Processing and oxidation behavior of Pt-diffused coatings
School of Materials Science and Engineering,Beihang University
Key Laboratory of Aerospace Materials and Performance (Ministry of Education),Beihang University
School of Energy and Power Engineering,Beihang University
摘 要：
Pt-diffused coatings were synthesized on Ni10 Al alloy by electroplating a thin layer of Pt(1-5 μm) followed by a diffusion treatment in vacuum at 1000-1100℃.Effects of processing parameters(Pt thickness and vacuum diffusion temperature) on the microstructures and chemical composition of Pt-diffused coatings were investigated.Also,the oxidation behavior of the coatings was studied at1100℃.Pt film reveals nano-sized grain size of 93-113 nm.Vacuum diffusion treatment leads to the grain growth.1-μm Pt film with 1000℃/1 h vacuum diffusion fabricates nano-sized Pt-diffused coating which exhibits the best oxidation resistance.And a single continuous Al2O3 scale forms on the coating with low Al(～9 at%)and Pt(～6 at%) contents.Increasing initial Pt thickness would introduce more defects and cracks,leading to a duplex scale and decreasing the oxidation resistance of Ptdiffused coatings.
作者简介：*Hong-Bo Guo,e-mail:guo.hongbo@buaa.edu.cn;
收稿日期：20 September 2016
基金：financially supported by the Nature Science Foundations of China (Nos.51590894,51425102 and 51231001);
Processing and oxidation behavior of Pt-diffused coatings
Liang-Liang Wei Hui Peng Lei Zheng Jing-Yong Sun Hong-Bo Guo
School of Materials Science and Engineering,Beihang University
Key Laboratory of Aerospace Materials and Performance (Ministry of Education),Beihang University
School of Energy and Power Engineering,Beihang University
Abstract：
Pt-diffused coatings were synthesized on Ni10 Al alloy by electroplating a thin layer of Pt(1-5 μm) followed by a diffusion treatment in vacuum at 1000-1100℃.Effects of processing parameters(Pt thickness and vacuum diffusion temperature) on the microstructures and chemical composition of Pt-diffused coatings were investigated.Also,the oxidation behavior of the coatings was studied at 1100℃.Pt film reveals nano-sized grain size of 93-113 nm.Vacuum diffusion treatment leads to the grain growth.1-μm Pt film with 1000℃/1 h vacuum diffusion fabricates nano-sized Pt-diffused coating which exhibits the best oxidation resistance.And a single continuous Al2O3 scale forms on the coating with low Al(～9 at%)and Pt(～6 at%) contents.Increasing initial Pt thickness would introduce more defects and cracks,leading to a duplex scale and decreasing the oxidation resistance of Ptdiffused coatings.
Keyword：
Platinum-modified γ+γ′coatings; Pt-diffused coatings; Electroplating; Oxidation; Nano-structure;
Received： 20 September 2016
1 Introduction
Platinum-modified aluminide (Pt-Al) coatings have been used to provide oxidation resistance for Ni-based superalloy components of gas turbine engines for decades
Recently,a new type Pt-Al coating with low Al content,which is called Pt-diffused coatings (sometimes called Ptmodifiedγ+γ'coatings or low-cost bond coats),has attracted increasing attention
Jiang and Gleeson
Niu et al.
2 Experimental
High-purity nickel and aluminum (purity higher than99.99 at%) were used.The Ni10Al button was produced by arc melting,casted in argon atmosphere and then annealed in vacuum at 1150℃for 24 h to ensure homogenization of the microstructure of the alloy.The chemical composition of annealed Ni10Al alloy is listed in Table 1.The concentration of sulfur impurity was also determined by inductively coupled plasma (ICP-AES,PHI700,Ulvacphi).Specimens were cut from the center of the button-shaped alloy into 10 mm×8 mm×3 mm,ground to 800-grit SiC finish and ultrasonic ally cleaned in alcohol and acetone prior to Pt plating.
Pt-diffused coatings were produced onto Ni10Al samples by electroplating Pt films with different thicknesses of1,3 and 5μm,followed by 1-h vacuum heat treatment(～1.1×10-4 Pa) at 1000 or 1100℃.Cyclic oxidation tests of the coated samples were carried out in an air furnace equipped with an automation system which allows the specimens moving in and out of the furnace automatically.Each specimen was hung in a pre-annealed alumina crucible by Pt wire to collect all oxide spalls.A cycle includes25-h heating in air furnace at 1100℃and subsequent10-min cooling out of the furnace to ambient temperature by compressed air.Mass changes were measured using an electronic balance (Sartorious CPA 225D,Germany) with a precision of 1×10-4 g.The mass changes for each coating were determined based on average weight changes of two specimens.
Table 1 Chemical composition of as-annealed Ni10Al alloy by ICP-AES
The surface and cross-sectional morphologies of the specimens were characterized by field emission scanning electron microscope (FE-SEM,Quanta 200F).The phases of Pt films and oxides were identified by X-ray diffractometer (XRD,Rigaku D/max2200 PC) using Cu Kαradiation.The chemical compositions of the coatings and oxides were determined by electron probe micro-analyzer(EPMA,JXA-8100).For cross-sectional observation,the specimens after 300-h oxidation were embedded in epoxy,ground and finely polished.
3 Results and discussion
3.1 Pt films by electroplating
After platinum electroplating,Pt films were characterized by XRD.The lattice constant of Ni crystal is 0.3524 nm(PDF 65-2865),while that of Pt is about 0.3923 nm (PDF65-2868).At the beginning of plating,Pt atoms deposited as Ni lattice constant like epitaxial growth.When Pt thickness is up to 5μm,the coating lattice changes to Pt lattice constant,as shown in Fig.1.Curve 3 has almost the same peak position as standard Pt PDF card,while Curves1 and 2 shift to right position,due to the influence of the substrate Ni10Al alloy.It can be inferred that the driving force of the mismatch between Ni10Al and 5-μm Pt film would introduce more defects than 1-and 3-μm Pt film specimens.
The grain size of Pt films was calculated from XRD data by Jade 5.0 software,as listed in Table 2.In order to calibrate the peak FWHM of XRD equipment,a standard Si powder XRD pattern was selected as reference.Pt films are composed of nano-sized grains.
3.2 Effect of Pt thickness
Vacuum diffusion treatment of three specimens with different Pt thicknesses was conducted at 1000℃.For comparison,the specimen with 5-μm Pt film was also heattreated in vacuum at 1100℃.Hereinafter,the specimens with 1-,3-and 5-μm Pt film diffused at 1000℃are named as No.1,No.2 and No.3,respectively,while the specimen with 5-μm Pt film diffused at 1100℃is named as No.4.
Fig.1 XRD patterns of standard platinum card and 1-,3-and 5-μm Pt films
Table 2 Average grain sizes of Pt films before and after 1000℃diffusion calculated from XRD
The coating surface elements concentrations were analyzed by EDS,as shown in Table 3.Al does not uphill diffuse from the substrate (10 at%Al) at 1000℃.However,a little higher Al content is obtained when the diffusion temperature increases to 1100℃.It has been shown by Glees on et al.
The oxidation of all specimens was performed in air at1100℃.The mass changes of Pt-diffused coatings on Ni10Al alloys are compared in Fig.2.It should be noted that the coating with 1-μm Pt film diffused at 1000℃reveals the lowest mass change among all four coatings.Surface and cross-sectional morphologies of the specimens after 300-h oxidation at 1100℃are shown in Figs.3 and4.The corresponding surface chemical composition is given in Table 4.According to XRD result in Fig.5,external Al2O3 scales are formed on No.1,2 and 4 specimens.Surface morphologies (Fig.3a,b,d) are very similar with the oxide scale grown onβ-NiAl coating,showing typical worm-like ridge structure.
The spallation of the oxide scale can be seen in No.3specimen (Fig.3c).Combining XRD (Fig.5) and surface chemical composition results,it can be determined that the bright area is NiAl2O4 with residual small Pt particles,while the dark area is Al2O3.As seen in Fig.4c,the scale of No.3 specimen consists of outer MAl2O4 spinel layer and inner Al2O3 layer.
Table 3 Processing parameters and surface element concentrations of Pt-diffused coatings
Fig.2 Weight changes of Pt-diffused coatings on Ni10Al alloy at 1100℃/300-h oxidation test in still air
Fig.3 Surface backscatter images of specimens after 1100℃/300-h oxidation test a 1-μm Pt with 1000℃/1-h diffusion,b 3-μm Pt with1000℃/1-h diffusion,c 5-μm Pt with 1000℃/1-h diffusion,d 5-μm Pt with 1100℃/1-h diffusion
Fig.4 Cross-sectional backscatter images of specimens after 1100℃/300-h oxidation test a 1-μm Pt with 1000℃/1-h diffusion,b 3-μm Pt with 1000℃/1-h diffusion,c 5-μm Pt with 1000℃/1-h diffusion,d 5-μm Pt with 1100℃/1-h diffusion
Table 4 Surface elements concentration of Pt-diffused coatings after1100℃/300-h oxidation (at%)
Fig.5 XRD patterns of specimens after 1100℃/300-h oxidation test
In the present study,with the same vacuum diffusion process,increasing Pt thickness has a negative effect in high-temperature oxidation,which could be attributed to the defects introduced by electroplating.During low-temperature vacuum diffusion process (1000℃),inward diffusion flux of Pt is higher than outward diffusion of Ni and Al from substrate,so these defects and cracks are generated near the surface,which could be the short path for oxygen and accelerate the oxidation.As shown in Fig.6b-d,the surface morphologies of Pt-diffused coatings become coarser with Pt thickness increasing.The cross-sectional back-scattered morphologies of Pt-diffused coatings are shown in Fig.7,which are typical darkγ-Ni and brightγ'-Ni3Al phase.
In summary,1-μm Pt film is sufficient to act as the third element for Al selective oxidation,and increasing Pt thickness does not show any advantage in oxidation resistance with low-temperature diffusion process.
3.3 Effect of diffusion temperature
In order to examine the effect of diffusion temperature,No.3 and 4 specimens with the same initial Pt thickness (5μm)were comparatively studied at different diffusion temperatures.As shown in Fig.2,No.4 specimen diffused at1100℃shows better oxidation resistance than No.3.In Fig.6d and e,the surface of No.4 specimen is smoother than that of No.3,which has less cracks and defects due to the high diffusion temperature.In summary,thick Pt film needs high-temperature diffusion process,which could diminish the defects in Pt electroplating and make a smoother coating surface.
In the present work,1-μm Pt-diffused coating has the lowest Al and Pt content in all four coatings and it also has the finest gain size.As shown in Fig.6,high-temperature diffusion treatment leads to the growth of grain.The grain size of 1-μm Pt-diffused coating on Ni10Al grows to～267 nm after 1-h diffusion treatment at 1000℃,whereas 5-μm Pt-diffused coating grows to 1-5μm after1-h diffusion treatment at 1100℃.Grain boundary is a short path for Al and oxygen diffusion.If the grain size is nano-sized,external Al2O3 scale could form on NiCrAl alloy even at low Al content
Fig.6 Surface secondary electron images of specimens a 1-μm Pt film,b 1-μm Pt with 1000℃/1-h diffusion,c 3-μm Pt with 1000℃/1-h diffusion,d 5-μm Pt with 1000℃/1-h diffusion,e 5-μm Pt with 1100℃/1-h diffusion
Fig.7 Cross-sectional back scatter images a 1-μm Pt with 1000℃/1-h diffusion,b 3-μm Pt with 1000℃/1-h diffusion,c 5-μm Pt with1000℃/1-h diffusion,d 5-μm Pt with 1100℃/1-h diffusion
4 Conclusion
Pt-diffused coatings were produced by electroplating 1-5-μm Pt films on Ni10Al alloy and then followed by vacuum diffusion treatment at 1000-1100℃.Pt film reveals nanosized grain of 93-113 nm,which diffused at 1000℃keeps nano-sized structure.1-μm Pt-diffused coating shows the best oxidation resistance even at such low Al (10 at%) and Pt (6 at%) content.Increasing initial Pt thickness would introduce more defects and cracks during vacuum diffusion process,leading to a duplex scale and decreasing the coating oxidation resistance.
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