GPa)is only reduced by 2 GPa,and the coating can still maintain a certain degree of hardness.After the nanoindentation test,an open crack(extending from the substrate to the middle of the coating)appears in the irradiated coating,indicating that its resistance to deformation has dropped sharply.Although it can maintain a certain degree of hardness,the formation of open cracks also means that the Cr37.6Al18.8Si6.3N38.3 coating is difficult to continue to provide the protection required by the reactor.In summary,through the analysis of the growth structure and mechanical properties of the three Cr-based thick coatings after irradiation,we found that the pure Cr coating has excellent radiation damage resistance.Specifically,after being irradiated by high-energy ions,it still exhibits the characteristics of high hardness and resistance to mechanical deformation.Considering the excellent characteristics of pure Cr coating in extremely harsh service environments(corrosion and oxidation)when the reactor is in normal operation.It can play multiple protective roles in the changeable,complex,and harsh service environments in nuclear reactors.At the same time,pure Cr coating,as a single-element pure coating,has low production and application costs and is easy to put into practical application on a large scale.For Cr62.8Al27.9Si9.3 and Cr37.6Al18.8Si6.3N38.3 coatings,their radiation damage resistance is still insufficient,although existing studies have shown that their resistance to high temperature steam oxidation is better than pure Cr Coatings,therefore,they cannot yet fully meet the actual operating conditions in the reactor.In the following work,we will simulate the normal operating conditions(360℃/18.6 MPa)in the reactor to conduct supercritical water corrosion experiments on the irradiated pure Cr coating,and further evaluate its practicability and reliability as a Zr based protective material.