- Email: [email protected]
A study of the diffusion and pre-oxidation treatment on the formation of Al2O3 ceramic scale on NiCrAlY bond-coat during initial oxidation process
Author’s Accepted Manuscript A study of the diffusion and pre-oxidation treatment on the formation of Al 2O3 ceramic scale on NiCrAlY bond-coat during initial oxidation process C. Zhu, P. Li, X.Y. Wu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)00231-5 http://dx.doi.org/10.1016/j.ceramint.2016.01.185 CERI12161
To appear in: Ceramics International Received date: 26 October 2015 Revised date: 28 December 2015 Accepted date: 25 January 2016 Cite this article as: C. Zhu, P. Li and X.Y. Wu, A study of the diffusion and preoxidation treatment on the formation of Al 2O3 ceramic scale on NiCrAlY bondcoat during initial oxidation process, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.185 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A study of the diffusion and pre-oxidation treatment on the formation of Al2O3 ceramic scale on NiCrAlY bond-coat during initial oxidation process
C. Zhu*1, P. Li2, X.Y. Wu3 1
Science and Technology on Thermostructural Composite Materials Laboratory,
Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, PR China 2
Northwest Institute for Nonferrous Metal Research, Xi'an, Shaanxi, 710016, PR
China 3
*
Luoyang Ship Material Research Institute, Luoyang, Henan, 471023, PR China
Corresponding author. Tel.: +86 29 88494914; Fax: +86 29 88494620.
E-mail address: [email protected] (C. Zhu)
Abstract In this study, the formation of thermally growth ceramic oxides during initial oxidation on treated NiCrAlY bond-coat was investigated. The oxidation resistance of NiCrAlY bond-coat was enhanced by two approaches: (i) an inter-diffusion treatment in a tube furnace with flowing argon at 600°C for 1 h, and (ii) a pre-oxidation treatment in static air at atmospheric pressure at 1000°C for 1 h. Experimental results showed that, after diffusion treatment, the content of Al in the bond-coat increased. This led to the growth of θ-Al2O3 during the pre-oxidation process, which was then helped in the formation of a dense and continuous α-Al2O3/Cr2O3 multilayer on the 1
surface of the bond-coat. Results on the cyclic oxidation at 1200°C (for time ≤ 204h) showed that the treated NiCrAlY bond-coat exhibited better oxidation resistance than the NiCrAlY bond-coat samples without diffusion treatment and the samples pre-oxidized at 800°C for 1 h. The formation of NiO, Cr2O3 and NiCr2O4 spinel oxides were suppressed due to the formation of α-Al2O3 scale during the initial oxidation on the surface of pre-treated NiCrAlY bond-coat.
Keywords: B. Surfaces; C. Diffusion; D. Al2O3; E. Thermal applications
1. Introduction Thermal barrier coatings (TBCs) are widely used in gas-turbine engines and blades. These protective coatings usually consist of an upper ceramic top-coat and an intermediate metallic bond-coat; both coatings are deposited on a Ni-cased superalloy substrate [1–3]. Top-coat provides the thermal insulation to the critical environment by its strain-tolerant yttria-stabilized zirconia component, but it is considered as “oxidation transparent” [4–6]. Thus, MCrAlY (M = Ni, Co or Ni+Co) alloys are of interest as a bond-coat to protect the substrate from oxidation and corrosion at high temperature. Moreover, the sprayed bond-coat provides enough surface roughness to mechanically bond the top-coat [7–9]. Specifically, the basic protection against high temperature oxidation is associated with the evolution of a thermally growth oxide (TGO) layer (especially α-Al2O3 scale) at the interface between the top-coat and the top-coat. A continuous and dense α-Al2O3 layer on a bond-coat plays a role as a barrier to retard the diffusion of cations 2
and oxygen through this stable oxide scale [10–12]. TGO growth on a bond-coat at high temperature environment usually involves the formation of transient oxides and non-protective oxides. The transient oxide products include some metastable alumina like θ-Al2O3, γ-Al2O3, and δ-Al2O3, which are transformed to α-Al2O3 during the heating treatment [13,14]. Non-protective oxides, such as Cr2O3, NiO, and Ni(Cr, Al)2O4 spinel oxides, are formed due to the fact that the diffusion of Al from the bond-coat to the surface is too slow to maintain an exclusive scale in the initial stage of oxidation, though Al is more active to react with oxygen than Cr and Ni [15–17]. Moreover, the small amount of Cr and Ni oxides exist in TGO is caused by the compositional inhomogeneity in the bond-coat during air plasma spraying [18]. The durability and reliability of TBCs largely depended on the formation of TGO [19,20]. This is because TGO provides the protection against high temperature oxidation of the bond-coat and the substrate, and TGO is also the main source of strain incompatibility; which drives the spallation of the top-coat during thermal cycling and initially causes defects leading to the final failure of TBCs [10,21–23]. Transition and other oxides of θ-Al2O3, Cr2O3, NiO and NiCr(Al)2O4 are less protective against oxidation and tend to possess cracks or voids within these oxides. Therefore, understanding the mechanism of TGO formation and the process of TBCs failure is essential for the formation of exclusive α-Al2O3 scale in TBCs system. The quality and microstructure of a bond-coat material for high temperature applications rely on several factors, such as coating method, choice of substrate and modification processes [7,8,15]. Several modification approaches have been adopted 3
to improve the quality of bond-coat materials by promoting the formation of α-Al2O3 scale [24–26]. The pre-oxidation is one post method to pre-form an exclusive α-Al2O3 scale before using samples in an oxidation and corrosion environment. In the work of Lih et al. [24], this method was aimed to promote the formation of α-Al2O3 and suppress the growth of spinel oxides. In Nijdam et al.’s work [25], they found that if the initial oxides at the onset of oxidation were θ-Al2O3 instead of α-Al2O3, then the α-Al2O3 scale grew on the bond-coat during thermal cycling at a lower rate than the normal TBCs samples, which involved the nucleation process on Al2O3. Consequently, the pretreated TBCs samples possessed longer lifetimes. Brossard et al. reported [26] that an Al film of thickness ~3 µm coated on NiCr alloy diffused when heat treated at 600°C and 900°C in an argon gas atmosphere. The weight gain recorded at 1100°C on the Al diffused coating sample was smaller than that of the uncoated-Al NiCr at 950°C [26]. This was due to the formation of Al2O3 scale on the coated surface, but Cr2O3 scale on the uncoated surface [26]. The kinetics of TBCs oxidation is usually quantified by the weight change per unit area per time during high temperature oxidation [27–29]. The kinetics curves generally consist of three stages: (i) a drastic increase at the initial stage; (ii) an extensive period of steady stage; and (iii) a sharp dramatic stage in the end. Stages (i) and (ii) indicate the parabolic weight gain behaviour of oxidized samples, while stage (iii) corresponds to the failure process of TBCs system. The formation of continuous and intact α-Al2O3 scale happens in the initial stage of oxidation, which determines the property and lifetime of the TGO at stage (ii) and (iii). Therefore, Stage (i) should 4
be deeply investigated. In this study, after the NiCrAlY bond-coat was deposited on Inconel 718 superalloys, a diffusion treatment was conducted in argon atmosphere at 600°C for 1 h. Then, pre-oxidation treatment was performed by heating the as-sprayed and diffused samples for 1 h at 800°C and 1000°C, respectively. These procedures were used to modify the surface composition of the bond-coat, and to improve the oxidation resistance of NiCrAlY bond-coat by forming a dense and continuous α-Al2O3 scale. The results on the initial oxidation behaviour of pre-oxidized TBCs sample are compared and discussed.
2. Experimental 2.1 Sample preparation Inconel 718 superalloys (Ci Dong Special Steel & Alloy Co., Ltd. Shanghai, China) were used as the substrate, on which mixture of NiCrAlY powder (KF-343, BGRIMM, Beijing, China; The nominal particle size distributions [D (0.1)–D (0.9)] = 10.1–40.2 μm, [D (0.5)] = 25.0 μm) was deposited by APS method. The chemical compositions of Inconel 718 and NiCrAlY bond-coat powders are listed in Table 1. Before deposition of bond-coat, Inconel 718 substrates were cut into disc shape (diameter = 25 mm, thickness = 5 mm), then were ultrasonically cleaned to ensure being free from contamination. The APS process was achieved by employing a commercial plasma spray system (GP-80, Jiujiang, China, 80 kW class), which is presented in Table 2 in detail. The achieved thickness of NiCrAlY bond-coat was 5
120–150 μm.
2.2 Thermal treatment A diffusion treatment was conducted in a tube furnace with flowing argon (0.2±0.004 L/min) at 600°C for 1 h. The treatment parameters were selected based on the work of Balmain et al. [26]. As-sprayed samples were placed into the furnace at room temperature (~ 25°C) and cooled down to the room temperature. Both of the heating rate and the cooling rate were 10°C/min. Then, pre-oxidation treatment was performed by heating the as-sprayed and diffused samples for 1 h at 800°C and 1000°C, respectively. This process was performed in static air at atmospheric pressure using high temperature furnace, consisting of a progressive heating up to targeted temperature, at the same heating and cooling rate of 10°C/min. In this paper, S1, S2 and S3 samples represent the as-sprayed samples after pre-oxidation for 1 h at 800°C, as-sprayed samples after pre-oxidation for 1 h at 1000°C and diffused samples (at 600°C) after pre-oxidation for 1 h at 1000°C, respectively. Isothermal oxidation test was conducted by heating the S1, S2 and S3 samples for 50 h at 1200°C in static air at atmospheric pressure. Thermal cyclic oxidation test was performed by treating the S1, S2 and S3 samples at 1200°C for 204 h. One thermal cycle of 12 h was consisted of 15 min ramp-up, 11 h isothermal dwelling at 1200°C, and 45 min air exposure to cool-down to room temperature (25°C).
2.3 Sample characterization 6
The surface morphology and polished cross-section of all samples were observed using scanning electron microscopy (SEM, JSM-7000F). Using energy-dispersive X-ray
spectroscopy
(EDX),
the
chemical
compositions
were
determined
semi-quantitatively. X-ray diffraction (XRD, Rigaku D, Cu Kα radiation, wavelength, λ = 1.5406 Å, 45 kV, 40 mA) technique was employed to study the phase composition. All XRD patterns were recorded by using X-ray diffractometer at a constant operating condition, i.e., the scanning step of 0.02° and the scanning range (2θ angle) of 20-80°. The roughness of bond-coat samples was quantified using a MicroXAM white light interferometer (ADE Phase Shift, U.S.A.). The amplitude was characterized by the root-mean-square roughness parameter Rq, which was calculated as the standard deviation of the height of each point on the surface of bond-coat relative to the average value. For each sample, Rq over eight different regions with an area of 805.5 ± 15.2 μm × 609.42 ± 8.3 μm was measured, and the average value of Rq was calculated. The mean area and length of TGO scale were measured/calculated using ImageJ software to estimate the thickness of TGO [27,30,31]. The accuracy of the measurement was ±0.1 µm. Twenty images were taken from each sample for TGO measurement, then the arithmetic mean of TGO length and area were obtained.
3. Results 3.1 Microstructure and elemental analysis of the as-sprayed samples and pre-oxidized samples Figure 1 shows the cross-sectional morphology of as-sprayed bond-coat on the 7
Ni-based substrate. The thickness of the bond-coat is ~150 μm. It can be seen that the splat NiCrAlY layers and the substrate are tightly connected. On the other hand, voids and microcracks are produced where the bond-coat is not fully dense. In addition, segmented Al2O3 veins can be observed, which are non-uniformly distributed in the NiCrAlY bond-coat. These Al2O3 oxides were formed during the air plasma sprayed process. Figures 2(a) and (b) display the surface morphology of the as-sprayed bond-coat before and after the diffusion treatment with flowing argon at 600°C for 1 h. Using the MicroXAM white light interferometer, the samples surface roughness, Rq, is quantified. The determined Rq of as-sprayed samples before and after the diffusion treatment are 12.80 ± 1.10 µm and 10.92 ± 0.80 µm, respectively. Protrudes and pores on the rough surface are the typical surface feature of plasma sprayed bond-coat [14,23]. In addition, the diffusion treatment has an effect on the elemental composition of NiCrAlY bond-coat. The EDX data show that the Al content increase from 7 wt.% for the as-sprayed bond-coat to 12 wt.% for the diffused bond-coat, which indicates that the diffusion treatment enhances the Al content on the surface of NiCrAlY bond-coat. Figures 3(a), (b), and (c) show the surface morphology of the pre-oxidized S1, S2 and S3 samples, respectively. It is found that S1 and S2 samples are covered with fine oxide particles combined with some protrudes, pores and pits. EDX spectra of the surface in Figures 3(d) and (e) indicate that the oxides are mainly Cr oxides and minor Al oxides. Whiskerlike/bladelike oxides are observed on the surface of S3 samples, 8
which are generally reported as θ-Al2O3 [13,14,28]. The EDX analysis of S3 also produces very strong Al peaks shown in Figure 3(e). The XRD patterns of S1, S2, and S3 NiCrAlY samples shown in Figure 4 confirm the oxides phases formed during the pre-oxidation process. The Cr2O3 phase is detected on the surface of as-sprayed samples after pre-oxidation for 1 h at 800°C and 1000°C (Figures 4(a) and (b)). In addition, both θ-Al2O3 and Cr2O3 phases are found on the surface of diffused samples after pre-oxidation for 1 h at 1000°C in Figure 4(c).
3.2 Microstructure and elemental analysis of the NiCrAlY samples after isothermal oxidation Figures 5(a), (b), and (c) show the morphology after isothermal oxidation for 50 h at 1200°C of S1, S2, and S3 samples, respectively; while Figure (d), (e), and (f) present the surface EDX spectra corresponding to these three groups of samples. Fine oxides particles formed on the surface of S1 and S2 samples, and these oxides are Cr oxides with trace Ni oxides and Al oxides. Moreover, sintering occurs during the high temperature environment on S2 samples. Pores are still on the surface of S1 and S2 samples, which indicates that the oxide scales are not fully dense. Equiaxed structure oxides are observed on the surface of S3 samples in Figure 5(a). EDX analysis show that the oxides mainly consist of Al and Cr oxides. Figure 6 shows the XRD patterns of S1, S2, and S3 samples after being isothermally oxidized at 1200°C for 50 h. The peaks and the results in Figure 5 show that the Cr2O3 with minor NiCr2O4 and α-Al2O3 forms on the surface of S1 and S2 9
samples; while the α-Al2O3 main phase with Cr2O3 and NiCr2O4 develops on the surface of S3 samples. Figures 7(a) and (b) show the cross-sectional morphology of S2 and S3 samples after isothermal oxidation at 1200°C for 50 h, respectively. They show that TGO formed on the surface of bond-coat of both samples, which consists of an outer layer and an inner layer. The composition of points were measured from sites A, B, C and D marked in the SEM images shown in Figures 7(a) and (b) for these pre-oxidized samples. Table 3 presents the EDX data on the four points, and Figures 7(c), (d), (e), and (f) show the EDX spectra of corresponding points. Combining these results with the XRD results in Figure 6, it can be found that the inner layer of oxides in both samples are α-Al2O3 layers, which is confirmed by Al contents of 70 and 88 wt.% at sites B and D; whereas the outer and bright layer of oxides consists of Cr2O3+Ni(Al,Cr)2O4 mixed oxides, which corresponds to a higher Cr content of 62 and 79 wt.% at sites A and C. Moreover, the total thickness of the TGO of S2 sample is in the range of ~1.0 μm to ~8.5 μm, which is not evenly distributed on the bond-coat surface. The average total thickness is ~6.5 μm; while the thickness of α-Al2O3 layer is <1.0 μm. For S3 samples, the average total thickness is ~6.1 μm, in which the average thickness of α-Al2O3 layer is ~4.3 μm. In addition, some pores are observed in the outer layer of oxides in S2 samples.
3.3 Thermal cycling oxidation behaviour of NiCrAlY samples Cyclic oxidation of NiCrAlY bond-coat samples after pre-oxidation treatment 10
was studied as a function of oxidation time. Figures 8(a), and (b) show the thickness of oxides as a function of oxidation time for S2 and S3 samples, respectively. It shows that the thickness of oxides of diffused samples after pre-oxidation for 1 h at 1000°C (S3 samples) is smaller than that of the S2 samples studied without diffusion treatment. The TGO thickness of S2 samples is ~3.0 μm, while that of S3 samples is ~2.5 μm after the first cyclic period oxidation (12 h). After oxidation for 204 h, the TGO thicknesses are ~9.5 μm and ~7.9 μm for S2 and S3 samples, respectively. After the sharp increase in TGO thickness at the initial oxidation stage, all the kinetic curves show an extensive period of slow trend (Figure 8).
4 Discussion Figures 9(a) and (b) show the schematics of S2 and S3 sample, respectively, which illustrate the diffusion and pre-oxidation treatment effect on the oxidation NiCrAlY bond-coat behavior. From the surface roughness data in Figure 2, it can be seen that the Rq of diffused sample is smaller than that of as-sprayed sample. This may be caused by the diffusion process which uniformed the surface morphology and composition at high temperature environment. Tolpygo and Clarke found that no wrinkling with smaller growth strain in TGO leads to an improved FeCrAlY alloys [32]; while Taylor et al. demonstrated that MCrAlY bond-coat with asperities of high aspect ratios can result in premature chemical failure during isothermal exposures [33]. Thus, the diffused sample with smaller surface roughness may possess improved life time as compared to the as-spayed sample. The EDX results of Figure 2 shows 11
that the Al content on the surface of NiCrAlY bond-coat increased after the diffusion treatment. This may be due to two reasons: i) The formation of Al2O3 veins during plasma spraying consumed the internal Al content, which led to a lower Al content of 7 wt.% on the surface of bond-coat compared to that of 10 wt.% for NiCrAlY powders; ii) The environment during diffusion was not in vacuum, but with low oxygen pressure. Al is more prone to diffuse to the bond-coat surface which is exposed to the air, because Al is more active than Cr and Ni. Nijdam et al. [34] also found that the values for the amounts of Al oxides increased very rapidly on the surface of NiCrAl alloys during the initial stages of oxidation. Chen et al. [30] observed that the Al concentration was highly non-uniform in the NiCoCrAlY APS bond-coat (8 wt.%Al), which was ranged from 9.6-17.9 at.%. This was attributed to the preferential oxidation and/or vaporization during spraying process. The Cr2O3 formed on the surface of S2 samples after pre-oxidation at 1000°C for 1 h. This is due to the high concentration and the high inter-diffusion coefficient of Cr, as well as the Al depletion caused by the oxidation during the production of samples. For S3 samples, both Cr2O3 and θ-Al2O3 were observed after pre-oxidation. Oxides of θ-Al2O3 are prone to grow on the surface of the bond-coat which possesses a higher Al content [13,28]. Moreover, the formation of θ-Al2O3 on the bond-coat during the initial stage of oxidation was reported at 950°C~1150°C [13] or in a low PO2 pressure environment [35]. During the high-temperature isothermal oxidation at 1200°C, two layers of oxides formed on the top of NiCrAlY bond-coat for S2 and S3 samples, which consisted of 12
the outer mixed oxides (Cr2O3+Ni(Al,Cr)2O4) layer and inner α-Al2O3 layer. The distribution of oxides is consistent with earlier studies [14,27,36]. Al was the most probable element in the TBC to react with oxygen at the initial oxidation stage according to thermodynamics [37]. After the Al depletion, a large content (20 wt.%) of Cr in the bond-coat and the high alloy inter-diffusion coefficient guaranteed the replenishment of Cr when the Cr2O3 formed [38]. The low diffusivity and solubility of oxygen in the NiCrAlY bond-coat made the diffusion of Cr from the deep part to the surface without forming internal oxides in the bond-coat [39]. Spinel oxides formed when the Ni diffused through microcracks in the Al2O3 and Cr2O3 layer. Thus, the two-layer structure was observed after isothermal oxidation. The formation of partial inner α-Al2O3 layer in diffusion treated S3 samples was from the θ-Al2O3 phase transformation during the high-temperature annealing [13,14,28]. The enhanced nucleation of θ-Al2O3 at the surface of bond-coat assisted the formation of α-Al2O3 and thus resulted in a thicker protective α-Al2O3 layer of S3 samples as compared to that of the S2 samples. The thick α-Al2O3 layer barrier to the oxygen environment was helpful to suppress the formation of Ni, Cr-oxides during the isothermal oxidation, and thus led to a smaller mixed oxides layer in the S3 samples than that of the S2 samples. Moreover, the pores and cracks formed in the mixed oxides layer, which accelerated the oxygen attack on the bond-coat and led to the uneven-thick TGO. During the cyclic oxidation test shown in Figure 8, the S3 samples also possessed a thinner TGO than the S2 samples. Compared to the work of Chen et al. [30], and Iwamoto et al. [40], the thickness of TGO (~6.1 μm) after 13
oxidation at 1200°C for 50 h in the S3 samples was larger than those of the Ni(Co)CrAlY samples produced with YSZ top-coat (~5.1 μm) and by laser cladding and then oxidized at 1200°C (~2 μm). It should be noted that the temperature (1200°C) used in this study is higher than the application of the Inconel 718 superalloy. Such high temperature may enhance the inter-diffusion between the substrate and the bon-coat. Figure 10 shows the interface of S3 sample between the Inconel 718 substrate and NiCrAlY bond-coat after cyclic oxidation at 1200°C for 204 h. It can be seen that an oxide layer formed at the interface. EDX mapping data inside Figure 10 indicate that the oxides are Al oxides. Moreover, this oxides layer contacts both bond-coat and substrate tightly. Few pores and cracks are observed. Therefore, in this case, the inter-diffusion between the substrate and the bon-coat does not lead to the failure of samples after cyclic oxidation within 204 h.
5. Conclusions In conclusion, the microstructure and initial oxidation behaviour of the NiCrAlY bond-coat before and after diffusion and pre-oxidation treatment were investigated. The conclusions drawn from this study is summarized as follows: 1. The Al content on the surface of NiCrAlY bond-coat increased from 7 wt.% to 12 wt.% after the samples were conducted in a tube furnace with flowing argon at 600°C for 1 h. The pre-oxidation treatment of these diffused samples under 1000°C for 1 h led to the formation of θ-Al2O3 and Cr2O3; while only Cr2O3 was 14
formed on the surface during the pre-oxidation of undiffused samples conducted under 800 and 1000°C for 1 h. 2. The TGO formed on the surface of pre-treated bond-coat after isothermally oxidized at 1200°C for 50 h consisted of an outer mixed oxides layer and an inner α-Al2O3 layer. The θ-Al2O3 phase transformed to the α-Al2O3 phase during this 50 h. The average total thickness of the TGO of un-diffused and pre-oxidized samples (S2 samples) was ~6.5 μm; while the thickness of α-Al2O3 layer was <1.0 μm. For diffused and pre-oxidized samples (S3 samples), the average total thickness was ~6.1 μm, in which the average thickness of α-Al2O3 layer was ~4.3 μm. The S3 samples with a thicker α-Al2O3 layer suppressed the formation of Ni, Cr-oxides during the isothermal oxidation, and thus provided better oxidation resistance compared to the S2 samples. 3. During the cyclic oxidation at 1200°C for 204 h, the TGO thickness of S3 samples was smaller than that of the S2 samples.
Acknowledgements This work was funded by the Chinese Natural Science Foundation (Grant No. 51401170 and 51032006). The authors would like to thank Mr. Li (Northwest Institute for Nonferrous Metal Research) for parts of the sample characterization, and Mr. Wu (Luoyang Ship Material Research Institute) for sample preparation, as well as their discussions and suggestions on this study.
15
References: [1] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas - turbine engine applications, Sci. 296 (2002) 280–284. [2] A.G. Evans, D.R. Clarke, C.G. Levi, The influence of oxides on the performance of advanced gas turbines, J. Eur. Ceram. Soc. 28 (2008) 1405–1419. [3] C.G. Levi, E. Sommer, S.G. Terry, A. Catanoiu, M. Ruhle, Alumina grown during deposition of thermal barrier coatings on NiCrAlY, J. Am. Ceram. Soc. 86 (2003) 676–685. [4] C. Chen, H.B. Guo, S.K. Gong, X.F. Zhao, X. Ping, Sintering of electron beam physical vapor deposited thermal barrier coatings under flame shock, Ceram. Int. 39 (2013) 5093–5102. [5] M.W. Bai, F.W. Guo, P. Xiao, Fabrication of thick YSZ thermal barrier coatings using electrophoretic deposition, Ceram. Int. 40 (2014) 16611–16616. [6] R. Sobhanverdi, A. Akbari, Porosity and microstructural features of plasma sprayed Yttria stabilized Zirconia thermal barrier coatings, Ceram. Int. 41 (2015) 14517–14528. [7] D. Mercier, B.D. Gauntt, M. Brochu, Thermal stability and oxidation behavior of nanostructured NiCoCrAlY coatings, Surf. Coat. Technol. 205 (2011) 4162–4168. [8] R. Panat, S.L. Zhang, K.J. Hsia, Bond coat surface rumpling in thermal barrier coatings, Acta. Mater. 51 (2003) 239–249. [9] B. Wang, J. Gong, A.Y. Wang, C. Sun, R.F. Huang, L.S. Wen, Oxidation behaviour of NiCrAlY coatings on Ni-based superalloy, Surf. Coat. Technol. 149 (2002) 70–75. [10] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater. Sci. 46 (2001) 505– 553. [11] P. Richer, M. Yandouzi, L. Beauvais, B. Jodoin, Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying, Surf. Coat. Technol. 204 (2010) 3962–3974. [12] J. Sun, Q.G. Fu, G.N. Liu, H.J. Li, Y.C. Shu, G. Fan, Thermal shock resistance of thermal barrier coatings for nickel-based superalloy by supersonic plasma spraying, Ceram. Int. 41 (2015) 9972–9979.
16
[13] T.F. An, H.R. Guan, X.F. Sun, Z.Q. Hu, Effect of the θ-α-Al2O3 transformation in scales on the oxidation behavior of a Nickel-base superalloy with an aluminide diffusion coating, Oxid. Met. 54 (2000) 301–316. [14] P. Puetz, X. Huang, Q. Yang, Z. Tang, Transient oxide formation on APS NiCrAlY after oxidation heat treatment, J. Therm. Spray. Technol. 20 (2011) 621– 629. [15] P. Niranatlumpong, C.B. Ponton, H.E. Evans, The failure of protective oxides on plasma-sprayed NiCrAlY overlay coatings, Oxid. Met. 53 (1997) 241–258. [16] T.J. Nijdam, L.P.H. Jeurgens, W.G. Sloof, Promoting exclusive α-Al2O3 growth upon high-temperature oxidation of NiCrAl alloys: experiment versus model predictions, Acta. Mater. 53 (2005) 1643–1653. [17] T.J. Nijdam, W.G. Sloof, Modelling of composition and phase changes in multiphase alloys due to growth of an oxide layer, Acta. Mater. 56 (2008) 4972–4983. [18] W.R. Chen, X. Wu, D. Dudzinski, P.C. Patnaik, Modification of oxide layer in plasma-sprayed thermal barrier coatings, Surf. Coat. Technol. 200 (2006) 5863–5868. [19] U. Schulz, C. Leyens, K. Fritscher, et al., Some recent trends in research and technology of advanced thermal barrier coatings, Aerosp. Sci. Technol. 7 (2003) 73– 80. [20] T.N. Rhys-Jones, Coatings for blade and vane applications in gas turbines, Corros. Sci. 29 (1989) 623–646. [21] B.A. Pint, On the formation of interfacial and internal voids in α-Al2O3 scales, Oxid. Met. 48 (1997) 303–328. [22] D. Straussa, G. Muller, G. Schumacher, V. Engelko, W. Stamm, D. Clemens, W.J. Quaddakers, Oxide scale growth on MCrAlY bond coatings after pulsed electron beam treatment and deposition of EBPVD-TBC, Surf. Coat. Technol. 135 (2001) 196–201. [23] A. Strawbridge, H.E. Evans, C.B. Ponton, spallation of oxide scales from NiCrAlY overlay coatings, Mater. Sci. Forum. 251–254 (2009) 365–372. [24] W. Lih, E. Chang, B.C. Wu, C.H. Chao, Effects of bond coat preoxidation on the properties of ZrO2-8wt.% Y2O3/Ni-22Cr-10Al-lY thermal barrier coatings, Oxid. Met. 36 (1991) 221–238. [25] T.J. Nijdam, G.H. Marijnissen, E. Vergeldt, A.B. Kloosterman, W.G. Sloof, Development of a pre-oxidation treatment to improve the adhesion between thermal barrier coatings and NiCoCrAlY bond coatings, Oxid. Met. 66 (2006) 269–294. 17
[26] J.M. Brossard, J. Balmain, F. Sanchette, G. Bonnet, High-temperature oxidation of an aluminized NiCr alloy formed by a magnetron-sputtered Al diffusion coating, Oxid. Met. 64 (2005) 43–61. [27] C. Zhu, P. Li, A. Javed, G.Y. Liang, P. Xiao, An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings. Surf. Coat. Technol. 206 (2012) 3739–3746. [28] C. Zhu, A. Javed, P. Li, G.Y. Liang, P. Xiao, Study of the effect of laser treatment on the initial oxidation behaviour of Al-coated NiCrAlY bond-coat, Surf. Interface Anal. 45 (2013) 1680–1689. [29] R. Ghasemi, R. Shoja-Razavi, R. Mozafarinia, H. Jamali, The influence of laser treatment on thermal shock resistance of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings, Ceram. Int. 40 (2014) 347–355. [30] W.R. Chen, X. Wu, B.R. Marple, R.S. Lima, P.C. Patnaik, Pre-oxidation and TGO growth behaviour of an air-plasma-sprayed thermal barrier coating, Surf. Coat. Technol. 202 (2008) 3787–3796. [31] W.R. Chen, X. Wu, D. Dudzinski, Influence of thermal cycle frequency on the TGO growth and cracking behaviours of a TBC, J. Therm. Spray Technol. 21 (2012) 1294–1299. [32] V.K. Tolpygo, J.R. Den, D.R. Clarke, Determination of the Growth Stress and Strain in α-Al2O3 Scales During the Oxidation of Fe−22Cr−4.8Al−0.3Y Alloy, Acta. Mater. 46 (1998) 927–937. [33] M.P. Taylor, W.M. Pragnell, H.E. Evans, The influence of bond coat surface roughness on chemical failure and delamination in TBC systems, Mater. Corros. 59 (2008) 508–513. [34] T.J. Nijdam, N. M. van der Pers, W.G. Sloof, Oxide phase development upon high temperature oxidation of γ-NiCrAl alloys, Mater. Corros. 57 (2006) 269–275. [35] T.J. Nijdam, L.P.H. Jeurgens, J.H. Chen, W.G. Sloof, On the microstructure of the initial oxide grown by controlled annealing and oxidation on a NiCoCrAlY bond coating, Oxid. Met. 64 (2005) 355–377. [36] W.R. Chen, Degradation of a TBC with HVOF-CoNiCrAlY bond coat, J. Therm. Spray Technol. 23 (2014) 876–884. [37] J. Moon, H. Chol, Y. Kim, C. Lee, Cooling rate effect on phase transformation of plasma sprayed partially stabilized zirconia, J. Mater. Sci. Lett. 20 (2001) 1611–1613. [38] R.A. Miller, C.C. Berndt, Performance of thermal barrier coatings in high heat flux environments, Thin Solid Films 119 (1984) 195–202. 18
[39] V. Lughi, V.K. Tolpygo, D.R. Clarke, Microstructural aspects of the sintering of thermal barrier coatings, Mater. Sci. Eng., A 368 (2004) 212–221. [40] H. Iwamoto, T. Sumikawa, K. Nishida, T. Asano, M. Nishida, T. Araki, High temperature oxidation behavior of laser clad NiCrAlY layer, Mater. Sci. Eng. A. 214 (1998) 251-258.
Figure captions: Fig. 1 Cross-sectional morphology of the as-sprayed APS bond-coat. Fig. 2 Surface morphology of the APS bond-coat (a) as-sprayed sample, (b) diffusion sample treated with flowing argon at 600°C for 1 h. Fig. 3 Surface morphology of the bond-coat samples after pre-oxidation for 1 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples; and EDX spectrum (d) S1 samples, (e) S2 samples, and (f) S3 samples. In the figure captions, S1, S2 and S3 samples represent the as-sprayed samples after pre-oxidation for 1 h at 800°C, as-sprayed samples after pre-oxidation for 1 h at 1000°C and diffused samples (at 600°C) after pre-oxidation for 1 h at 1000°C, respectively. Fig. 4 XRD patterns of bond-coat samples after pre-oxidation: (a) S1 samples, (b) S2 samples, and (c) S3 samples. Fig. 5 Surface morphology of bond-coat samples after isothermal oxidation at 1200°C for 50 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples; and EDX spectrum (d) S1 samples, (e) S2 samples, and (f) S3 samples. Fig. 6 XRD patterns of bond-coat samples after isothermal oxidation at 1200°C for 50 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples. Fig. 7 Cross-section morphology of bond-coat samples after isothermally oxidized at 19
1200°C for 50 h: (a) S2 samples, (b) S3 samples; and EDX spectrum (c) point A, (d) point B, (e) point C, and (f) point D. Fig. 8 TGO thickness as a function of oxidation time for (a) S2 samples, and (b) S3 samples. Fig. 9 Schematic diagram illustrating the initial oxidation behaviour of NiCrAlY bond-coat oxidized at 1200°C: (a) S2 samples, and (b) S3 samples. Fig. 10 Cross-section morphology of the interface of S3 sample between NiCrAlY bond-coat and Inconel 718 substrate oxidized at 1200°C for 204 h. The inside EDX mapping is the elemental map of Al.
20
21
22
23
24
25
26
Table 1 Chemical compositions of the substrate and the NiCrAlY bond-coat powders Unit wt. %
Ni
Cr
Al
Y
Zr
O
Fe
Mo
Nb
Ti
C
IN718
Balance
17.9
0.5
-
-
-
18.4
3.0
5.4
0.9
0.02
Bond-coat
Balance
20
10
1
-
-
-
-
-
-
-
Table 2 Spray Processing Parameters for the NiCrAlY bond-coat powders Parameter
Unit
NiCrAlY powder
Nozzle diameter
mm
6
Arc current
A
650
Carrier gas flow rate
L/min
10
Primary plasma forming gas flow rate (Ar)
L/min
55
Secondary plasma forming gas flow rate (H2)
L/min
4.5
Feedstock mass flow rate
g/min
40
Spray distance
mm
80
Traverse speed
mm/s
380
Scanning Step
mm
3-4
Number of passes
2-3
Table 3 Composition data measured by EDX on the cross-section of S2 and S3 samples Composition
S2 sample
S3 sample
(wt.%)
Site A
Site B
Site C
Site D
Al
26
70
15
88
Cr
62
19
79
10
Ni
12
11
6
2
27
PII: DOI: Reference:
S0272-8842(16)00231-5 http://dx.doi.org/10.1016/j.ceramint.2016.01.185 CERI12161
To appear in: Ceramics International Received date: 26 October 2015 Revised date: 28 December 2015 Accepted date: 25 January 2016 Cite this article as: C. Zhu, P. Li and X.Y. Wu, A study of the diffusion and preoxidation treatment on the formation of Al 2O3 ceramic scale on NiCrAlY bondcoat during initial oxidation process, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.185 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A study of the diffusion and pre-oxidation treatment on the formation of Al2O3 ceramic scale on NiCrAlY bond-coat during initial oxidation process
C. Zhu*1, P. Li2, X.Y. Wu3 1
Science and Technology on Thermostructural Composite Materials Laboratory,
Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, PR China 2
Northwest Institute for Nonferrous Metal Research, Xi'an, Shaanxi, 710016, PR
China 3
*
Luoyang Ship Material Research Institute, Luoyang, Henan, 471023, PR China
Corresponding author. Tel.: +86 29 88494914; Fax: +86 29 88494620.
E-mail address: [email protected] (C. Zhu)
Abstract In this study, the formation of thermally growth ceramic oxides during initial oxidation on treated NiCrAlY bond-coat was investigated. The oxidation resistance of NiCrAlY bond-coat was enhanced by two approaches: (i) an inter-diffusion treatment in a tube furnace with flowing argon at 600°C for 1 h, and (ii) a pre-oxidation treatment in static air at atmospheric pressure at 1000°C for 1 h. Experimental results showed that, after diffusion treatment, the content of Al in the bond-coat increased. This led to the growth of θ-Al2O3 during the pre-oxidation process, which was then helped in the formation of a dense and continuous α-Al2O3/Cr2O3 multilayer on the 1
surface of the bond-coat. Results on the cyclic oxidation at 1200°C (for time ≤ 204h) showed that the treated NiCrAlY bond-coat exhibited better oxidation resistance than the NiCrAlY bond-coat samples without diffusion treatment and the samples pre-oxidized at 800°C for 1 h. The formation of NiO, Cr2O3 and NiCr2O4 spinel oxides were suppressed due to the formation of α-Al2O3 scale during the initial oxidation on the surface of pre-treated NiCrAlY bond-coat.
Keywords: B. Surfaces; C. Diffusion; D. Al2O3; E. Thermal applications
1. Introduction Thermal barrier coatings (TBCs) are widely used in gas-turbine engines and blades. These protective coatings usually consist of an upper ceramic top-coat and an intermediate metallic bond-coat; both coatings are deposited on a Ni-cased superalloy substrate [1–3]. Top-coat provides the thermal insulation to the critical environment by its strain-tolerant yttria-stabilized zirconia component, but it is considered as “oxidation transparent” [4–6]. Thus, MCrAlY (M = Ni, Co or Ni+Co) alloys are of interest as a bond-coat to protect the substrate from oxidation and corrosion at high temperature. Moreover, the sprayed bond-coat provides enough surface roughness to mechanically bond the top-coat [7–9]. Specifically, the basic protection against high temperature oxidation is associated with the evolution of a thermally growth oxide (TGO) layer (especially α-Al2O3 scale) at the interface between the top-coat and the top-coat. A continuous and dense α-Al2O3 layer on a bond-coat plays a role as a barrier to retard the diffusion of cations 2
and oxygen through this stable oxide scale [10–12]. TGO growth on a bond-coat at high temperature environment usually involves the formation of transient oxides and non-protective oxides. The transient oxide products include some metastable alumina like θ-Al2O3, γ-Al2O3, and δ-Al2O3, which are transformed to α-Al2O3 during the heating treatment [13,14]. Non-protective oxides, such as Cr2O3, NiO, and Ni(Cr, Al)2O4 spinel oxides, are formed due to the fact that the diffusion of Al from the bond-coat to the surface is too slow to maintain an exclusive scale in the initial stage of oxidation, though Al is more active to react with oxygen than Cr and Ni [15–17]. Moreover, the small amount of Cr and Ni oxides exist in TGO is caused by the compositional inhomogeneity in the bond-coat during air plasma spraying [18]. The durability and reliability of TBCs largely depended on the formation of TGO [19,20]. This is because TGO provides the protection against high temperature oxidation of the bond-coat and the substrate, and TGO is also the main source of strain incompatibility; which drives the spallation of the top-coat during thermal cycling and initially causes defects leading to the final failure of TBCs [10,21–23]. Transition and other oxides of θ-Al2O3, Cr2O3, NiO and NiCr(Al)2O4 are less protective against oxidation and tend to possess cracks or voids within these oxides. Therefore, understanding the mechanism of TGO formation and the process of TBCs failure is essential for the formation of exclusive α-Al2O3 scale in TBCs system. The quality and microstructure of a bond-coat material for high temperature applications rely on several factors, such as coating method, choice of substrate and modification processes [7,8,15]. Several modification approaches have been adopted 3
to improve the quality of bond-coat materials by promoting the formation of α-Al2O3 scale [24–26]. The pre-oxidation is one post method to pre-form an exclusive α-Al2O3 scale before using samples in an oxidation and corrosion environment. In the work of Lih et al. [24], this method was aimed to promote the formation of α-Al2O3 and suppress the growth of spinel oxides. In Nijdam et al.’s work [25], they found that if the initial oxides at the onset of oxidation were θ-Al2O3 instead of α-Al2O3, then the α-Al2O3 scale grew on the bond-coat during thermal cycling at a lower rate than the normal TBCs samples, which involved the nucleation process on Al2O3. Consequently, the pretreated TBCs samples possessed longer lifetimes. Brossard et al. reported [26] that an Al film of thickness ~3 µm coated on NiCr alloy diffused when heat treated at 600°C and 900°C in an argon gas atmosphere. The weight gain recorded at 1100°C on the Al diffused coating sample was smaller than that of the uncoated-Al NiCr at 950°C [26]. This was due to the formation of Al2O3 scale on the coated surface, but Cr2O3 scale on the uncoated surface [26]. The kinetics of TBCs oxidation is usually quantified by the weight change per unit area per time during high temperature oxidation [27–29]. The kinetics curves generally consist of three stages: (i) a drastic increase at the initial stage; (ii) an extensive period of steady stage; and (iii) a sharp dramatic stage in the end. Stages (i) and (ii) indicate the parabolic weight gain behaviour of oxidized samples, while stage (iii) corresponds to the failure process of TBCs system. The formation of continuous and intact α-Al2O3 scale happens in the initial stage of oxidation, which determines the property and lifetime of the TGO at stage (ii) and (iii). Therefore, Stage (i) should 4
be deeply investigated. In this study, after the NiCrAlY bond-coat was deposited on Inconel 718 superalloys, a diffusion treatment was conducted in argon atmosphere at 600°C for 1 h. Then, pre-oxidation treatment was performed by heating the as-sprayed and diffused samples for 1 h at 800°C and 1000°C, respectively. These procedures were used to modify the surface composition of the bond-coat, and to improve the oxidation resistance of NiCrAlY bond-coat by forming a dense and continuous α-Al2O3 scale. The results on the initial oxidation behaviour of pre-oxidized TBCs sample are compared and discussed.
2. Experimental 2.1 Sample preparation Inconel 718 superalloys (Ci Dong Special Steel & Alloy Co., Ltd. Shanghai, China) were used as the substrate, on which mixture of NiCrAlY powder (KF-343, BGRIMM, Beijing, China; The nominal particle size distributions [D (0.1)–D (0.9)] = 10.1–40.2 μm, [D (0.5)] = 25.0 μm) was deposited by APS method. The chemical compositions of Inconel 718 and NiCrAlY bond-coat powders are listed in Table 1. Before deposition of bond-coat, Inconel 718 substrates were cut into disc shape (diameter = 25 mm, thickness = 5 mm), then were ultrasonically cleaned to ensure being free from contamination. The APS process was achieved by employing a commercial plasma spray system (GP-80, Jiujiang, China, 80 kW class), which is presented in Table 2 in detail. The achieved thickness of NiCrAlY bond-coat was 5
120–150 μm.
2.2 Thermal treatment A diffusion treatment was conducted in a tube furnace with flowing argon (0.2±0.004 L/min) at 600°C for 1 h. The treatment parameters were selected based on the work of Balmain et al. [26]. As-sprayed samples were placed into the furnace at room temperature (~ 25°C) and cooled down to the room temperature. Both of the heating rate and the cooling rate were 10°C/min. Then, pre-oxidation treatment was performed by heating the as-sprayed and diffused samples for 1 h at 800°C and 1000°C, respectively. This process was performed in static air at atmospheric pressure using high temperature furnace, consisting of a progressive heating up to targeted temperature, at the same heating and cooling rate of 10°C/min. In this paper, S1, S2 and S3 samples represent the as-sprayed samples after pre-oxidation for 1 h at 800°C, as-sprayed samples after pre-oxidation for 1 h at 1000°C and diffused samples (at 600°C) after pre-oxidation for 1 h at 1000°C, respectively. Isothermal oxidation test was conducted by heating the S1, S2 and S3 samples for 50 h at 1200°C in static air at atmospheric pressure. Thermal cyclic oxidation test was performed by treating the S1, S2 and S3 samples at 1200°C for 204 h. One thermal cycle of 12 h was consisted of 15 min ramp-up, 11 h isothermal dwelling at 1200°C, and 45 min air exposure to cool-down to room temperature (25°C).
2.3 Sample characterization 6
The surface morphology and polished cross-section of all samples were observed using scanning electron microscopy (SEM, JSM-7000F). Using energy-dispersive X-ray
spectroscopy
(EDX),
the
chemical
compositions
were
determined
semi-quantitatively. X-ray diffraction (XRD, Rigaku D, Cu Kα radiation, wavelength, λ = 1.5406 Å, 45 kV, 40 mA) technique was employed to study the phase composition. All XRD patterns were recorded by using X-ray diffractometer at a constant operating condition, i.e., the scanning step of 0.02° and the scanning range (2θ angle) of 20-80°. The roughness of bond-coat samples was quantified using a MicroXAM white light interferometer (ADE Phase Shift, U.S.A.). The amplitude was characterized by the root-mean-square roughness parameter Rq, which was calculated as the standard deviation of the height of each point on the surface of bond-coat relative to the average value. For each sample, Rq over eight different regions with an area of 805.5 ± 15.2 μm × 609.42 ± 8.3 μm was measured, and the average value of Rq was calculated. The mean area and length of TGO scale were measured/calculated using ImageJ software to estimate the thickness of TGO [27,30,31]. The accuracy of the measurement was ±0.1 µm. Twenty images were taken from each sample for TGO measurement, then the arithmetic mean of TGO length and area were obtained.
3. Results 3.1 Microstructure and elemental analysis of the as-sprayed samples and pre-oxidized samples Figure 1 shows the cross-sectional morphology of as-sprayed bond-coat on the 7
Ni-based substrate. The thickness of the bond-coat is ~150 μm. It can be seen that the splat NiCrAlY layers and the substrate are tightly connected. On the other hand, voids and microcracks are produced where the bond-coat is not fully dense. In addition, segmented Al2O3 veins can be observed, which are non-uniformly distributed in the NiCrAlY bond-coat. These Al2O3 oxides were formed during the air plasma sprayed process. Figures 2(a) and (b) display the surface morphology of the as-sprayed bond-coat before and after the diffusion treatment with flowing argon at 600°C for 1 h. Using the MicroXAM white light interferometer, the samples surface roughness, Rq, is quantified. The determined Rq of as-sprayed samples before and after the diffusion treatment are 12.80 ± 1.10 µm and 10.92 ± 0.80 µm, respectively. Protrudes and pores on the rough surface are the typical surface feature of plasma sprayed bond-coat [14,23]. In addition, the diffusion treatment has an effect on the elemental composition of NiCrAlY bond-coat. The EDX data show that the Al content increase from 7 wt.% for the as-sprayed bond-coat to 12 wt.% for the diffused bond-coat, which indicates that the diffusion treatment enhances the Al content on the surface of NiCrAlY bond-coat. Figures 3(a), (b), and (c) show the surface morphology of the pre-oxidized S1, S2 and S3 samples, respectively. It is found that S1 and S2 samples are covered with fine oxide particles combined with some protrudes, pores and pits. EDX spectra of the surface in Figures 3(d) and (e) indicate that the oxides are mainly Cr oxides and minor Al oxides. Whiskerlike/bladelike oxides are observed on the surface of S3 samples, 8
which are generally reported as θ-Al2O3 [13,14,28]. The EDX analysis of S3 also produces very strong Al peaks shown in Figure 3(e). The XRD patterns of S1, S2, and S3 NiCrAlY samples shown in Figure 4 confirm the oxides phases formed during the pre-oxidation process. The Cr2O3 phase is detected on the surface of as-sprayed samples after pre-oxidation for 1 h at 800°C and 1000°C (Figures 4(a) and (b)). In addition, both θ-Al2O3 and Cr2O3 phases are found on the surface of diffused samples after pre-oxidation for 1 h at 1000°C in Figure 4(c).
3.2 Microstructure and elemental analysis of the NiCrAlY samples after isothermal oxidation Figures 5(a), (b), and (c) show the morphology after isothermal oxidation for 50 h at 1200°C of S1, S2, and S3 samples, respectively; while Figure (d), (e), and (f) present the surface EDX spectra corresponding to these three groups of samples. Fine oxides particles formed on the surface of S1 and S2 samples, and these oxides are Cr oxides with trace Ni oxides and Al oxides. Moreover, sintering occurs during the high temperature environment on S2 samples. Pores are still on the surface of S1 and S2 samples, which indicates that the oxide scales are not fully dense. Equiaxed structure oxides are observed on the surface of S3 samples in Figure 5(a). EDX analysis show that the oxides mainly consist of Al and Cr oxides. Figure 6 shows the XRD patterns of S1, S2, and S3 samples after being isothermally oxidized at 1200°C for 50 h. The peaks and the results in Figure 5 show that the Cr2O3 with minor NiCr2O4 and α-Al2O3 forms on the surface of S1 and S2 9
samples; while the α-Al2O3 main phase with Cr2O3 and NiCr2O4 develops on the surface of S3 samples. Figures 7(a) and (b) show the cross-sectional morphology of S2 and S3 samples after isothermal oxidation at 1200°C for 50 h, respectively. They show that TGO formed on the surface of bond-coat of both samples, which consists of an outer layer and an inner layer. The composition of points were measured from sites A, B, C and D marked in the SEM images shown in Figures 7(a) and (b) for these pre-oxidized samples. Table 3 presents the EDX data on the four points, and Figures 7(c), (d), (e), and (f) show the EDX spectra of corresponding points. Combining these results with the XRD results in Figure 6, it can be found that the inner layer of oxides in both samples are α-Al2O3 layers, which is confirmed by Al contents of 70 and 88 wt.% at sites B and D; whereas the outer and bright layer of oxides consists of Cr2O3+Ni(Al,Cr)2O4 mixed oxides, which corresponds to a higher Cr content of 62 and 79 wt.% at sites A and C. Moreover, the total thickness of the TGO of S2 sample is in the range of ~1.0 μm to ~8.5 μm, which is not evenly distributed on the bond-coat surface. The average total thickness is ~6.5 μm; while the thickness of α-Al2O3 layer is <1.0 μm. For S3 samples, the average total thickness is ~6.1 μm, in which the average thickness of α-Al2O3 layer is ~4.3 μm. In addition, some pores are observed in the outer layer of oxides in S2 samples.
3.3 Thermal cycling oxidation behaviour of NiCrAlY samples Cyclic oxidation of NiCrAlY bond-coat samples after pre-oxidation treatment 10
was studied as a function of oxidation time. Figures 8(a), and (b) show the thickness of oxides as a function of oxidation time for S2 and S3 samples, respectively. It shows that the thickness of oxides of diffused samples after pre-oxidation for 1 h at 1000°C (S3 samples) is smaller than that of the S2 samples studied without diffusion treatment. The TGO thickness of S2 samples is ~3.0 μm, while that of S3 samples is ~2.5 μm after the first cyclic period oxidation (12 h). After oxidation for 204 h, the TGO thicknesses are ~9.5 μm and ~7.9 μm for S2 and S3 samples, respectively. After the sharp increase in TGO thickness at the initial oxidation stage, all the kinetic curves show an extensive period of slow trend (Figure 8).
4 Discussion Figures 9(a) and (b) show the schematics of S2 and S3 sample, respectively, which illustrate the diffusion and pre-oxidation treatment effect on the oxidation NiCrAlY bond-coat behavior. From the surface roughness data in Figure 2, it can be seen that the Rq of diffused sample is smaller than that of as-sprayed sample. This may be caused by the diffusion process which uniformed the surface morphology and composition at high temperature environment. Tolpygo and Clarke found that no wrinkling with smaller growth strain in TGO leads to an improved FeCrAlY alloys [32]; while Taylor et al. demonstrated that MCrAlY bond-coat with asperities of high aspect ratios can result in premature chemical failure during isothermal exposures [33]. Thus, the diffused sample with smaller surface roughness may possess improved life time as compared to the as-spayed sample. The EDX results of Figure 2 shows 11
that the Al content on the surface of NiCrAlY bond-coat increased after the diffusion treatment. This may be due to two reasons: i) The formation of Al2O3 veins during plasma spraying consumed the internal Al content, which led to a lower Al content of 7 wt.% on the surface of bond-coat compared to that of 10 wt.% for NiCrAlY powders; ii) The environment during diffusion was not in vacuum, but with low oxygen pressure. Al is more prone to diffuse to the bond-coat surface which is exposed to the air, because Al is more active than Cr and Ni. Nijdam et al. [34] also found that the values for the amounts of Al oxides increased very rapidly on the surface of NiCrAl alloys during the initial stages of oxidation. Chen et al. [30] observed that the Al concentration was highly non-uniform in the NiCoCrAlY APS bond-coat (8 wt.%Al), which was ranged from 9.6-17.9 at.%. This was attributed to the preferential oxidation and/or vaporization during spraying process. The Cr2O3 formed on the surface of S2 samples after pre-oxidation at 1000°C for 1 h. This is due to the high concentration and the high inter-diffusion coefficient of Cr, as well as the Al depletion caused by the oxidation during the production of samples. For S3 samples, both Cr2O3 and θ-Al2O3 were observed after pre-oxidation. Oxides of θ-Al2O3 are prone to grow on the surface of the bond-coat which possesses a higher Al content [13,28]. Moreover, the formation of θ-Al2O3 on the bond-coat during the initial stage of oxidation was reported at 950°C~1150°C [13] or in a low PO2 pressure environment [35]. During the high-temperature isothermal oxidation at 1200°C, two layers of oxides formed on the top of NiCrAlY bond-coat for S2 and S3 samples, which consisted of 12
the outer mixed oxides (Cr2O3+Ni(Al,Cr)2O4) layer and inner α-Al2O3 layer. The distribution of oxides is consistent with earlier studies [14,27,36]. Al was the most probable element in the TBC to react with oxygen at the initial oxidation stage according to thermodynamics [37]. After the Al depletion, a large content (20 wt.%) of Cr in the bond-coat and the high alloy inter-diffusion coefficient guaranteed the replenishment of Cr when the Cr2O3 formed [38]. The low diffusivity and solubility of oxygen in the NiCrAlY bond-coat made the diffusion of Cr from the deep part to the surface without forming internal oxides in the bond-coat [39]. Spinel oxides formed when the Ni diffused through microcracks in the Al2O3 and Cr2O3 layer. Thus, the two-layer structure was observed after isothermal oxidation. The formation of partial inner α-Al2O3 layer in diffusion treated S3 samples was from the θ-Al2O3 phase transformation during the high-temperature annealing [13,14,28]. The enhanced nucleation of θ-Al2O3 at the surface of bond-coat assisted the formation of α-Al2O3 and thus resulted in a thicker protective α-Al2O3 layer of S3 samples as compared to that of the S2 samples. The thick α-Al2O3 layer barrier to the oxygen environment was helpful to suppress the formation of Ni, Cr-oxides during the isothermal oxidation, and thus led to a smaller mixed oxides layer in the S3 samples than that of the S2 samples. Moreover, the pores and cracks formed in the mixed oxides layer, which accelerated the oxygen attack on the bond-coat and led to the uneven-thick TGO. During the cyclic oxidation test shown in Figure 8, the S3 samples also possessed a thinner TGO than the S2 samples. Compared to the work of Chen et al. [30], and Iwamoto et al. [40], the thickness of TGO (~6.1 μm) after 13
oxidation at 1200°C for 50 h in the S3 samples was larger than those of the Ni(Co)CrAlY samples produced with YSZ top-coat (~5.1 μm) and by laser cladding and then oxidized at 1200°C (~2 μm). It should be noted that the temperature (1200°C) used in this study is higher than the application of the Inconel 718 superalloy. Such high temperature may enhance the inter-diffusion between the substrate and the bon-coat. Figure 10 shows the interface of S3 sample between the Inconel 718 substrate and NiCrAlY bond-coat after cyclic oxidation at 1200°C for 204 h. It can be seen that an oxide layer formed at the interface. EDX mapping data inside Figure 10 indicate that the oxides are Al oxides. Moreover, this oxides layer contacts both bond-coat and substrate tightly. Few pores and cracks are observed. Therefore, in this case, the inter-diffusion between the substrate and the bon-coat does not lead to the failure of samples after cyclic oxidation within 204 h.
5. Conclusions In conclusion, the microstructure and initial oxidation behaviour of the NiCrAlY bond-coat before and after diffusion and pre-oxidation treatment were investigated. The conclusions drawn from this study is summarized as follows: 1. The Al content on the surface of NiCrAlY bond-coat increased from 7 wt.% to 12 wt.% after the samples were conducted in a tube furnace with flowing argon at 600°C for 1 h. The pre-oxidation treatment of these diffused samples under 1000°C for 1 h led to the formation of θ-Al2O3 and Cr2O3; while only Cr2O3 was 14
formed on the surface during the pre-oxidation of undiffused samples conducted under 800 and 1000°C for 1 h. 2. The TGO formed on the surface of pre-treated bond-coat after isothermally oxidized at 1200°C for 50 h consisted of an outer mixed oxides layer and an inner α-Al2O3 layer. The θ-Al2O3 phase transformed to the α-Al2O3 phase during this 50 h. The average total thickness of the TGO of un-diffused and pre-oxidized samples (S2 samples) was ~6.5 μm; while the thickness of α-Al2O3 layer was <1.0 μm. For diffused and pre-oxidized samples (S3 samples), the average total thickness was ~6.1 μm, in which the average thickness of α-Al2O3 layer was ~4.3 μm. The S3 samples with a thicker α-Al2O3 layer suppressed the formation of Ni, Cr-oxides during the isothermal oxidation, and thus provided better oxidation resistance compared to the S2 samples. 3. During the cyclic oxidation at 1200°C for 204 h, the TGO thickness of S3 samples was smaller than that of the S2 samples.
Acknowledgements This work was funded by the Chinese Natural Science Foundation (Grant No. 51401170 and 51032006). The authors would like to thank Mr. Li (Northwest Institute for Nonferrous Metal Research) for parts of the sample characterization, and Mr. Wu (Luoyang Ship Material Research Institute) for sample preparation, as well as their discussions and suggestions on this study.
15
References: [1] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas - turbine engine applications, Sci. 296 (2002) 280–284. [2] A.G. Evans, D.R. Clarke, C.G. Levi, The influence of oxides on the performance of advanced gas turbines, J. Eur. Ceram. Soc. 28 (2008) 1405–1419. [3] C.G. Levi, E. Sommer, S.G. Terry, A. Catanoiu, M. Ruhle, Alumina grown during deposition of thermal barrier coatings on NiCrAlY, J. Am. Ceram. Soc. 86 (2003) 676–685. [4] C. Chen, H.B. Guo, S.K. Gong, X.F. Zhao, X. Ping, Sintering of electron beam physical vapor deposited thermal barrier coatings under flame shock, Ceram. Int. 39 (2013) 5093–5102. [5] M.W. Bai, F.W. Guo, P. Xiao, Fabrication of thick YSZ thermal barrier coatings using electrophoretic deposition, Ceram. Int. 40 (2014) 16611–16616. [6] R. Sobhanverdi, A. Akbari, Porosity and microstructural features of plasma sprayed Yttria stabilized Zirconia thermal barrier coatings, Ceram. Int. 41 (2015) 14517–14528. [7] D. Mercier, B.D. Gauntt, M. Brochu, Thermal stability and oxidation behavior of nanostructured NiCoCrAlY coatings, Surf. Coat. Technol. 205 (2011) 4162–4168. [8] R. Panat, S.L. Zhang, K.J. Hsia, Bond coat surface rumpling in thermal barrier coatings, Acta. Mater. 51 (2003) 239–249. [9] B. Wang, J. Gong, A.Y. Wang, C. Sun, R.F. Huang, L.S. Wen, Oxidation behaviour of NiCrAlY coatings on Ni-based superalloy, Surf. Coat. Technol. 149 (2002) 70–75. [10] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater. Sci. 46 (2001) 505– 553. [11] P. Richer, M. Yandouzi, L. Beauvais, B. Jodoin, Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying, Surf. Coat. Technol. 204 (2010) 3962–3974. [12] J. Sun, Q.G. Fu, G.N. Liu, H.J. Li, Y.C. Shu, G. Fan, Thermal shock resistance of thermal barrier coatings for nickel-based superalloy by supersonic plasma spraying, Ceram. Int. 41 (2015) 9972–9979.
16
[13] T.F. An, H.R. Guan, X.F. Sun, Z.Q. Hu, Effect of the θ-α-Al2O3 transformation in scales on the oxidation behavior of a Nickel-base superalloy with an aluminide diffusion coating, Oxid. Met. 54 (2000) 301–316. [14] P. Puetz, X. Huang, Q. Yang, Z. Tang, Transient oxide formation on APS NiCrAlY after oxidation heat treatment, J. Therm. Spray. Technol. 20 (2011) 621– 629. [15] P. Niranatlumpong, C.B. Ponton, H.E. Evans, The failure of protective oxides on plasma-sprayed NiCrAlY overlay coatings, Oxid. Met. 53 (1997) 241–258. [16] T.J. Nijdam, L.P.H. Jeurgens, W.G. Sloof, Promoting exclusive α-Al2O3 growth upon high-temperature oxidation of NiCrAl alloys: experiment versus model predictions, Acta. Mater. 53 (2005) 1643–1653. [17] T.J. Nijdam, W.G. Sloof, Modelling of composition and phase changes in multiphase alloys due to growth of an oxide layer, Acta. Mater. 56 (2008) 4972–4983. [18] W.R. Chen, X. Wu, D. Dudzinski, P.C. Patnaik, Modification of oxide layer in plasma-sprayed thermal barrier coatings, Surf. Coat. Technol. 200 (2006) 5863–5868. [19] U. Schulz, C. Leyens, K. Fritscher, et al., Some recent trends in research and technology of advanced thermal barrier coatings, Aerosp. Sci. Technol. 7 (2003) 73– 80. [20] T.N. Rhys-Jones, Coatings for blade and vane applications in gas turbines, Corros. Sci. 29 (1989) 623–646. [21] B.A. Pint, On the formation of interfacial and internal voids in α-Al2O3 scales, Oxid. Met. 48 (1997) 303–328. [22] D. Straussa, G. Muller, G. Schumacher, V. Engelko, W. Stamm, D. Clemens, W.J. Quaddakers, Oxide scale growth on MCrAlY bond coatings after pulsed electron beam treatment and deposition of EBPVD-TBC, Surf. Coat. Technol. 135 (2001) 196–201. [23] A. Strawbridge, H.E. Evans, C.B. Ponton, spallation of oxide scales from NiCrAlY overlay coatings, Mater. Sci. Forum. 251–254 (2009) 365–372. [24] W. Lih, E. Chang, B.C. Wu, C.H. Chao, Effects of bond coat preoxidation on the properties of ZrO2-8wt.% Y2O3/Ni-22Cr-10Al-lY thermal barrier coatings, Oxid. Met. 36 (1991) 221–238. [25] T.J. Nijdam, G.H. Marijnissen, E. Vergeldt, A.B. Kloosterman, W.G. Sloof, Development of a pre-oxidation treatment to improve the adhesion between thermal barrier coatings and NiCoCrAlY bond coatings, Oxid. Met. 66 (2006) 269–294. 17
[26] J.M. Brossard, J. Balmain, F. Sanchette, G. Bonnet, High-temperature oxidation of an aluminized NiCr alloy formed by a magnetron-sputtered Al diffusion coating, Oxid. Met. 64 (2005) 43–61. [27] C. Zhu, P. Li, A. Javed, G.Y. Liang, P. Xiao, An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings. Surf. Coat. Technol. 206 (2012) 3739–3746. [28] C. Zhu, A. Javed, P. Li, G.Y. Liang, P. Xiao, Study of the effect of laser treatment on the initial oxidation behaviour of Al-coated NiCrAlY bond-coat, Surf. Interface Anal. 45 (2013) 1680–1689. [29] R. Ghasemi, R. Shoja-Razavi, R. Mozafarinia, H. Jamali, The influence of laser treatment on thermal shock resistance of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings, Ceram. Int. 40 (2014) 347–355. [30] W.R. Chen, X. Wu, B.R. Marple, R.S. Lima, P.C. Patnaik, Pre-oxidation and TGO growth behaviour of an air-plasma-sprayed thermal barrier coating, Surf. Coat. Technol. 202 (2008) 3787–3796. [31] W.R. Chen, X. Wu, D. Dudzinski, Influence of thermal cycle frequency on the TGO growth and cracking behaviours of a TBC, J. Therm. Spray Technol. 21 (2012) 1294–1299. [32] V.K. Tolpygo, J.R. Den, D.R. Clarke, Determination of the Growth Stress and Strain in α-Al2O3 Scales During the Oxidation of Fe−22Cr−4.8Al−0.3Y Alloy, Acta. Mater. 46 (1998) 927–937. [33] M.P. Taylor, W.M. Pragnell, H.E. Evans, The influence of bond coat surface roughness on chemical failure and delamination in TBC systems, Mater. Corros. 59 (2008) 508–513. [34] T.J. Nijdam, N. M. van der Pers, W.G. Sloof, Oxide phase development upon high temperature oxidation of γ-NiCrAl alloys, Mater. Corros. 57 (2006) 269–275. [35] T.J. Nijdam, L.P.H. Jeurgens, J.H. Chen, W.G. Sloof, On the microstructure of the initial oxide grown by controlled annealing and oxidation on a NiCoCrAlY bond coating, Oxid. Met. 64 (2005) 355–377. [36] W.R. Chen, Degradation of a TBC with HVOF-CoNiCrAlY bond coat, J. Therm. Spray Technol. 23 (2014) 876–884. [37] J. Moon, H. Chol, Y. Kim, C. Lee, Cooling rate effect on phase transformation of plasma sprayed partially stabilized zirconia, J. Mater. Sci. Lett. 20 (2001) 1611–1613. [38] R.A. Miller, C.C. Berndt, Performance of thermal barrier coatings in high heat flux environments, Thin Solid Films 119 (1984) 195–202. 18
[39] V. Lughi, V.K. Tolpygo, D.R. Clarke, Microstructural aspects of the sintering of thermal barrier coatings, Mater. Sci. Eng., A 368 (2004) 212–221. [40] H. Iwamoto, T. Sumikawa, K. Nishida, T. Asano, M. Nishida, T. Araki, High temperature oxidation behavior of laser clad NiCrAlY layer, Mater. Sci. Eng. A. 214 (1998) 251-258.
Figure captions: Fig. 1 Cross-sectional morphology of the as-sprayed APS bond-coat. Fig. 2 Surface morphology of the APS bond-coat (a) as-sprayed sample, (b) diffusion sample treated with flowing argon at 600°C for 1 h. Fig. 3 Surface morphology of the bond-coat samples after pre-oxidation for 1 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples; and EDX spectrum (d) S1 samples, (e) S2 samples, and (f) S3 samples. In the figure captions, S1, S2 and S3 samples represent the as-sprayed samples after pre-oxidation for 1 h at 800°C, as-sprayed samples after pre-oxidation for 1 h at 1000°C and diffused samples (at 600°C) after pre-oxidation for 1 h at 1000°C, respectively. Fig. 4 XRD patterns of bond-coat samples after pre-oxidation: (a) S1 samples, (b) S2 samples, and (c) S3 samples. Fig. 5 Surface morphology of bond-coat samples after isothermal oxidation at 1200°C for 50 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples; and EDX spectrum (d) S1 samples, (e) S2 samples, and (f) S3 samples. Fig. 6 XRD patterns of bond-coat samples after isothermal oxidation at 1200°C for 50 h: (a) S1 samples, (b) S2 samples, and (c) S3 samples. Fig. 7 Cross-section morphology of bond-coat samples after isothermally oxidized at 19
1200°C for 50 h: (a) S2 samples, (b) S3 samples; and EDX spectrum (c) point A, (d) point B, (e) point C, and (f) point D. Fig. 8 TGO thickness as a function of oxidation time for (a) S2 samples, and (b) S3 samples. Fig. 9 Schematic diagram illustrating the initial oxidation behaviour of NiCrAlY bond-coat oxidized at 1200°C: (a) S2 samples, and (b) S3 samples. Fig. 10 Cross-section morphology of the interface of S3 sample between NiCrAlY bond-coat and Inconel 718 substrate oxidized at 1200°C for 204 h. The inside EDX mapping is the elemental map of Al.
20
21
22
23
24
25
26
Table 1 Chemical compositions of the substrate and the NiCrAlY bond-coat powders Unit wt. %
Ni
Cr
Al
Y
Zr
O
Fe
Mo
Nb
Ti
C
IN718
Balance
17.9
0.5
-
-
-
18.4
3.0
5.4
0.9
0.02
Bond-coat
Balance
20
10
1
-
-
-
-
-
-
-
Table 2 Spray Processing Parameters for the NiCrAlY bond-coat powders Parameter
Unit
NiCrAlY powder
Nozzle diameter
mm
6
Arc current
A
650
Carrier gas flow rate
L/min
10
Primary plasma forming gas flow rate (Ar)
L/min
55
Secondary plasma forming gas flow rate (H2)
L/min
4.5
Feedstock mass flow rate
g/min
40
Spray distance
mm
80
Traverse speed
mm/s
380
Scanning Step
mm
3-4
Number of passes
2-3
Table 3 Composition data measured by EDX on the cross-section of S2 and S3 samples Composition
S2 sample
S3 sample
(wt.%)
Site A
Site B
Site C
Site D
Al
26
70
15
88
Cr
62
19
79
10
Ni
12
11
6
2
27