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Microstructure degradation of simple, Pt- and Pt + Pd-modified aluminide coatings on CMSX-4 superalloy under cyclic oxidation conditions
Surface & Coatings Technology 215 (2013) 16–23
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Microstructure degradation of simple, Pt- and Pt + Pd-modified aluminide coatings on CMSX-4 superalloy under cyclic oxidation conditions Radosław Swadźba a,⁎, Marek Hetmańczyk b, Jerzy Wiedermann a, Lucjan Swadźba b, Grzegorz Moskal b, Bartosz Witala b, Krzysztof Radwański a a b
Institute for Ferrous Metallurgy, Gliwice, Poland Silesian University of Technology, Katowice, Poland
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Aluminide coatings Pt/Pd aluminide coatings Cyclic oxidation Superalloys
a b s t r a c t The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings' cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt + Pd electroplating, followed by vapor phase aluminizing at 1050 °C. Cyclic oxidation tests were performed at 1100 °C in 23 h cycles. Microstructural and phase analysis conducted using XRD, SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diffusion aluminide coatings are applied on jet engine turbine blades in order to provide high temperature oxidation and corrosion resistance. Incorporation of Pt to aluminide coatings makes it possible to apply them as bond coatings for thermal barrier coating (TBC) systems on blades and vanes operating under the most severe conditions [1–4]. Since the performance of the whole TBC system relies directly on the durability of the bond coat, its degradation mechanisms are widely studied in the literature. Reversible transformation from a β phase (B2) to a tetragonally distorted martensitic phase (L10) was observed in single-phase NiAl and (Ni,Pt)Al bond coats during quenching from high temperature both in isothermal and cyclic oxidation tests by many researchers [5–7]. This phenomena occurs once Al concentration in the β phase drops to 32–38 at.% and when the cooling rate is high enough to suppress the β–>ɣ′ transformation, as well as due to interdiffusion with the substrate alloy and alloying elements present in the coating, such as Pt, Cr and Co [6–8]. Chen et al. [6,9] have concluded that the strain produced by martensitic transformation during each cycle is 0.7% which is comparable to the thermal strain caused by the coefficient of thermal expansion (CTE) mismatch. This may lead to elastic stresses in the surface layer, destruction of the heat resistant oxide film and
⁎ Corresponding author. E-mail address: [email protected] (R. Swadźba). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.06.093
intensification of diffusion processes at high temperatures [10]. Zhang suggested that the volume changes probably contribute to the early stage of surface roughening [5]. Another type of transformation occurring in the β-NiAl coatings during oxidation is the alumina transformation studied in a number of articles [11,12]. While α-Al2O3 provides protective properties because of its thermodynamic stability and slow growth rate, other types such as θ and ɣ-Al2O3 exhibit low adhesion, porosity and volumetric changes with exposure time, hence they are not desirable [13,14]. The amount of α-alumina in the oxide layer during oxidation increases with exposure time and temperature and leads to reduction of parabolic rate constant by one order of magnitude [15]. Modifying aluminide coatings with Pt significantly improves the formation rate and stability of α-alumina, both in single and double phase PtAl coatings [16–18]. Other beneficial effect attributed to Pt is improvement of oxide scale adherence during cyclic and isothermal oxidation tests as well as mitigating the detrimental effects of sulfur impurities on scale adherence and inhibition of void growth at the scale–metal interface [19]. Li et al. [20,21] examined the influence of Pd on the transformations occurring in the oxide during high temperature exposure. Apart from the fact that palladium itself facilitates the θ to α transformation, it has been shown that it enhances the outward diffusion of Ti from the substrate to the coating surface which in turn forms TiO2 that accelerates the transformation from θ to α-alumina. Oxide layer that forms during high temperature oxidation is also susceptible to degradation due to void formation along the
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metal-oxide interface which has been studied in a number of papers [18,19,22,23]. This phenomenon is attributed to nonuniform Al/Ni diffusion, vacancy formation and/or Kirkendall effect [17]. Angenete et al. [23] proposed a void formation mechanism associated with volumetric changes accompanying the β–> ɣ′ phase transformation. Other studies [22] suggest that pores and voids tend to collapse during cyclic oxidation tests due to thermal stresses, while they coalesce during long-term oxidation. Incorporation of Pt significantly retards the formation of voids due to enhanced Al diffusion which leads to a twofold advantage: it levels the Al/Ni diffusivity and promotes
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Al2O3 formation filling the voids and increasing the contact area between the oxide and the metal [17,18]. Surface roughening during high temperature cyclic oxidation, referred to as rumpling, has been observed both in Pt-modified and simple aluminide coatings on Ni-based superalloys and extensively studied by Tolpygo et al. [22,24–26]. It is also considered as a significant degradation form since it leads to initiation of cracking and eventual separation between the TBC and the superalloy. Alperine et al. [27,28] studied high temperature performance of palladium modified aluminide coatings obtained applying various
Fig. 1. Surface microstructure of aluminide coatings after cyclic oxidation test: (a and b) simple aluminide coating, (c and d) Pt-modified aluminide coating, and (e and f) Pt/Pdmodified aluminide coating.
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Fig. 2. XRD analysis results from the surface of simple aluminide coating after cyclic oxidation test.
Pd–M (M = Ni, Cr or Co) predeposits which lead to the formation of defect-free coatings. The authors proved that all studied kinds of Pd-modified aluminide coatings exhibited excellent high temperature behavior in comparison to conventional aluminide coatings [31]. This paper is a continuation of our previous work [29] where we analyzed cyclic oxidation resistance and microstructures of as-deposited coatings and proved that incorporation of both Pt and Pd to aluminide coatings provides superior cyclic oxidation resistance over simple and Pt-modified aluminide coatings. 2. Experimental procedure The investigation concerned three samples of CMSX-4 Ni-based single crystal superalloy. The alloy's composition was [wt.%]: Al-5.13, B-0.007, C-0.01, Co-9.02, Cr-6.07, Hf-0.09, Mo-0.64, Re-3.21, Ta-6.57, Ti-0.92, W-6.09, and Zr-0.002. The samples of CMSX-4 superalloy (cylinder shaped, 10 mm diameter, 80 mm height) were put under heat treatment that consisted of solution and aging in order to obtain γ–γ′ microstructure. The two of the three samples were electroplated with 5 μm of Pt and 3 μm of Pd + 2 μm of Pt, respectively. After such a preparation all the samples were annealed at 1010 °C for 2 h and underwent “out of pack” diffusion aluminizing in CrAl alloy granules at 1050 °C in argon atmosphere. Simple and Pt/Pd-modified aluminide coatings consisted of β-NiAl phase containing 55.3 at.% (36.3 wt.%) and 53.6 at.% (27.7 wt.%) of Al, respectively. The Pt-modified aluminide coating exhibited double phase microstructure consisting of PtAl2 phase precipitates on grain boundaries of β-NiAl phase, which contained 57.7 at.% (34.2 wt.%) of Al [29]. Cyclic oxidation tests were performed at 1100 °C in 23 h long cycles in laboratory air environment. After failure the samples were cut, chemically coated with nickel plate, grinded and polished in order to examine their cross-section microstructure using a scanning electron microscope (FEI Inspect F device). The nickel plate was applied to the samples to protect the coating from damage that may be caused during the grinding process. Chemical composition in microareas and element distribution were examined using X-ray energy dispersive spectrometry (EDS). Phase compositions of the samples were identified using an X-ray diffractometer (XRD) JEOL JDX-7S and electron backscatter diffraction (EBSD). 3. Results and discussion 3.1. Simple aluminide coating after cyclic oxidation test The surface microstructure of simple aluminide coating after cyclic oxidation test is presented in Fig. 1a. Fig. 2b illustrates its characteristic
features along with areas analyzed using EDS, marked as 1–4. The results of chemical composition microanalysis are presented in Table 1. The surface of simple aluminide coating after cyclic oxidation test, presented in Fig. 1a, consists mostly of an oxide layer formed during the test, visible as gray-scale areas. What is more there are also bright areas of the substrate exposed by oxide layer spallation. There are three distinctive types of oxides, marked as 1, 2 and 3 in Fig. 1b. They differ by chemical composition and therefore by brightness on the image captured using backscattered electrons (BSE). Microarea 1 marked in Fig. 1b relates to the darkest and major oxide layer present on the coating's surface, which consists of two elements — Al and O, 39.5 at.% and 60.6 at.% respectively. Chemical composition of this oxide corresponds to Al2O3, which was detected using XRD (Fig. 2). Above this area there is a slightly brighter oxide layer, marked as 2. The predominant elements present in this area are Al, O and Ni. There are also slight amounts of Ti, Cr Co and Ta. However, the concentration of the main elements indicates the presence of NiAl2O4 spinel, which was also detected using XRD (Fig. 2). On the top of the described oxide layers there are fine and bright oxide precipitates, marked as 3 in Fig. 1b, containing, apart from Al and O, Ti and Ta. Their chemical composition refers to TiO2 oxide corresponding to XRD analysis. Chemical composition of the brightest and distinct areas, marked as 4 in Fig. 1b, is very similar to that of the substrate alloy which indicates that it was exposed by oxide layer spallation. What is more it has been identified as Ni3Al phase by XRD (Fig. 2). The cross-sectional microstructure of the simple aluminide coating after cyclic oxidation test along with EDS analysis markers is presented in Fig. 3a, the results of which are placed in Table 2. Fig. 3b illustrates Al-depleted NiAl phase grains after etching. The results of EBSD analysis in microarea 1 (Fig. 3a) are presented in Fig. 3c–d. After cyclic oxidation test the coating consist of ɣ′-Ni3Al phase grains and Al-depleted β-NiAl phase grains. The results of EBSD analysis conducted in microarea 1 (Fig. 3a) confirm the results obtained using XRD from the coating's surface. The presence of ɣ′-Ni3Al is a
Table 1 Chemical composition [at.%] in microareas 1, 2, 3 and 4, Fig. 1b. [at.%]
Al
O
1 2 3 4
39.5 27 22.9 12.1
60.5 53.7 65.8
Ni
Ti
Cr
Co
Ta
Mo
W
Re
11.1 2.3 58.4
2.1 6 0.5
2.3 1.1 13.4
3.1 0.6 12.6
0.8 1.4 1.4
0.4
0.3
0.9
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Fig. 3. Cross-sectional microstructure of simple aluminide coating after cyclic oxidation test (a), features revealed after etching (b), and Kikuchi pattern and corresponding indexes for Ni3Al phase (c and d).
result of β–> ɣ′ transformation which occurred due to Al depletion from the β phase during oxidation and repeatable spallation of the oxide layer. The ɣ′-Ni3Al phase contains 22 at.% Al, 61.5 at.% Ni, Co, and Cr and minor amounts of Ti, Ta and W. The Al-depleted β-NiAl phase grains contain 34.2 at.% Al, 53.4 at.% Ni and small amounts of Cr and Co (Table 2). What is more, etching of these grains revealed the presence of stripes along the Al-depleted β-NiAl phase, which are characteristic for martensitic transformation occurring between oxidation cycles, when the cooling rate is high enough to stop the β–> ɣ′ transformation [5–7]. Microstructural investigation results of the oxide layer that formed on the coating during cyclic oxidation test are presented in Fig. 4a and Table 3. Kikuchi patterns and corresponding indexes obtained for microareas 1 and 2 (Fig. 4a) are presented in Fig. 4b–e. Cross-sectional microstructure of the oxide layer formed on the investigated coating consists of three zones visible in various gray-scale colors on BSE image. Directly on the substrate alloy's surface there is a continuous layer of Al2O3, identified using EBSD. It contains 44.6 at.% Al and 55.4 at.% O and no other elements. What is more, above this layer a brighter oxide layer consisting of NiAl2O4
Table 2 Chemical composition of microareas 1 and 2 (Fig. 3a) in the cross-section of the simple aluminide coating after cyclic oxidation test. [at.%]
Al
Ni
Co
Cr
Mo
Ti
Ta
W
1 2
22 34.2
61.5 53.4
7.9 6.6
4.9 4.7
0.3
0.7 0.3
1.8 0.6
0.9 0.3
spinel is present, which was also detected using EBSD. Apart from the main elements it also contains Cr, Co, Ti and Ta. Between the Al2O3 and NiAl2O4 layers, as well as at the very top of the oxides, there are clusters of fine and bright oxide precipitates. They contain 14.4 at.% Al, 71.2 at.% O, 6.8 at.% Ti and other elements such as Ni, Cr, Co, W and Ta. Taking into consideration that signals from some of these elements may be coming from surrounding phases that were excited by the electron beam it is plausible that this chemical composition corresponds to the TiO2 oxide detected on the coating's surface. On the basis of the oxide layer's cross-section analysis a particular regularity can be observed, which has also been reported by other authors [30]. As a matter of fact, the oxide layer that forms above the ɣ′-Ni3Al phase does not consist exclusively of Al2O3, but contains other oxides and elements, namely Ni, Ti and Ta. This phenomena can be explained by the fact that the ɣ′-Ni3Al phase forms due to Al depletion from β-NiAl phase, and therefore in this area the Al concentration is too low to maintain an oxide layer consisting only of Al2O3 hence oxides containing other elements are formed. What is more, above the areas where β-NiAl phase is present there is a significantly lower number of oxides containing Ti and Ta.
3.2. Platinum modified aluminide coating after cyclic oxidation test The surface microstructure of platinum modified aluminide coating after cyclic oxidation test is presented in Fig. 1c. Characteristic features of the investigated surface along with EDS analysis microareas are presented in Fig. 1d.
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Fig. 4. Cross-sectional microstructure of the oxide layer formed on aluminide coating during cyclic oxidation test (a), Kikuchi pattern for: α-Al2O3 (b) and NiAl2O4 (d), and corresponding indexes (c and e).
The surface of the investigated coating after cyclic oxidation tests consists mainly of an oxide layer which appears gray on BSE image and bright areas of the exposed substrate alloy. The characteristic features presented in Fig. 1d indicate that the oxide layer consists of distinctive types of oxides characterized by different gray-scale colors
on BSE image. The darkest oxide layer which is the predominant kind on the surface, contains 41.4 at.% Al and 58.6 at.% O (Table 4), which is consistent with XRD analysis results (Fig. 5) indicating the presence of Al2O3 oxide. Another type of oxides was small bright precipitates, marked as 2 in Fig. 1d, containing 26.6 at.% Al, 65.3 at.% O and other
Table 3 Chemical composition in the cross-section of the oxide layer formed on simple aluminide coating during cyclic oxidation test (microareas 1, 2 and 3, Fig. 4a).
Table 4 Chemical composition on the surface of platinum modified aluminide coating after cyclic oxidation test (microareas 1, 2 and 3, Fig. 1d).
[at.%]
Al
O
1 2 3
44.6 26.8 14.4
55.4 63.4 71.2
Ni 5.6 3
Cr 1.3 1.8
Co 2 1
Ti 0.8 6.8
Ta 0.2 1.5
W
[at.%]
Al
O
41.4 26.6 12.5
58.6 65.3
0.4
1 2 3
Ni
Cr
Co
Ti
Ta
W
Pt
Re
Mo
2.1 52.7
0.7 16.5
0.4 13.5
2.6 1
2.3 0.6
0.6
1.2
0.9
0.5
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Fig. 5. XRD analysis results from the platinum modified aluminide coating after cyclic oxidation test.
elements such as Ni, Ti and Ta. In spite of the fact that the differences in chemical composition between the oxides were not so distinct, on the basis of XRD analysis (Fig. 5) it is clear that the phase composition of the formed oxide layer is the same as that of simple aluminide coating. It is, however, evident that the quantitative composition differs significantly. In fact, the amount of signals belonging to Al2O3 oxide appears to be larger on the platinum modified coating in comparison to simple aluminide coating. What is more, signals from other oxides (NiAl2O4 and TiO2) are smaller in this type of a coating. This proves that the platinum modification enhances the growth of the oxide layer in respect of protective properties, since among detected oxides the only desirable type is Al2O3. However, the presence of Ta and Ti indicates that the addition of Pt does not inhibit diffusion processes of these elements from the substrate. Cross-sectional microstructure of platinum modified aluminide coating after failure during cyclic oxidation test is presented in Fig. 6a. Characteristic features of the analyzed microstructure are presented in Fig. 6b–c along with EDS analysis markers. After failure the investigated coating consists of alternately arranged gray and dark-gray grains. The ɣ′-Ni3Al grains, marked as 1 in Fig. 6a, that formed due to Al depletion from the initial β-NiAl phase during repeated oxide scale spallation and interdiffusion, contain 20.7 at.% Al, 60.4 at.% Ni and among others 1.3 at.% Pt (Table 5). Marked as 2 are the Al-depleted β-NiAl phase grains that contain 32.8 at.% Al and 51.7 at.% Ni, as well as a higher amount of Pt — 1.7 at.%. The Al content indicates that this phase has not yet transformed into ɣ′ phase. Taking into account the amount and size of these grains it is evident that this type of a coating was more stable during high temperature exposure in comparison to simple aluminide coating. The remaining β phase grains remain as a reservoir of Al providing protective Al2O3 oxide scale and further oxidation protection. The bright precipitates marked as 3 in Fig. 6b contain high amount of Re (18.3 at.%) which proves outward diffusion of this element. Apart from that, these precipitates contain other elements present in the substrate alloy, such as Mo, W and Ta (Table 5). SEM investigation after chemical etching (Fig. 6c) revealed the presence of martensitic transformation stripes protruding through the Al-depleted β phase grains. Their extent is noticeably higher than in simple aluminide coating therefore it can be suspected that the stresses forming in the investigated coating are also higher.
3.3. Platinum and palladium modified aluminide coatings after cyclic oxidation test Surface microstructure of platinum and palladium modified aluminide coatings after failure during cyclic oxidation test is presented in Fig. 1e, while Fig. 1f presents its characteristic features along with marked EDS microareas. The results of EDS analysis are presented in Table 6. The degradation mechanism of Pt/Pd-modified aluminide coating is very similar to that noted for Pd-modified aluminide coatings by Alperine et al. [27,28]. Similarly to Pt-modified and simple aluminide coating, the surface of Pt–Pd-modified aluminide coating after oxidation is composed of Al2O3 matrix and brighter regions of NiAl2O4 spinel, as well as finely grained and bright precipitates of TiO2. Each type of an oxide is visible in different gray-scale on the BSE image which is a result of different chemical compositions. The oxides were identified using XRD analysis, the results of which are presented in Fig. 7. Chemical composition analysis in the darkest microarea 1 in Fig. 1f indicates that no additional elements are present in the Al2O3 oxide matrix. The NiAl2O4 and finely grained TiO2 oxides present on top of the Al2O3 layer also contained different amounts of Co, Cr and Ta. A significantly higher amount of Ta was found in the TiO2 precipitates, while more Co segregated in NiAl2O4. Apart from the oxide layer, the surface of the investigated coating is characterized by the presence of exposed substrate regions, identified as Ni3Al phase by XRD analysis (Fig. 7). Elements such as Ta, Re and W most probably diffused from the substrate alloy to the outer zone of the transformed coating. It is noteworthy that no Pt was found in this region, while the concentration of Pd is relatively small (0.7 at.%). Cross-sectional microstructure analysis of platinum and palladium modified aluminide coatings after cyclic oxidation test along with marked EDS analysis microareas is presented in Fig. 8a. Characteristic features visible in the cross-section after chemical etching are visible in Fig. 8b. Table 7 contains EDS microanalysis results from areas 1, 2 and 3 in Fig. 8a. The cross-sectional microstructure of the investigated coating consists of ɣ′-Ni3Al grains in the outer zone of the coating (marked as 1 in Fig. 8a) and Al-depleted β-NiAl grains (marked as 2 in Fig. 8a). The transformed ɣ′ phase contains higher amount of Ti and Ta (1.0 at.% and 2.5 at.% respectively) than the Al-depleted β phase. However, the concentration of Pt and Pd is significantly higher in the latter — 0.8 at.% and 6.1 at.% respectively. This proves higher
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R. Swadźba et al. / Surface & Coatings Technology 215 (2013) 16–23 Table 6 Chemical composition [at.%] in microareas 1, 2, 3 and 4, Fig. 1f. [at.%]
Al
O
1 2 3 4
60 25.3 15.5 18.8
40 59.6 65.6
Ni
Co
Cr
Ti
Ta
Pd
Re
W
6.5 2.9 48.4
2.1 0.6 12.5
1.5 1.7 15.6
3.9 8 1
1.1 5.6 1
0.7
1.4
0.6
Pd in the ɣ′ phase after cyclic oxidation test (1.9 at.%) is very close to that reported by Li et al. for Pd-modified aluminide coatings (2–3 at.%). It is noteworthy that the β-grains which can be considered as Al reservoir seem to be slightly smaller than those in the Pt-modified aluminide coating, however they are significantly larger in comparison to simple aluminide coating after failure. This type of a coating also exhibited the presence of Re and Cr-rich precipitates (marked as 3 in Fig. 8a) that formed underneath the Al-depleted β phase grains. What is more, chemical etching revealed lamellar structure of the Al-depleted β-phase grains (Fig. 8b). Among the three presented coatings in this particular case the delamination between the lamellas was the most distinct which presumably may be related to highest stresses inside the grains.
4. Summary
solubility of elements diffusing from the substrate in the ɣ′-Ni3Al phase and concurrently higher solubility of Pt and Pd in the β phase, as it was suggested by Alperine et al. [27,28]. The concentration of
After failure during cyclic oxidation tests all three of the investigated coatings exhibited similar microstructural degradation, both on the surface and in the cross-sections. The oxide layer that formed on the surface of the coatings consisted of α-Al2O3 matrix along with top layer of NiAl2O4 and fine precipitates of TiO2. It is noteworthy that the two latter types of oxides seem to form mainly above ɣ′-Ni3Al regions where not enough Al is available for Al2O3 to form exclusively. In case of Pt and Pt–Pd modified aluminide coatings, the amount of these oxides seems to be smaller than on simple aluminide coating. Since Ti and Ta were found in the oxide layer, while W and Re were present in the exposed area it is clear that neither Pt nor Pd directly inhibits outward diffusion of these elements. On the other hand, it can be agreed that there is an indirect impact of Pt and Pd on the outward diffusion of these elements since they are less soluble in the Pt, Pd-stabilized β-NiAl phase. Cross-sectional microstructure of the investigated coatings after cyclic oxidation tests consists of three distinctive regions: outer ɣ′-Ni3Al layer, middle Al-depleted (32–34 at.%) β-NiAl layer and rich in refractory elements precipitates at the bottom. It has been proven that, in comparison to simple aluminide coatings, both Pt and Pt–Pd modified aluminide coatings exhibit significantly higher β-NiAl phase stability and Al reservoir as well as maintain more stable α-alumina oxide layer. It appears that the β phase depletion was less severe on the Pt-modified aluminide coating, as compared to the Pt/Pd-modified coating. Last but not least, the presence of Pt and Pt/Pd additions in aluminide coatings seems to introduce considerable stresses that lead to cracking along Al-depleted β phase grains. Further work should concern the influence of these stresses on the performance of the coatings as well as the ratio of Pt and Pd layer thickness in order to reach a compromise between the performance and costs of the coatings.
Table 5 Chemical composition [at.%] in microareas 1, 2 and 3, Fig. 6a and b.
Table 7 Chemical composition [at.%] in microareas 1, 2 and 3, Fig. 8a.
Fig. 6. Cross-sectional microstructure of platinum modified aluminide coating after failure during cyclic oxidation test (a). Re-rich precipitate underneath the outer zone (b), and features revealed after etching (c).
[at.%]
Al
Ni
Cr
Co
Ti
Ta
W
Pt
Re
Mo
[at.%]
Al
Ni
Cr
Co
Ti
Ta
W
Pt
Pd
1 2 3
20.7 32.8 3.9
60.4 51.7 19.6
4.8 5.6 27.7
8.1 6.4 17
1.2 0.6 0.8
2.8 0.9 0.6
0.4
1.3 1.7
0.2 0.1 18.3
0.2 0.2 2.8
1 2 3
21.5 34.1 3.5
59.7 47.7 20
4.3 4.2 25.1
7.8 5.7 16.4
1 0.4
2.5 0.9 2.2
0.9 0.1 9.2
0.4 0.8
1.9 6.1
9.3
Mo
Re
2.2
21.4
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Fig. 7. XRD analysis results from Pt–Pd-modified aluminide coating's surface after cyclic oxidation test.
Fig. 8. Cross-sectional microstructure of platinum and palladium modified aluminide coatings after failure during cyclic oxidation test (a), and features revealed after etching (b).
Acknowledgments The financial support of the Structural Funds in the Operational Programme — Innovative Economy (IE OP) from the European Regional Development Fund — Project No. POIG.01.01.02-00-015/08-00 “Modern Material Technologies in Aerospace Industry”, Task: “Modern Thermal Barrier Coatings for Engine Parts”, is gratefully acknowledged. References [1] N. Birks, G.H. Meier, F.S. Pettit, High Temperature Oxidation of Metals, Cambridge University Press, 2006. [2] Y. Tamarin, ASM Int. (2002) 161. [3] R.C. Reed, in: The Superalloys: Fundamentals and Application, Cambridge University Press, 2008, p. 322. [4] S. Bose, in: High Temperature Coatings, Elsevier, 2007, p. 3. [5] Y. Zhang, J.A. Haynes, B.A. Pint, I.G. Wright, W.Y. Lee, Surf. Coat. Technol. 163–164 (2003) 19. [6] M.W. Chen, R.T. Ott, T.C. Hufnagel, P.K. Wright, K.J. Hemker, Surf. Coat. Technol. 163–164 (2003) 25. [7] R.J. Thompson, J.C. Zhao, K.J. Hemker, Intermetallics 18 (2010) 796. [8] S.J. Hong, G.H. Hwang, W.K. Han, S.G. Kang, Intermetallics 17 (2009) 381. [9] M.W. Chen, M.L. Glynn, R.T. Ott, T.C. Hufnagel, K.J. Hemker, Acta Mater. 51 (2003) 4279. [10] V.S. Litvinov, E.G. Pantsyreva, I.L. Kupriyanov, Metalloved. Term. Obrab. Met. 6 (1973) 71.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Surf. Coat. Technol. 185 (2004) 172. S. Dryepondt, D.R. Clarke, Scr. Mater. 60 (2009) 917. N. Mu, J. Liu, J.W. Byeon, Y.H. Sohn, Y.L. Nava, Surf. Coat. Technol. 188–189 (2004) 27. V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1276. T.F. An, H.R. Guan, X.F. Sun, Z.Q. Hu, Oxid. Met. 54 (2000) 301. H. Svensson, J. Angenete, K. Stiller, Surf. Coat. Technol. 177–178 (2004) 152. H. Svensson, M. Christensen, P. Knutsson, G. Wahnstrom, K. Stiller, Corros. Sci. 51 (Jan. 1 2009) 539. P. Hou, V.K. Tolpygo, Surf. Coat. Technol. 202 (2007) 623. Y. Zhang, J.A. Haynes, W.Y. Lee, I.G. Wright, B.A. Pint, K.M. Cooley, P.K. Liaw, Metall. Mater. Trans. 32A (2001) 1727. M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Oxid. Met. 59 (2003) 483. M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Oxid. Met. 61 (2004) 91. V.K. Tolpygo, D.R. Clarke, Acta Mater. 48 (2000) 3283. J. Angenete, K. Stiller, E. Bakchinova, Surf. Coat. Technol. 176 (2004) 272. V.K. Tolpygo, D.R. Clarke, Acta Mater. 52 (2004) 5129. V.K. Tolpygo, D.R. Clarke, Scr. Mater. 57 (2007) 563. V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 203 (2009) 3278. S. Alpérine, P. Steinmetz, P. Josso, A. Constantini, Mater. Sci. Eng. A 121 (1989) 367. S. Alpérine, P. Steinmetz, A. Friant-Constantini, P. Josso, Surf. Coat. Technol. 43/44 (1990) 347. R. Swadźba, M. Hetmańczyk, M. Sozańska, B. Witala, L. Swadźba, Surf. Coat. Technol. 206 (2011) 1538. J.A. Haynes, Y. Zhang, W.Y. Lee, B.A. Pint, I.G. Wright, K.M. Cooley, Elev. Temp. Coat. Sci. Technol. II (1998) 185 (TMS). M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Surf. Coat. Technol. 167 (2003) 106.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure degradation of simple, Pt- and Pt + Pd-modified aluminide coatings on CMSX-4 superalloy under cyclic oxidation conditions Radosław Swadźba a,⁎, Marek Hetmańczyk b, Jerzy Wiedermann a, Lucjan Swadźba b, Grzegorz Moskal b, Bartosz Witala b, Krzysztof Radwański a a b
Institute for Ferrous Metallurgy, Gliwice, Poland Silesian University of Technology, Katowice, Poland
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Aluminide coatings Pt/Pd aluminide coatings Cyclic oxidation Superalloys
a b s t r a c t The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings' cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt + Pd electroplating, followed by vapor phase aluminizing at 1050 °C. Cyclic oxidation tests were performed at 1100 °C in 23 h cycles. Microstructural and phase analysis conducted using XRD, SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diffusion aluminide coatings are applied on jet engine turbine blades in order to provide high temperature oxidation and corrosion resistance. Incorporation of Pt to aluminide coatings makes it possible to apply them as bond coatings for thermal barrier coating (TBC) systems on blades and vanes operating under the most severe conditions [1–4]. Since the performance of the whole TBC system relies directly on the durability of the bond coat, its degradation mechanisms are widely studied in the literature. Reversible transformation from a β phase (B2) to a tetragonally distorted martensitic phase (L10) was observed in single-phase NiAl and (Ni,Pt)Al bond coats during quenching from high temperature both in isothermal and cyclic oxidation tests by many researchers [5–7]. This phenomena occurs once Al concentration in the β phase drops to 32–38 at.% and when the cooling rate is high enough to suppress the β–>ɣ′ transformation, as well as due to interdiffusion with the substrate alloy and alloying elements present in the coating, such as Pt, Cr and Co [6–8]. Chen et al. [6,9] have concluded that the strain produced by martensitic transformation during each cycle is 0.7% which is comparable to the thermal strain caused by the coefficient of thermal expansion (CTE) mismatch. This may lead to elastic stresses in the surface layer, destruction of the heat resistant oxide film and
⁎ Corresponding author. E-mail address: [email protected] (R. Swadźba). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.06.093
intensification of diffusion processes at high temperatures [10]. Zhang suggested that the volume changes probably contribute to the early stage of surface roughening [5]. Another type of transformation occurring in the β-NiAl coatings during oxidation is the alumina transformation studied in a number of articles [11,12]. While α-Al2O3 provides protective properties because of its thermodynamic stability and slow growth rate, other types such as θ and ɣ-Al2O3 exhibit low adhesion, porosity and volumetric changes with exposure time, hence they are not desirable [13,14]. The amount of α-alumina in the oxide layer during oxidation increases with exposure time and temperature and leads to reduction of parabolic rate constant by one order of magnitude [15]. Modifying aluminide coatings with Pt significantly improves the formation rate and stability of α-alumina, both in single and double phase PtAl coatings [16–18]. Other beneficial effect attributed to Pt is improvement of oxide scale adherence during cyclic and isothermal oxidation tests as well as mitigating the detrimental effects of sulfur impurities on scale adherence and inhibition of void growth at the scale–metal interface [19]. Li et al. [20,21] examined the influence of Pd on the transformations occurring in the oxide during high temperature exposure. Apart from the fact that palladium itself facilitates the θ to α transformation, it has been shown that it enhances the outward diffusion of Ti from the substrate to the coating surface which in turn forms TiO2 that accelerates the transformation from θ to α-alumina. Oxide layer that forms during high temperature oxidation is also susceptible to degradation due to void formation along the
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metal-oxide interface which has been studied in a number of papers [18,19,22,23]. This phenomenon is attributed to nonuniform Al/Ni diffusion, vacancy formation and/or Kirkendall effect [17]. Angenete et al. [23] proposed a void formation mechanism associated with volumetric changes accompanying the β–> ɣ′ phase transformation. Other studies [22] suggest that pores and voids tend to collapse during cyclic oxidation tests due to thermal stresses, while they coalesce during long-term oxidation. Incorporation of Pt significantly retards the formation of voids due to enhanced Al diffusion which leads to a twofold advantage: it levels the Al/Ni diffusivity and promotes
17
Al2O3 formation filling the voids and increasing the contact area between the oxide and the metal [17,18]. Surface roughening during high temperature cyclic oxidation, referred to as rumpling, has been observed both in Pt-modified and simple aluminide coatings on Ni-based superalloys and extensively studied by Tolpygo et al. [22,24–26]. It is also considered as a significant degradation form since it leads to initiation of cracking and eventual separation between the TBC and the superalloy. Alperine et al. [27,28] studied high temperature performance of palladium modified aluminide coatings obtained applying various
Fig. 1. Surface microstructure of aluminide coatings after cyclic oxidation test: (a and b) simple aluminide coating, (c and d) Pt-modified aluminide coating, and (e and f) Pt/Pdmodified aluminide coating.
18
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Fig. 2. XRD analysis results from the surface of simple aluminide coating after cyclic oxidation test.
Pd–M (M = Ni, Cr or Co) predeposits which lead to the formation of defect-free coatings. The authors proved that all studied kinds of Pd-modified aluminide coatings exhibited excellent high temperature behavior in comparison to conventional aluminide coatings [31]. This paper is a continuation of our previous work [29] where we analyzed cyclic oxidation resistance and microstructures of as-deposited coatings and proved that incorporation of both Pt and Pd to aluminide coatings provides superior cyclic oxidation resistance over simple and Pt-modified aluminide coatings. 2. Experimental procedure The investigation concerned three samples of CMSX-4 Ni-based single crystal superalloy. The alloy's composition was [wt.%]: Al-5.13, B-0.007, C-0.01, Co-9.02, Cr-6.07, Hf-0.09, Mo-0.64, Re-3.21, Ta-6.57, Ti-0.92, W-6.09, and Zr-0.002. The samples of CMSX-4 superalloy (cylinder shaped, 10 mm diameter, 80 mm height) were put under heat treatment that consisted of solution and aging in order to obtain γ–γ′ microstructure. The two of the three samples were electroplated with 5 μm of Pt and 3 μm of Pd + 2 μm of Pt, respectively. After such a preparation all the samples were annealed at 1010 °C for 2 h and underwent “out of pack” diffusion aluminizing in CrAl alloy granules at 1050 °C in argon atmosphere. Simple and Pt/Pd-modified aluminide coatings consisted of β-NiAl phase containing 55.3 at.% (36.3 wt.%) and 53.6 at.% (27.7 wt.%) of Al, respectively. The Pt-modified aluminide coating exhibited double phase microstructure consisting of PtAl2 phase precipitates on grain boundaries of β-NiAl phase, which contained 57.7 at.% (34.2 wt.%) of Al [29]. Cyclic oxidation tests were performed at 1100 °C in 23 h long cycles in laboratory air environment. After failure the samples were cut, chemically coated with nickel plate, grinded and polished in order to examine their cross-section microstructure using a scanning electron microscope (FEI Inspect F device). The nickel plate was applied to the samples to protect the coating from damage that may be caused during the grinding process. Chemical composition in microareas and element distribution were examined using X-ray energy dispersive spectrometry (EDS). Phase compositions of the samples were identified using an X-ray diffractometer (XRD) JEOL JDX-7S and electron backscatter diffraction (EBSD). 3. Results and discussion 3.1. Simple aluminide coating after cyclic oxidation test The surface microstructure of simple aluminide coating after cyclic oxidation test is presented in Fig. 1a. Fig. 2b illustrates its characteristic
features along with areas analyzed using EDS, marked as 1–4. The results of chemical composition microanalysis are presented in Table 1. The surface of simple aluminide coating after cyclic oxidation test, presented in Fig. 1a, consists mostly of an oxide layer formed during the test, visible as gray-scale areas. What is more there are also bright areas of the substrate exposed by oxide layer spallation. There are three distinctive types of oxides, marked as 1, 2 and 3 in Fig. 1b. They differ by chemical composition and therefore by brightness on the image captured using backscattered electrons (BSE). Microarea 1 marked in Fig. 1b relates to the darkest and major oxide layer present on the coating's surface, which consists of two elements — Al and O, 39.5 at.% and 60.6 at.% respectively. Chemical composition of this oxide corresponds to Al2O3, which was detected using XRD (Fig. 2). Above this area there is a slightly brighter oxide layer, marked as 2. The predominant elements present in this area are Al, O and Ni. There are also slight amounts of Ti, Cr Co and Ta. However, the concentration of the main elements indicates the presence of NiAl2O4 spinel, which was also detected using XRD (Fig. 2). On the top of the described oxide layers there are fine and bright oxide precipitates, marked as 3 in Fig. 1b, containing, apart from Al and O, Ti and Ta. Their chemical composition refers to TiO2 oxide corresponding to XRD analysis. Chemical composition of the brightest and distinct areas, marked as 4 in Fig. 1b, is very similar to that of the substrate alloy which indicates that it was exposed by oxide layer spallation. What is more it has been identified as Ni3Al phase by XRD (Fig. 2). The cross-sectional microstructure of the simple aluminide coating after cyclic oxidation test along with EDS analysis markers is presented in Fig. 3a, the results of which are placed in Table 2. Fig. 3b illustrates Al-depleted NiAl phase grains after etching. The results of EBSD analysis in microarea 1 (Fig. 3a) are presented in Fig. 3c–d. After cyclic oxidation test the coating consist of ɣ′-Ni3Al phase grains and Al-depleted β-NiAl phase grains. The results of EBSD analysis conducted in microarea 1 (Fig. 3a) confirm the results obtained using XRD from the coating's surface. The presence of ɣ′-Ni3Al is a
Table 1 Chemical composition [at.%] in microareas 1, 2, 3 and 4, Fig. 1b. [at.%]
Al
O
1 2 3 4
39.5 27 22.9 12.1
60.5 53.7 65.8
Ni
Ti
Cr
Co
Ta
Mo
W
Re
11.1 2.3 58.4
2.1 6 0.5
2.3 1.1 13.4
3.1 0.6 12.6
0.8 1.4 1.4
0.4
0.3
0.9
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Fig. 3. Cross-sectional microstructure of simple aluminide coating after cyclic oxidation test (a), features revealed after etching (b), and Kikuchi pattern and corresponding indexes for Ni3Al phase (c and d).
result of β–> ɣ′ transformation which occurred due to Al depletion from the β phase during oxidation and repeatable spallation of the oxide layer. The ɣ′-Ni3Al phase contains 22 at.% Al, 61.5 at.% Ni, Co, and Cr and minor amounts of Ti, Ta and W. The Al-depleted β-NiAl phase grains contain 34.2 at.% Al, 53.4 at.% Ni and small amounts of Cr and Co (Table 2). What is more, etching of these grains revealed the presence of stripes along the Al-depleted β-NiAl phase, which are characteristic for martensitic transformation occurring between oxidation cycles, when the cooling rate is high enough to stop the β–> ɣ′ transformation [5–7]. Microstructural investigation results of the oxide layer that formed on the coating during cyclic oxidation test are presented in Fig. 4a and Table 3. Kikuchi patterns and corresponding indexes obtained for microareas 1 and 2 (Fig. 4a) are presented in Fig. 4b–e. Cross-sectional microstructure of the oxide layer formed on the investigated coating consists of three zones visible in various gray-scale colors on BSE image. Directly on the substrate alloy's surface there is a continuous layer of Al2O3, identified using EBSD. It contains 44.6 at.% Al and 55.4 at.% O and no other elements. What is more, above this layer a brighter oxide layer consisting of NiAl2O4
Table 2 Chemical composition of microareas 1 and 2 (Fig. 3a) in the cross-section of the simple aluminide coating after cyclic oxidation test. [at.%]
Al
Ni
Co
Cr
Mo
Ti
Ta
W
1 2
22 34.2
61.5 53.4
7.9 6.6
4.9 4.7
0.3
0.7 0.3
1.8 0.6
0.9 0.3
spinel is present, which was also detected using EBSD. Apart from the main elements it also contains Cr, Co, Ti and Ta. Between the Al2O3 and NiAl2O4 layers, as well as at the very top of the oxides, there are clusters of fine and bright oxide precipitates. They contain 14.4 at.% Al, 71.2 at.% O, 6.8 at.% Ti and other elements such as Ni, Cr, Co, W and Ta. Taking into consideration that signals from some of these elements may be coming from surrounding phases that were excited by the electron beam it is plausible that this chemical composition corresponds to the TiO2 oxide detected on the coating's surface. On the basis of the oxide layer's cross-section analysis a particular regularity can be observed, which has also been reported by other authors [30]. As a matter of fact, the oxide layer that forms above the ɣ′-Ni3Al phase does not consist exclusively of Al2O3, but contains other oxides and elements, namely Ni, Ti and Ta. This phenomena can be explained by the fact that the ɣ′-Ni3Al phase forms due to Al depletion from β-NiAl phase, and therefore in this area the Al concentration is too low to maintain an oxide layer consisting only of Al2O3 hence oxides containing other elements are formed. What is more, above the areas where β-NiAl phase is present there is a significantly lower number of oxides containing Ti and Ta.
3.2. Platinum modified aluminide coating after cyclic oxidation test The surface microstructure of platinum modified aluminide coating after cyclic oxidation test is presented in Fig. 1c. Characteristic features of the investigated surface along with EDS analysis microareas are presented in Fig. 1d.
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Fig. 4. Cross-sectional microstructure of the oxide layer formed on aluminide coating during cyclic oxidation test (a), Kikuchi pattern for: α-Al2O3 (b) and NiAl2O4 (d), and corresponding indexes (c and e).
The surface of the investigated coating after cyclic oxidation tests consists mainly of an oxide layer which appears gray on BSE image and bright areas of the exposed substrate alloy. The characteristic features presented in Fig. 1d indicate that the oxide layer consists of distinctive types of oxides characterized by different gray-scale colors
on BSE image. The darkest oxide layer which is the predominant kind on the surface, contains 41.4 at.% Al and 58.6 at.% O (Table 4), which is consistent with XRD analysis results (Fig. 5) indicating the presence of Al2O3 oxide. Another type of oxides was small bright precipitates, marked as 2 in Fig. 1d, containing 26.6 at.% Al, 65.3 at.% O and other
Table 3 Chemical composition in the cross-section of the oxide layer formed on simple aluminide coating during cyclic oxidation test (microareas 1, 2 and 3, Fig. 4a).
Table 4 Chemical composition on the surface of platinum modified aluminide coating after cyclic oxidation test (microareas 1, 2 and 3, Fig. 1d).
[at.%]
Al
O
1 2 3
44.6 26.8 14.4
55.4 63.4 71.2
Ni 5.6 3
Cr 1.3 1.8
Co 2 1
Ti 0.8 6.8
Ta 0.2 1.5
W
[at.%]
Al
O
41.4 26.6 12.5
58.6 65.3
0.4
1 2 3
Ni
Cr
Co
Ti
Ta
W
Pt
Re
Mo
2.1 52.7
0.7 16.5
0.4 13.5
2.6 1
2.3 0.6
0.6
1.2
0.9
0.5
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21
Fig. 5. XRD analysis results from the platinum modified aluminide coating after cyclic oxidation test.
elements such as Ni, Ti and Ta. In spite of the fact that the differences in chemical composition between the oxides were not so distinct, on the basis of XRD analysis (Fig. 5) it is clear that the phase composition of the formed oxide layer is the same as that of simple aluminide coating. It is, however, evident that the quantitative composition differs significantly. In fact, the amount of signals belonging to Al2O3 oxide appears to be larger on the platinum modified coating in comparison to simple aluminide coating. What is more, signals from other oxides (NiAl2O4 and TiO2) are smaller in this type of a coating. This proves that the platinum modification enhances the growth of the oxide layer in respect of protective properties, since among detected oxides the only desirable type is Al2O3. However, the presence of Ta and Ti indicates that the addition of Pt does not inhibit diffusion processes of these elements from the substrate. Cross-sectional microstructure of platinum modified aluminide coating after failure during cyclic oxidation test is presented in Fig. 6a. Characteristic features of the analyzed microstructure are presented in Fig. 6b–c along with EDS analysis markers. After failure the investigated coating consists of alternately arranged gray and dark-gray grains. The ɣ′-Ni3Al grains, marked as 1 in Fig. 6a, that formed due to Al depletion from the initial β-NiAl phase during repeated oxide scale spallation and interdiffusion, contain 20.7 at.% Al, 60.4 at.% Ni and among others 1.3 at.% Pt (Table 5). Marked as 2 are the Al-depleted β-NiAl phase grains that contain 32.8 at.% Al and 51.7 at.% Ni, as well as a higher amount of Pt — 1.7 at.%. The Al content indicates that this phase has not yet transformed into ɣ′ phase. Taking into account the amount and size of these grains it is evident that this type of a coating was more stable during high temperature exposure in comparison to simple aluminide coating. The remaining β phase grains remain as a reservoir of Al providing protective Al2O3 oxide scale and further oxidation protection. The bright precipitates marked as 3 in Fig. 6b contain high amount of Re (18.3 at.%) which proves outward diffusion of this element. Apart from that, these precipitates contain other elements present in the substrate alloy, such as Mo, W and Ta (Table 5). SEM investigation after chemical etching (Fig. 6c) revealed the presence of martensitic transformation stripes protruding through the Al-depleted β phase grains. Their extent is noticeably higher than in simple aluminide coating therefore it can be suspected that the stresses forming in the investigated coating are also higher.
3.3. Platinum and palladium modified aluminide coatings after cyclic oxidation test Surface microstructure of platinum and palladium modified aluminide coatings after failure during cyclic oxidation test is presented in Fig. 1e, while Fig. 1f presents its characteristic features along with marked EDS microareas. The results of EDS analysis are presented in Table 6. The degradation mechanism of Pt/Pd-modified aluminide coating is very similar to that noted for Pd-modified aluminide coatings by Alperine et al. [27,28]. Similarly to Pt-modified and simple aluminide coating, the surface of Pt–Pd-modified aluminide coating after oxidation is composed of Al2O3 matrix and brighter regions of NiAl2O4 spinel, as well as finely grained and bright precipitates of TiO2. Each type of an oxide is visible in different gray-scale on the BSE image which is a result of different chemical compositions. The oxides were identified using XRD analysis, the results of which are presented in Fig. 7. Chemical composition analysis in the darkest microarea 1 in Fig. 1f indicates that no additional elements are present in the Al2O3 oxide matrix. The NiAl2O4 and finely grained TiO2 oxides present on top of the Al2O3 layer also contained different amounts of Co, Cr and Ta. A significantly higher amount of Ta was found in the TiO2 precipitates, while more Co segregated in NiAl2O4. Apart from the oxide layer, the surface of the investigated coating is characterized by the presence of exposed substrate regions, identified as Ni3Al phase by XRD analysis (Fig. 7). Elements such as Ta, Re and W most probably diffused from the substrate alloy to the outer zone of the transformed coating. It is noteworthy that no Pt was found in this region, while the concentration of Pd is relatively small (0.7 at.%). Cross-sectional microstructure analysis of platinum and palladium modified aluminide coatings after cyclic oxidation test along with marked EDS analysis microareas is presented in Fig. 8a. Characteristic features visible in the cross-section after chemical etching are visible in Fig. 8b. Table 7 contains EDS microanalysis results from areas 1, 2 and 3 in Fig. 8a. The cross-sectional microstructure of the investigated coating consists of ɣ′-Ni3Al grains in the outer zone of the coating (marked as 1 in Fig. 8a) and Al-depleted β-NiAl grains (marked as 2 in Fig. 8a). The transformed ɣ′ phase contains higher amount of Ti and Ta (1.0 at.% and 2.5 at.% respectively) than the Al-depleted β phase. However, the concentration of Pt and Pd is significantly higher in the latter — 0.8 at.% and 6.1 at.% respectively. This proves higher
22
R. Swadźba et al. / Surface & Coatings Technology 215 (2013) 16–23 Table 6 Chemical composition [at.%] in microareas 1, 2, 3 and 4, Fig. 1f. [at.%]
Al
O
1 2 3 4
60 25.3 15.5 18.8
40 59.6 65.6
Ni
Co
Cr
Ti
Ta
Pd
Re
W
6.5 2.9 48.4
2.1 0.6 12.5
1.5 1.7 15.6
3.9 8 1
1.1 5.6 1
0.7
1.4
0.6
Pd in the ɣ′ phase after cyclic oxidation test (1.9 at.%) is very close to that reported by Li et al. for Pd-modified aluminide coatings (2–3 at.%). It is noteworthy that the β-grains which can be considered as Al reservoir seem to be slightly smaller than those in the Pt-modified aluminide coating, however they are significantly larger in comparison to simple aluminide coating after failure. This type of a coating also exhibited the presence of Re and Cr-rich precipitates (marked as 3 in Fig. 8a) that formed underneath the Al-depleted β phase grains. What is more, chemical etching revealed lamellar structure of the Al-depleted β-phase grains (Fig. 8b). Among the three presented coatings in this particular case the delamination between the lamellas was the most distinct which presumably may be related to highest stresses inside the grains.
4. Summary
solubility of elements diffusing from the substrate in the ɣ′-Ni3Al phase and concurrently higher solubility of Pt and Pd in the β phase, as it was suggested by Alperine et al. [27,28]. The concentration of
After failure during cyclic oxidation tests all three of the investigated coatings exhibited similar microstructural degradation, both on the surface and in the cross-sections. The oxide layer that formed on the surface of the coatings consisted of α-Al2O3 matrix along with top layer of NiAl2O4 and fine precipitates of TiO2. It is noteworthy that the two latter types of oxides seem to form mainly above ɣ′-Ni3Al regions where not enough Al is available for Al2O3 to form exclusively. In case of Pt and Pt–Pd modified aluminide coatings, the amount of these oxides seems to be smaller than on simple aluminide coating. Since Ti and Ta were found in the oxide layer, while W and Re were present in the exposed area it is clear that neither Pt nor Pd directly inhibits outward diffusion of these elements. On the other hand, it can be agreed that there is an indirect impact of Pt and Pd on the outward diffusion of these elements since they are less soluble in the Pt, Pd-stabilized β-NiAl phase. Cross-sectional microstructure of the investigated coatings after cyclic oxidation tests consists of three distinctive regions: outer ɣ′-Ni3Al layer, middle Al-depleted (32–34 at.%) β-NiAl layer and rich in refractory elements precipitates at the bottom. It has been proven that, in comparison to simple aluminide coatings, both Pt and Pt–Pd modified aluminide coatings exhibit significantly higher β-NiAl phase stability and Al reservoir as well as maintain more stable α-alumina oxide layer. It appears that the β phase depletion was less severe on the Pt-modified aluminide coating, as compared to the Pt/Pd-modified coating. Last but not least, the presence of Pt and Pt/Pd additions in aluminide coatings seems to introduce considerable stresses that lead to cracking along Al-depleted β phase grains. Further work should concern the influence of these stresses on the performance of the coatings as well as the ratio of Pt and Pd layer thickness in order to reach a compromise between the performance and costs of the coatings.
Table 5 Chemical composition [at.%] in microareas 1, 2 and 3, Fig. 6a and b.
Table 7 Chemical composition [at.%] in microareas 1, 2 and 3, Fig. 8a.
Fig. 6. Cross-sectional microstructure of platinum modified aluminide coating after failure during cyclic oxidation test (a). Re-rich precipitate underneath the outer zone (b), and features revealed after etching (c).
[at.%]
Al
Ni
Cr
Co
Ti
Ta
W
Pt
Re
Mo
[at.%]
Al
Ni
Cr
Co
Ti
Ta
W
Pt
Pd
1 2 3
20.7 32.8 3.9
60.4 51.7 19.6
4.8 5.6 27.7
8.1 6.4 17
1.2 0.6 0.8
2.8 0.9 0.6
0.4
1.3 1.7
0.2 0.1 18.3
0.2 0.2 2.8
1 2 3
21.5 34.1 3.5
59.7 47.7 20
4.3 4.2 25.1
7.8 5.7 16.4
1 0.4
2.5 0.9 2.2
0.9 0.1 9.2
0.4 0.8
1.9 6.1
9.3
Mo
Re
2.2
21.4
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Fig. 7. XRD analysis results from Pt–Pd-modified aluminide coating's surface after cyclic oxidation test.
Fig. 8. Cross-sectional microstructure of platinum and palladium modified aluminide coatings after failure during cyclic oxidation test (a), and features revealed after etching (b).
Acknowledgments The financial support of the Structural Funds in the Operational Programme — Innovative Economy (IE OP) from the European Regional Development Fund — Project No. POIG.01.01.02-00-015/08-00 “Modern Material Technologies in Aerospace Industry”, Task: “Modern Thermal Barrier Coatings for Engine Parts”, is gratefully acknowledged. References [1] N. Birks, G.H. Meier, F.S. Pettit, High Temperature Oxidation of Metals, Cambridge University Press, 2006. [2] Y. Tamarin, ASM Int. (2002) 161. [3] R.C. Reed, in: The Superalloys: Fundamentals and Application, Cambridge University Press, 2008, p. 322. [4] S. Bose, in: High Temperature Coatings, Elsevier, 2007, p. 3. [5] Y. Zhang, J.A. Haynes, B.A. Pint, I.G. Wright, W.Y. Lee, Surf. Coat. Technol. 163–164 (2003) 19. [6] M.W. Chen, R.T. Ott, T.C. Hufnagel, P.K. Wright, K.J. Hemker, Surf. Coat. Technol. 163–164 (2003) 25. [7] R.J. Thompson, J.C. Zhao, K.J. Hemker, Intermetallics 18 (2010) 796. [8] S.J. Hong, G.H. Hwang, W.K. Han, S.G. Kang, Intermetallics 17 (2009) 381. [9] M.W. Chen, M.L. Glynn, R.T. Ott, T.C. Hufnagel, K.J. Hemker, Acta Mater. 51 (2003) 4279. [10] V.S. Litvinov, E.G. Pantsyreva, I.L. Kupriyanov, Metalloved. Term. Obrab. Met. 6 (1973) 71.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Surf. Coat. Technol. 185 (2004) 172. S. Dryepondt, D.R. Clarke, Scr. Mater. 60 (2009) 917. N. Mu, J. Liu, J.W. Byeon, Y.H. Sohn, Y.L. Nava, Surf. Coat. Technol. 188–189 (2004) 27. V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1276. T.F. An, H.R. Guan, X.F. Sun, Z.Q. Hu, Oxid. Met. 54 (2000) 301. H. Svensson, J. Angenete, K. Stiller, Surf. Coat. Technol. 177–178 (2004) 152. H. Svensson, M. Christensen, P. Knutsson, G. Wahnstrom, K. Stiller, Corros. Sci. 51 (Jan. 1 2009) 539. P. Hou, V.K. Tolpygo, Surf. Coat. Technol. 202 (2007) 623. Y. Zhang, J.A. Haynes, W.Y. Lee, I.G. Wright, B.A. Pint, K.M. Cooley, P.K. Liaw, Metall. Mater. Trans. 32A (2001) 1727. M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Oxid. Met. 59 (2003) 483. M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Oxid. Met. 61 (2004) 91. V.K. Tolpygo, D.R. Clarke, Acta Mater. 48 (2000) 3283. J. Angenete, K. Stiller, E. Bakchinova, Surf. Coat. Technol. 176 (2004) 272. V.K. Tolpygo, D.R. Clarke, Acta Mater. 52 (2004) 5129. V.K. Tolpygo, D.R. Clarke, Scr. Mater. 57 (2007) 563. V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 203 (2009) 3278. S. Alpérine, P. Steinmetz, P. Josso, A. Constantini, Mater. Sci. Eng. A 121 (1989) 367. S. Alpérine, P. Steinmetz, A. Friant-Constantini, P. Josso, Surf. Coat. Technol. 43/44 (1990) 347. R. Swadźba, M. Hetmańczyk, M. Sozańska, B. Witala, L. Swadźba, Surf. Coat. Technol. 206 (2011) 1538. J.A. Haynes, Y. Zhang, W.Y. Lee, B.A. Pint, I.G. Wright, K.M. Cooley, Elev. Temp. Coat. Sci. Technol. II (1998) 185 (TMS). M.J. Li, X.F. Sun, H.R. Guan, X.X. Jiang, Z.Q. Hu, Surf. Coat. Technol. 167 (2003) 106.