YSZ micro-laminated coating on high-temperature oxidation resistance

Applied Surface Science 279 (2013) 85–91

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Size effect of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coating on high-temperature oxidation resistance Junqi Yao, Lili Lv, Yedong He ∗ , Deren Wang Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China

a r t i c l e

i n f o

Article history: Received 2 February 2013 Received in revised form 8 April 2013 Accepted 12 April 2013 Available online 18 April 2013 Keywords: Micro-laminated coating Oxidation resistance Cyclic oxidation Thermal stress Toughening effects

a b s t r a c t The size effect of structure plays an important role in the high-temperature oxidation and spallation resistance of micro-laminated coating on alloy substrate. In this study, (Al2 O3 –Y2 O3 )/YSZ micro-laminated coatings with different thickness ratios and layer numbers were prepared on MCrAlY alloy by electrolytic deposition and microwave sintering under pressure. The oxidation and spallation resistance of the microlaminated coatings were investigated by cyclic oxidation tests in air at 1050 ◦ C for 200 h. Results indicate that both oxidation resistance and spallation resistance of the coating have been improved significantly, by increasing the thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer and layer number in a specific range. In such micro-laminated coating, increasing the thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer can improve the apparent thermal expansion coefficient of coating and decrease the thermal stresses. While, increasing the layer number can extend the crack propagation paths and improve the plasticity and fracture toughness. They can improve the mechanical properties of micro-laminated coating and exhibit excellent cracking and spallation resistance. This would give rise to the beneficial effects on suppressing the oxygen inward diffusion and consequently improve the high-temperature oxidation resistance of micro-laminated coating. Such size effect would provide reference values in the structural design of micro-laminated coating in high-temperature application. © 2013 Elsevier B.V. All rights reserved.

1. Introduction It has been widely acknowledged that oxidation is an important factor of the degradation of alloys in high temperature application [1]. Generally, there are two ways to resist high-temperature oxidation of alloy. The first one is forming protective oxide scales by the selective oxidation of alloy or depositing metallic coatings [2–4]. The second one is depositing ceramic coatings [5–7]. The excessive stresses are often generated in the oxide scales and ceramic coatings, due to which the cracking and spallation of oxide scales and ceramic coatings take place. Therefore, cracking and spallation of oxide scales and ceramic coatings are the key factors to influence the lifetime of alloy coatings and ceramic coatings. There have been built a number of models on the processes of cracking and spallation of oxide scale. And these models generally suppose that the oxide scale is a homogeneous single phase material [8,9]. Consequently, single phase oxide scale or coating cannot be avoided the cracking and spallation completely, which would lead to the further oxidation. It has been demonstrated that composite ceramic can merge the advantages of components together and improve the strength,

∗ Corresponding author. Tel.: +86 010 62332715; fax: +86 010 62332715. E-mail address: [email protected] (Y. He). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.039

durability and fracture toughness of ceramic effectively [10]. As a representing composite structure material, laminated ceramic is designed to surmount the brittleness of single phase material deriving from simulating the nacreous layers of shell. As early as 1990s, Clegg et al. [11] have fabricated SiC–graphite laminar composites that exhibit an apparent toughness (calculated from the ultimate √ strength) of 15 MPa m and a work of fracture of 4625 J m−2 . For comparison, the toughness and the work of fracture of monolithic √ SiC were only 3.6 MPa m and 25 J m−2 , respectively. In recent years, alumina–zirconia layered systems have been extensively investigated as an alternative route for enhancing the mechanical response of alumina-based monolithic ceramics in terms of strength and fracture toughness [12]. Thus it is reasonable that the oxide scale or coating with laminated composite structure exhibits more excellent mechanical stability than single phase material. Recent researches have indicated that multilayer ceramic coatings can possess excellent mechanical properties and also excellent oxidation resistance of alloy [13,14]. Our laboratory has been working on micro-laminated composite coating to resist hightemperature oxidation and corrosion in recent years [15–22]. And excellent properties have been acquired from the experimental results. From the point view of coating structure, there is a large thickness range of each layer from hundreds of micrometers to sub-micron level [23,24]. Nevertheless, little attention is paid on the systematical researches on structure factors.

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In the application of laminated ceramic materials, components selection, preparation technologies and treatment processes are generally chosen as the starting points of researches [25–27]. It is suggested that optimizing the laminated structure can also be proposed as a considerable way to improve the durability and mechanical properties of coating, which has great application potential in protection of alloy during its high-temperature service. Here the thickness ratio of different components and layer number are considered as the main influence structural factors. In this work, (Al2 O3 –Y2 O3 )/YSZ micro-laminated coatings with different thickness ratios and layer numbers were prepared on MCrAlY alloys by cathodic electrolytic deposition and microwave sintering. The size effect of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coating on high-temperature oxidation resistance of MCrAlY alloy and spallation resistance of coating was investigated and mechanisms accounting for such effects were discussed. 2. Experimental procedure 2.1. Preparation of the coatings MCrAlY alloy substrate (Ni–32Co–20Cr–8Al–0.5Y, wt.%) was cut into 15 mm × 10 mm × 3 mm samples and precisely polished (Ra = 0.03 ␮m), followed by ultrasonic cleaned in acetone and ethanol for use. Al2 O3 –Y2 O3 layers were deposited in Al(NO3 )3 and Y(NO3 )3 ethanol solution with a molar ratio of 1:0.01 for Al3+ /Y3+ and a total concentration of 0.1 mol/L. And YSZ layers were deposited in Zr(NO3 )4 and Y(NO3 )3 ethanol solution with a molar ratio of 1:0.16 for Zr4+ /Y3+ and a total concentration of 0.1 mol/L. The deposition process was performed under constant current of 5 mA/cm2 . The electrochemical cell comprised a cathodic MCrAlY substrate centered between two parallel graphite counter electrodes (25 mm × 15 mm) with the distance of 15 mm. Al2 O3 –Y2 O3 and YSZ layers were deposited on the substrates alternately. Details in electrolytic deposition processes are shown in Table 1, where N is layer number and the subscripts A and Z indicate Al2 O3 –Y2 O3 layer and the YSZ layer, RZA , t and n are thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer, every deposition time, and number of deposition in every layer respectively. After every deposition, pre-heat treatment was performed in air at 300 ◦ C for 20 min. The final samples with laminated coating embedded in graphite powder were sintered by microwave under hydrostatic pressure of 5 MPa for 20 min. The microwave frequency was 2.45 GHz and the average power of microwave furnace was 900 W. 2.2. High-temperature cyclic oxidation tests High-temperature cyclic oxidation tests were carried out to study the influence of micro-laminated coatings on oxidation and thermal cyclic spallation resistance of the samples. The tests were performed in tube type resistance furnace at 1050 ◦ C in air for 200 h. Quartz crucibles were used to accommodate different samples respectively and have been pre-heated to a constant weight. Table 1 Details in electrolytic deposition processes of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coatings. Abbreviation

6-layer 1:1 6-layer 2:1 6-layer 3:1 8-layer 3:1 10-layer 3:1

N

6 6 6 8 10

RZA

1:1 2:1 3:1 3:1 3:1

Al2 O3 –Y2 O3 layers

YSZ layers

t (s)

NA × n

t (s)

NZ × n

60 60 60 45 36

3×2 3×2 3×2 4×2 5×2

60 60 60 54 54

3×2 3×4 3×6 4×5 5×4

Before the tests, the samples and crucibles were weighed by the electronic balance with an accuracy of 10−5 g. At first, the samples were exposed to the condition of high-temperature oxidation for a period of 10 h. And then, the samples were removed from the furnace, cooled to room temperature by natural cooling for 30 min. Thirdly, the samples were reweighed to obtain the weight gain of the oxidized samples and weight loss of the stripped samples before they were put back to the furnace again for another cycle. The cyclic oxidation tests provided 20 times thermal cycles. At last, the data weighed were divided by the surface area of corresponding samples to plot the kinetic curves as a function of time.

2.3. Scratching tests The scratching tests were carried out to evaluate the adhesion of coatings and formed oxide scales at room temperature after cyclic oxidation by an adhesion automatic scratch tester (WS-2005, China) in double model of the friction force (Ff) and acoustic emission (AE) signals. The test load, loading rate and scratch length were 50 N, 50 N/min and 5 mm, respectively.

2.4. Characterization The surface and cross-section morphologies of coatings were observed by a high-resolution field emission scanning electron microscope (FE-SEM, ZEISS SUPRA 55). Phases of the coatings were characterized by X-ray diffraction (XRD, Cu K␣, PW 3710, Philips, step wise of 0.02◦ , continuous scanning) in the 2 range of 20–80◦ .

3. Results and discussion 3.1. Microstructure of the as-prepared micro-laminated coating Fig. 1 shows the surface and cross-section morphologies of micro-laminated coating after microwave sintering under pressure. It can be seen in Fig. 1(a) that the surface of laminated coating is homogeneous, smooth and compact without any cracks. The cross-section of 10-layer 3:1 laminated coating has been shown in Fig. 1(b), where the thickness of every layer is controlled at the submicron level with the total thickness of 1.95 ␮m. The Al2 O3 –Y2 O3 layers are displayed with the same thickness at almost 100 nm and the YSZ layers follow the designed thickness ratio roughly. The interfaces of layers reveal tight bound and no flaws or cracks emerge in the coating. It should be responsible for the blurry interfaces that some complicated processes such as dehydration, phase transformation and diffusion may come up during sintering process [18]. Fig. 2 shows the XRD patterns of the micro-laminated coating on MCrAlY substrate after pre-heat treatment at 300 ◦ C of every deposition and subsequent microwave sintering. The only diffraction peaks are observed for the main phase of substrate (PDF Card No. 65-6613, also referred in Ref. [28]) in Fig. 2(a). Also, a slightly broad hump in the 2 range of 20–35◦ can be observed. This suggests the presence of amorphous phases in the green coating after pre-heat treatment, which can be ascribed to the low pre-heat temperature just for dehydration. Besides, ␣-Al2 O3 (JCPDS 46-1212, also referred in Ref. [29]) and t-Zr0.92 Y0.08 O1.96 (JCPDS 48-0224, also referred in Ref. [30]) are detected in the micro-laminated coating, as can be demonstrated in Fig. 2(b). It illustrates that phase transformations have taken place during the microwave sintering. These phases can be stable in the whole temperature region of cyclic oxidation, which would release the transformation stress largely and thereby improve the durability of coating.

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Fig. 1. SEM images of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coating after microwave sintering: (a) surface and (b) cross-section.

Fig. 2. XRD patterns of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coating on MCrAlY substrate: (a) pre-heat treatment at 300 ◦ C and (b) microwave sintering.

3.2. High-temperature cyclic oxidation behavior Fig. 3 shows the weight gain and spallation mass per unit area as a function of time for the cyclic oxidation test of different samples in air at 1050 ◦ C. It is shown that the blank sample displays high weight gain and spallation relatively, which implies the substrate has been oxidized seriously and the thermally grown oxide (TGO)

has been destroyed. The poor adhesion of TGO can be attributed to the excessive growth stress and thermal stress in cyclic oxidation induced by the thermal expansion mismatch between the TGO and substrate. Consequently, the uncontinuous TGO cannot suppress further oxidation of substrate, leading to its worse oxidation resistance. Besides, all of the (Al2 O3 –Y2 O3 )/YSZ micro-laminated coatings with different thickness ratios and different layer numbers exhibit relatively low weight gain and spallation at different levels after cyclic oxidation at 1050 ◦ C for 200 h. Also, it can be concluded that there is an ascensive trend of oxidation and spallation resistance with the increase of thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer and layer number. The coating of 10-layer 3:1 exhibits only 0.22 mg/cm2 weight gain and 0.06 mg/cm2 spallation after cyclic oxidation. Fig. 4 reveals the surface images of samples after cyclic oxidation at 1050 ◦ C for 200 h. The blank sample shows a relatively rough surface with uncontinuous oxide particles, which are mainly ␣Al2 O3 and NiO (JCPDS 65-2901, also referred in Ref. [21]) phases analyzed by the XRD patterns in Fig. 5(a). They cannot suppress the further oxidation, as demonstrated in the oxidation kinetics of blank sample in Fig. 3. In comparison, all of the laminated coatings are still preserved in the most region of the surface, with cracks in different sizes rather than loose oxide scale particles. It is also displayed that, as the thickness ratio of YSZ layer to Al2 O3 layer and layer number increase, the width of cracks gets smaller and surface of coating becomes denser and smoother, which is consistent to the downward trend of spallation curve (Fig. 3(b)). XRD patterns (Fig. 5) indicate that after cyclic oxidation at 1050 ◦ C for 200 h, ␣-Al2 O3 and t-Zr0.92 Y0.08 O1.96 phases still remain well and no other oxide phases can be identified. They ensure the excellent oxidation resistance of

Fig. 3. Oxidation kinetic curves of samples coated with different coatings at 1050 ◦ C: (a) weight gain versus time and (b) spallation versus time.

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Fig. 4. SEM images of surface morphologies of samples coated with different coatings after cyclic oxidation: (a) blank; (b) 6-layer 1:1; (c) 6-layer 2:1; (d) 6-layer 3:1; (e) 8-layer 3:1; (f) 10-layer 3:1.

samples coated with laminated coatings. Besides, as can be seen in Fig. 4(b), some oxide particles are formed in the crack gaps of 6layer 1:1 coating after cyclic oxidation, which cannot be observed in other coatings. This implies that as the increase of thickness ratio of YSZ layer to Al2 O3 layer and layer number, the laminated coating can endure larger stress and maintain better structural integrality of coating. And then the coating can suppress the inward diffusion of oxygen and keep the oxidation resistance for a longer period. It has been widely recognized that the (Al2 O3 –Y2 O3 )/YSZ microlaminated coating exhibits excellent high-temperature oxidation resistance owing to the stability of the materials at high temperature, the low oxygen diffusion coefficient of ␣-Al2 O3 layers and also the designed multi-sealed structure, which can suppress the oxygen diffusion stepwise [18,31]. However, cracking and spallation of coating can also accelerate localized oxidation by providing fast oxygen-diffusion paths generally [32]. And then exhaustive failure would occur when cracking and spallation exist in large area of

Fig. 5. XRD patterns of samples coated with different coatings after cyclic oxidation at 1050 ◦ C for 200 h: (a) blank; (b) 6-layer 1:1; (c) 6-layer 2:1; (d) 6-layer 3:1; (e) 8-layer 3:1; (f) 10-layer 3:1.

coating. Thus oxidation resistance of the alloy is closely related to the integrality of the coating with sealed structure, which is guaranteed by the excellent cracking and spallation resistance of the coating. According to the criterion for failure analyzed by Evans [33], decreasing the stress and increasing the fracture toughness of coating are the two practical ways to avoid coating spallation. The influence of thickness ratio on the spallation and oxidation resistance of micro-laminated coating is discussed briefly as follows. The relationship of thermal stress () in oxide scales or coating was deduced by Timoshenko [34]: =

−Ecoating (˛coating − ˛m )T 1−

(1)

where the Ecoating and  are the elastic modulus and poisson’s ratio of coating, ˛coating and ˛m are the linear coefficient of thermal expansion (CTE) for the surface coating and metal matrix, and T is the change in temperature. Thus, thermal stress can be decreased by reducing the difference between the ˛coating and ˛m , which

Fig. 6. The coefficient of thermal expansion of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coatings with different thickness ratios.

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Fig. 7. SEM images of cross-section morphologies of samples after cyclic oxidation: (a) blank; (b) 10-layer 3:1.

means increasing the CTE of the coating in most cases. In the laminated coating system, we define the longitudinal CTE (˛l ) as the CTE in the x or y direction and the transverse CTE (˛t ) as the coefficient of thermal expansion in the z or thickness direction. The Schapery equations modified for laminated composite give the CTE in longitudinal and transverse directions as [35]: ˛x = ˛y = ˛l =

˛1 E1 V1 + ˛2 E2 V2 E1 V1 + E2 V2

˛z = ˛t ∼ = (1 + 1 )˛1 V1 + (1 + 2 )˛2 V2 − ˛l ¯

(2) (3)

where ¯ = 1 V1 + 2 V2 and E,  and V are Young’s modulus, poisson’s ratio and the volume fraction respectively. The subscripts 1 and 2 indicate the components of the composite. As we know, ZrO2 exhibits a higher CTE to match the metal substrate than Al2 O3 . Consequently, it would lead to an improved apparent CTE of micro-laminated coating than that of Al2 O3 , by increasing the thickness ratio of YSZ layer to Al2 O3 layer suitably in the specific temperature, as shown in Fig. 6. Thus the difference of CTE between the laminated coating and alloy substrate in longitudinal and transverse directions will be reduced generally. In fact, this trend can be expected for application in the whole service temperature region by Eqs. (2) and (3). And then the thermal stress can be decreased, which brings more excellent cracking and spallation resistance as shown in Fig. 4. Consequently, the oxidation resistance of the micro-laminated coating can be also improved. The influence of layer number on the spallation and oxidation resistance is mainly based on the toughening effect of laminated

structure. This toughening mechanism can be attributed to the release of stress energy in various forms, such as crack deflection or crack bifurcation, while there is no macroscopic damage occurred in the material [12]. Fig. 7 reveals the cross-section SEM images of samples after cyclic oxidation. It can be seen from Fig. 7(a) that the thickness of thermally grown oxide (TGO) formed on blank sample is about 3 ␮m. And the TGO contains cracks and voids, which provide defect paths for a fast diffusion of oxygen, alumina and other elements. Thus they cannot provide durable protection against the further oxidation. Comparatively, the sample coated with 10-layer 3:1 laminated coating has a thin and compact TGO layer as shown in Fig. 7(b). However, the laminated structure of composite coating cannot be identified due to element diffusion in the thin coating itself during high-temperature cyclic oxidation. As we know, it can decrease the thickness of each layer by increasing layer number under the constant total thickness. So the single transverse crack can be limited within a shorter length, no longer than the thickness of the single layer. And the crack deflection caused by the residual fracture energy will occur more easily. Also, more interfaces in the coating can enable more crack propagation paths to be extended and more energy to be released. Besides, catastrophic failure of material occurs at a load  c given by the expression as follows [36]: c =

KIc √ Y a

(4)

where KIc is the critical stress intensity factor (fracture toughness), c denotes critical, a and Y are the half length of crack and a geometrical factor related to the shape and loading modes of crack, which is constant in certain condition. From this expression, with

Fig. 8. Results of scratching tests on the samples coated with different coatings after cyclic oxidation: (a) schematic diagram of critical load in 10-layer 3:1 coating; (b) critical load regions of samples coated with different coatings.

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limited shorter initial cracks in laminated coating and higher fracture toughness, critical stress which the coating can stand without brittle fracture would be higher. Moreover, a hump of the laminated coating was formed during oxidation, as can be observed in Fig. 7(b). It may be attributed to the localized stress concentration and the stress relaxing by the superplasticity of micro-laminated coating along with the thickness of each layer reaches to nanoscale with refined structure. Consequently, the fracture toughness and thereby the cracking and spallation resistance of micro-laminated coating can be improved significantly by increasing layer number. Thus, it can enhance the durability of the coating and improve its oxidation resistance. 3.3. Scratching tests of the samples after cyclic oxidation Fig. 8 shows the results of scratching test on the samples coated with different coatings after cyclic oxidation. The schematic diagram of critical load in 10-layer 3:1 coating has been taken for example in Fig. 8(a), where the Ff and AE signals are collected during the scratching tests. It can be seen that there exists a huge inflection point of the Ff curves relatively with different slopes in the two sides, and the AE signal curves coincide at the load region of 36–39 N. That is because the sound generated by the remarkable breakage of the coating leads to the abrupt change of the received AE signal. And the critical load can be confirmed in the scratch image by the location of the start of the crack in the scratch where substrate has been exposed. Thus it is rigorous to identify the critical load between the region of 36–39 N. Fig. 8(b) displays the critical load regions of samples coated with different coatings after cyclic oxidation. It can be seen that with the increase of thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer and layer number in a specific range, the critical load to destroy the coating becomes larger. As we know, in scratching test, the critical load presents the damage resistance of coating. And increasing the thickness or interface of coating can promote the crack propagation paths to be extended and more fracture energy to be consumed. Consequently, the critical load to destroy the coating can be improved and more excellent mechanical properties can be achieved. 4. Conclusions In this paper, size effect of (Al2 O3 –Y2 O3 )/YSZ micro-laminated coating to resist high-temperature oxidation has been investigated from the point view of changing the thickness ratio and layer number. The results of cyclic oxidation experiments reveal that the high-temperature oxidation and spallation resistance can be improved with the increase of thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer and layer number in a specific range. Generally, the high-temperature oxidation resistance can be attributing to the stability of the materials at high temperature, the low oxygen diffusion coefficient of ␣-Al2 O3 layers and also the designed multi-sealed structure, which can suppress the oxygen diffusion stepwise. It can be concluded that the thickness ratio and layer number play an important role in the mechanical properties of the micro-laminated coating and consequently in its oxidation resistance. Increasing the thickness ratio of YSZ layer to Al2 O3 –Y2 O3 layer can improve the apparent thermal expansion coefficient of the micro-laminated coating and mitigate the thermal expansion mismatch between the coating and substrate. Hence, it can decrease the thermal stress, and thereby improve the spallation resistance and the durability of coating in high-temperature service. Besides, increasing layer number can extend the crack propagation paths and consume more fracture energy. This can lead to the improvement of fracture toughness and tolerance to cracking and spallation of coating. Consequently, the size effect of (Al2 O3 –Y2 O3 )/YSZ

micro-laminated coating on high-temperature oxidation resistance has reference values in application of the protective coating in hightemperature service environments.

Acknowledgment The authors thank the financial support from the Chinese National Nature Science Foundation (Grant No. 51071030).

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