a comparative study of four modified al coatings on ni3al ... · a comparative study of four...

A comparative study of four modified Al coatings on Ni3Al-based single crystal superalloy

Qiong WU, Rui-bo YANG, Yu-xiao WU, Shu-suo LI, Yue MA, Sheng-kai GONG

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

Received 10 October 2011; Accepted 10 December 2011

Abstract: Four modified Al diffusion coatings (Al, Cr−Al, Al−Si and Cr−Al−Si coatings) were prepared on Ni3Al based single crystal superalloy IC20. The oxidation tests were carried out at 1 150 °C for up to 100 h. Cyclic hot corrosion tests were carried out at 950 °C for 50 h. The results indicate that the oxidation and corrosion resistances of IC20 alloy are improved significantly by the coatings, and both oxidation and hot corrosion resistances of the four coatings are rated in the order (from worst to best) of Cr−Al, Al, Cr−Al−Si, Al−Si coatings. It is found that the degradations of Al and Cr−Al coatings are very quick due to the serious inter-diffusion between the coatings and substrates. The inter-diffusion between Si-containing coatings and substrates is reduced since Si effectively retards the outward diffusion of Mo. The weak effects of Cr and benefit effects of Si to the oxidation and hot corrosion resistance were discussed. The hot corrosion degradation mechanism of superalloy IC20 was analyzed. Key words: superalloy; oxidation; coating; chromium; molybdenum; Ni3Al; microstructure 1 Introduction

The increasing efficiency of advanced turbine engines requires higher operation temperatures for more complete combustion of fuels. Ni or Ni3Al based superalloys, as one kind of the most important materials in modern aviation industry, require both mechanical strength and oxidation resistance [1]. At the high working temperature above 1 000 °C, surface oxidation becomes the life limiting factor [2]. Various surface coating technologies have been developed to overcome the high-temperature oxidation problem. High Mo-contained Ni3Al based superalloys, such as IC6 [3−5], exhibit a high yield strength and fairly good ductility from room temperature to 1 100 °C, high creep resistance up to 1 100 °C, lower density, and lower production cost. However, the poor oxidation and corrosion resistance inhibited the application of the alloy at the high temperature due to the high-Mo content [6, 7]. Protective coatings should be appended for its application. Various kinds of coatings were prepared on IC6 superalloys, such as MCrAlY [8], MCrAlYSi [9−11], two-layer (MCrAlYSi+MCrAlY) bond coating [12], Re-based diffusion barrier coating [6, 13, 14], etc. It has been found that intense inter-diffusion would happen in

the IC6/MCrAlY coating system, especially the occurrence of Mo in coating surface, which is very detrimental to the anti-oxidation property. The small amount of Si addition to the coatings inhibits the diffusion of refractory elements (Ti, Mo, W, V) toward the surface and enhances the selective oxidation of Al, thus improving the coating performance [7, 15]. However, MCrAlY can withstand temperature no more than 1 100 °C due to the high Cr content [16−19]. NiAl or Ni3Al based coating can withstand the temperature more than 1 150 °C.

In the present work, four modified Al coatings of Al, Cr−Al, Al−Si and Cr−Al−Si coatings were developed and studied for the same Ni3Al-based superalloy IC20 which is a modified IC6SX alloy with the certain amount of the element Re. Oxidations were carried out at the temperatures up to 1 150 °C. Cyclic hot corrosion behavior was also studied at 950 °C. The purpose of the present study was to evaluate the oxidation behavior of the four coatings, and hopefully to make sense of the effects of Cr and Si on the oxidation property and diffusion behavior of the coatings. 2 Experimental

Ni3Al-based single crystal superalloy IC20 with the

Foundation item: Project (50971005) supported by the National Natural Science Foundation of China Corresponding author: Sheng-kai GONG; Tel: +86-10-82339003; Fax: +86-10-82338200; E-mail: [email protected]; [email protected]

Qiong WU, et al/Progress in Natural Science: Materials International 21(2011) 496−505

497 nominal composition (mass fraction) of 8% Al, 19%−15% Mo, 0.55%−3% Re, and balanced Ni was used as substrates. Samples with the diameter of 10 mm and the thickness of 3 mm were subsequently coated with four different aluminum coatings. All the coatings were prepared by powder pack cementation. The coating preparation technologies used in the present work were as follows.

1) Al pack cementation High Al activity pack cementation was carried out

at 850 °C for 90 min in an argon protected atmosphere using an Al2O3 crucible containing the specimens and a powder mixture of m(Al)׃m(Al2O3)׃m(NiH4Cl)=103׃87׃.

2) Al−Si pack cementation Al−Si pack cementation was carried out at 1 100 °C

for 90 min and the powder mixture was m(Al)׃m(Si)׃ m(Al2O3)׃m(NaF)=105׃75׃10׃.

3) Cr pack cementation The Cr pack cementation was carried out at 1 300

°C for 5 h in a mixture of m(Cr)׃m(Al2O3)׃m(NaF)= .3׃73׃24

Al coating was prepared by using technology 1, Al−Si coating was prepared by technology 2, Cr−Al coating was prepared by technology 3 and then 1, and Cr−Al−Si coating was prepared by technology 3 and then 2.

The oxidation tests were performed at 1 150 °C for 100 h. The specimens were air-quenched and weighed every 10 h. Cyclic hot-corrosion tests were performed in a furnace equipped with gas burner set-up. During the testing, the diluted sea-salt solution (NaCl 0.02 mol/L; MgCl2 0.01 mol/L; KCl 0.005 mol/L; CaCl2 0.002 mol/L) and aviation kerosene were atomized into the burner to simulate the hot-corrosion condition of advanced sea-based gas turbines. Each cycle consisted of 55 min heating at 950 °C in the furnace with burning gas corrosion environment and 5 min cooling in air. The hot-corrosion tests were carried out for 50 h to evaluate the hot-corrosion behavior of the four coatings. The mass change of the specimens was measured using a balance with 0.01 mg accuracy at room temperature. The oxidized and hot corroded samples were investigated by using an Apollo 300 field emission scanning electron microscope (FE-SEM) equipped with an Oxford energy-dispersive X-ray spectrometry system (EDS), and X-ray diffractometer (XRD). 3 Results 3.1 As-prepared coatings

The as-prepared coatings mainly consisted of δ-Ni2Al3, as identified by XRD results shown in Fig. 1. The α-Al2O3 peaks appearing in Al and Cr−Al coatings demonstrated that the coating surface was slightly

oxidized during coating preparation. Also, α-Mo phase could be detected in Al and Cr−Al coatings. α-Cr should exist in the Cr−Al and Cr−Al−Si coatings. However, the α-Cr and β-NiAl have a nearly identical lattice parameter that could not be identified according to the XRD results [20].

Fig. 1 XRD spectra of as-prepared coatings: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating

Figure 2 shows the cross-sectional microstructure of

the coatings in the as-prepared condition. All the coatings were relatively compact. The Al coating was composed of an outer layer and a thin interdiffusion zone (IDZ). The outer layer of Cr−Al coating consisted of δ-Ni2Al3 phase with dispersed complex Cr(Mo)-rich phase (α-(Cr,Mo)). Both Al−Si and Cr−Al−Si coatings had a two-layered structure with an outer layer mainly composed of δ-Ni2Al3 phase, and an inner layer composed of δ-Ni2Al3 and β-NiAl phase. The result of EDS analysis showed that the precipitates in the inner layer of Cr−Al−Si coatings were mainly chromium silicide phases. The precipitates in IDZ were enriched of refractory elements, such as Mo, Re, and/or Cr, and/or Si.

The average contents of the elements in each layer are given in Table 1. It was shown that the content of Al in the coating by high-activity Al pack cementation with the preparing method at the low temperature of 850 °C was much higher than that by low-activity Al−Si pack cementation at 1 100 °C. It has been observed that the contents of Mo in Al and Cr−Al coatings were very high, while the Mo contents were very low in Si-contained Al−Si and Cr−Al−Si coatings. This indicated that the diffusion of Mo into the coatings was very intense in the coatings without Si, i.e., the presence of Si in the coating could effectively retard the diffusion of Mo into the coatings. The average contents in the outer and inner layers of the Al−Si and Cr−Al−Si coatings demonstrated the presence of a lower amount of Si and higher amount of Al in the outer zone. It could be seen that the diffusion


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Fig. 2 Cross-sectional back-scattered electron micrographs of as-prepared coatings: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating Table 1 Average composition of coatings in as-deposited conditions (mass fraction, %)

Coating Ni Al Mo Si Cr Re

Al 45.44 46.32 6.43 − − 1.81

Cr−Al 42.18 43.15 5.84 − 7.20 1.64

Al−Si outer layer 61.30 33.86 0.43 3.39 − 1.02

Al−Si inner layer 64.24 30.01 0.52 3.97 − 1.26

Cr−Al−Si outer layer 57.45 33.53 0.37 3.57 4.02 1.06

Cr−Al−Si inner layer 59.84 29.83 0.54 4.44 4.49 0.86

rate of silicon was higher than that of Al during the Al−Si pack cementation process. 3.2 Oxidation behavior of coatings and substrates at

1 150 °C 3.2.1 Oxidation kinetics and products

Typical mass gain vs time plots for the oxidation of the coatings and substrates are shown in Fig. 3. All the specimens with coatings exhibited much better oxidation resistance than the bare alloy. The anti-oxidation property of the coatings increased in the following order: Cr−Al→Al→Cr−Al−Si→Al−Si. Due to the high Mo content in the prepared coatings as given in Table 1, Cr−Al and Al coatings exhibited poorer oxidation resistance. Coatings with Si and little Mo exhibited better

oxidation resistance. The coatings containing Cr performed poorer than coatings without Cr.

The comparison of the diffraction patterns for the four coatings in Fig. 4 shows that Al2O3 phase formed in all the coatings. However, there were some NiAl2O4 phases in Al and Cr−Al coatings. In addition, spinel Ni2SiO4 and a little SiO2 could be detected in Al−Si and Cr−Al−Si coatings. 3.2.2 Microstructures after oxidation tests

Figure 5 shows the surface microstructure of the four coatings after oxidized for 20 h. Al2O3 phase formed at all the three surfaces. It could be found that the surface morphologies of the four coating were different. The morphologies of both Al and Cr−Al coatings exhibited rough surfaces, many cracks formed in the surface Al2O3,


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Fig. 3 Mass gains as function of exposure time for alloys and coatings

Fig. 4 XRD patterns of coatings after oxidized at 1 150 °C for 100 h: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating and large blocks of spalled Al2O3 remained on the surface. And the surface of Cr−Al coating was much rougher than Al coatings, which is consistent with the poorer oxidation resistance of Cr−Al than Al coatings. However, both Al−Si and Cr−Al−Si coatings exhibited comparably much smoother surfaces, and the relatively fine and compact Al2O3 phases formed at the surface. The Al−Si coating showed much finer Al2O3 grains than Cr−Al−Si coatings.

Figure 6 shows the microstructures of the coating surfaces after oxidized for 100 h. The oxides of Al and Cr−Al coatings were multi-layered. The results of EDS and XRD showed that there was some Al2O3 grown on bare alloy, and some NiAl2O4 and NiO on the Al2O3 layer. For the Al−Si and Cr−Al−Si coating surfaces, the grain of Al2O3 grew much bigger after 100 h than Al2O3 that formed after oxidized for 20 h, which may be due to a sintering effect.

Figure 7 shows the cross-sectional microstructure after oxidized for 100 h. The oxides forming on Al and

Cr−Al coatings exhibited two layers: outer layer of NiAl2O4, and inner layer of Al2O3 which was not continuous. The cross-sectional microstructures were consistent with the surface microstructures in Fig. 6. The average compositions of the coatings after oxidized for 100 h are listed in Table 2. From Fig. 7, it could be observed that the δ and β phases in Al and Cr−Al coatings degraded into γ and γ' phases. Figures 7(c) and (d) show the cross-sectional microstructures of Al−Si and Cr−Al−Si coatings, respectively. A continuous Al2O3 layer formed on the Al−Si and Cr−Al−Si coating surface. The thickness of Al2O3 on Cr−Al−Si coating was much larger (~8 μm) than Al−Si coatings (3−5 μm). The structures of the coating changed into γ′ in outer layer and β phases in inner layer β. The β phase remaining in the coatings showed that the coatings still had some protective properties. Therefore, it might be considered that the interdiffusion layer in Si-containing coatings could act as an Al diffusion barrier compared with Si-free coatings.

It has been found that SiO2 did not form at 1 100 °C, while SiO2 formed at 1 150 °C. The reason may be as follows. The solubility of Si in NiO is low, and the slow diffusion of Si in the alloy results in a super saturation of Si in the coatings, which promotes the reaction [21, 22]: Si+2NiO=SiO2+2Ni (1)

Since the reaction rate increased with increasing temperatures, the SiO2 was not detected when the coatings were oxidized at 1 100 °C [23]. On the other hand, the NiO might incorporate SiO2 to form Ni2SiO4, which was spinel having the similar crystallographic and thermodynamic properties as Ni(Al,Cr)2O4. The SiO2 and Ni2SiO4 were detected by XRD, as shown in Fig. 4. 3.3 Hot corrosion behavior

The cyclic hot-corrosion tests were carried out for


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Fig. 5 SEM micrographs of surface of four coatings after oxidized at 1 150 °C for 20 h: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating

Fig. 6 SEM micrographs of surface of four coatings after oxidized at 1 150 °C for 100 h: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating


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Fig. 7 Cross-sectional microstructures of coatings after oxidized at 1 150 °C for 100 h: (a) Al coating; (b) Cr−Al coating; (c) Al−Si coating; (d) Cr−Al−Si coating Table 2 Average compositions of coatings after oxidized for 100 h

Composition (molar fraction)/% Coating Phase

Ni Al Mo Si Cr

Al outer layer γ 86.43 12.11 1.46 − −

Al inner layer γ′ 77.94 20.34 1.72 − −

Cr−Al outer layer γ 82.52 11.59 1.52 − 4.37

Cr−Al inner layer γ′ 77.78 19.15 1.39 − 1.68

Al−Si outer layer γ′ 74.40 23.01 − 2.59 −

Al−Si inner layer β 67.09 30.97 − 1.94 −

Cr−Al−Si outer layer γ′ 72.14 23.45 − 2.27 2.14

Cr−Al−Si inner layer β 65.80 30.44 − 2.15 1.61

50 h at 950 °C to evaluate the hot-corrosion behavior of the four coatings. The mass changes of the alloys and coatings after cyclic hot corrosion at 950 °C for 50 cycles were −163.26 mg/cm2 for IC20, −29.66 mg/cm2 for Al coating, −61.28 mg/cm2 for Cr−Al coating, 0.55 mg/cm2 for Al−Si coating, and 1.14 mg/cm2 for Cr−Al−Si coating, respectively, showing that the Al−Si coating had the best hot-corrosion resistance. Figure 8 shows the cross-sectional microstructures of the alloys

and coatings after cyclic hot corrosion at 950 °C for 50 cycles. Figure 8(a) shows that the bare alloy had a catastrophic corrosion, a porous layer of 40−50 μm formed on the surface, the oxide scale mainly consisted of NiAl2O4 and NiO, and internal oxidation was terrible. These features corresponded to Type II corrosion mechanism (acid dissolution). Figures 8(b) and (c) show that the hot corrosion resistance of Al and Cr−Al coatings was poor, and the whole coatings had been


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Fig. 8 Cross-sectional microstructures of coatings cyclic-hot-corroded at 950 °C for 50 h: (a) IC20 alloy; (b) Al coating; (c) Cr−Al coating; (d) Al−Si coating; (e) Cr−Al-Si coating corroded. It has been observed that serious internal oxidation happened in the alloy substrate, and the oxides in the coating mainly consisted of NiAl2O4. Figures 8(d) and (e) show that a continuous Al2O3 formed on the Al−Si and Cr−Al−Si coating surfaces, the protective layer of alumina was stable on the coating surface, and oxidation took place as the same in the absence of molten salt. 4 Discussion 4.1 Effects of Cr on oxidation behavior of coatings

Cr has been reported to accelerate the θ to α-Al2O3 phase transformation [24, 25] and result in a finer α-Al2O3 grain size [26], which may be due to the

increasing number of α nuclei after Cr addition. However, PINT et al [27] pointed out the addition of Cr to the NiAl alloy resulted in a decline of oxidation resistance. The experimental results in the present work show that the addition of Cr in the NiAl-based coating results in a detrimental effect both in Al coatings and Al−Si coatings. The reasons of the weak effect of the Cr addition to the oxidation resistance of NiAl may be as follows.

Firstly, it may be due to the low solubility of Cr in β-NiAl. It is known that the solubility of Cr increases with increasing Ni content in β-NiAl [28−30]. Since the Al-sublattice is almost completely occupied by Al, and the Ni-sublattice by Ni and Va (vacancy) in Al-rich side of B2-NiAl, large positive interactions between Cr and Ni (or Va) make the Cr solubility in the Al-rich B2 phase


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very low [31]. Due to the low solubility of Cr in β-NiAl, Cr leads to the formation of α-Cr precipitates at or near to the metal scale interface [15, 32]. Therefore, the α-Cr particles may weaken the metal-oxide adhesion and cause the oxide spallation [33]. It can be seen from Fig. 7 that lots of white precipitates (α-Cr,Mo) form at the interface between Al2O3 and coatings, which decreases the metal-oxide adhesion. It may be the reason why the oxidation resistance of Cr−Al−Si coating is more adversely declined than Al−Si coatings.

Secondly, the mixed (Al,Cr)2O3 has poorer protective properties than pure Al2O3. The atomic radius of Cr (1.85 Å) is slightly larger than that of Al (1.82 Å) [34]. The previous studies show that Cr substitutes for Al both in the spinel NiAl2O4 and in Al2O3 [35, 36]. The lattice energy of Cr2O3 is lower than that of Al2O3 [37], the reactive species inherently transport slower through the alumina than Cr2O3 [38] and the diffusion coefficient of Ni is 3.5 times more in NiCr2O4 than in NiAl2O4 at 1 200 °C [36]. All the above results suggest that metal ions diffuse faster through the mixed oxide (Al,Cr)2O3 layer than Al2O3 and also transport faster through Ni(Al, Cr)2O4 when it contains more Cr, and hence the mixed oxide has poorer protective effect.

The another reason for the negative effect of Cr in the β-NiAl coatings may be due to the instability of chromia in air at service temperatures above 1 000 °C. 4.2 Effects of Si on oxidation behavior of coatings

By comparison of Si-free and Si-containing coatings, the degradation of the coating was much slowed down in Si-containing coatings. After oxidized at 1 150 °C for 100 h, the δ-Ni2Al3 and β-NiAl phases in Al and Cr−Al coatings degraded into γ and γ′ phases, while Al−Si and Cr−Al−Si coatings remained to contain a large amount of β phases, showing good protective property of Si containing coatings. This may be attributed to the fact that Si can effectively retard the interdiffusion between coatings and substrates. Furthermore, since the formation energy of Mo−Si is low (130 kJ/mol at 1173−1623 K) [39], the Mo atoms can be strongly attracted by the Si atoms, which increases the diffusion activation energy of Mo.

It has been reported that Si can promote the selective oxidation of the protective Al2O3 scale forming in the initial corrosion stage and improve the adherence of oxide scale [40−46]. And NICKEL et al [47] reported that silicon addition could promote the formation of α-Cr phase. Therefore, the Si-containing coatings showed good oxidation and hot corrosion resistances in the present work. 4.3 Hot corrosion mechanism

The experimental results showed that IC20 alloys

with high content of Mo suffered catastrophic hot corrosions, which may be due to the reason that the Mo in the IC20 alloy has a very strong affinity with O2−. In the initial hot corrosion stage, MoO3 would form at the same time with NiO, Al2O3, and NiAl2O4. The reactive ability of MoO3 with O2− in the salt solution was very strong. The following reaction may happen: MoO3+O2−=MoO4

2− (2)

This reaction consumed the O2− in the salt/alloy interface, which made the salt solution near the interface become acid. At this time, the oxides began to decompose at the alloy surface, for example: NiO=Ni2++O2− (3) Al2O3=2Al3++3O2− (4)

At the same time, the ions like Ni2+, Al3+ and MoO4

2− would diffuse out into the salt/gas interface. When they reached the surface, MoO4

2− would be volatile in the form of MoO3 and O2− due to the high vapor pressure of refractory elements, and NiO and Al2O3 would precipitate as well. And these oxides would form a porous oxide layer at the surface. During this process, the activity of O2− could be kept by the dissolving of MoO3 at the alloy/salt interface and the volatile of MoO3 at the salt/gas interface, which made the reaction continue to proceed. This hot corrosion happened in the situation where the activity of O2− in the salt film deposited on the oxide surface was very low. The alloy, therefore, was oxidized with an accelerated rate.

By comparing the hot corrosion behavior of Al and Al−Si coatings, it can be seen that after a certain time of the hot corrosion, the Mo is still enriched in the alloys in Si-containing coating. While for Si-free coatings, Mo is enriched in the coating surface, which creates the acidic melt conditions in the subsequent hot corrosion process. Therefore, Si in the coating can fix Mo, so that Mo is not concentrated at the surface. Thus, the acid melting conditions can be destroyed, and Si plays a role in inhibition of acid melting damage. 5 Conclusions

1) The protection capability of the four developed coatings (from worst to best) is rated as follows: Cr−Al coatings, Al coatings, Cr−Al−Si coatings, and Al−Si coatings.

2) The addition of Cr has a detrimental effect on the oxidation and hot corrosion behavior of the coatings, which is attributed to the following reasons. The low solubility of Cr in β-NiAl causes the formation of α-Cr precipitates at or near to metal scale interface, which weakens the metal-oxide adhesion and causes the oxide


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spallation. The mixed (Al,Cr)2O3 has poorer protective properties than pure Al2O3. The chromium is instable in the air at a temperature above 1000 °C.

3) The addition of Si has a beneficial effect on the oxidation and hot corrosion behavior of the coatings, which may be attributed to the following reasons. Si can effectively retard the diffusion of Mo into the coatings, the interdiffusion between coatings and substrates slows down, and the degradation of the coating reduces. Si promotes the formation of the protective Al2O3 layer and improves the adherence of oxide scale.

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a comparative study of four modified al coatings on ni3al ... · a comparative study of four...

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