Preparation And Performance Investigation Of Low-diffusion Platinum-Modified Aluminide Coating On Single Crystal Superalloys

Due to excellent resistance to both high temperature oxidation and hot corrosion,Pt-modified nickel aluminide coatings(mainly ?-(Ni,Pt)Al)are extensively used to protect key structureal components(e.g.turbine blades made of Ni-base single crystal superalloy)in both aero and land-based gas turbine engines.In contrast to good oxidation resistance,at high temperature the Pt-modified nickel aluminide coatings would generally cause elemental interdiffusion because there exist notable concentration differences(e.g.,Al and Ni)between the coating and the substrate,which usually boosts the coating to premature failure by phase degradation;besides,the elemental interdiffusion would induce formation of harmful needle-like topologically-close-packed(TCP)phases and rapid growth of secondary reaction zone(SRZ)in the subsrate of single crystal(SC)superalloy.It is deemed that the formation of SRZ could deteriorate mechanical properties(especially creep rupture life and fatigue)of SC alloy by destroying the coherent structure of ?/?'.In this study,two methods are adopted to alleviate the interdiffusion problem between ?-(Ni,Pt)Al coating and SC substrate.One is to add a layer of Ni prior to depositing ?-(Ni,Pt)Al coating,using the precedent Ni-plating as Ni source for(Ni,Pt)Al growth during aluminization,which aims to relieve the TCP precipitation in SC and hold good oxidation resistance;the other is to prepare Ni-Re film as precursor of Re-base diffusion barrier for ?-(Ni,Pt)Al coating on Ni-and Ni3Al-base SC superalloys to acquir a low-diffusion NiRePtAl coating.The following results have been obtained:A(Ni,Pt)Al/Ni composite coating was prepared by sequential treatments of electroplating Ni and Pt,and successive gaseous aluminization.In comparison with normal(Ni,Pt)Al,high temperature performance of the composite coating was evaluated in isothermal oxidation test at 1100?.There was no interdiffusion zone(IDZ)formed beneath the ?-(Ni,Pt)Al/Ni coating after aluminization due to sufficient Ni supply by the precedent Ni-plating.The formation of SRZ under ?-(Ni,Pt)Al/Ni coating has been postponed to being oxidized after 100 h.Both the two coatings exhibited good resistance against isothermal oxidation,but the interdiffusion of elements between composite coating and SC superalloy substrate was greatly relieved,in which the thickness of SRZ and the amount of precipitated TCP phase in the SC alloy matrix were significantly decreased.A low-diffusion(Ni,Pt)Al coating with Re-base diffusion barrier(DB)was prepared on a second-generation single-crystal superalloy via electroplating and gaseous aluminizing,in which the electroplating consisted of depositing Ni-Re and Pt,sequentially.Compared with normal(Ni,Pt)Al,cyclic and isothermal oxidation performances of the coating with Re-base DB were evaluated at 1100? in static air.It is indicated that the Re-base DB,with ?-Re(Cr)crystal structure,contributed to better oxidation resistance and lower inter-diffusion extent,where the thickness of secondary reaction zone decreased by 49%after 1000 h isothermal oxidation.Since 100 h,the thickness and morphology of the Re-base DB layer retained with little change till 1000 h,which confirms that at high temperature the Re-base DB had good stability to the Ni-base SC superalloy substrate and(Ni,Pt)Al coating.The solution of Re-base DB can be applied to Ni3Al-base SC superalloy as well,in which the intermetallic compound was designated for serving at higher temperature with prolongable service life.A low-diffusion NiRePtAl coating((Ni,Pt)Al out layer plus Re-rich diffusion barrier layer)was prepared on a Ni3Al-base single crystal superalloy via similar electroplating and gaseous aluminizing treatments.In comparison with conventional NiAl and(Ni,Pt)Al,cyclic and isothermal oxidation performance of the NiRePtAl coating was evaluated at 1100 to 1200?.It is indicated that the oxidation resistance of NiRePtAl coating was significantly improved by higher presence of residual ?-NiAl phase in the outer layer and lesser outward diffusion of Mo from substrate,where more adherent and intact alumina scale was formed on the NiRePtAl coating after oxidation.In addition,the coating with Re-rich diffusion barrier showed lower interdiffusion extent to the substrate,where the thickness of second reaction zone(SRZ)in substrate alloy decreased by 25%.All these confirm that the low diffusion(Ni,Pt)Al coating showed better oxidation performance,while effectively inhibited the elemental diffusion between coating and substrate.