Study On Structural Stability And Antioxidant Properties Of Amorphous Zr-Si-O Thin Films

Surface coating technology is an effective means to improve the oxidation resistance of nuclear zirconium alloys.The in-situ zirconia protective layer of zirconium alloy has its limitations.First,there will be a tetragonal phase to a monoclinic phase transition inside the zirconia,accompanied by the appearance of micropores and microcracks.The crystal defects provide a fast and short-range diffusion path,which will accelerate the diffusion of oxygen in the zirconia layer.Si can inhibit the formation of crystalline phases in a multi-element system,and Si O₂ is mostly in an amorphous state in some multi-oxide systems.Its fluidity can repair micropores and cracks generated during the corrosion oxidation process.In this experiment,an amorphous Zr-Si-O-coated zirconium alloy was prepared by doping Si in zirconia by magnetron sputtering.By exploring the reaction parameters?working pressure,target power,etc.?in a reactive co-sputtering system,DC/RF)on the structure of thin film coatings.It is found that when the Si/Zr ratio in Zr-Si-O reaches more than 0.26:1,the thin film is amorphous.Based on this,Zr-Si-O films with different Si contents were prepared,and the stability of the coating and zirconium alloy was investigated by introducing a Zr-4layer under the film.At the same time,the oxide film thickness of the Zr-4 layer was used as the standard.Protective effect of coating on zirconium alloy.A crystalline zirconia coating was first prepared as a comparative reference,and then a silicon element was added to the zirconia.When the Si/Zr was 0.37,the Zr-Si-O film was subjected to oxidation at 400?for 30 hours.It clearly shows better protection.However,after undergoing air-baking oxidation at 400?for different times,the silicon element in the coating gradually diffused into the interior,mainly because the atomic arrangement inside the coating was relatively loose.This indicates that the coating itself is not stable in an oxidizing environment,and the activation energy of element diffusion is small.Due to the chemical potential,the silicon element diffuses inward through the solid phase interface.As the silicon content decreases,the coating gradually evolves into a crystalline zirconia coating,which in turn loses its ability to block oxygen.Considering that the silicon-oxygen bond is shorter than the zirconium-oxygen bond,when the silicon atom content is increased to 0.63,the internal diffusion capacity of silicon is weakened,and the structural stability and oxygen blocking capacity of the coating are improved to a certain extent.As the oxidation progressed,the coating evolved into a two-layer structure-layer 1 and layer 2,mainly due to interfacial reactions—Zr in the Zr substrate reacted with oxygen in the Zr-Si-O coating,so Zr-Si-An O-poor layer of Zr-Si-O is formed at the bottom of the O layer,which is layer 2.The oxidation source of the Zr-4 layer mainly comes from the oxygen diffused from the outside and the oxygen in the Zr-Si-O layer.It should be noted that when the silicon content is very low,since the diffusion of external oxygen easily occurs and combines with the zirconium substrate,the oxygen partial pressure at the interface of the oxide matrix is high,so no oxygen-depleted layer layer 2 appears.In order to further improve the stability and atomic arrangement density of the coating,a coating with Si/Zr of 1 was prepared.During the oxidation of the sample,the internal diffusion of Si in the coating almost disappeared,mainly because the bonding in Zr-Si-O gradually evolved into a three-dimensional network mode with the increase of the silicon content,and the bond energy was enhanced.At this time,the diffusion activation energy of the element in the coating increases,and the thickness of the oxide film at the bottom of the Zr-4 layer changes parabolically and gradually stabilizes.It should be noted that part of the oxygen in the coating at the initial stage of oxidation also diffused to the Zr-4 layer through the interface,quickly evolved into a double-layer structure,and the thickness of layer 2 tended to be stable,and the interface of layer1/layer2 no longer moved.The thickness of layer 2 was slightly reduced in the subsequent oxidation because a small amount of oxygen in layer 2 continued to undergo slight internal diffusion.When the oxygen content in the layer 2 reaches 10%,it basically evolves into an amorphous zirconium silicon layer.At this time,the internal diffusion of oxygen atoms basically stops,and a stable oxygen blocking effect is achieved.By curve fitting the oxidation kinetics,it is found that it is significantly better than the conventional zirconium alloys reported in the literature.Analysis of coatings of different thicknesses found that the difference in thickness between 200 nm and 400nm did not affect the oxygen barrier ability of Zr-Si-O films.Further increase the Si content of the Zr-Si-O film?Si/Zr:1.3,1.8?,the internal atomic bond energy of Zr-Si-O increases,the interface reaction between the Zr substrate and the Zr-Si-O layer becomes difficult,and the initial reaction The thickness of the formed stable layer 2tends to decrease and eventually to zero.And because of its excellent oxygen barrier properties,when the Si/Zr is 1.8,the coating structure is very stable,no loss of silicon and oxygen elements occurs,and the inner zirconium alloy does not have layer 2 and Zr O₂ layers.The Zr substrate only contains partially dissolved oxygen.The changes of the internal microstructure of Zr-Si-O thin films under high temperature rapid annealing conditions are discussed next.When annealing above600?,the internal structure of Zr-Si-O thin film is no longer a single phase,but consists of two phases of zirconia nanocrystalline particles and amorphous silicon oxide,where the zirconia nanocrystals are surrounded by amorphous silicon oxide.After rapid annealing of Zr-Si-O films with different compositions,the zirconia nanocrystalline particles increased with the increase of zirconium content in the films.The same is that all the nanocrystalline particles are tetragonal zirconia,on the one hand,because the ternary covalent bond of Si-O-Zr exists at the interface between amorphous silicon oxide and zirconia,and the external Si-O and Zr form Strong interaction.The interfacial anchoring effect brought by this kind of force makes the tetragonal zirconia not transform to monoclinic.On the other hand,the amorphous silicon oxide restricts the diffusion and aggregation of the zirconium element and the tetragonal zirconia grains.It is too large to reach the critical grain size for the transition to monoclinic zirconia.Therefore,the addition of Si can make the tetragonal zirconia stable.This result has room for expansion in the design of nuclear power coatings.