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The effect of ion implantation on the oxidation resistance of vacuum plasma sprayed CoNiCrAlY coatings
Applied Surface Science 261 (2012) 422–430
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of ion implantation on the oxidation resistance of vacuum plasma sprayed CoNiCrAlY coatings Jie Jiang a,b,c , Huayu Zhao a,b , Xiaming Zhou a,b , Shunyan Tao a,b,∗ , Chuanxian Ding a,b a b c
The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China Shanghai Institute of Ceramic, Chinese Academy of Sciences, Shanghai 200050, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 27 July 2012 Accepted 6 August 2012 Available online 13 August 2012 Keywords: CoNiCrAlY coating Ion implantation High temperature oxidation resistance Thermally grown oxide
a b s t r a c t CoNiCrAlY coatings prepared by vacuum plasma spraying (VPS) were implanted with Nb and Al ions at a fluence of 1017 atoms/cm2 . The effects of ion implantation on the oxidation resistance of CoNiCrAlY coatings were investigated. The thermally grown oxide (TGO) formed on each specimen was characterized by XRD, SEM and EDS, respectively. The results showed that the oxidation process of CoNiCrAlY coatings could be divided into four stages and the key to obtaining good oxidation resistance was to remain high enough amount of Al and promote the lateral growth of TGO. The implantation of Nb resulted in the formation of continuous and dense Al2 O3 scale to improve the oxidation resistance. The Al implanted coating could form Al2 O3 scale at the initial stage, however, the scale was soon broken and TGO transformed to non-protective spinel. © 2012 Published by Elsevier B.V.
1. Introduction Thermal barrier coatings (TBCs) have been widely used for thermal protection of hot-sectional metal components in gas turbines [1]. The current TBCs typically consist of an Y2 O3 stabilized ZrO2 (YSZ) top coat and a MCrAlY (M = Co or/and Ni) bond coat. It is generally known that the TBCs’ life strongly depends on the thermally grown oxide (TGO) formed between bond coat and YSZ top coat at high temperatures. The overgrowth of TGO leads to an increase in the residual stress, which accelerates the spalling of TBCs [2]. The oxidation resistance of MCrAlY coatings relies on the ability of the coating to form a stable continuous, slow growing and adherent TGO on its surface. The formation of a pure ␣-Al2 O3 scale as a protective layer on the bond coat surface is effective in suppressing the oxidation. However, the oxidation of the bond coat usually accompanies fast growing, non-protective oxide phases such as (Co,Ni)(Al,Cr)2 O4 , which are believed to promote the spalling of TBCs [3–5]. Many attempts have been made to suppress the TGO growth in the bond coat. The MCrAlY coatings with rare element addition such as Nb, Ce, Re, Pt and Nd, are preferred to form a protective Al2 O3 scale [6–9]. The other reports revealed that the bond coat
∗ Corresponding author at: The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China. Tel.: +86 21 52414101; fax: +86 21 52413903. E-mail address: [email protected] (S. Tao). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.08.026
with aluminizing by pack cementation reserved a much longer lifetime due to the formation of a dense and continuous Al2 O3 scale with the sufficient Al supplement [10–12]. Ion implantation is a well-controlled and precise tool that can add various elements only to the surface layer. However, few reports on the effect of ion implanted coatings have been published [13]. In the present work, the vacuum plasma sprayed (VPS) CoNiCrAlY coatings with Nb/Al ion implantation were prepared and the high-temperature oxidation behaviors of the VPS coatings with and without ion implantation were investigated, respectively. The microstructures and phase composition of the coatings were also discussed. 2. Materials and methods A Ni-based superalloy GH3128 was cut into specimens with a dimension of 10 mm × 20 mm × 1 mm as a substrate. The CoNiCrAlY coating was deposited to a thickness of about 100 m by the vacuum plasma spray (VPS) technique (F4-VB, Sulzer Metco), with powders of Co29Ni21Cr16Al0.3Y (at.%) (Amdry 995), onto both sides of the substrate. The plasma spraying parameters are shown in Table 1. Two types of ion species, Nb and Al, were implanted into bond coat with a fluence of 1017 atoms/cm2 at 30 kV for each element, using a novel multi-functional ion implantation/deposition system (PIIIS-700). Pre-annealing treatment was performed at 1100 ◦ C in a furnace for 1 h. During the specimen were being put into the furnace, the temperature of the furnace at this point was 1050 ◦ C and then it rose
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Table 1 Summary of vacuum plasma spray parameters. Parameters
Unit
Value
Arc current Primary plasma gas Ar Secondary plasma gas H2 Carrier gas Ar Spray distance Pressure of spraying atmosphere
A slpm slpm slpm mm mbar
650–725 45–55 7–12 2.0–2.5 250–300 75–125
to 1100 ◦ C in 10 min. After 1 h annealing at 1100 ◦ C, the specimens were cooled under flowing air. Isothermal oxidation experiments of coatings with and without ion implantation were also carried out at 1100 ◦ C in a furnace. The specimens were taken out after different duration periods, cooled in air, and weighted. The mass changes of the specimens were measured using an electro-balance with a detection limit of 0.1 mg. X-ray photoelectron spectroscopy (XPS, ESCAlab250) was used to detect the element distribution in the modified layer. The morphologies and the chemical compositions of the specimens were characterized by scanning electron microscopy (SEM, JXA 8100) with an energy dispersive spectrometry (EDS). The phases of the coatings and the oxide scales were also identified by X-ray diffraction (XRD, D/Max 2550 V). 3. Results 3.1. Element distribution in the ion implanted layer profiled by XPS Fig. 1(a) shows the element distribution in the layer of the assprayed coating profiled by XPS. The Al concentration is about 19 at.% at the depths of 5–30 nm, which is slightly higher than its atomic fraction in the powder Amdry 995. The depth profiles of relevant elements after the implantation of Nb are shown in Fig. 1(b). The maximum concentration of Nb is about 19 at.% at the depth of 1.6 nm and the maximum penetration depth of Nb is about 32 nm, indicating that Nb has been successfully implanted into the coating. On the other hand, the concentration of Al remains 19 at.% at the depth of about 30 nm, without obviously change after the implantation of Nb. The depth profiles of relevant elements after the implantation of Al are also shown in Fig. 1(c). The Al concentration is higher than any other elements over a depth range of 0–30 nm due to the Al implantation. The maximum concentration of Al is about 42 at.% at the depth of 1.6 nm and remains almost 30 at.% at the subsurface. The penetration depth of Al seems deeper than that of Nb, due to its better solubility for the original existence of Al in the coating. 3.2. Isothermal oxidation kinetics As shown in Fig. 2, at 1100 ◦ C for 100 h in air, a parabolic law is obeyed for the three coatings. There is not much difference among the mass gains of coatings with and without ion implantation. However, in the initial stage of oxidation, the implanted coating shows a little higher oxidation rate than the as-sprayed one. It has been proven that Al2 O3 will be formed first in the CoNiCrAlY coating [14]. The result indicates that ion implantation of Nb and Al can accelerate the formation of Al2 O3 to some extent. Fig. 3 shows the isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 1000 h in air. It can be found that the Nb implanted coating shows excellent oxidation resistance after long-term oxidation for 600 h at 1100 ◦ C. On the other hand, it is noted that the Al implanted coating exhibits similar good
Fig. 1. Element distribution at the surface of (a) as-sprayed, (b) Nb implanted, (c) Al implanted coatings profiled by XPS.
oxidation resistance as that of Nb implanted one within the first 70 h. Its oxidation rate, however, accelerates quickly after 300 h, which results in a higher overall weight gain than that of the Nb implanted coating and even the as-sprayed one. After 600 h, all the three specimens oxidize seriously.
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Fig. 2. Isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 100 h in air.
3.3. Microstructures and phase composition of the coatings 3.3.1. Characterization of the short-term oxidation scale Fig. 4(a)–(c) shows the cross-section morphology of the three coatings oxidized at 1100 ◦ C for 1 h in air. Examinations with EDS reveals that the initial TGO formed at the surface mainly consists of Al2 O3 . For the as-sprayed coating, the Al2 O3 scale is scattered while the TGO formed in the Nb implanted coating is dense and the scale of Al implanted coating is thin but continuous. Fig. 5 shows the element constitution in the initial oxidation scale of the three coatings after oxidation at 1100 ◦ C for 1 h. An Al2 O3 scale is formed on all the three coatings. For the Nb implanted one, the Nb shows a relatively high concentration of about 8 at.% in the outer part of the scale and then it is dispersed homogeneously with the low concentration of about 2 at.%. For the Al implanted coating, the concentration of Al is a little higher than the as-sprayed and Nb implanted ones at the surface after oxidation at 1100 ◦ C for 1 h. Fig. 6 shows the XRD patterns of the coatings oxidized at 1100 ◦ C for 50 h. The peaks of ␥ -AlNi3 , ␣-Al2 O3 and (Co,Ni)(Cr,Al)2 O4 can all be found in the coatings. The intensity of ␣-Al2 O3 is strong in both Nb and Al implanted coatings and the intensity of (Co,Ni)(Cr,Al)2 O4 is so weak, implying that the oxide scales formed in ion implanted coatings are mainly ␣-Al2 O3 . Besides the peak of
Fig. 4. Cross-section morphology of the scale after oxidation at 1100 ◦ C for 1 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
Fig. 3. Isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 1000 h in air.
␣-Al2 O3 , (Co,Ni)(Cr,Al)2 O4 is significantly identified in as-sprayed coating, showing that its TGO consists of these two phases. Fig. 7(a) shows the cross-sectional morphology of the assprayed coating after oxidation at 1100 ◦ C for 50 h. The TGO consists of two layers: the upper layer has a relatively bright gray contrast, and the bottom layer has dark contrast, corresponding
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Fig. 6. XRD patterns of the three coatings oxidized at 1100 ◦ C for 50 h.
still discontinuous. Fig. 7(d) shows the EDS line scanning analysis of the as-sprayed coating after oxidation at 1100 ◦ C for 50 h. The composition of the two layers of TGO is Cr-rich oxides and Al2 O3 , respectively, coinciding with the results of the EDS point scanning analysis. Fig. 7(b) shows the cross-sectional morphology of the Nb implanted coating after oxidation at 1100 ◦ C for 50 h. The two layers are also obviously detected. EDS analysis shows that the upper layer of the gray oxides consists of 7 at.% Ni + 9 at.% Co and 12 at.% Cr + 28 at.% Al. It seems that the upper layer is composed mainly of Al2 O3 and Cr2 O3 . However, the Al2 O3 portion in this layer of the Nb implanted coating is higher than that of the assprayed one. The bottom layer consists of 52 at.% Al and other oxides scarcely exists. It can be inferred that this layer is composed of pure Al2 O3 . Fig. 7(c) shows the cross-sectional morphology of the Al ion implanted coating after oxidation at 1100 ◦ C for 50 h. It is different from Fig. 7(a) and (b) that there is only one layer of TGO. EDS analysis shows that the layer of the dark oxides mainly consist of pure Al2 O3 and other oxides are scarcely found. Fig. 7(d)–(f) shows that beneath the TGO, the concentrations of Al all drop obviously in the three coatings.
Fig. 5. Element distribution in the oxide scale formed after oxidation at 1100 ◦ C for 1 h profiled by XPS: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
with the results of XRD. EDS point scanning analysis shows that the upper layer of the gray oxides consists of 9 at.% Ni + 12 at.% Co and 16 at.% Al + 28 at.% Cr. It is believed that the gray oxide mainly consists of Al2 O3 , Cr2 O3 and some other composited oxides. The EDS analysis also reveals that 49 at.% Al and low concentrations of other atoms in the bottom layer, suggesting that this layer is mainly composed of Al2 O3 . The formed Al2 O3 scale is thin but
3.3.2. Characterization of the long-term oxidation scale As the oxidation continues, the TGO layers of the three coatings grow thicker. For the as-sprayed coating, the outer Cr-rich layer develops continuously after 200 h (Fig. 8(a)) and then reacts with Al2 O3 , forming spinel, while the Al2 O3 scale underneath the spinel layer is continuous and dense (Fig. 9(a) and (d)). The spinel grows thicker and will cause the spallation of the coating (Fig. 10(a)). For the Nb implanted coating, the over growth of Cr-rich oxide stops due to the formed continuous Al2 O3 scale (Fig. 8(b)). As the oxidation duration prolongs to 500 h, the continuous Al2 O3 scale grow thicker but the amount of spinel is little (Fig. 9(b) and (e)). As oxidation proceeds, transient oxidation will occur and consequently, the formation and growth of spinel will dominate the afterward TGO thickening (Fig. 10(b)). For the Al implanted coating, the Cr-rich oxide forms underneath the Al2 O3 scale (Fig. 8(c)). As the oxidation continues, the Al2 O3 scale is broken, leading to severe oxidation and the formation of a large quantity of spinels (Fig. 9(c) and (f)). The spinels grow fast and form around the Al2 O3 scale (Fig. 10(c)), leading to the thickest TGO of the three coatings.
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Fig. 7. (a)–(c) Cross-section SEM morphology and (d)–(f) EDS line scanning analysis of the coatings after oxidation at 1100 ◦ C for 50 h: (a) and (d) as-sprayed coating, (b) and (e) Nb implanted coating, (c) and (f) Al implanted coating.
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(1) An instantaneous Al2 O3 growth (Fig. 2). It is believed that Al2 O3 would form first in the CoNiCrAlY alloys for the lowest Gibbs free energy (101.325 kPa, 298.15 K) of all the possible oxides [17,18]. 2Al + 3/2O2 → Al2 O3 ,
f Gm (Al2 O3 ) = −1690.46 kJ/mol
(1)
2Cr + 3/2O2 → Cr2 O3 ,
f Gm (Cr2 O3 ) = −1153.88 kJ/mol
(2)
Ni + 1/2O2 → NiO,
f Gm (NiO) = −251.93 kJ/mol
(3)
Co + 1/2O2 → CoO,
f Gm (CoO) = −254.60 kJ/mol
(4)
As long as the local equilibrium exists at the surface, the oxidation of CoNiCrAlY coating at high temperature is associated with the preferential oxidation of Al in the initial stage of oxidation as shown in Fig. 4(a). The preferential oxidation of Al will result in a severe depletion of Al near the coating surface, because the flux of Al from the surface to the formation of Al2 O3 is much higher than the flux of Al from the coating towards the surface. When the concentration of Al drops to an extent, the second stable oxide Cr2 O3 will be formed. (2) Internal oxidation of Al and simultaneous overgrowth of Cr2 O3 , lasting for about 80 h (Fig. 2). The flux of oxygen that dissolves and diffuses into the coating is higher than the flux of Al that diffuses from the coating towards the surface [15]. As a result, the internal oxidation of Al occurs accompanying with the formation of Cr2 O3 out of the Al2 O3 as it is shown in Fig. 7(a) and (d). If the growth of Cr2 O3 leads to a severe Cr depletion at the surface, other stable oxide, such as NiO and CoO, will be formed. Provided that the activity of oxygen is not decreased by the continuous layer of Cr2 O3 , the internal oxidation of Al will continue and extend much deeper. The oxidation rate of this stage exhibits a decelerating growth. (3) The formation of continuous Cr2 O3 and Al2 O3 layer, lasting for about 300 h (Fig. 3). As soon as a continuous Cr2 O3 layer is formed at the surface, the activity of oxygen drops rapidly because of the slow inward diffusion of oxygen through the whole Cr2 O3 layer. As a result, the internal oxidation of Al is inhibited. The dominated growth of Al2 O3 turns to lateral growth until the Al2 O3 precipitates coalesce into a continuous and dense layer which stops the growth of other oxides as shown in Fig. 8(a). The oxidation rate of this stage keeps steady for a long time and the coating and substrate are prevented from deep oxidation. (4) The spinel occurs and the oxide layer spalls after the oxidation, 300 h later (Fig. 3). As the oxidation processes, the Al2 O3 layer will react with the Cr2 O3 layer to be a spinel such as (Co,Ni)(Cr,Al)2 O4 (Fig. 9(a) and (d)). It is believed that the spinel grows fast and the transformation accompanies with great volume change, which will cause the emergence and propagation of the crack in the TGO, eventually leading to the spallation of the TBCsand (Fig. 10(a)). The oxidation rate of this stage shows an accelerating growth.
Fig. 8. Cross-section SEM morphology of the coatings after oxidation at 1100 ◦ C for 200 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
The high temperature oxidation resistance of CoNiCrAlY coating depends on the formation of a continuous and dense Al2 O3 layer. On the other hand, the amount of Cr2 O3 should be controlled to avoid the transformation to spinel. The key to protect the coating can be considered from the two followings: (a) remain high enough concentration of Al in the coating at the surface; (b) promote the Al2 O3 rapid lateral growth to form continuous Al2 O3 scale on the surface.
4. Discussion 4.2. The influence of Nb implantation into VPS CoNiCrAlY coating 4.1. The process of TGO growth of CoNiCrAlY coatings According to the isothermal oxidation kinetics of as-sprayed coatings shown in Figs. 2 and 3, the growth of TGO exhibits a fourstage growth behavior [15,16]:
For the Nb implanted coating, it exhibits better oxidation resistance than that of as-sprayed one. At the stage 1, a thin and protective Al2 O3 scale formed at the surface (Fig. 4(b)). As the oxidation continues, a few Cr-rich oxides form out of the initial formed
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Fig. 9. (a)–(c) Cross-section SEM morphology and (d)–(f) EDS line scanning analysis of the coatings after oxidation at 1100 ◦ C for 500 h: (a) and (d) as-sprayed coating, (b) and (e) Nb implanted coating, (c) and (f) Al implanted coating.
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stage and extends the stable stage in the whole oxidation resulting in the better oxidation resistance. Ion implantation is rarely used for promoting the oxidation resistance of CoNiCrAlY coatings. Referring to the work of Nb implantation into TiAl alloys [19–23], some mechanisms have been proposed to explain the beneficial effect of Nb implantation: i. A decrease in the Cr concentration at the surface ii. Doping of Nb to the initially formed Cr2 O3 iii. Change in the process of the initially formed scale (1) As shown in Fig. 1(b), it reveals that the Nb ions have successfully implanted in the coating surface and the concentration of each element is decreased including Cr. At the stage 2, the formation of Cr2 O3 is delayed due to the decreasing of Cr. Because of this delay, Al can diffuse to the interface from the coating and the Al2 O3 can be enriched there. As shown in Fig. 7(b), the continuous Al2 O3 scale has been formed after the oxidation for 50 h. In addition, as soon as the scale forms, the diffusion of oxygen into the coating is prohibited. The third stage of the oxidation process comes earlier and the weight gain can remain stable for a long time. (2) If Nb ions are doped into the outer formed Cr2 O3 at stage 2, it should be the replacement of Cr2 O3 lattice by Nb2 O5 . Doping of the Cr2 O3 by Nb5+ will reduce the concentration of oxygen vacancies and interstitial Cr ions [20], resulting in a decrease in the diffusion rate of cations and anions, which will enhance the enrichment of Al2 O3 in the initially formed scale. As shown in Fig. 5(b), the Nb is dispersed homogeneously at the surface. The dissolution of Nb at the initial stage reduces the solubility of Al in Cr2 O3 , decreasing the possibility of the formation of spinel. However, the Nb2 O5 is not confirmed by the XRD patterns. It may attribute to the rare amount or the thin distribution for only a few nanometers. But in Fig. 5(b), Nb2 O5 may exist at the surface of about 30 nm at least. (3) If the formation of Al2 O3 develops to the lateral growth earlier by the effect of Nb implantation, the oxidation of other elements will be stopped. Compared with the as-sprayed coating, the TGO formed at the Nb implanted coating surface is continuous after the oxidation for only 50 h (Fig. 7(b)). The formation of Al2 O3 prefers lateral growth to grain growth after the Nb implantation. The oxidation resistance of CoNiCrAlY coating depends on two conditions. Though the implantation cannot increase the concentration of Al at the surface, the oxidation process has the trend to lateral growth. The effect of Nb implantation is believed to be very complex and it may have more than one mechanism responsible for the beneficial effects. In spite of the indeterminate reason for the effect, the implantation of Nb exactly improves the oxidation resistance of CoNiCrAlY coating. 4.3. The influence of Al implantation into CoNiCrAlY coating
Fig. 10. Cross-section SEM morphology of the coatings after oxidation at 1100 ◦ C for 1000 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
Al2 O3 scale (Fig. 7(b) and (e)). Compared with the as-sprayed coating, the Al concentration in TGO enhances after Nb implantation. Therefore, the stage 3 is prolonged to 600 h due to the continuous and dense TGO mainly composes of Al2 O3 (Figs. 8(b) and 9(b)). The implantation of Nb promotes the formation of Al2 O3 at the initial
For the Al implanted coating, the oxidation behavior is different from the as-sprayed coating and Nb implanted one. At the first 80 h (stage 1 and stage 2), the oxidation process is similar to the others. As shown in Figs. 4(c) and 7(c), the TGO consists of single Al2 O3 and other oxides are rarely found. But the stage 3 is so short lasting only to 250 h and it cannot be the strictly steady stage for the obvious weight gain. From the cross-section morphology of the scale after oxidation at 1100 ◦ C for 200 h (Fig. 8(c)), the Cr-rich oxide is formed underneath the initial Al2 O3 scale, associating with a large volume increase. As a result, the stage 4 prematurely comes due to the broken scale and spinel occurs surrounding the Al2 O3 scale (Fig. 9(c)).
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After the Al implantation, the Al concentration at the thin surface is really slightly increased (Fig. 1(c)). This enrichment is effective to maintain the Al2 O3 formation at stage 1 (Fig. 5(c)), which refrains from the early Al depleting [24]. At the same time, the Cr concentration is observably decreased (Fig. 1(c)) and Cr2 O3 would not be formed out of the Al2 O3 even Al has been depleted. After the oxidation for 50 h, the TGO still mainly consists of Al2 O3 (Fig. 7(c)). On the other hand, the formation of single Al2 O3 leads to the severe depleting of Al at the surface region and the internal oxidation becomes the oxidation of Cr. The formation of Cr-rich oxides underneath the Al2 O3 scale (Fig. 8(c)) associates with large volume change, resulting in breaking the scale and the emergence of the fast growing spinel (Fig. 9(c)). The stage 4 of accelerating growth comes too early. In addition, the implantation of Al introduces lattice defect of Al2 O3 as ordinary ion implantation dose and breaks the continuity of Al2 O3 scale. For the Nb implanted coating, the outer layer is Cr2 O3 , the lattice defect introduced by ion implantation was in Cr2 O3 and the integrity of Al2 O3 has not been influenced. Though the Al implantation partly enhances the formation of Al2 O3 at the initial stage, the defects cannot be neglected. 5. Conclusion 1. The oxidation process of vacuum plasma sprayed CoNiCrAlY coating can be divided into four stages: (1) an instantaneous Al2 O3 growth; (2) internal oxidation of Al and simultaneous overgrowth of Cr2 O3 ; (3) the formation of continuous Cr2 O3 and Al2 O3 layer; (4) the spinel occurs and the oxide layer spalls. The key point to protect the coating was to remain high enough concentration of Al in the coating at the surface and to promote the Al2 O3 rapid lateral growth to form Al2 O3 continuous scale on the surface of the CoNiCrAlY coating. 2. The ion implantation of Nb can improve the oxidation resistance of the CoNiCrAlY coating. A continuous and dense Al2 O3 scale is formed on the surface earlier and the steady stage has been prolonged, which prevents other elements from oxidation. 3. For the Al implanted coating, a single Al2 O3 scale is formed at the initial stage due to the slight increase of Al concentration. However, the scale is not continuous and will be broken by the underneath formed Cr-rich oxides, leading to the formation of non-protective spinel. References [1] N.P. Padture, M. Cell, E.H. Jordan, Materials science—thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280. [2] A.G. Evans, D.R. Mumm, J.W. Jutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Progress in Materials Science 46 (2001) 505. [3] J. Toscano, R. Vaen, A. Gil, M. Subanovic, D. Naumenko, L. Singheiser, W.J. Quadakkers, Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC’s, Surface & Coatings Technology 201 (2006) 3906.
[4] T.J. Nijdam, W.G. Sloof, Combined pre-annealing and pre-oxidation treatment for processing of thermal barrier coatings on NiCoCrAlY bond coatings, Surface & Coatings Technology 201 (2006) 3894. [5] Zhenhua Xu, Shimei He, Limin He, Rende Mu, Guanghong Huang, Xueqiang Cao, Novel thermal barrier coatings based on La2 (Zr0.7 Ce0.3 )2 O7 /8YSZ doubleceramic-layer systems deposited by electron beam physical vapor deposition, Journal of Alloys and Compounds 509 (2011) 4273. [6] N. Czech, F. Schmitz, W. Stamm, Improvement of MCrAlY coatings by addition of rhenium, Surface & Coatings Technology 68/69 (1994) 17. [7] Hongyu Wang, Dunwen Zuo, Xiangfeng Li, Kangmin Chen, Mingmin Huang, Effects of CeO2 nanoparticles on microstructure and properties of laser cladded NiCoCrAlY coatings, Journal of Rare Earths 28 (2010) 246. [8] G.W. Goward, Progress in coatings for gas turbine airfoils, Surface & Coatings Technology 108 (1998) 73. [9] K.A. Khor, J. Yang, Rapidly solidified neodymia-stabilised zirconia coatings prepared by DC plasma spraying, Surface & Coatings Technology 96 (1997) 313. [10] Xin Ren, Fuhui Wang, High-temperature oxidation and hot-corrosion behavior of a sputtered NiCrAlY coating with and without aluminizing, Surface & Coatings Technology 201 (2006) 30. [11] Lu Jintao, Zhu Shenglong, Wang Fuhui, High temperature corrosion behavior of an AIP NiCoCrAlY coating modified by aluminizing, Surface & Coatings Technology 205 (2011) 5053. [12] B.C. Wu, C.H. Chao, E. Chang, Effects of bond coat pre-aluminizing treatment on the properties of ZrO2 –8 wt.%Y2 O3 /Co–29Cr–6Al–1Y thermal barrier coatings, Materials Science and Engineering A 124 (1990) 215. [13] Eungsun Byon, Shihong Zhang, Sung-Hun Lee, Goo-Hyun Lee, Sik-Chol Kwon, Effect of ion implantation on growth of thermally grown oxide in MCrAlY coating for TBC, Surface & Coatings Technology 205 (2010) S435. [14] H. Hindam, D.P. Whittle, Microstructure, adhesion and growth kinetics of protective scales on metals and alloys, Oxidation of Metals 18 (1982) 245. [15] T.J. Nijdam, L.P.H. Jeurgens, W.G. Sloof, Promoting exclusive ␣-Al2 O3 growth upon high-temperature oxidation of NiCrAl alloys: experiment versus model predictions, Acta Materialia 53 (2005) 1643. [16] W.R. Chen, X. Wu, B.R. Marple, R.S. Lima, P.C. Patnaik, Pre-oxidation and TGO growth behavior of an air-plasma-sprayed thermal barrier coating, Surface & Coatings Technology 202 (2008) 3787. [17] G.H. Meier, F.S. Pettit, High temperature oxidation and corrosion of intermetallic compounds, Materials Science and Technology 8 (1992) 331. [18] Yingjiao Liang, Yinchang Che, Xiaoxia Liu, The Thermodynamic Data Manual of Inorganic Materials, first ed., NEU Press, Shenyang, 1994. [19] S. Taniguchi, K. Uesaki, Y.-C. Zhu, H.-X. Zhang, T. Shibata, Influence of niobium ion implantation on the oxidation behavior of TiAl under thermal cycle conditions, Materials Science and Engineering A 249 (1998) 223. [20] Yao-Can Zhu, Y. Zhang, X.Y. Li, K. Fujita, N. Iwamoto, The effect of niobium-ion implantation on the oxidation behavior of (-TiAl alloys in static and flowing air, Oxidation of Metals 55 (2001) 119. [21] X.Y. Li, S. Taniguchi, Y.-C. Zhu, K. Fujita, N. Iwamoto, Y. Matsunaga, K. Nakagawa, Oxidation behavior of TiAl protected by Al and Nb combined ion implantation at high temperature, Nuclear Instruments and Methods in Physics Research B 187 (2002) 207. [22] M.F. Stroosnijder, N. Zheng, W.J. Quadakkers, R. Hofman, A. Gil, F. Lanza, The effect of niobium ion implantation on the oxidation behavior of a (-TiAl-based intermetallic, Oxidation of Metals 46 (1996) 19. [23] Shigeji Taniguchi, Toshio Shibata, Takehiko Saeki, Huixing Zhang, Xianghuai Liu, Effect of Nb-ion implantation on the oxidation resistance of TiAl, Materials Transactions, JIM 37 (1996) 998. [24] S. Taniguchi, K. Uesaki, Y.-C. Zhu, Y. Matsumoto, T. Shibata, Influence of implantation of Al, Si, Cr or Mo ions on the oxidation behavior of TiAl under thermal cycle conditions, Materials Science and Engineering A 206 (1999) 267.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of ion implantation on the oxidation resistance of vacuum plasma sprayed CoNiCrAlY coatings Jie Jiang a,b,c , Huayu Zhao a,b , Xiaming Zhou a,b , Shunyan Tao a,b,∗ , Chuanxian Ding a,b a b c
The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China Shanghai Institute of Ceramic, Chinese Academy of Sciences, Shanghai 200050, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
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Article history: Received 1 February 2012 Received in revised form 27 July 2012 Accepted 6 August 2012 Available online 13 August 2012 Keywords: CoNiCrAlY coating Ion implantation High temperature oxidation resistance Thermally grown oxide
a b s t r a c t CoNiCrAlY coatings prepared by vacuum plasma spraying (VPS) were implanted with Nb and Al ions at a fluence of 1017 atoms/cm2 . The effects of ion implantation on the oxidation resistance of CoNiCrAlY coatings were investigated. The thermally grown oxide (TGO) formed on each specimen was characterized by XRD, SEM and EDS, respectively. The results showed that the oxidation process of CoNiCrAlY coatings could be divided into four stages and the key to obtaining good oxidation resistance was to remain high enough amount of Al and promote the lateral growth of TGO. The implantation of Nb resulted in the formation of continuous and dense Al2 O3 scale to improve the oxidation resistance. The Al implanted coating could form Al2 O3 scale at the initial stage, however, the scale was soon broken and TGO transformed to non-protective spinel. © 2012 Published by Elsevier B.V.
1. Introduction Thermal barrier coatings (TBCs) have been widely used for thermal protection of hot-sectional metal components in gas turbines [1]. The current TBCs typically consist of an Y2 O3 stabilized ZrO2 (YSZ) top coat and a MCrAlY (M = Co or/and Ni) bond coat. It is generally known that the TBCs’ life strongly depends on the thermally grown oxide (TGO) formed between bond coat and YSZ top coat at high temperatures. The overgrowth of TGO leads to an increase in the residual stress, which accelerates the spalling of TBCs [2]. The oxidation resistance of MCrAlY coatings relies on the ability of the coating to form a stable continuous, slow growing and adherent TGO on its surface. The formation of a pure ␣-Al2 O3 scale as a protective layer on the bond coat surface is effective in suppressing the oxidation. However, the oxidation of the bond coat usually accompanies fast growing, non-protective oxide phases such as (Co,Ni)(Al,Cr)2 O4 , which are believed to promote the spalling of TBCs [3–5]. Many attempts have been made to suppress the TGO growth in the bond coat. The MCrAlY coatings with rare element addition such as Nb, Ce, Re, Pt and Nd, are preferred to form a protective Al2 O3 scale [6–9]. The other reports revealed that the bond coat
∗ Corresponding author at: The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 200050, China. Tel.: +86 21 52414101; fax: +86 21 52413903. E-mail address: [email protected] (S. Tao). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.08.026
with aluminizing by pack cementation reserved a much longer lifetime due to the formation of a dense and continuous Al2 O3 scale with the sufficient Al supplement [10–12]. Ion implantation is a well-controlled and precise tool that can add various elements only to the surface layer. However, few reports on the effect of ion implanted coatings have been published [13]. In the present work, the vacuum plasma sprayed (VPS) CoNiCrAlY coatings with Nb/Al ion implantation were prepared and the high-temperature oxidation behaviors of the VPS coatings with and without ion implantation were investigated, respectively. The microstructures and phase composition of the coatings were also discussed. 2. Materials and methods A Ni-based superalloy GH3128 was cut into specimens with a dimension of 10 mm × 20 mm × 1 mm as a substrate. The CoNiCrAlY coating was deposited to a thickness of about 100 m by the vacuum plasma spray (VPS) technique (F4-VB, Sulzer Metco), with powders of Co29Ni21Cr16Al0.3Y (at.%) (Amdry 995), onto both sides of the substrate. The plasma spraying parameters are shown in Table 1. Two types of ion species, Nb and Al, were implanted into bond coat with a fluence of 1017 atoms/cm2 at 30 kV for each element, using a novel multi-functional ion implantation/deposition system (PIIIS-700). Pre-annealing treatment was performed at 1100 ◦ C in a furnace for 1 h. During the specimen were being put into the furnace, the temperature of the furnace at this point was 1050 ◦ C and then it rose
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Table 1 Summary of vacuum plasma spray parameters. Parameters
Unit
Value
Arc current Primary plasma gas Ar Secondary plasma gas H2 Carrier gas Ar Spray distance Pressure of spraying atmosphere
A slpm slpm slpm mm mbar
650–725 45–55 7–12 2.0–2.5 250–300 75–125
to 1100 ◦ C in 10 min. After 1 h annealing at 1100 ◦ C, the specimens were cooled under flowing air. Isothermal oxidation experiments of coatings with and without ion implantation were also carried out at 1100 ◦ C in a furnace. The specimens were taken out after different duration periods, cooled in air, and weighted. The mass changes of the specimens were measured using an electro-balance with a detection limit of 0.1 mg. X-ray photoelectron spectroscopy (XPS, ESCAlab250) was used to detect the element distribution in the modified layer. The morphologies and the chemical compositions of the specimens were characterized by scanning electron microscopy (SEM, JXA 8100) with an energy dispersive spectrometry (EDS). The phases of the coatings and the oxide scales were also identified by X-ray diffraction (XRD, D/Max 2550 V). 3. Results 3.1. Element distribution in the ion implanted layer profiled by XPS Fig. 1(a) shows the element distribution in the layer of the assprayed coating profiled by XPS. The Al concentration is about 19 at.% at the depths of 5–30 nm, which is slightly higher than its atomic fraction in the powder Amdry 995. The depth profiles of relevant elements after the implantation of Nb are shown in Fig. 1(b). The maximum concentration of Nb is about 19 at.% at the depth of 1.6 nm and the maximum penetration depth of Nb is about 32 nm, indicating that Nb has been successfully implanted into the coating. On the other hand, the concentration of Al remains 19 at.% at the depth of about 30 nm, without obviously change after the implantation of Nb. The depth profiles of relevant elements after the implantation of Al are also shown in Fig. 1(c). The Al concentration is higher than any other elements over a depth range of 0–30 nm due to the Al implantation. The maximum concentration of Al is about 42 at.% at the depth of 1.6 nm and remains almost 30 at.% at the subsurface. The penetration depth of Al seems deeper than that of Nb, due to its better solubility for the original existence of Al in the coating. 3.2. Isothermal oxidation kinetics As shown in Fig. 2, at 1100 ◦ C for 100 h in air, a parabolic law is obeyed for the three coatings. There is not much difference among the mass gains of coatings with and without ion implantation. However, in the initial stage of oxidation, the implanted coating shows a little higher oxidation rate than the as-sprayed one. It has been proven that Al2 O3 will be formed first in the CoNiCrAlY coating [14]. The result indicates that ion implantation of Nb and Al can accelerate the formation of Al2 O3 to some extent. Fig. 3 shows the isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 1000 h in air. It can be found that the Nb implanted coating shows excellent oxidation resistance after long-term oxidation for 600 h at 1100 ◦ C. On the other hand, it is noted that the Al implanted coating exhibits similar good
Fig. 1. Element distribution at the surface of (a) as-sprayed, (b) Nb implanted, (c) Al implanted coatings profiled by XPS.
oxidation resistance as that of Nb implanted one within the first 70 h. Its oxidation rate, however, accelerates quickly after 300 h, which results in a higher overall weight gain than that of the Nb implanted coating and even the as-sprayed one. After 600 h, all the three specimens oxidize seriously.
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Fig. 2. Isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 100 h in air.
3.3. Microstructures and phase composition of the coatings 3.3.1. Characterization of the short-term oxidation scale Fig. 4(a)–(c) shows the cross-section morphology of the three coatings oxidized at 1100 ◦ C for 1 h in air. Examinations with EDS reveals that the initial TGO formed at the surface mainly consists of Al2 O3 . For the as-sprayed coating, the Al2 O3 scale is scattered while the TGO formed in the Nb implanted coating is dense and the scale of Al implanted coating is thin but continuous. Fig. 5 shows the element constitution in the initial oxidation scale of the three coatings after oxidation at 1100 ◦ C for 1 h. An Al2 O3 scale is formed on all the three coatings. For the Nb implanted one, the Nb shows a relatively high concentration of about 8 at.% in the outer part of the scale and then it is dispersed homogeneously with the low concentration of about 2 at.%. For the Al implanted coating, the concentration of Al is a little higher than the as-sprayed and Nb implanted ones at the surface after oxidation at 1100 ◦ C for 1 h. Fig. 6 shows the XRD patterns of the coatings oxidized at 1100 ◦ C for 50 h. The peaks of ␥ -AlNi3 , ␣-Al2 O3 and (Co,Ni)(Cr,Al)2 O4 can all be found in the coatings. The intensity of ␣-Al2 O3 is strong in both Nb and Al implanted coatings and the intensity of (Co,Ni)(Cr,Al)2 O4 is so weak, implying that the oxide scales formed in ion implanted coatings are mainly ␣-Al2 O3 . Besides the peak of
Fig. 4. Cross-section morphology of the scale after oxidation at 1100 ◦ C for 1 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
Fig. 3. Isothermal oxidation kinetics of the coatings oxidized at 1100 ◦ C for 1000 h in air.
␣-Al2 O3 , (Co,Ni)(Cr,Al)2 O4 is significantly identified in as-sprayed coating, showing that its TGO consists of these two phases. Fig. 7(a) shows the cross-sectional morphology of the assprayed coating after oxidation at 1100 ◦ C for 50 h. The TGO consists of two layers: the upper layer has a relatively bright gray contrast, and the bottom layer has dark contrast, corresponding
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Fig. 6. XRD patterns of the three coatings oxidized at 1100 ◦ C for 50 h.
still discontinuous. Fig. 7(d) shows the EDS line scanning analysis of the as-sprayed coating after oxidation at 1100 ◦ C for 50 h. The composition of the two layers of TGO is Cr-rich oxides and Al2 O3 , respectively, coinciding with the results of the EDS point scanning analysis. Fig. 7(b) shows the cross-sectional morphology of the Nb implanted coating after oxidation at 1100 ◦ C for 50 h. The two layers are also obviously detected. EDS analysis shows that the upper layer of the gray oxides consists of 7 at.% Ni + 9 at.% Co and 12 at.% Cr + 28 at.% Al. It seems that the upper layer is composed mainly of Al2 O3 and Cr2 O3 . However, the Al2 O3 portion in this layer of the Nb implanted coating is higher than that of the assprayed one. The bottom layer consists of 52 at.% Al and other oxides scarcely exists. It can be inferred that this layer is composed of pure Al2 O3 . Fig. 7(c) shows the cross-sectional morphology of the Al ion implanted coating after oxidation at 1100 ◦ C for 50 h. It is different from Fig. 7(a) and (b) that there is only one layer of TGO. EDS analysis shows that the layer of the dark oxides mainly consist of pure Al2 O3 and other oxides are scarcely found. Fig. 7(d)–(f) shows that beneath the TGO, the concentrations of Al all drop obviously in the three coatings.
Fig. 5. Element distribution in the oxide scale formed after oxidation at 1100 ◦ C for 1 h profiled by XPS: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
with the results of XRD. EDS point scanning analysis shows that the upper layer of the gray oxides consists of 9 at.% Ni + 12 at.% Co and 16 at.% Al + 28 at.% Cr. It is believed that the gray oxide mainly consists of Al2 O3 , Cr2 O3 and some other composited oxides. The EDS analysis also reveals that 49 at.% Al and low concentrations of other atoms in the bottom layer, suggesting that this layer is mainly composed of Al2 O3 . The formed Al2 O3 scale is thin but
3.3.2. Characterization of the long-term oxidation scale As the oxidation continues, the TGO layers of the three coatings grow thicker. For the as-sprayed coating, the outer Cr-rich layer develops continuously after 200 h (Fig. 8(a)) and then reacts with Al2 O3 , forming spinel, while the Al2 O3 scale underneath the spinel layer is continuous and dense (Fig. 9(a) and (d)). The spinel grows thicker and will cause the spallation of the coating (Fig. 10(a)). For the Nb implanted coating, the over growth of Cr-rich oxide stops due to the formed continuous Al2 O3 scale (Fig. 8(b)). As the oxidation duration prolongs to 500 h, the continuous Al2 O3 scale grow thicker but the amount of spinel is little (Fig. 9(b) and (e)). As oxidation proceeds, transient oxidation will occur and consequently, the formation and growth of spinel will dominate the afterward TGO thickening (Fig. 10(b)). For the Al implanted coating, the Cr-rich oxide forms underneath the Al2 O3 scale (Fig. 8(c)). As the oxidation continues, the Al2 O3 scale is broken, leading to severe oxidation and the formation of a large quantity of spinels (Fig. 9(c) and (f)). The spinels grow fast and form around the Al2 O3 scale (Fig. 10(c)), leading to the thickest TGO of the three coatings.
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Fig. 7. (a)–(c) Cross-section SEM morphology and (d)–(f) EDS line scanning analysis of the coatings after oxidation at 1100 ◦ C for 50 h: (a) and (d) as-sprayed coating, (b) and (e) Nb implanted coating, (c) and (f) Al implanted coating.
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(1) An instantaneous Al2 O3 growth (Fig. 2). It is believed that Al2 O3 would form first in the CoNiCrAlY alloys for the lowest Gibbs free energy (101.325 kPa, 298.15 K) of all the possible oxides [17,18]. 2Al + 3/2O2 → Al2 O3 ,
f Gm (Al2 O3 ) = −1690.46 kJ/mol
(1)
2Cr + 3/2O2 → Cr2 O3 ,
f Gm (Cr2 O3 ) = −1153.88 kJ/mol
(2)
Ni + 1/2O2 → NiO,
f Gm (NiO) = −251.93 kJ/mol
(3)
Co + 1/2O2 → CoO,
f Gm (CoO) = −254.60 kJ/mol
(4)
As long as the local equilibrium exists at the surface, the oxidation of CoNiCrAlY coating at high temperature is associated with the preferential oxidation of Al in the initial stage of oxidation as shown in Fig. 4(a). The preferential oxidation of Al will result in a severe depletion of Al near the coating surface, because the flux of Al from the surface to the formation of Al2 O3 is much higher than the flux of Al from the coating towards the surface. When the concentration of Al drops to an extent, the second stable oxide Cr2 O3 will be formed. (2) Internal oxidation of Al and simultaneous overgrowth of Cr2 O3 , lasting for about 80 h (Fig. 2). The flux of oxygen that dissolves and diffuses into the coating is higher than the flux of Al that diffuses from the coating towards the surface [15]. As a result, the internal oxidation of Al occurs accompanying with the formation of Cr2 O3 out of the Al2 O3 as it is shown in Fig. 7(a) and (d). If the growth of Cr2 O3 leads to a severe Cr depletion at the surface, other stable oxide, such as NiO and CoO, will be formed. Provided that the activity of oxygen is not decreased by the continuous layer of Cr2 O3 , the internal oxidation of Al will continue and extend much deeper. The oxidation rate of this stage exhibits a decelerating growth. (3) The formation of continuous Cr2 O3 and Al2 O3 layer, lasting for about 300 h (Fig. 3). As soon as a continuous Cr2 O3 layer is formed at the surface, the activity of oxygen drops rapidly because of the slow inward diffusion of oxygen through the whole Cr2 O3 layer. As a result, the internal oxidation of Al is inhibited. The dominated growth of Al2 O3 turns to lateral growth until the Al2 O3 precipitates coalesce into a continuous and dense layer which stops the growth of other oxides as shown in Fig. 8(a). The oxidation rate of this stage keeps steady for a long time and the coating and substrate are prevented from deep oxidation. (4) The spinel occurs and the oxide layer spalls after the oxidation, 300 h later (Fig. 3). As the oxidation processes, the Al2 O3 layer will react with the Cr2 O3 layer to be a spinel such as (Co,Ni)(Cr,Al)2 O4 (Fig. 9(a) and (d)). It is believed that the spinel grows fast and the transformation accompanies with great volume change, which will cause the emergence and propagation of the crack in the TGO, eventually leading to the spallation of the TBCsand (Fig. 10(a)). The oxidation rate of this stage shows an accelerating growth.
Fig. 8. Cross-section SEM morphology of the coatings after oxidation at 1100 ◦ C for 200 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
The high temperature oxidation resistance of CoNiCrAlY coating depends on the formation of a continuous and dense Al2 O3 layer. On the other hand, the amount of Cr2 O3 should be controlled to avoid the transformation to spinel. The key to protect the coating can be considered from the two followings: (a) remain high enough concentration of Al in the coating at the surface; (b) promote the Al2 O3 rapid lateral growth to form continuous Al2 O3 scale on the surface.
4. Discussion 4.2. The influence of Nb implantation into VPS CoNiCrAlY coating 4.1. The process of TGO growth of CoNiCrAlY coatings According to the isothermal oxidation kinetics of as-sprayed coatings shown in Figs. 2 and 3, the growth of TGO exhibits a fourstage growth behavior [15,16]:
For the Nb implanted coating, it exhibits better oxidation resistance than that of as-sprayed one. At the stage 1, a thin and protective Al2 O3 scale formed at the surface (Fig. 4(b)). As the oxidation continues, a few Cr-rich oxides form out of the initial formed
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Fig. 9. (a)–(c) Cross-section SEM morphology and (d)–(f) EDS line scanning analysis of the coatings after oxidation at 1100 ◦ C for 500 h: (a) and (d) as-sprayed coating, (b) and (e) Nb implanted coating, (c) and (f) Al implanted coating.
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stage and extends the stable stage in the whole oxidation resulting in the better oxidation resistance. Ion implantation is rarely used for promoting the oxidation resistance of CoNiCrAlY coatings. Referring to the work of Nb implantation into TiAl alloys [19–23], some mechanisms have been proposed to explain the beneficial effect of Nb implantation: i. A decrease in the Cr concentration at the surface ii. Doping of Nb to the initially formed Cr2 O3 iii. Change in the process of the initially formed scale (1) As shown in Fig. 1(b), it reveals that the Nb ions have successfully implanted in the coating surface and the concentration of each element is decreased including Cr. At the stage 2, the formation of Cr2 O3 is delayed due to the decreasing of Cr. Because of this delay, Al can diffuse to the interface from the coating and the Al2 O3 can be enriched there. As shown in Fig. 7(b), the continuous Al2 O3 scale has been formed after the oxidation for 50 h. In addition, as soon as the scale forms, the diffusion of oxygen into the coating is prohibited. The third stage of the oxidation process comes earlier and the weight gain can remain stable for a long time. (2) If Nb ions are doped into the outer formed Cr2 O3 at stage 2, it should be the replacement of Cr2 O3 lattice by Nb2 O5 . Doping of the Cr2 O3 by Nb5+ will reduce the concentration of oxygen vacancies and interstitial Cr ions [20], resulting in a decrease in the diffusion rate of cations and anions, which will enhance the enrichment of Al2 O3 in the initially formed scale. As shown in Fig. 5(b), the Nb is dispersed homogeneously at the surface. The dissolution of Nb at the initial stage reduces the solubility of Al in Cr2 O3 , decreasing the possibility of the formation of spinel. However, the Nb2 O5 is not confirmed by the XRD patterns. It may attribute to the rare amount or the thin distribution for only a few nanometers. But in Fig. 5(b), Nb2 O5 may exist at the surface of about 30 nm at least. (3) If the formation of Al2 O3 develops to the lateral growth earlier by the effect of Nb implantation, the oxidation of other elements will be stopped. Compared with the as-sprayed coating, the TGO formed at the Nb implanted coating surface is continuous after the oxidation for only 50 h (Fig. 7(b)). The formation of Al2 O3 prefers lateral growth to grain growth after the Nb implantation. The oxidation resistance of CoNiCrAlY coating depends on two conditions. Though the implantation cannot increase the concentration of Al at the surface, the oxidation process has the trend to lateral growth. The effect of Nb implantation is believed to be very complex and it may have more than one mechanism responsible for the beneficial effects. In spite of the indeterminate reason for the effect, the implantation of Nb exactly improves the oxidation resistance of CoNiCrAlY coating. 4.3. The influence of Al implantation into CoNiCrAlY coating
Fig. 10. Cross-section SEM morphology of the coatings after oxidation at 1100 ◦ C for 1000 h: (a) as-sprayed coating, (b) Nb implanted coating, (c) Al implanted coating.
Al2 O3 scale (Fig. 7(b) and (e)). Compared with the as-sprayed coating, the Al concentration in TGO enhances after Nb implantation. Therefore, the stage 3 is prolonged to 600 h due to the continuous and dense TGO mainly composes of Al2 O3 (Figs. 8(b) and 9(b)). The implantation of Nb promotes the formation of Al2 O3 at the initial
For the Al implanted coating, the oxidation behavior is different from the as-sprayed coating and Nb implanted one. At the first 80 h (stage 1 and stage 2), the oxidation process is similar to the others. As shown in Figs. 4(c) and 7(c), the TGO consists of single Al2 O3 and other oxides are rarely found. But the stage 3 is so short lasting only to 250 h and it cannot be the strictly steady stage for the obvious weight gain. From the cross-section morphology of the scale after oxidation at 1100 ◦ C for 200 h (Fig. 8(c)), the Cr-rich oxide is formed underneath the initial Al2 O3 scale, associating with a large volume increase. As a result, the stage 4 prematurely comes due to the broken scale and spinel occurs surrounding the Al2 O3 scale (Fig. 9(c)).
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After the Al implantation, the Al concentration at the thin surface is really slightly increased (Fig. 1(c)). This enrichment is effective to maintain the Al2 O3 formation at stage 1 (Fig. 5(c)), which refrains from the early Al depleting [24]. At the same time, the Cr concentration is observably decreased (Fig. 1(c)) and Cr2 O3 would not be formed out of the Al2 O3 even Al has been depleted. After the oxidation for 50 h, the TGO still mainly consists of Al2 O3 (Fig. 7(c)). On the other hand, the formation of single Al2 O3 leads to the severe depleting of Al at the surface region and the internal oxidation becomes the oxidation of Cr. The formation of Cr-rich oxides underneath the Al2 O3 scale (Fig. 8(c)) associates with large volume change, resulting in breaking the scale and the emergence of the fast growing spinel (Fig. 9(c)). The stage 4 of accelerating growth comes too early. In addition, the implantation of Al introduces lattice defect of Al2 O3 as ordinary ion implantation dose and breaks the continuity of Al2 O3 scale. For the Nb implanted coating, the outer layer is Cr2 O3 , the lattice defect introduced by ion implantation was in Cr2 O3 and the integrity of Al2 O3 has not been influenced. Though the Al implantation partly enhances the formation of Al2 O3 at the initial stage, the defects cannot be neglected. 5. Conclusion 1. The oxidation process of vacuum plasma sprayed CoNiCrAlY coating can be divided into four stages: (1) an instantaneous Al2 O3 growth; (2) internal oxidation of Al and simultaneous overgrowth of Cr2 O3 ; (3) the formation of continuous Cr2 O3 and Al2 O3 layer; (4) the spinel occurs and the oxide layer spalls. The key point to protect the coating was to remain high enough concentration of Al in the coating at the surface and to promote the Al2 O3 rapid lateral growth to form Al2 O3 continuous scale on the surface of the CoNiCrAlY coating. 2. The ion implantation of Nb can improve the oxidation resistance of the CoNiCrAlY coating. A continuous and dense Al2 O3 scale is formed on the surface earlier and the steady stage has been prolonged, which prevents other elements from oxidation. 3. For the Al implanted coating, a single Al2 O3 scale is formed at the initial stage due to the slight increase of Al concentration. However, the scale is not continuous and will be broken by the underneath formed Cr-rich oxides, leading to the formation of non-protective spinel. References [1] N.P. Padture, M. Cell, E.H. Jordan, Materials science—thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280. [2] A.G. Evans, D.R. Mumm, J.W. Jutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Progress in Materials Science 46 (2001) 505. [3] J. Toscano, R. Vaen, A. Gil, M. Subanovic, D. Naumenko, L. Singheiser, W.J. Quadakkers, Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC’s, Surface & Coatings Technology 201 (2006) 3906.
[4] T.J. Nijdam, W.G. Sloof, Combined pre-annealing and pre-oxidation treatment for processing of thermal barrier coatings on NiCoCrAlY bond coatings, Surface & Coatings Technology 201 (2006) 3894. [5] Zhenhua Xu, Shimei He, Limin He, Rende Mu, Guanghong Huang, Xueqiang Cao, Novel thermal barrier coatings based on La2 (Zr0.7 Ce0.3 )2 O7 /8YSZ doubleceramic-layer systems deposited by electron beam physical vapor deposition, Journal of Alloys and Compounds 509 (2011) 4273. [6] N. Czech, F. Schmitz, W. Stamm, Improvement of MCrAlY coatings by addition of rhenium, Surface & Coatings Technology 68/69 (1994) 17. [7] Hongyu Wang, Dunwen Zuo, Xiangfeng Li, Kangmin Chen, Mingmin Huang, Effects of CeO2 nanoparticles on microstructure and properties of laser cladded NiCoCrAlY coatings, Journal of Rare Earths 28 (2010) 246. [8] G.W. Goward, Progress in coatings for gas turbine airfoils, Surface & Coatings Technology 108 (1998) 73. [9] K.A. Khor, J. Yang, Rapidly solidified neodymia-stabilised zirconia coatings prepared by DC plasma spraying, Surface & Coatings Technology 96 (1997) 313. [10] Xin Ren, Fuhui Wang, High-temperature oxidation and hot-corrosion behavior of a sputtered NiCrAlY coating with and without aluminizing, Surface & Coatings Technology 201 (2006) 30. [11] Lu Jintao, Zhu Shenglong, Wang Fuhui, High temperature corrosion behavior of an AIP NiCoCrAlY coating modified by aluminizing, Surface & Coatings Technology 205 (2011) 5053. [12] B.C. Wu, C.H. Chao, E. Chang, Effects of bond coat pre-aluminizing treatment on the properties of ZrO2 –8 wt.%Y2 O3 /Co–29Cr–6Al–1Y thermal barrier coatings, Materials Science and Engineering A 124 (1990) 215. [13] Eungsun Byon, Shihong Zhang, Sung-Hun Lee, Goo-Hyun Lee, Sik-Chol Kwon, Effect of ion implantation on growth of thermally grown oxide in MCrAlY coating for TBC, Surface & Coatings Technology 205 (2010) S435. [14] H. Hindam, D.P. Whittle, Microstructure, adhesion and growth kinetics of protective scales on metals and alloys, Oxidation of Metals 18 (1982) 245. [15] T.J. Nijdam, L.P.H. Jeurgens, W.G. Sloof, Promoting exclusive ␣-Al2 O3 growth upon high-temperature oxidation of NiCrAl alloys: experiment versus model predictions, Acta Materialia 53 (2005) 1643. [16] W.R. Chen, X. Wu, B.R. Marple, R.S. Lima, P.C. Patnaik, Pre-oxidation and TGO growth behavior of an air-plasma-sprayed thermal barrier coating, Surface & Coatings Technology 202 (2008) 3787. [17] G.H. Meier, F.S. Pettit, High temperature oxidation and corrosion of intermetallic compounds, Materials Science and Technology 8 (1992) 331. [18] Yingjiao Liang, Yinchang Che, Xiaoxia Liu, The Thermodynamic Data Manual of Inorganic Materials, first ed., NEU Press, Shenyang, 1994. [19] S. Taniguchi, K. Uesaki, Y.-C. Zhu, H.-X. Zhang, T. Shibata, Influence of niobium ion implantation on the oxidation behavior of TiAl under thermal cycle conditions, Materials Science and Engineering A 249 (1998) 223. [20] Yao-Can Zhu, Y. Zhang, X.Y. Li, K. Fujita, N. Iwamoto, The effect of niobium-ion implantation on the oxidation behavior of (-TiAl alloys in static and flowing air, Oxidation of Metals 55 (2001) 119. [21] X.Y. Li, S. Taniguchi, Y.-C. Zhu, K. Fujita, N. Iwamoto, Y. Matsunaga, K. Nakagawa, Oxidation behavior of TiAl protected by Al and Nb combined ion implantation at high temperature, Nuclear Instruments and Methods in Physics Research B 187 (2002) 207. [22] M.F. Stroosnijder, N. Zheng, W.J. Quadakkers, R. Hofman, A. Gil, F. Lanza, The effect of niobium ion implantation on the oxidation behavior of a (-TiAl-based intermetallic, Oxidation of Metals 46 (1996) 19. [23] Shigeji Taniguchi, Toshio Shibata, Takehiko Saeki, Huixing Zhang, Xianghuai Liu, Effect of Nb-ion implantation on the oxidation resistance of TiAl, Materials Transactions, JIM 37 (1996) 998. [24] S. Taniguchi, K. Uesaki, Y.-C. Zhu, Y. Matsumoto, T. Shibata, Influence of implantation of Al, Si, Cr or Mo ions on the oxidation behavior of TiAl under thermal cycle conditions, Materials Science and Engineering A 206 (1999) 267.