| Nuclear energy is a kind of efficient and clean energy.The development and use of nuclear energy is one of the energy themes in the new era.At the same time,the nuclear security issue has drawn increasing attention from all society.Zr alloy,as one of the main nuclear materials,is easy to be damaged and fail in the harsh environment such as high temperature,high pressure steam.Depositing Cr coating is one of the most promising ways to improve the high temperature oxidation resistance of Zr substrate.At present research,the Cr coatings are thicker and the Cr crystal is bulky,which damaged the thermal neutron economy and increased defects.The preparation of equiaxed dense Cr coating has become the focus of current research.Multilayer structure is an effective way to improve the structure and properties of coatings.Cr multilayer coatings with different structures were successfully prepared on the surface of Zr-4 alloy substrates by two techniques of alternating deposition of High power impulse magnetron sputtering(Hi PIMS)and DC magnetron sputtering(DCMS)and Magnetron sputtering assisted by ion source cyclic bombardment in this study.The thickness of each sublayer is adjusted by changing different period and the bombardment times of ion source.The effects of different sublayer thickness on microstructure,mechanical properties and corrosion resistance of Cr coating were studied.At the same time,the high temperature oxidation resistance and oxidation kinetics of Cr coatings were characterized and analyzed.And the main research results are as follows:(1)All Cr coatings prepared have a preferred orientation of(110).The surface structure of Cr coating becomes smoother,crystal clusters become smaller,and the density increases with the increase of alternating deposition period.The Cr coating with period T=8 has the minimum surface roughness of 25 nm and the minimum grain size of 30.9 nm,which corresponds to the highest Vickers hardness of 278 HV.All the coatings have good bonding properties,which increase with the increase of cycle number,and the maximum is 47.5 N.With the increase of the number of periods,the contact Angle of the coating surface increases.To some extent,the critical heat flux(CHF)of the pool boiling is reduced.Also,corrosion resistance of the coating increases with the increase of the number of alternating deposition period.The Cr coating with period T=8 has the minimum corrosion current density of 4.741?A·cm-2 and the maximum polarization resistance of 67997?.(2)Circular bombardment of ion source is beneficial to improve the structure and properties of the coating.As the number of bombardments increases,the coating becomes denser.Ion source bombardment etched the unstable structure and material on the sublayer surface,which introduced more interfaces and effectively improved the corrosion resistance of Cr coating.Cr coating with n=7 has the minimum corrosion current density of 1.142?A·cm-2 and the maximum polarization resistance of 17891?.However,the hardness of the coating is not improved obviously.All Cr coatings have good bonding properties,which increase with the increase of bombardment times.When the bombardment times n=7,the maximum bonding force of Cr coating was obtained,which was 45.4 N.At the same time,the roughness is minimal and the coating is the smoothest.(3)The deposited Cr coating has a good protective effect on the substrate oxidation at high temperature,and the effect increases with the decrease of the thickness of the sublayers(the increase of alternate deposition period and the increase of bombardment times).The thickness of oxide layer decreases with the decrease of sublayer thickness during the same oxidation time.The Cr coatings with the maximum deposition period T=8 and the maximum bombardment times n=7 have the smallest oxide thickness and the best oxidation resistance at high temperature.The oxidation kinetics results show that the oxidation kinetics of the coating conforms to the rule that the square of the oxide layer thickness is proportional to the oxidation time.It shows that the coating is in the initial oxidation stage and has a good protective effect.The parabolic rate constants kp fitted of Cr coating oxidation kinetics curve of period T=8 and bombardment times n=7 are also the smallest,which are 15.12?m2·h-1 and 8.61?m2·h-1,respectively.The design of multilayer structure can interrupt the growth of columnar crystal,reduce the columnar gap,reduce the grain size,improve the compactness of the coating and improve the high temperature oxidation resistance.It provides a new idea to further design the structure of Cr coating and improve the high temperature oxidation resistance of Cr coating,and has certain guiding significance. |
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