Oxidation-resistant Nanocrystalline Coating For Second-generation Single Crystal Ni-base Superalloy

In the oxidation environment,nanocrystalline structure can promote the selective oxidation of superalloys and improve the adhesion of oxide scale,which has attracted extensive attention in the field of material corrosion.The composition of nanocrystalline coating is the same as that of the substrate alloy.The trend of element interdiffusion between the coating and superalloy is low,which has little effect on the spueralloy.Moreover,the strength of the nanocrystalline coating is higher than that of MCrAlY and NiPtAl coatings,and it is not easy for surface rumpling occuring.The existing thermal barrier coatings?TBCs?are mainly the NiPtAl bonding layer/YSZ surface layer system,which is difficult to overcome the side effects caused by their mechanical properties.Then the above two characteristics of the nanocrystalline coatings are rare advantages for the single crystal nickel base superalloy turbine blades coated with TBCs,whose service temperature is 1050 ? or higher.Therefore,the widely used second-generation single crystal nickel-based superalloy N5 is adopted as the substrate alloy,and the effects of preparation,composition and structure design of the nanocrystalline coating on the high temperature oxidation mechanism have been studied in this paper.The following results have been obtained:The effect of the sand-blasting,mechanically polishing,and grinding of the N5 alloy on the cyclic oxidation behavior of the nanocrystalline coatings deposited by magnetron sputtering was studied.Surface finishing of the alloy substrate can affect the deposition process of the N5 nanocrystalline coatings,leading to a significant change in the size of columnar grains,thereby affecting the oxidation behavior.The columnar grain of the N5 nanocrystalline coating deposited on the sand-blasted substrate is very heterogeneous,and their size at some locations has reached to micro level.After cyclic oxidation at 1050 ?,obvious surface rumpling occurred.While,the columnar grains deposited on the mechanically polished and ground nanocrystalline coatings were very uniform,all of them were in nano-sized.No surface rumpling occured.Similarly,all the N5 nanocrystalline coatings have good cyclic oxidation resistance;Ta in the coatings participates in the oxidation process,ruins the purity of the oxide scale on the N5 nanocrystalline coatings,and changes the spalling mode of the oxide scale.Effect of chenmical composition on the oxidation behavior of the N5 nanocrystalline coating was studied.And the modifided nanocrystalline coatings suitable for different service temperatures were designed and prepared.When the N5 nanocrystalline coating with the same chemical composition of the alloy substrate was oxidized at 1050?,phase transformation occurred,resulting in the precipitation of Ta-rich ?'.Ta in Ta-rich ?'phase participates in the oxidation process.And a variety of oxides appear in the oxide scale,such as TaOx?X=2 and/or 2.5?and AlTa04,which damage the structure integrity of the oxide scale.Adding appropriate amount of Y into N5 nanocrystalline coating can effectively inhibit the movement of Ta from coating to oxide scale,thus improving the oxidation resistance.When the oxidation temperature was increased to 1150?,the side effects of Ta on the oxidation behavior of the N5 nanocrystalline coating are more obvious.While,the retarding effect of Y on Ta was also decreased significantly.With the increase of A1 content in N5 nanocrystalline coating,no tantalum oxides were observed on the oxide scale and no element diffusion between the coating and N5 single crystal alloy was found.This N5 nanocrystalline coating with high contents of A1 is more suitable for service at 1150?.Based on the advantages of the nanocrystalline coating and the traditional NiCrAlY coating,the nanocrystalline/NiCrAlY coating with excellent oxidation resistance and hot corrosion properties were designed and prepared.The coating consists of thick inner nanocrystalline layer and thin outer NiCrAlY layer.The grain sizes of both layers are nano-scale,which contribute to the formation of Al2O3.The inner nanocrystalline layer can eliminate the element diffusion between NiCrAlY coating and alloy substrate,while the outer NiCrAlY layer hinders the diffusion of S and O along the grain boundary of the coating.After oxidation at 1050? for 1000 h,400 cycles at 1050 0C and corrosion at 850 0C in 75 wt%Na2SO4 +25 wt%K2SO4 for100 h,a continuous and densed Al2O3 was formed on the nanocrystalline/NiCrAlY coating.Based on the above research,the influence of coating ration on oxidation behavior of nanocrystalline/NiCrAlY coating was studided.The nanocrystalline/NiCrAlY coating consisting of inner nanocrystalline layer and outer NiCrAlY layer was deposited on the N5 single crystal superalloy.The inner nanocrystaline layer can not only weaken the interdiffusion between NiCrAlY layer and alloy substrate,but also improves the spalling resistance of NiCrAlY coating.During oxidation,Ta in the inner nanocrystalline layer can diffuse to the coating surface through the outer NiCrAlY layer and be oxidized into Ta2O5.The content of Ta₂O₅ in the oxide scale increases with the increase of the thickness of inner nanocrystalline layer.