| Nuclear-grade zirconium materials have been widely used in cladding materials of reactor core,but it has become an urgent problem solved to improve the high-temperature oxidation resistance of fuel cladding after the Fukushima nuclear accident.For nuclear fuel cladding,the key is to reduce the oxidation kinetics between the fuel cladding and high-temperature steam in order to decrease the generation of heat and the release of hydrogen.Based on this concept,two main strategies for developing new high-temperature oxidation-resistant cladding materials and coatings with excellent high-temperature oxidation and corrosion resistance have been proposed.However,the development of new cladding materials requires a longer period.Surface coating technology is the most effective way in the short time to make zirconium alloys have better resistance to high temperature,corrosion,and thorium and helium in long-term service condition.The formation of a dense oxide layer and the stability of the coating structure are of great significance to improve the safety and reliability of zirconium alloys in the reactor environment.At present,the research on anti-oxidation coatings mainly involves Fe Cr Al,MAX,ceramics,silicides,and Cr-based coatings.A large number of studies have also proved that the coating can improve the high temperature oxidation resistance of the zirconium alloy substrate.However,the single coatings in the high temperature oxidation environment formed some defects such as segregation,element diffusion,cracks,dissolution and the formation of non-protective oxide layer and so on.Therefore,multilayer coating combines the advantages of a single coating such as mechanical properties and oxidation resistance.So multilayer is designed to improve the oxidation resistance of the substrate.In addition,it is important that the improvement of the oxidation and corrosion resistance of the coating with lateral interface can hinder the internal diffusion of oxygen,the development of cracks,and the migration of elements.The quality of the coating is important to improve the application performance of the cladding in complex environments.In this paper,an advanced high target utilization sputtering?Hi TUS?was used to prepare dense Zr/X?X is Si,Cr,Ni?multilayer,in addition to,a Cr/Si multilayer was also prepared,and the oxidation and corrosion behavior of coatings in different environments was studied by adjusting the coating structure.The oxidation study of the Zr/Cr multilayer coating at 400°Cfor 60 h in air showed that the Cr layer with a lower diffusion coefficient of oxygen provided a channel for rapid diffusion of oxygen due to the large number of columnar crystals,thus leading to the oxidation of the internal Zr layer while the metal Cr layer was not completely oxidized.Although the structure of the oxidized Zr/Cr multilayer is stable,the Zr/Cr multilayer coating does not effectively hinder the internal diffusion of oxygen,indicating that the layer contains the columnar crystal structure exhibits weak ability to block oxygen.The oxidation study of the Zr/Ni multilayer coating at 400°C for 60 h in air found that the severe diffusion behavior of the Ni layer during the oxidation process resulted in the formation of larger holes at the interface,and the strong adhesion between the Zr and Ni interfaces was destroyed.The reason for the damage of the multilayer structure is that Ni has a higher diffusion coefficient in Zr,therefore the Ni layer has a serious diffusion to the Zr layer.However,it can be noted that only the outermost Zr layer was oxidized to form zirconia,and the remaining layers were not oxidized since oxygen has a lower diffusion coefficient in the Ni layer.Although the Zr/Ni multilayer hinders the internal diffusion of oxygen,but the serious diffusion behavior between Zr and Ni is occured,resulting in the multilayer structure is damaged,which indicates that the design of the coating structure needs to consider the interdiffusion coefficient between the elements.Since oxygen has a lower diffusion coefficient in Si,and Si has a lower diffusion coefficient in the Zr layer,so the Zr/Si multilayers were oxidized in air at 400°C for different time?10?200 h?,the study indicated that only the outermost Zr layer was completely oxidized,and a dense zirconia film was formed on the surface.The Si layer is still amorphous and the thickness was not changed much.With the increase of oxidation time,the original mixed phase of tetragonal zirconia and monoclinic zirconia changed into monoclinic zirconia phase.The microstructure shows that diffusion occurs at the Zr/Si interface to form a Zr-Si amorphous layer.Composition shows that Zr Si phase is formed in the Zr-Si diffusion region;a Zr-Si-O amorphous layer was formed at the Zr O2/Si interface.The Zr-Si-O amorphous layer maintains a stable system of Zr O2/ZSO/Si during the oxidation process.The thickness of the Zr-Si amorphous layer initially increases linearly,then increases parabolically with the increase of the oxidation time,and finally stabilizes.The mechanical properties of the multilayer coating after oxidation was studied by nano-indentation.When the outermost Zr layer is completely oxidized,the hardness and elastic modulus of the coating significantly increase,and then slowly increase with the increase of oxidation time.The trend,through nano-indentation simulation experiments,proves that the significant changes in hardness and elastic modulus values depend on the oxidation of the Zr layer,when the two Zr layers are completely oxidized,the simulated experimental values show that the elastic modulus increases linearly with the oxidation time,while the experimental results in fact show that the elastic modulus changes linearly during the initial oxidation time,and then increases slowly with the increase of the oxidation time,which indicates that the linear increase of the elastic modulus is only reflected by the oxidation of the first Zr layer.The increase of the value of the elastic modulus is due to the diffusion reaction between Zr and Si that has improved its mechanical properties to some extent.The oxidation of Zr/Si at 400°C for different time shows that the formation of the ZSO amorphous layer and the role of the Si layer make the coating maintain structural stability and improve excellent oxidation resistance throughout the oxidation process.When Zr/Si multilayers with different modulation structures were oxidized in autoclave?320°C,15.5 MPa?for different times?10?200 h?,the same phenomenon is that only the outermost Zr layer was completely oxidized.The others were not oxidized.During the corrosion process of the coating,since the inter-diffusion of Zr and Si,a Zr-Si-O amorphous layer was formed at the Zr O2/Si interface,which formed a stable Zr O2/ZSO/Si system.And the diffusion of Zr and Si inside the coating resulted in the formation of Zr-Si amorphous layer.However,when the thickness of the Zr and Si layers was adjusted to about 20 nm,it can be seen that a complete mixing phenomenon occured between the Zr and Si layers inside the coating after a long-term corrosion,which resulted in the multilayer structure was completely destroyed.This is because the thinner Si layer diffused into the Zr layer and was completely consumed.Multilayers corrosion experiments with different modulation structures shows that the thickness of the Si layer affects its structural stability.When the thickness of the Si layer was increased,the air oxidation experiment of the Zr/Si multilayers under high temperature?600°C,800°C?was explored.And the study found that after oxidation at 600°C for 20 h,the structure of the multilayer remained stable,and the interface was still flat,with only the outermost Zr layer was oxidized,and the internal Zr layer completely formed a Zr-Si mixed layer due to the diffusion of Si,which was mainly composed of the Zr3Si phase.After oxidation at 800°C for20 h,it can be observed that the increase of temperature promotes the diffusion between Zr and Si.Though the interface is uneven,t the multilayer film structure is still intact.For the structural analysis of the Zr layer,not only the outermost layer Zr was oxidized,but also the Zr layer inside the coating layer was oxidized,forming a Zr-Si-O layer that mainly composed of Zirconium silicate.But the Zr layer near the substrate was not oxidized,which formed a Zr-Si layer.For the change of the structure of the Si layer,it is known that the first layer of Si layer formed different structures with amorphous Si O2 and crystalline Si during the oxidation,the remaining Si layers were not oxidized.The same thing is that the outermost Zr layer at both temperatures was rapidly oxidized due to thinner Zr layer,so that the amorphous Zr-Si-O was not formed at the Zr O2/Si interface.High temperature oxidation experiments have again proved that the thickness of Si can not only stabilize the multilayer structure,but also effectively block the internal diffusion of oxygen and improve the oxidation resistance of the coating.As a comparative study of Zr/Si multilayers,the oxidation behavior of Cr/Si multilayer in high-temperature environment was studied.It can be seen that the element diffusion between Cr and Si was more intense after at 800°C for 20 h.The structure of the Cr layer becomed uneven due to diffusion,it can be seen that part of the Cr layers was consumed so that the structure was destroyed.The Cr-Si layer was formed by diffusion between Cr and Si.Noticing that the surface of the coating was oxidized to form Si O2due to the external diffusion of the Si layer during the oxidation was not chromium oxide.But the oxide layer of the coating is only Si O2on the surface,and the internal layer has not been oxidized.The reason for the analysis may be that the formation of the Cr-Si layer has a shielding effect on the internal diffusion of oxygen,so the coating shows good resistance to high temperature oxidation,but the Si O2 formed on the surface will dissolve in corrosive environment.,the Cr layer does not block the contact between Si and the aqueous solution,so it is not suitable for the reactor environment.A large number of experimental studies have shown that the thickness of a single Zr and Si layer has an important effect on the thermal stability and oxidation resistance of the multilayer structure.Therefore,a better Zr/Si multilayer structure should be designed based on the diffusion behavior of the elements,the evolution of the structure,and the oxidation behavior. |
PDF Full Text Request