High Temperature Oxidation Behavior Of Nanocrystalline Coaing On A Nickel-base Single-crystal Superalloy N5

With increasing performance and oxidation resistacne of a second generation single-crystal nickel-base superalloy N5,has attracted intesests in advanced thrust-weight ratio and potentially using as blade material.To prolong the lifetime of the superalloy,high temperature protective coating is frequently used for protecting the substrate alloy under the extremely harsh environment.Usually,the coated samples show great improvement on oxidation and hot corrosion resistance.However,a series of problems are reported in using the traditional protective coatings,such as serious spallation of the oxide scales and interdiffusion between the coating and substrate,resulting in harmful effects on the oxidation performance and decreasing the mechanical properties under high temperature enviroment.Recently,sputtering nanocrystalline coating is designed as a new protective coating with better performance on oxidation resistance and perfectly avoided the interdiffusion between coating and substate alloy causing by differences amount of the intrinsic elements.In this study,a second generation of single-crystal nickel-base superalloy N5 is denfined as the substate alloy,the oxidation behavior of the sputtered nanocrystalline coating with same compositon of the substrate is investigated,using X-ray diffraction?XRD?,scanning electron microscopy?SEM?equipped with energy dispersive spectroscopy?EDS?and transmission electron microscopy?TEM?.A traditional overlay Ni-27Cr-11Al-0.5Y?wt.%?coating by arc ion palting is introduced as a contrast in the comparative study.Moreover,yttrium is added in the sputterd nanocrystalline coating for study the reactive elements effects on improving the oxidation behavior.The main research results are summarized as follows:Complex and non-protective oxides of NiO and NiAl₂O₄ formed on the surface of the substrate alloy N5 when oxidized at 900,1000 and 1100^oC.The oxide scale was thin and together with lower change in mass after a short term oxidation.With the oxidation temperature and time increasing,severe cracking and spallation was occurred on the oxide scale which shown obviously degradation on oxidation resistance of the substrate.The sputtering nanocrystalline coating enhanced the oxidation resistance of the single-crystal superalloy N5 at 900 and 1000?C,and the spallation resistance of oxide scale at 1000 and1100?C.The oxide scale was dense and adherent with?-Al₂O₃.Interdiffusion was not observed at the interface of the coating/substrate.The refractory element Ta acted as an reactive element,which segregated at the grain boundaries of alumina scale after oxidation at1100?C,resulting in the formation of columnar?-Al₂O₃ grains that grows at a lower rate than the equiaxed?-Al₂O₃ grains.The single-crystal superalloy N5 had a high oxidation resistance due to the formation of an inner?-Al₂O₃ layer after a long term oxidation at 1000 and 1100^oC.However,its spallation resistance of oxide scale was low.AIP NiCrAlY coating did not enhance the oxidation resistance,but enhance the spallation resistance to some extent of oxide scale of the superalloy substrate at both 1000 and 1100?C.However,interdiffusion occurred at this case after 1000 h exposure.A large number of Cr-,Re-,Mo-and W-rich TCP phases precipitated out from the IDZ and SRZ.The sputtered nanocrystalline coating enhanced both the oxidation resistance and spallation resistance of the superalloy substrate at 1000?C.In spite the isothermal oxidation rate was a little higher than the superalloy substrate at 1100?C,the nanocrystalline coating enhanced substantially the spallation resistance of oxide scale.Moreover,neither interdiffusion nor TCP phases were found at the interfacial area between the nanocrystalline coating and the single-crystal superalloy substrate even after exposure at1100?C for as long as 1000 h.The oxidation behavior of the yttium modified nanocrystalline coating was studied on isothermal oxidation and cyclic oxidation at 1050^oC.Yttrium addition delays transformation of aluminum oxide from metastable?phase to the stable?one formed on the nanocrystalline coating.The yttrium modified nanocrystalline coating?SNY?possesses a higher rate at the initial oxidation stage than the ordinary one?SN?.However,at the stable oxidation stage the rate constant of the SNY coating is one third lower.Yttrium addition prevents rumpling of oxide scale but promotes the formation of oxide pegs at interface between the oxide scale and the underlying nanocrystalline coating.Yttrium segregates at grain boundaries of the underlying nanocrystalline coating.It retards the transportation of tantalum from the nancrystalline coating to the oxide scale,greatly reducing the amount of tantalum oxides developed in the oxide scale on the nancrystalline coating.