Study On The Preparation And Oxidation Properties For The NiAl And RE Zr-doped NiAl Coating On Nickel-based Superalloy

NiAl and Pt modified NiAl coatings are widely used as high-temperature protective coatings or bond coat of the thermal barrier coatings for nickel-based alloys.They are widely used in high-temperature components of ground gas turbines and aeroengines,such as combustion chambers,blades and so on.However,on one hand the cost of the Pt is too high.On the other hand,during the high temperature service process,the continuous oxidation of the coating,the oxide scale stress and the interface rumpling of the oxide scale and coating will lead to the spallation failure of the protective oxide sacle.Reactive elements?RE?,such as Zr,not only can save the cost,but also can improve the oxidation performance,are potential materials that are expected to replace Pt.When prepares a RE?e.g.,Zr?doped coating,the content of RE needs to be controlled.It poses a problem for the preparation of the RE doped coating.The pack cementation method is widely used in industrial production compared with other preparation methods because of its low production cost and simple process.However,the pack cementation method can cause the diffusion of the alloy elements from the substrate into the coating,which can affect the oxidation performance of the coating.Therefore,it is of great significance to study the preparation method of the RE-doped NiAl coating and study the effect of RE on the oxidation performance of the coating to reduce the cost of the coating and improve the oxidation resistance of the coating.In this paper,first,the NiAl coating was prepared by pack cementation method.The Al content in the pack powder was changed?10%Al,15%Al and 20%Al?to produce three kinds of NiAl coatings on the Hastelloy X substrate.The effects of Al content in the pack powder on the microstructure and oxidation properties of the coating were studied.It was found that with the increase of the Al content in the pack powder,the distribution of Al in the coating was basically the same,and the thickness of the coating was changed.The increase of the thickness resulted in the increase of the available amount of Al in the coating,which improved the ability of oxidation spallation resistant.However,when the coating thickness increased to a certain extent and increasing the thickness did not significantly enhance oxidation spallation resistant.Such as the thickness of 20%Al coating was more than 15%Al,but both ability of oxidation spallation resistant was similar.So from the oxidation performance and coating thickness,15%Al coating was a better choice.Secondly,the NiAl coatings were prepared on the substrates of pure Ni,Hastelloy X,DZ125,DZ411 and DD6 with 15%Al pack aluminizing process.It was found that:?1?The composition in the NiAl coating depended on the composition of the substrate,and the difference of the composition of the coating affected its oxidation performance.?2?The DZ125 and DD6 coating systems exhibited relatively good oxidation resistance owing to the presence of the RE Hf in the substrate.Especially DD6 coating system displayed the lowest scale growth rate,and the oxide pegs which improved oxide scale adhesion were detected in the DZ125coating system.?3?For the DZ411 coating system,the formation of TiO₂and the Ti/Ta-rich oxide in the oxide sacle was one of the reasons for the oxide spallation.A large number of?-NiAl to?'-Ni₃Al transformation leaded to more interface rumpling.It was the other important reason for the spallation failure of the DZ411 coating system.?4?For pure Ni and Hastelloy X coating systems,the elements?such as Hf,Ta,et al.?which played an important suppressive role in the formation of voids,did not exist in these substrates and the corresponding coatings.During oxidation process,a numerous formations of the interfacial voids decreased the adherence of the oxide scale,and then resulted in the large oxide scale spallation.In order to reduce the effect of substrate composition on the oxidation performance of the coating and to improve the oxidation performace,the Zr-doped NiAl coatings were prepared on the Hastelloy X substrate by electroplating and pack cementation method.It was observed that:?1?With the increase of Zr particles in the plating solution,the Zr particles were gradually uniform dispersed in the Ni-Zr coatings and the volume fraction of Zr in the Ni-Zr coating increased.?2?The electrodeposited samples were aluminized by pack cementation and annealed under argon atmosphere to get the different content Zr-doped NiAl coatings.Besides,the coatings were without elements from the substrate.The oxidation performace of different content Zr doped NiAl coatings prepared by the composite electrodepositing and pack cementation were investigated.The results showed that:?1?The ability of oxidation spallation resistance for Zr-doped NiAl coatings was better than the coating without Zr,and the coating of 0.5 g/L Zr loading exhibited the best spallation resistance.?2?During the transient oxidation stage,the amount of?-Al₂O₃ in the oxide scale decreased with an increase of the Zr content in the samples.That means the presence of Zr in the nickel aluminide coatings can accelerate the?to?-Al₂O₃ phase transformation.?3?The growth rates of oxide scale of Zr-doped coatings were lower than that of undoped Zr,0.3 g/L Zr and 0.5 g/L Zr-doped coating samples showed a lower oxide scale growth rate compared to other Zr-doped coating samples.?4?The stresses for the Zr-doped and the un-doped NiAl coatings were all approximately the same at the stage before the oxide scale had spalled.?5?The appropriate oxide pegs were the most important reason for a better spallation resistance of the 0.5 g/L Zr coating sample.