Research On The Tuning Of Defects In Nanocrystalline Zirconia And Its Performance As Coatings For Zirconium Cladding Material

As an important protection method for nuclear industry,surface engineering plays an increasingly important role in improving the anti-oxidation and anti-irradiation properties for zirconium alloys.Oxides naturally form as a dense layer on the surface of the zirconium alloy during service.The phase stability and martensitic transformation of zirconia will have a significant effect on the coating quality,affecting the corrosion rate and irradiation process of the protected cladding tube.In this dissertation,ZrO₂ coating was artificially prepared using High Target Utilization Sputtering System?HiTUS?.The effects of reactive sputtering parameters such as the plasma launch system power?PLS?,the pulsed direct current target power?DC?,and the partial pressure of oxygen on the microstructure of the coating were studied.The content of tetragonal phase increases with the increase of the ratio of PLS/DC and the decrease of oxygen flow rate.Therefore,by adjusting the parameters of the HiTUS system,we deliberately controlled the phase composition and defect content in the zirconia coating.The oxidation behavior and irradiation behavior of the optimized nanocrystalline Zr O₂ coatings were also investigated.In the oxidative corrosion experiments,we found that the accommodation of oxygen during the oxidation process fills up the oxygen vacancies generated during the preparation of nanocrystalline ZrO₂ coatings,which results in parts of the tetragonal phase that stabilized due to the presence of oxygen vacancies,breaks through the bondage and turned into the monoclinic phase.Moreover,we found that the phenomenon of reverse optimum growth of some grain on the surface of the coating.At the same time,the anoxic system of the coating material makes the zirconium enriched on the coating surface reacted with the oxygen atoms enriched in the environment to produce new zirconia crystal grains.The newly generated disordered zirconia particles and the volume shrinkage during the martensitic phase transformation cause loosening and cracking of the coating,providing a path for the diffusion of oxygen atoms in the coating,which greatly accelerates the oxidative corrosion process.In the irradiation experiment,Ar⁸⁺was introduced as bombardment source for nanocrystalline ZrO₂ coatings.It was found that irradiation could introduce oxygen defects and lattice distortions,leading to a decrease in the average coordination number of zirconium,which enhances the Zr-O ionic bonding.The the elastic modulus of the coating was also increased.With the increase of irradiation damage,the high-energy particles“kicked off”the atoms at the edge of the monoclinic grain from the equilibrium position,leading to the rearrangement along with the crystal plane of the tetragonal phase,which constantly consumes the monoclinic phase and the tetragonal phase was formed around it.In order to investigate the structural evolution of the irradiated sample during oxidation,this paper innovatively uses Ar⁸⁺irradiation as coating modification treatment to further introduce different degrees of oxygen defects and lattice distortions in the coating.The effect of irradiation-induced different defect states on the oxidation corrosion process of the coating was also investigated.It was found that the samples modified by irradiation showed high retention of tetragonal phase after oxidation,and the oxide coating was still dense and uniform overall without large cracks.The oxidation resistance of the nanocrystalline ZrO₂ coating under different irradiation conditions was evaluated by measuring the thickness of the oxide film.It was found that the oxidation rate of the sample modified by irradiation was greatly reduced.One should be noted that the structure of tetragonal zirconia formed in the preparation process is not stable.It will reversibly transform back to monoclinic zirconia along with the volume shrinkage.The volume shrinkage during the transformation will destroy the integrity of the coating and accelerate the permeation of oxygen.In this work,the method of ion irradiation modification is innovative.It can not only promote the transformation of monoclinic phase to tetragonal phase,but also stabilize it to room temperature.Under normal working conditions in the reactor,the radiation will stabilize the tetragonal phase of the ZrO₂ coating due to its introduced defects and distortions,thereby acting as a barrier to the oxidative corrosion process,which is a self-reinforcing and self-stabilizing process that greatly improved the anti-oxidation properties of nanocrystalline ZrO₂ coatings.At this moment,it is difficult to say how long the self-reinforcing and self-stabilizing phenomenon will last.However,it will help us to elucidate the complex oxidation behavior in nuclear reactors under the synergistic action of oxidation and irradiation.