Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

Corrosion Science xxx (xxxx) xxxx Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci Pore...

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Corrosion Science xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack Xiao Shana, Wenfu Chena, Lixia Yangb, Fangwei Guoa, Xiaofeng Zhaoa,*, Ping Xiaoa a Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China b College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermal barrier coating (TBC) Yttria-stabilized zirconia (YSZ) Plasma spraying Calcium-Magnesium-Alumino-Silicate (CMAS) Hot corrosion Pore filling

The pore filling behavior in yttria-stabilized zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated. After fully infiltration, the total porosity of the coating decreased. However, the total porosity of the top region increased due to severe microstructural change. For the bottom region, whose microstructure did not change significantly, its total porosity dropped and almost all crack network disappeared, but 48 vol. % of globular pores still existed and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.

1. Introduction Thermal barrier coatings (TBCs) usually made of low-thermal conductivity ceramics are widely applied to the surfaces of metallic components in hot sections of gas turbine engines employed in transportation, energy, and defense sectors, because they can thermally insulate metallic components from hot gas stream due to their excellent thermal insulation performance [1–3]. For TBCs, however, environmental degradation caused by molten silicates corrosion is becoming a more critical issue because of the increasing demand for higher operating temperatures in future high-efficiency gas-turbine engines [2,4]. Molten silicates, generally known as calcium-magnesium-alumino-silicate (CMAS), usually stem from siliceous particles (e.g., sand, volcanic dust, and fuel residue) [5,6]. Generally, most of the larger siliceous particles (larger than ∼80 μm) can be filtered out from the inlet air by the state-of-the-art particle separators equipped in gas turbine engines [7]. However, small particles can still enter gas turbine engines. And some of these particles can adhere to TBCs, and then form siliceous deposits [8]. When the siliceous deposits on a TBC melt, the molten silicates will infiltrate into the pores and the cracks in the TBC because of their excellent wetting ability [9–11]. The porosity of the TBC will decrease [12]. Consequently, the thermal conductivity of the TBC will increase, which can result in degradation of the thermal insulation performance of the TBC [13]. In addition, some of the tetragonal yttriastabilized zirconia (YSZ) in the CMAS-infiltrated region will dissolve



and then will reprecipitate as yttria-depleted tetragonal zirconia in the molten CMAS, which can alter the microstructure of the TBC [6]. And the yttria-depleted tetragonal zirconia will transform to monoclinic zirconia during cooling process [14]. This transformation will be accompanied by a volume expansion (∼5 %), which may cause premature failure of the CMAS-infiltrated TBC [14]. Besides, CMAS may result in volume expansion of TBCs, which possibly leads to buckling failure of TBCs at high temperatures [15]. CMAS attack can also change other properties of TBCs such as the coefficient of thermal expansion [12], but from the vantage point of failure, the primary issue of CMAS is the degradation of strain tolerance [4]. At high temperatures, liquid CMAS can fill pores in YSZ TBCs. As a result, the in-plane modulus of the CMAS-infiltrated TBCs will increase, and the in-plane compliance will be degraded. Therefore, the degradation of strain tolerance is closely related to pore filling by CMAS. It was reported that after YSZ air plasma sprayed (APS) TBCs were infiltrated by CMAS, not all the pores in the CMAS-infiltrated zones were filled by CMAS [16]. This indicates that some pores in APS TBCs may be able to resist filling by CMAS. If it is known what kinds of pores are “CMAS-proof”, tailoring pores in TBCs to enable them to resist filling by CMAS may be a promising way to mitigate CMAS attack. More recently, Zhu et al. studied the evolution of pores in APS TBCs under volcanic ash corrosion using X-ray computed tomography combined with scanning electron microscopy (SEM) and mercury infiltration porosimetry [17]. However, a comparison of the pore filling behavior

Corresponding author. E-mail address: [email protected] (X. Zhao).

https://doi.org/10.1016/j.corsci.2020.108478 Received 24 September 2019; Received in revised form 13 January 2020; Accepted 16 January 2020 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Xiao Shan, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2020.108478

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Fig. 1. SEM images illustrating the image acquisition procedures for the CMAS-infiltrated APS TBCs. First, a BSE image (A) was obtained in the top half region, followed by an SE image (B) in exactly the same region. Next, an SE image (Fig. 1C) and then a BSE (Fig. 1D) image were obtained in the bottom half region. Afterwards, image stitching was performed, thus through-thickness images (Fig. 1E and F) were obtained. The ellipses indicate a relatively big pore, the left part of which can hardly be identified in the BSE image but can be easily identified in the SE image. The rectangle indicates the position of the cropped image on which image analysis was performed.

study the pore filling behavior. Then these TBCs were heat treated to 1250 oC for 0.5, 1, and 3 h, respectively; the heating/cooling rate was 10 oC/min. This corrosion temperature was selected because it represents the operating temperature (∼1250 oC [28]) of TBCs and exceeds the melting point of CMAS [19]. Three hours was selected as the longest corrosion duration because it can enable molten CMAS to fully infiltrate the TBC based on our experience. Detailed image analysis was conducted on the 3hCMAS-infiltrated (fully-infiltrated) TBC. TBCs corroded for 0.5 and 1 h were prepared, such that the change of pores in the TBCs during the corrosion process can traced.

between different types of pores (e.g., intersplat and globular pores) is still not clear. In this research, the pore filling behavior in typical YSZ APS TBCs during CMAS infiltration process was investigated using SEM and image analysis method. The pore filling behaviors of different types of pores were compared, and then possible mechanisms governing the pore changing behavior observed in this research were discussed. 2. Materials and methods 2.1. Preparation of TBCs The APS TBCs were deposited on a grit-blasted stainless steel substrate (50 × 50 × 3 mm3). The ceramic powder used was ZrO2–8 wt. % Y2O3 (Metco 204C-NS); this composition was selected because it has been widely used and studied in high-temperature applications [18]. The thickness of the top coat was ∼310 μm, which is close to the thickness of a typical APS TBC [1]. After deposition, the as-sprayed coating was cut into pieces (5 × 5 mm2). Then the substrates of the pieces were removed using aqua regia, which can avoid severe degradation of the metal substrates at high corrosion temperatures.

2.4. Characterization 2.4.1. Metallographic preparation All samples were prepared according to ASTM Standard E1920-03 [29], a standard for metallographic preparation of thermal sprayed coatings. Then one as-sprayed and the three CMAS-infiltrated TBCs were vacuum impregnated with low viscosity epoxy resin. These samples were then subjected to grinding and polishing. After final polishing (50-nm alumina suspension), many pores (especially small pores) in these TBCs were filled by polishing debris. Therefore, ultrasonic cleaning was used. Since the specimens were fragile ceramic coatings, coating particles may be lost during this energetic process [29]. Therefore, the ultrasonic cleaning time was kept to only one minute.

2.2. Preparation of CMAS The chemical composition of CMAS used was 33CaO–9MgO–13AlO1.5–45SiO2 mol. % (glass transition and melting temperatures are 764 and 1233 oC [19], respectively). This composition was selected because it was based on the average of CMAS in TBCs on turboshaft shrouds operated in a desert environment [20]. And it has been selected by many CMAS-related studies, including our own [6,11,15,16,19,21–23], so using it is convenient for comparison. The CMAS powder was prepared by mixing fine reagent- grade nanoscale powders of individual oxides and milling them in isopropanol to form a thick paste.

2.4.2. Image acquisition Before SEM image acquisition, a total of ten locations on the crosssection of each sample were randomly selected. And several parameters were fixed such as magnification (1600×, corresponding to a field width of 161 × 187 μ m2) and image size (3536 × 4096 pixels), such that each image has a numerical resolution of 0.046 μm/pixel. This resolution exceeds the recommended value (0.2 μm/pixel) for quantitative image analysis for plasma sprayed TBCs [30], so it was expected to be enough to resolve even fine pores. For the as-sprayed TBC, one backscatter electron (BSE) image was taken at each location. BSE imaging mode was selected because it can exhibit higher contrast between pores and other phases than secondary electron (SE) imaging mode [30], as can be seen in Fig. 2A. For the CMAS-infiltrated TBCs, some relatively big pores can hardly be identified using only BSE images. For example, the left part of the pore indicated by the ellipses in Fig. 1A and B can hardly be identified in the BSE image, but it can be easily identified in the SE image. The reason why the right part of the pore can be easily identified in the BSE

2.3. CMAS corrosion of TBCs The CMAS paste was applied on three freestanding TBCs, according to other CMAS-related studies [6,24]. CMAS paste instead of dry CMAS powder was used because this allows relatively easy application of CMAS onto the coatings and can make distribution of CMAS on the coatings more uniformly. The amount of CMAS on each piece of TBC was ∼16 mg/cm2. Similar CMAS loading has been used by some researchers [25–27]. It can provide enough volume of molten CMAS to 2

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Fig. 2. Images illustrating the image analysis procedures: (A) gray-scale BSE image, (B) binary image with all pores, (C) binary image with globular pores, and (D) binary image with crack network.

objectives of this research was to investigate the pore filling behavior of different types of pores in APS TBCs, so porosities of different types of pores need to be obtained. Generally, pores in APS TBCs can be divided into two categories: (1) globular pores, and (2) crack network (including intersplat and intrasplat pores) [32]. Globular pores were segmented from the binary images with all pores using a common binary image processing function known as opening [32,33]. Then binary images with globular pores (termed BIglo) were obtained. Next, binary images with crack network (termed BIcra) were obtained by substracting BIglo from BIall. Note that these binary images with crack network included small globular pores. These small globular pores were added back to BIglo using an additional filtering according to pore shape (circularity) [30]. The final versions of BIglo and BIcra are shown in Fig. 2C and D, respectively.

image is that it was filled by polishing debris, which were confirmed using energy dispersive spectroscopy (EDS). For the CMAS-infiltrated TBCs, therefore, SE images were also obtained. The image acquisition procedures are as follows. First, a BSE image (Fig. 1A) was obtained in the top half region, followed by an SE image (Fig. 1B) in exactly the same region. Next, an SE image (Fig. 1C) and then a BSE (Fig. 1D) image were obtained in the bottom half region. Afterwards, image stitching was performed, such that through-thickness images (Fig. 1E and F) were obtained. Through-thickness images were obtained because the porosities were different for the top and bottom regions, as can be seen in Fig. 1E.

2.4.3. Image analysis Before image analysis, for the CMAS-infiltrated TBCs, each BSE image was cropped, such that a rectangular image with a width of 150 μm and just excluding the epoxy resin was obtained, as indicated by the rectangle in Fig. 1E. Image analysis was performed on these gray-scale BSE images based on the method described by Lavigne et al. [30]. First, image segmentation was performed to segment pores from these images. Since intensity levels of most pores were markedly lower than those of other regions, global thresholding (using Otsu’s method [31]) was adopted to implement image segmentation. However, for the cropped BSE images of CMAS-infiltrated TBCs, some pores (e.g., the left part of the pore indicated by the ellipse in Fig. 1A) have similar intensity levels with other regions. Therefore, these pores were blacked manually before image segmentation. Although manual processing was adopted, the results were expected to be accurate; the reason is that the boundaries of these pores can be easily identified using SE images. Afterwards, image segmentation was performed, thus binary images showing all pores (termed BIall) were obtained, as shown in Fig. 2B. With these images, total porosities can be calculated. One of the

3. Results and discussion 3.1. Pores in the as-sprayed TBCs The microstructure of the as-sprayed APS TBC is shown in Fig. 3. It is seen that the APS TBC used in this research show features of typical APS TBCs. Intersplat, intrasplat, and globular pores can be observed in the coating. Most of the pores were filled by resin epoxy, which indicates that most of the pores are open pores. In addition, a few pores remained unfilled (e.g., the globular pore in the inset in Fig. 3B), possibly because they were closed pores. The volume fraction of all the pores in the as-sprayed TBC measured using image analysis is 11.4 %, as shown in Fig. 4A. To study the pore filling behavior of different types of pores, the volume fractions of different types of pores were also measured using image analysis method. As shown in Fig. 4A, the volume fractions of globular pores and crack network are about 5.4 % and 6.0 %, respectively. 3

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Fig. 3. Cross-sectional BSE images showing the microstructure of the as-sprayed APS TBC: (A) low and (B) high magnification. The inset in 3B is a cross-sectional BSE image showing a globular pore that was not filled by resin.

microstructures changed significantly due to dissolution-reprecipitation of the YSZ grains. These alterations can be clearly observed by comparing the microstructures of the severe interaction zones with that of the as-sprayed APS TBC (Fig. 3). By contrast, for the slight interaction zones, the microstructures did not change significantly. After the APS TBC was fully infiltrated by CMAS after corrosion for 3 h, there were still many unfilled pores in the coating. Preliminary inspection of the BSE images of the fully-infiltrated TBC indicated that almost all the unfilled pores are globular pores. These unfilled globular pores can be divided into two categories: (1) relatively big unfilled globular pores in the severe interaction zone (Fig. 6A), and (2) normal unfilled globular pores in the slight interaction zone (Fig. 6B). It is worth mentioning that similar big pores that were formed in severe interaction zones of CMAS-infiltrated APS TBCs were also observed in previous studies [15,34]. Besides unfilled globular pores, globular pores that were filled by CMAS can also be observed in this coating, as shown in Fig. 6C. On the other hand, preliminary inspection indicated that almost all crack network pores disappeared; it was difficult to observe unfilled crack network pores in the BSE images. Examples of the CMASfilled crack network pores were given in Fig. 6D. To quantitatively characterize the unfilled pores in the fully-infiltrated TBC, image analysis was performed. The results are shown in

The distribution of globular pore diameter was determined based on the 10 binary images with globular pores (e.g., Fig. 2C). Note that the diameter represents the two-dimensional (2D) diameter of a circle with the same area as a globular pore. The result is shown in Fig. 4B. It is seen that 33.6 vol. % of the globular pores in the as-sprayed coatings are no larger than 1.5 μm in 2D diameter, and almost 80 vol.% of globular pores are no larger than 6.0 μm in 2D diameter. 3.2. Pores in the CMAS-infiltrated TBCs The microstructures of the APS TBCs after CMAS corrosion are shown in Fig. 5. First, residual CMAS can be observed on the 0.5h- and 1h-CMAS-infiltrated TBCs, and all the CMAS infiltrated into the APS TBC after heat treatment for 3 h. The CMAS-infiltrated zones were identified using BSE images coupled with EDS analysis, and the approximate positions of CMAS fronts are indicated in Fig. 5. It was found that the APS TBC was fully infiltrated by CMAS after corrosion for 3 h, which indicates that the amount of CMAS applied was enough. In these CMAS-infiltrated TBCs, each CMAS-infiltrated zone can be divided into two parts, based on the degree of corrosion. In this study, the upper and lower parts are termed severe and slight interaction zones, respectively. For the severe interaction zones, the

Fig. 4. (A) Porosities of different types of pores and (B) globular pore size distribution of the as-sprayed APS TBCs. Error bars represent the standard deviations (SD). 4

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Fig. 7. The total porosity of the APS TBC decreased from 11.4 % (Fig. 4A) to 4.8 %. This appears to indicate that 42 vol. % of the pores in the pristine APS TBC were not filled by CMAS after the coating was fully infiltrated. However, this is not the case, as will be explained later. For the globular pores, its volume fraction decreased from 5.4 % to 4.6 %. By contrast, the volume fraction of crack network dropped from 6.0 % to 0.1 %, which is in agreement with the BSE images. The distribution of the 2D diameters of the globular pores is shown in Fig. 7B. Recall that in the pristine APS TBC, almost 80 vol. % of globular pores are no larger than 6.0 μm in 2D diameter. However, 64 vol. % of the globular pores are larger than 6.0 μm in 2D diameter after fully infiltration. This indicates that the proportion of relatively small globular pores decreased after CMAS corrosion. It seems that the pores in the severe and slight interactions are markedly different in terms of total porosity and globular pore size distribution. And the mechanisms by which pores changed are different for the severe and the slight interaction zones. In the severe interaction zone, the microstructure of this zone indicated that the dissolution of YSZ grains played an important role in changing the pores. By contrast, it seems that no severe dissolution occurred in the slight interaction zone. Therefore, it is necessary to characterize the pores in the severe and the slight interaction zones separately. The selections of the locations of the severe and slight interaction zones on which image analysis was performed would influence the quantitative characterization of the pores in these zones, because these two zones were quite different and it was hard to determine a clear boundary between them. To obtain accurate results, the following method was adopted. For the characterization of the severe interaction zone, image analysis was performed on the top 50 μm of the cropped BSE images; this zone contained only severe interaction zone. For the slight interaction zone, image analysis was performed on the bottom 200 μm of the cropped BSE image; this zone was away from the severe interaction zone, and thus contained only slight interaction zone. The results are shown in Fig. 8. First, the most noticeable result is that after fully infiltration the total porosity of the severe interaction zone increased by 25 %, from 11.4 %–14.3 %. Almost all the unfilled

Fig. 5. Cross-sectional BSE images showing the microstructures of APS TBC after CMAS corrosion at 1250 °C for different times: (A) 0.5 h, (B) 1 h, and (C) 3 h. The solid lines indicate the approximate positions of CMAS front; the dashed lines indicate the approximate interfaces between the severe and the slight interaction zones.

Fig. 6. Cross-sectional BSE images showing the microstructures of APS TBC after CMAS corrosion at 1250 °C for 3 h: (A) relatively big unfilled globular pores in the severe interaction zone, (B) normal unfilled globular pores, (C) CMAS-filled globular pores, and (D) CMASfilled crack network pores. (Unfilled pores refer to the pores that are not filled by CMAS, while some of the unfilled pores are still filled by polishing debris even after ultrasonic cleaning, as mentioned in Section 2.4.1).

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Fig. 7. (A) Porosities of different types of pores and (B) globular pore size distribution of the fully-infiltrated APS TBC. Error bars represent the standard deviations (SD).

mainly caused by CMAS infiltration rather than solid state sintering. Third, the porosity of globular pores dropped from 5.4 % to 3.6 % after sintering for 0.5 h, and then it almost remained steady at this level with further heat treatment. For the fully-infiltrated TBC, there was still ∼145-μm-thick region at the bottom that had not been infiltrated by CMAS after corrosion for 0.5 h. This means that the porosity of globular pores in this region had already dropped from 5.4 % to 3.6 % due to solid state sintering before CMAS reached this region. For this region, the effect of subsequent CMAS infiltration was only to reduce the porosity of globular pores from 3.6 % to 2.6 % (Fig. 8C). Therefore, solid state sintering is a critical factor causing the globular pores to disappear in the slight interaction zone of the fully-infiltrated TBC.

pores are globular pores, and almost 80 vol. % of the globular pores are larger than 6.0 μm in 2D diameter (Fig. 8B). On the other hand, the total porosity of the slight interaction dropped from 11.4 % to 2.7 %, which means that 76 vol. % of pores disappeared after CMAS infiltration. In terms of pore types, the porosity of crack network dropped from 6.0 % to 0.1 %, which indicates that almost all crack network disappeared after CMAS infiltration. By contrast, the porosity of globular pores dropped from 5.4 % to 2.6 %, which means that 48 vol. % of globular pores still existed after CMAS corrosion. To investigate the influence of pore size on pore changing behavior, globular pores are classified into four categories based on 2D diameter, and the porosities of the four groups of pores in the as-sprayed TBC and in the slight interaction zone of the fully-infiltrated TBC were determined using image analysis. The result is shown in Fig. 9. It is seen that the porosities of globular pores having a 2D diameter smaller than 4.5 μm decreased after CMAS infiltration. And this figure also indicates that larger globular pores are more likely to remain after CMAS infiltration.

3.4. Mechanisms governing the pore changing behavior After the APS TBC was fully infiltrated by CMAS, the total porosity of the severe interaction zone increased from 11.4 %–14.3 %. For the slight interaction zone of this coating, the total porosity dropped from 11.4 % to 2.7 %; almost all crack network in this zone disappeared after CMAS infiltration, while some globular pores still remained. Possible reasons for these phenomena are discussed below.

3.3. Pores in the sintered TBCs For the fully-infiltrated TBC, many pores (including crack network and globular pores) disappeared after CMAS corrosion. It appears that this was caused by CMAS infiltration. However, it is possible that solid state sintering also caused many pores to disappear before CMAS reach them. In order to investigate whether solid state sintering is one reason causing pores to disappear, APS TBCs without CMAS were heat treated at 1250 °C for 0.5, 1 and 3 h. And image analysis was conducted on these samples; the results are shown in Fig. 10. First, the total porosity decreased from 11.4 %–9.5 % after heat treatment for 0.5 h, then it further decreased to 8.7 % when the heat treatment time reached 1 h; after that, the total porosity almost did not change with further heat treatment. By contrast, for the fully-infiltrated TBC, the total porosity of the slight interaction zone dropped from 11.4 % to 2.7 %. This comparison indicates that CMAS infiltration is the main reason why the total porosity decreased so much after CMAS corrosion rather than solid state sintering. Second, let us consider crack network. After heat treatment for 3 h, the porosity of crack network dropped slightly from 6.0 %–5.4 %. However, for the fully-infiltrated TBC, the porosity of crack network of the slight interaction zone dropped to 0.1 %. This indicates that the disappearance of crack network in the slight interaction in this TBC was

3.4.1. Mechanisms for the porosity increase in the severe interaction zone Two mechanisms for the increase of the porosity of the severe interaction zone are possible: (1) splat separation, and (2) pore coalescence. 3.4.1.1. Splat separation. During the CMAS corrosion process, some separated splats can be observed in the top region of the APS TBCs, as shown in Fig. 11. Note that the largest separation distance reaches ∼3.9 μm. Such large separation between splats can hardly be found in the as-sprayed APS TBC. This indicates that the relatively large separations between splats occurred as a result of CMAS corrosion. According to a recent study, such splat separations should be caused by the large stresses and the thermal-mechanical-chemical coupling effect during CMAS corrosion (see Ref. [35] for detailed mechanisms of this phenomenon). Once such separations have been formed, it is possible that the liquid CMAS in these separations will flow outwards to other capillaries, since CMAS tends to fill small globular pores, thin crack network, and intrasplat grain boundaries. Consequently, horizontal pores that did not exist in the as-sprayed TBCs occurred, which can increase the total porosity of the severe interaction zone. Note that 6

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Fig. 8. (A & C) Porosities of different types of pores for the severe (A) and slight (C) interaction zones of the fully-infiltrated APS TBC. (B & D) Globular pore size distributions for the severe (B) and slight (D) interaction zones of the fully-infiltrated APS TBC. Error bars represent the standard deviations (SD).

Fig. 9. Porosities of four groups of globular pores in the as-sprayed TBC and in the slight interaction zone of the fully-infiltrated TBC. Error bars represent the 95 % confidence intervals (95 % CI).

Fig. 10. Porosities of different types of pores of the as-sprayed (termed 0 h) and the sintered APS TBCs. Error bars represent the 95 % confidence intervals (95 % CI).

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Assume that the effect of gravity is neglected. The capillary pressure ΔP can be calculated by the following equation [39]:

ΔP =

−γLV cos (θ) d

(3)

where γLV is the liquid-vapor interfacial energy, θ is the contact angle between the liquid and the solid material, and d equals half of the distance between the two plates. The capillary pressure does not decrease during the infiltration process since the contact angle and the distance between the two plates are constant. Because the driving force for liquid infiltration into the gap comes from the capillary pressure, the liquid will not stop infiltrating the gap between the two plates. In APS TBCs, it seems that the crack network, including intersplat and intrasplat pores (Fig. 3B), is similar to the pore in Fig. 12. This may explain why crack network can easily be filled by CMAS. Fig. 11. Cross-sectional BSE image showing a splat separation in the severe interaction zone of an APS TBC after CMAS corrosion at 1250 °C for 0.5 h.

3.4.2.2. Globular pores. After CMAS corrosion for 3 h, the porosity of globular pores in the slight interaction zone dropped from 5.4 % to 2.6 %. The comparison between CMAS-infiltrated and sintered TBCs indicates that solid state sintering is a critical factor causing the porosity decrease. Besides, CMAS can cause volume expansion of APS TBCs [15]. Therefore, volume expansion may be another reason. Finally, CMAS infiltration is also one reason, since many globular pores were filled by CMAS, as shown in Fig. 6C. In addition to the CMAS-filled pores, many globular pores in the slight interaction zone remained unfilled after CMAS corrosion. Possible reasons for this phenomenon are as follows. On one hand, some globular pores in the as-sprayed coating may be closed pores (e.g., the globular pore in the inset in Fig. 3B). There are only two ways CMAS can fill such globular pores: CMAS reach them (1) by infiltrating through grain boundaries or (2) by directly dissolving YSZ grains. However, both need longer time than the way CMAS reach them by infiltrating through cracks connected to these globular pores. Therefore, some globular pores were not filled possibly because they were closed pores and thus CMAS was not able to reach them without infiltrating grain boundaries and dissolving their surrounding YSZ grains due to insufficient time. On the other hand, it can be observed that many unfilled globular pores in the slight interaction zone of the fully-infiltrated coating were connected to CMAS, as seen in Fig. 6B. Therefore, the reason that CMAS could not reach them is ruled out. It was noted that the surfaces of the unfilled globular pores were not completely wetted by CMAS even in CMAS infiltrated zone. However, filling a pore requires complete wetting of the pore surface [40]. Therefore, non-complete wetting of the pore surface is one reason why many globular pores remained unfilled.

similar process—outward liquid flow from liquid lake to other small capillaries can cause formation of pores—has also been observed in the liquid phase sintering process of W-Ni [36]. 3.4.1.2. Pore coalescence. The viscosity of CMAS at 1250 °C is 8.1 Pa·s (calculated using the composition-based viscosity model by Giordano et al. [37]), which indicates the severe interaction zone may behave like a viscous liquid at high temperatures and liquid phase sintering may occur in this zone. Besides, it can be seen in Fig. 5 that the number of pores in the severe interaction zone decreased with increasing corrosion time. Therefore, pore coalescence possibly occurred during the corrosion process. This can cause an increase of pore volume due to the reduction of pore gas pressure. To understand how pore coalescence increases pore volume, consider a simple case. In this case, assuming that all of the spherical pores have the same size and are immersed in liquid CMAS. The volume increase can be estimated by applying the condition of mass conservation. The pressure of ideal gas in the pore which is in balance with the liquid capillary pressure leads to the following equations [38]

4γ 4γ 3 3 N ′pore ⎜⎛ LV + PL ⎟⎞ D′pore = Npore ⎛⎜ LV + PL ⎞⎟ DPore ′ D D pore ⎝ ⎠ ⎝ pore ⎠

V ′pore Vpore

4γLV

=

D pore 4γLV D ′pore

(1)

+ PL + PL

(2)

where Npore is the total number of pores, γLV is the liquid-vapor interfacial energy, Dpore is the diameter of pores, PL is the liquid pressure which is assumed to be 1 atm, Vpore is the total volume of the pores; N ′pore , D′pore , and V ′pore are the values after pores coalescence. The γLV is ∼0.4 J/m2 [12]. Assume that Dpore and D′pore are 3 and 12 μm, respectively. Then V ′pore Vpore is calculated to be 2.7, which means that volume of pores can increase by 170 % due to pore coalescence. Therefore, pore coalescence may be one reason for the increase of porosity of the severe interaction zone.

3.5. Implications This study shows that crack network in APS TBCs can be easily filled by CMAS, and larger globular pores are more likely to remain after CMAS infiltration. Although this study was performed based on YSZ APS TBCs, APS TBCs deposited using other oxide materials may exhibit a similar pore filling behavior, because the pore filling behavior depends largely on wetting properties and characteristics of the pores in the TBCs. In addition to infiltration cracks and pores, CMAS also damages APS TBCs by chemical corrosion, such as dissolution and intergranular corrosion. Once an APS TBC with good chemical corrosion resistance has been fabricated, it is likely that CMAS infiltrating into its cracks and pores and degrading its in-plane compliance will play the major role in damaging the TBC. In such situation, introducing relatively large globular pores into such APS TBC may enable it more resistant to CMAS damage, because the degree of CMAS infiltration will be lowered and thus the degradation of strain tolerance will be reduced. A recent study has shown that an apatite-type silicate material having excellent

3.4.2. Mechanisms for the porosity decrease in the slight interaction zone 3.4.2.1. Crack network. For the slight interaction zone of this coating, almost all crack network in this zone disappeared after CMAS infiltration, and the comparison between CMAS-infiltrated and sintered TBCs indicates that the disappearance of the crack network was mainly caused by CMAS infiltration. The phenomenon that CMAS can easily fill crack network may be understood by analyzing the liquid flow process between parallel plates. Fig. 12 is a two-dimentional schematic representation showing the liquid infiltration into a horizontal pore between two parallel plates. 8

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Fig. 12. Two-dimentional schematic representation showing liquid infiltration into a horizontal pore between two parallel plates.

Appendix A. Supplementary data

comprehensive properties for TBC applications exhibit excellent resistance to chemical corrosion by CMAS [41]. Future work will include introducing large globular pores into APS TBCs deposited using such material to investigate whether it is possible to increase the CMAS resistant performance of the APS TBCs using the tailoring-pore method. Finally, it is worth noting that introducing large globular pores into APS TBCs may lower their strength. As a result, using large globular pores as a design strategy for resisting CMAS may result in undesirable penalties for other design requirements. Therefore, this issue should be included in future work.

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4. Conclusions In this research, the pore filling behavior in typical yttria-stabilized zirconia (YSZ) air-plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated using scanning electron microscopy and image analysis. After CMAS corrosion at 1250 °C for 3 h, the APS TBC was fully infiltrated by CMAS. Its total porosity decreased from 11.4 % to 4.8 %. However, the porosities and microstructures in the top and bottom regions of this fully infiltrated TBC were different. For the top region, the microstructure changed significantly due to severe dissolution-reprecipitation of the YSZ grains. Rather than decrease, the total porosity of this zone increased by 25 %, from 11.4% to 14.3 %. Almost all the unfilled pores were globular pores, and nearly all the crack network disappeared. For the bottom region, the microstructures did not change significantly. The total porosity of this zone dropped from 11.4 % to 2.7 %. In terms of pore types, almost all crack network disappeared. By contrast, 48 vol.% of globular pores still existed after CMAS corrosion, and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage. CRediT authorship contribution statement Xiao Shan: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft. Wenfu Chen: Resources, Validation, Writing - review & editing. Lixia Yang: Resources, Writing - review & editing. Fangwei Guo: Writing - review & editing. Xiaofeng Zhao: Visualization, Project administration, Funding acquisition. Ping Xiao: Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Inner Mongolia Natural Science Foundation [No. 2017MS0538] and Shanghai Jiao Tong University Baotou Institute of Material Research. 9

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