The Effect Of Hf Diffusion In Nickel-based Alloys On The Growth Of Al₂O₃ On The Surface Of Modified Aluminide Coatings

With the increasing of service temperature and trust-weight ratio of aeroengine combustion chamber,higher requirements for turbine blade are put forward.As blade materials,high-temperature superalloy cannot meet the need of performances.Therefore,it is necessary to utilize protective coatings on the superalloys.Among various protective coatings,the application of thermal barrier coating system is extensive.Thermally grown oxide（TGO）will be formed at the top-coat/bond-coat interface when TBC system is applied.The ability to form TGO which is continuous,slow growing and adherent controls the whole performances of thermal barrier coating system.Therefore,it is important to reduce the growth rate and adhesion of oxide scale for improving the service life of thermal barrier coating system.The addition of element in the aluminide coating,such as Pt,Y,Hf,Si,is beneficial to reduce the growth rate and improve the adhesion of oxide scale formed on the surface of bond coat.The formation of lower oxidation rate and excellent adhesion of oxide scale is promoted by the pre-oxidation treatment.At high temperature,Hf can affect the growth of oxide scale on the bond coat,such as growth rate and adhesion.The mechanism of the influence of HfO₂ distribution in oxide scale on the growth of oxide scale is not clear.The effect of pre-oxidation and Pt on Hf diffusion in the aluminide coating is not definite.In this paper,the effect of Hf diffusion on growth of oxide scale formed on several modified aluminide coatings deposited on DZ125 superalloy is studied.The growth of oxide scale of Pt modified aluminide coating after 144 h cyclic oxidation in air is studied.The growth of oxide scale of pre-oxidation modified aluminide coating after 144 h cyclic oxidation in air is studied,and 48 h pre-oxidation is performed in air plus water vapour and Ar plus water vapour.The growth of oxide scale of pre-oxidation modified Pt-doped-coating after 144 h cyclic oxidation in air is studied,and 48 h preoxidation is performed in air plus water vapour and Ar plus water vapour.Aluminide coating is prepared by pack cementation and Pt is deposited by electroplating.The surface composition phase of the specimens is examined by X-ray diffraction（XRD）.Surface and cross-sectional microstructures of aluminide coatings are analyzed by scanning electron microscopy（SEM）.Electron probe microanalysis（EPMA）with energy dispersive spectroscopy（EDS）is applied to characterize the coating composition and element mapping profiles.The main results of the experiments are as following:（1）No HfO₂ could be observed at the oxide-sale/simple-aluminide-coating interface.Local HfO₂ is formed at the coating/oxide-scale interface for the Pt modified aluminide coating.Owing to the occupied diffusion paths of Al by Hf segregation,the outward diffusion of Al is restrained.Therefore,the growth rate of oxide scale is reduced.The thickness of oxide scale formed on the surface of Pt modified aluminide coating is thinner.The rumpling of the interface between oxide scale and Pt modified aluminide coating is improved due to the formation of local HfO₂.Compared with the occurrence of voids in the oxide scale for Pt free aluminide coating,the oxide scale on the Pt modified aluminide coating is quite compact,indicating the excellent adhesion of Pt modified aluminide coating.（2）For the cyclic oxidation experiment of pre-oxidation modified aluminide coating,pre-oxidation in different atmosphere results in the different distribution of HfO₂ at the oxide-scale/coating interface.Local HfO₂ particle is formed at the oxidescale/coating interface for the aluminide coating after pre-oxidation in air plus water vapour.However,continuous HfO₂ layer is formed at the oxide-scale/coating interface for the coating after pre-oxidation in Ar plus water vapour.Continuous HfO₂ at the oxide-scale/coating interface reduces the growth rate of oxide scale,so the thinner oxide scale is formed on the aluminide coating after pre-oxidation in Ar plus water vapour.The formation of continuous HfO₂ improves the rumpling of the interface of oxidescale/coating.Compared with the formation of cracks in the oxide scale on the aluminide coating after pre-oxidation in air plus water vapour,the oxide scale is extremely compact for the coating after pre-oxidation in Ar plus water vapour,meaning the enhanced adhesion of the coating after pre-oxidation in Ar plus water vapour.（3）The difference of the growth rate of oxide scale on Pt-doped coating after preoxidation in air plus water vapour and in Ar plus water vapour is little.Continuous HfO₂ layer is formed at the oxide-scale/coating interface for Pt-doped coating after preoxidation in air plus water vapour.No HfO₂ is formed at the oxide-scale/coating interface for Pt modified aluminide coating after pre-oxidation in Ar plus water vapour.Internal diffusion barrier layer is formed in Pt modified aluminide coating after preoxidation in Ar plus water vapour.For the two coating samples,the interfaces of oxidescale/coating are smooth.（4）Compared with simple aluminide coating,Pt modified aluminide coating and pre-oxidation modified aluminide coating,the oxidation rate of oxide scale on the surface of pre-oxidation modified Pt-doped-coating is lowest.