Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings

Journal Pre-proof Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings Feifei Zhou, Chunming Deng, You Wang, Min Liu, Liang Wang, Yaming Wang, Xiaofeng Zhang

PII:

S0955-2219(19)30818-0

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.11.073

Reference:

JECS 12898

To appear in:

Journal of the European Ceramic Society

Received Date:

17 October 2018

Revised Date:

22 November 2019

Accepted Date:

25 November 2019

Please cite this article as: Zhou F, Deng C, Wang Y, Liu M, Wang L, Wang Y, Zhang X, Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.11.073

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Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings Feifei Zhoua,b, Chunming Dengb, You Wanga,*, Min Liub,**, Liang Wangc, Yaming Wanga, Xiaofeng Zhangb a

Department of Materials Science, School of Materials Science and Engineering, Harbin Institute

of Technology, Harbin 150001, PR China b

National Engineering Laboratory for Modern Materials Surface Engineering Technology&The

Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510651, China

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c

Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics, Chinese

Academy of Sciences, Shanghai 201899, PR China

*Corresponding author at: Department of Materials Science, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

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**Corresponding author at: National Engineering Laboratory for Modern Materials Surface

Engineering Technology&The Key Lab of Guangdong for Modern Surface Engineering Technology,

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Guangdong Institute of New Materials, Guangzhou 510651, China

E-mail address:[email protected] (Y. Wang), [email protected] (M. Liu).

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Highlights  The surface roughness of the nanostructured 8YSZ TBCs are lower than that of conventional counterparts.  The nanostructured 8YSZ coating demonstrates typical bi-modal microstructure.  The Weibull distribution is a method to analyse the bi-modal microstructure in nanostructured 8YSZ coating.  The fracture toughness and bonding strength of the nanostructured 8YSZ coating are superior to the conventional 8YSZ coating.

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Abstract

The nanostructured 8YSZ thermal barrier coatings were deposited by atmospheric plasma spraying onto K417G nickel-based superalloy with high velocity oxygen fuel sprayed NiCoCrAlYCe bond-coat using as-prepared nanostructured t´-Zr0.9Y0.1O1.95 feedstocks for the first time. The microstructure and mechanical properties of nanostructured and conventional 8YSZ coatings were comparatively investigated systematically. The results revealed that both coatings were composed of t´Zr0.9Y0.1O1.95 phase and the formation mechanism of t´ phase was elucidated. The

nanostructured 8YSZ coatings demonstrated typical bi-modal microstructure, whereas the conventional 8YSZ coatings exhibited mono-modal microstructure. Furthermore, the bi-modal microstructure of nanostructured 8YSZ coatings was analysed by elastic modulus and nanohardness Weibull distribution plots. The high and low slopes in Weibull distribution plots corresponded to unmelted and melted regions of nanostructured 8YSZ coatings, respectively. The fracture toughness and bonding strength of nanostructured coatings were higher than that of conventional 8YSZ coatings. Finally the reasons were explained in detail. Keywords: Nanostructured 8YSZ; bi-modal microstructure; mechanical properties;

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plasma spraying; thermal barrier coatings

1. Introduction

Thermal barrier coatings (TBCs), which belong to advanced high temperature protective coatings with low thermal conductivity, have played a key role in turbine

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blades to enhance energy efficiency of gas turbine engines and protect the superalloys

from hostile environments such as oxidation, corrosion, wear and erosion over the last few decades [1-6]. Generally speaking, the TBCs system consists of bond coat and

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ceramic top coat. For plasma-sprayed systems, the bond coat is typically made of MCrAlYX (M=Ni and/or Co, X=Si, Ta, Hf, Ce) for oxidation resistance and improving

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bonding strength between the substrate and ceramic top coat [2,7-10]. The ceramic top coat is typically composed of 6-8 wt.% Y2O3 stabilized ZrO2 (YSZ) for heat insulation [2,11,12].

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There are various methods to fabricate TBCs, such as air plasma spraying (APS), electron beam-physical vapor deposition (EB-PVD), plasma spraying-physical vapor deposition (PS-PVD) and suspension plasma spraying (SPS) [2,12-16]. Among these

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methods, the APS is the main method to fabricate TBCs due to its lower cost, higher efficiency and better flexibility. As for gas turbine engines, the unprecedented demands

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about high performance and efficiency of engines from ever-increasing operating temperature are insatiable [1,17]. Therefore, developing the advanced TBCs is necessary to satisfy the above-mentioned demands. Recently, the nanostructured YSZ thermal barrier coatings have caught great

attention due to their interesting mechanical properties, good thermal insulation capability and thermal shock resistance which are superior to conventional counterparts [18-24]. Based on these advantages of nanostructured TBCs, a number of articles have investigated and reported the nanostructured YSZ TBCs [25-30]. Typical for

nanostructured APS TBCs, the coatings exhibit bi-modal distribution, i.e., the molten lamellae and semi-molten or unmelted particles preserving the properties of the original nanostructure [24,25,31,32]. The bi-modal microstructure in nanostructured YSZ TBCs would result in different behavior of mechanical properties compared with the conventional YSZ TBCs. However, there is little systematic multi-scale investigation about microstructure and mechanical properties in nanostructured YSZ TBCs. In this paper, the nanostructured 8YSZ TBCs derived from nanostructured t´Zr0.9Y0.1O1.95 YSZ feedstocks and conventional 8YSZ TBCs were fabricated by atmospheric plasma spraying. The microstructure and phase composition of nanostructured and conventional 8YSZ coatings were analysed and micromechanical

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properties measured by nano-indentation were discussed via fitting the experimental data including elastic modulus and nanohardness with probability density function of

Weibull distribution in detail. Furthermore, the fracture toughness and bonding strength

2. Experimental procedure

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Preparation of feedstocks and coatings

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of nanostructured and conventional 8YSZ coatings were also evaluated systematically.

2.1 The NiCoCrAlYCe spherical powders (Institute of Metal Research, Chinese

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Academy of Sciences, Shenyang) whose particle size was between 15 μm and 45 μm were used as bond-coat materials, as shown in Fig.1(a). The composition of NiCoCrAlYCe powders is Ni-20Co-19.75Cr-11.4Al-0.68Y-0.79Ce (wt.%). The

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nanostructured spherical 8YSZ thermal sprayed feedstocks with t´ phase were prepared by the nanopowder regranulation technique shown in Fig.1(b). Fig.1(c) is high magnification SEM image from granulated powders in Fig.1(b). The scanning

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transmission electron microscopy of as-prepared 8YSZ feedstocks are shown in the inset of Fig.1(c). According the Fig.1(c) and the inset, it can be observed that the

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feedstocks are porous and composed of nanostructured agglomerates. The detailed preparation and characterization about nanostructured t´-YSZ spherical feedstocks can be seen in our previous work [33]. Meanwhile, the conventional 8YSZ feedstocks (Jinzhou Jinjiang spraying material Co., Ltd, Jinzhou) were also selected as a comparison. The conventional 8YSZ powders with the particle size between 30 μm and 85 μm were shown in Fig.1(d) and the inset. The K417G nickel-based superalloy (Φ25 mm×6 mm) was used as the substrate. The substrate was cleaned ultrasonically with alcohol for 10 min and then grit-blasted

in order to roughen the surface and enhance the bonding strength. Subsequently, the NiCoCrAlYCe coatings with the thickness about 100 μm and 8YSZ coatings with the thickness about 300 μm were deposited by high velocity oxygen fuel (HVOF, JP-5000) spraying and atmospheric plasma spraying (APS, Metco 9MC), respectively. The parameters about HVOF-sprayed NiCoCrAlYCe coatings and APS 8YSZ coatings are listed in Table 1 and Table 2. Fig.2 shows the XRD patterns of NiCoCrAlYCe feedstocks and corresponding coatings. It can be seen that the phase composition is γNi and β-NiAl in NiCoCrAlYCe feedstocks and coatings. 2.2 Characterization

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The microstructure and phase composition of coatings were examined by scanning

electron microscopy (SEM, Nova NanoSEM 430), transmission electron microscopy (TEM, JEM-2100F) and X-ray diffraction (XRD, D/max-rB, RIGAKU, Japan) with Cu Kα radiation. Furthermore, the surface roughness (Ra) of coatings was measured by a

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3D profiler (DEKTAK XT) and five different scanning areas were selected to calculate the average of surface roughness.

The elastic modulus (E) and nanohardness (H) of as-sprayed nanostructured and

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conventional 8YSZ coatings were determined by the nano-indentation method. The nano-indentation (Anton-Paar) was performed under 98 mN with the holding time of

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10 s on the polished cross-section of 8YSZ coatings. For each sample, 20 indents were taken randomly for statistical analysis. The fracture toughness of coatings can be evaluated by Vickers indentation with 5 kgf (98 N) load. The bonding strength of

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nanostructured and conventional 8YSZ coatings was measured according to ASTMC633 standard. Here six as-sprayed coatings samples were used for the tensile test and

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the adhesive was FM-1000 glue.

3. Results and discussion

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Microstructure of nanostructured 8YSZ coatings

3.1 The surface morphologies of nanostructured 8YSZ and conventional 8YSZ coatings are shown in Fig.3. As for nanostructured 8YSZ coatings in Fig.3(a), there is a typical bi-modal distribution, i.e., unmelted particles and lamellae resulting from molten particles. In addition, there are also some voids and fine cracks. The unmelted regions of Fig.3(a) with higher magnification are displayed in Fig.3(b). The Fig.3(b) indicates that partially unmelted zones in coatings show the nanostructure, inheriting the characteristics of nanostructured 8YSZ feedstocks. However, the conventional

8YSZ coatings exhibit denser microstructure and there are lamellae, voids and coarse cracks in Fig.3(c). The distribution of cracks in nanostructured 8YSZ coatings shows much more scattered whereas the distribution of cracks in conventional 8YSZ coatings presents more denser. Fig.4 displays the cross-sectional morphologies of nanostructured and conventional 8YSZ TBCs. It is evident that the interfaces of the bond-coat/ceramic coating and bond-coat/substrate are rough. The roughness of the former is due to the splat stacking during plasma spraying while the roughness of the latter is attributed to sand-blasting [23,34]. Furthermore, there are pores in both coatings. As for nanostructured 8YSZ coatings, there is a distinct bi-modal microstructure shown in

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Fig.4(b). The inset in Fig.4(b) shows that the unmelted particles similar to

nanostructured 8YSZ feedstocks keep original nanostructure properties. These nonmolten or semi-molten regions are also called nanozones, which are loose and porous. The microcracks in nanostructured 8YSZ coatings are fewer and finer. However, there

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are more and coarser cracks including interlamellar cracks and intrasplat cracks in conventional 8YSZ coatings. These results are consistent with the analysis in Fig.3.

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Fig.5 shows the scanning transmission electron microscopy (STEM) morphologies of nanostructured and conventional 8YSZ coatings. From Fig.5(a,b), it

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can be noted that the grain size of unmelted nanostructured powders ranges from 10 nm to 30 nm while the grain size of melted parts ranges from 50 nm to 110 nm. Compared with the unmelted particles, the grain size of the melted parts has grown to a certain

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extent during plasma spraying. However, the as-spayed coatings still keep the nanostructure. Fig.5(c) indicates that the splats consist of fine columnar nano-grains. Therefore, it can be inferred that the nanostructure in coatings comes from the following

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two aspects, i.e., unmelted nanostructured powders and the recrystallized nano-grains from the melted powders due to the rapid cooling in the process of plasma spraying.

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The grain size of the conventional 8YSZ coatings is above 400 nm, as shown in Fig.5(c). In order to evaluate the surface roughness of nanostructured and conventional

8YSZ coatings, herein the 3D profiler is carried out, as displayed in Fig.6. And the result is listed in Table 3. The surface roughness of nanostructured 8YSZ coatings is about 6.953 μm, which is significantly lower than that of conventional 8YSZ coatings (6.953 μm). Thus, it can be inferred that the nanostructured 8YSZ feedstocks melt and spread more fully than conventional 8YSZ feedstocks during plasma spraying. Fig.7 shows the XRD patterns of nanostructured and conventional 8YSZ

feedstocks and corresponding coatings. In order to confirm whether the nanostructured and conventional 8YSZ feedstocks and corresponding coatings contain monoclinic phase (m), tetragonal phase (t), non-transformable tetragonal phase (t´) and cubic phase (c), the local scanning regions (27.5-32.5° and 72.5-75.5°) with the rate of 0.2 °/min are selected. From Fig.7, it can be seen that the nanostructured 8YSZ feedstocks consist of t´-Zr0.9Y0.1O1.95 phase while the conventional 8YSZ feedstocks consist of t phase and c phase. In as-sprayed coatings, both coatings are composed of t´-Zr0.9Y0.1O1.95 phase. What is the difference between the formation of t´-Zr0.9Y0.1O1.95 in nanostructured and conventional 8YSZ coatings? Fig.8 displays the schematic illustration of nanostructured t´-8YSZ coatings. In fact, concerning nanostructured 8YSZ coatings,

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the phase composition of nanostructured 8YSZ feedstocks is t´phase. The t´ phase in nanostructured coatings is composed of two parts, one is due to the melting and

solidification of original t´ feedstocks during plasma spraying, i.e., only one phase

transformation from liquid to solid, the other is remaining original t´ feedstocks.

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However, as for conventional 8YSZ coatings, the formation of t´ phase is attributed to two types of phase transformation. Due to the rapid solidification, the diffusionless transition from liquid to solid.

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phase transformation from t phase to t´ phase is accompanied by the other phase

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Mechanical properties of nanostructured 8YSZ coatings 3.2 The mechanical properties are vital for TBCs to evaluate their durability and reliability. The micromechanical properties such as elastic modulus and nanohardness

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are one of the important mechanical properties for TBCs [35-38]. Fig.9 shows typical load-displacement curves derived from nano-indentation for nanostructured and conventional 8YSZ coatings. The elastic modulus (E) and nanohardness (H) of coatings

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can be obtained by each unloading curve. According to Fig.9, it can be noticed that the maximum penetration depth of nanostructured 8YSZ coatings is higher than that of

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conventional 8YSZ coatings in load-displacement curves (1151.21 nm versus 970.24 nm in the molten region). This indicates that the regions in nanostructured 8YSZ coatings where the penetration depth reaches to maximum (1151.21 nm) can be considered as unmelted or partially melted regions. In fact, the regions are indeed unmelted or partially melted whose morphology image is not shown here. Meanwhile, the slope of unloading curves at maximum penetration depth of nanostructured 8YSZ coatings (unmelted or partially melted regions) is lower than that of conventional 8YSZ coatings (0.4917 mN/nm versus 0.5204 mN/nm in the melted regions). R.S. Lima et al

[31] have pointed out that the properties of molten regions in nanostructured YSZ coatings are similar to conventional YSZ coatings. Therefore, it is possible to infer that the elastic modulus and nanohardness of unmelted regions is lower than that of melted regions for nanostructured 8YSZ coatings. In order to further understand the effect of microstructure on mechanical properties such as elastic modulus and nanohardness of nanostructured 8YSZ coatings and verify the above inference, the statistical analysis of measured values is necessary. As discussed previouly, the nanostructured 8YSZ coatings show typical bi-modal microstructure while the conventional 8YSZ coating primarily performed fully molten lamellae. The characteristics of bi-modal microstructure would directly determine the

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distribution of micromechanical properties.

Commonly speaking, the statistical analysis such as Weibull distribution can describe the differences in microstructure and properties of different coatings [39]. The Weibull distribution can be expressed as the following equation:

i - 0.5 N

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pi 

-p

 x  p  1 - exp - ( ) m   x0 

(1)

(2)

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Where p is cumulative probability density function, x is the value of elastic modulus (E) or nanohardness (H), xo is the eigenvalue of the cumulative probability of 63.2%, m is Weibull modulus, i is the order number (i=1, 2, 3..., 20), N is the sample capacity (N=20).

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m can represent the scatter of data. The larger the m, the smaller the scatter of the test value is. The equation (3) can be derived from equation (1) as follows: ]  m ln x  m ln x0 (1  p)

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ln ln[ 1

(3)

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Based on equation (3), the m can be obtained with the method of least square for fitting. The Weibull plots about E and H of nanostructured and conventional 8YSZ coatings are displayed in Fig.10. The information based on Fig.10 is summarized in Table 4. As for nanostructured and conventional 8YSZ coatings, the range of elastic modulus and nanohardness values measured by nano-indentation coincides with references [34,40]. And this indicates that the measured result is convincing and reliable. As shown in Fig.10, the nanostructured 8YSZ coatings demonstrate a significant bimodal distribution in E and H Weibull plots. The selection of the boundary points for

fitting curves with two different slopes can be referred to in reference [31]. However, the conventional 8YSZ coatings also here one can clearly see mono-modal distribution in E and H Weibull plots and these are in agreement with other investigations [31,41]. Although there are non-molten parts in the conventional coatings, it is not so important here. When combining Fig.10 with Table 4, it can be found that the range of elastic modulus and nanohardness in the low slope region of nanostructured 8YSZ coatings is similar to that of conventional 8YSZ coatings. Considering that the conventional 8YSZ coatings primarily consist of fully molten lamellae, it reveals that the low slope region with higher elastic modulus or nanohardness values represents the molten particles in the E or H Weibull plot. Meanwhile, the high slope region with lower elastic modulus

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or nanohardness values represents the unmelted or semi-molten particles. It is further demonstrated that the elastic modulus and nanohardness of melted regions are higher

than that of unmelted regions for nanostructured 8YSZ coatings. The low slope region

(low Weibull modulus) in the E or H Weibull plot shows the values of elastic modulus

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or nanohardness in molten regions are dispersed and fluctuant, whereas the high slope

region (high Weibull modulus) in the E or H Weibull plot displays the values of elastic

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modulus or nanohardness in unmelted or semi-molten regions are concentrated and stable. This indicates that the elastic modulus or nanohardness of nanostructured 8YSZ

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coatings is strongly sensitive to molten regions where there are some defects including pores, microcracks and splat boundaries [40,42]. It has been pointed out that properties such as hardness of nanostructured feedstocks depend largely on the agglomeration

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condition rather than the grain size [31]. Furthermore, the elastic modulus of coatings decreases with increasing porosity [43-46]. As mentioned previously, the unmelted regions (nanozones) are porous and loosely restricted. Therefore, the nanohardness and

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elastic modulus of nanozones are lower than that of molten regions in nanostructured 8YSZ coatings. The bi-modal distribution of micromechanical properties for

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nanostructured 8YSZ coatings and the mono-modal distribution for conventional 8YSZ coatings are in line with the previous analysis about the microstructure of coatings. Fig.11 shows the Vickers-indentation morphologies of nanostructured and

conventional 8YSZ coatings where the applied load is 5 kgf (49 N). It can be seen that the induced cracks in nanostructured 8YSZ coatings are finer and shorter whereas the induced cracks in conventional 8YSZ coatings are coarser and longer. Based on the induced cracks in the same applied load, the fracture toughness of coatings can be evaluated. The fracture toughness indicates the resistance of cracks propagation for

materials. From the perspective of induced cracks propagation, the toughness of nanostructured 8YSZ coatings is superior to conventional 8YSZ coatings. In nanostructured 8YSZ coatings, the nanozones play a role in arresting microcracks at the stage of microcracks propagation. However, the propagation of microcracks in molten regions is easier than that in nanozones [40,42,47]. Therefore, the nanostructured 8YSZ coatings exhibit higher fracture toughness than conventional 8YSZ coatings. The bonding strength is an another important mechanical property for as-sprayed coatings. It should be noticed that the bonding strength of coatings can be divided into cohesive strength and adhesive strength. The cohesive failure means that the fracture

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occurs entirely within the coating and the adhesive failure refers to the facture occurring

at the interface between bond coat and top coat or substrate and bond coat [48,49]. Fig.12 shows the macro photos of nanostructured and conventional 8YSZ coatings after tensile testing. The fracture of coatings mainly occurs at the interface between bond

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coats and 8YSZ coatings, which is classified as adhesive failure. The measured bonding

strength of both coatings is shown in Fig.13. The average bonding strength of

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nanostructured 8YSZ coatings is 29.2 MPa, which is about 1.4 times higher than that of conventional 8YSZ coatings (about 21.3 MPa).

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There are some causes about the higher bonding strength for nanostructured 8YSZ coatings. One is the dependence on the microstructure of nanostructured 8YSZ coatings. The fracture microstructure of both coatings is displayed in Fig.14. The unmelted or

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partially molten nanostructured feedstocks, i.e., nano-zones in nanostructured 8YSZ coatings would further prevent the cracks propagation during the tensile test [24]. However, the crack propagation in the conventional 8YSZ coating is relatively easy

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[34,47,50]. A second reason is that the thermal expansion coefficient (TEC) of crystalline materials is strongly correlated to the grain size. The TEC increases with the

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decrease of grain size. During plasma spraying, the higher thermal expansion coefficient of the ceramic coating would result in lower thermal mismatch between the substrate and ceramic coatings. M. Mutter et al. [44] have already pointed out that the residual stress in coatings is mainly affected by the evolution of thermal stress. Consequently, the lower residual stress might occur in nanostructured 8YSZ coatings. This can be the reason why the nanostructured 8YSZ coatings show higher bonding strength compared with conventional 8YSZ coatings.

4. Conclusions

In this paper, the nanostructured and conventional 8YSZ coatings were prepared by atmospheric plasma spraying and the microstructure was characterized by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), Xray diffraction (XRD) and a 3D profiler. Furthermore, the mechanical properties of coatings, including elastic modulus, nanohardness, fracture toughness and bonding strength, were also investigated systematically. Some meaningful conclusions can be gained as follows: (1) The nanostructured 8YSZ coatings have a lower surface roughness compared with conventional 8YSZ coatings. The phase compositions of both coatings are t´Zr0.9Y0.1O1.95 phase. As for nanostructured 8YSZ coatings, the t´-Zr0.9Y0.1O1.95 phase is

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the melting and solidification of nanostructured 8YSZ feedstocks as well as the

remaining t´-Zr0.9Y0.1O1.95 feedstocks. As for conventional 8YSZ coatings, the formation of t´-Zr0.9Y0.1O1.95 phase is attributed to the diffusionless transformation from t/c phases to t´ phase in the process of rapid solidification. nanostructured

8YSZ

coatings

demonstrate

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(2) The

typical

bi-modal

microstructure while conventional 8YSZ coatings show noticeable mono-modal

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microstructure. The Weibull distribution is a method to analyse the bi-modal microstructure in nanostructured 8YSZ coatings. The low slope with high E or H

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reflects molten regions and the high slope with low E or H indicates unmelted or partially melted regions in the E or H Weibull plot. Based on the analysis of Weibull distribution, the elastic modulus and nanohardness are sensitive to molten regions

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where there are some defects such as pores and microcracks. (3) The average bonding strength of nanostructured and conventional 8YSZ coatings is 29.2 MPa and 21.3 MPa, respectively. The fracture toughness and bonding

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strength of nanostructured 8YSZ coatings are superior to conventional 8YSZ coatings and these are mainly due to nano effects in nanostructured 8YSZ coatings.

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Declaration of interests 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.

Acknowledgements This work was supported by the National Key Research Program (2017YFB0306100), the National Natural Science Foundation of China (No.51771059), Guangdong Academy of Sciences Program (Nos. 2018GDASCX-0402, 2018GDASCX-0950,

2017GDASCX-0111), Guangdong Natural Science Foundation (No.2016A030312015), Science and Technology Planning Project of Guangdong province (Nos. 2017A070701027, 2016A030312015, 2014B070705007).

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plasma spraying, Scr. Mater. 66 (2012) 109-112. [23] R. Ghasemi, H. Vakilifard, Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength, Ceram. Int. 43 (2017) 8556-8563.

[24] J.S. Wang, J.B. Sun, H. Zhang, S.J. Dong, J.N. Jiang, L.H. Deng, X. Zhou, X.Q. Cao, Effect of spraying power on microstructure and property of nanostructured YSZ thermal barrier coatings, J. Alloys Compd. 730 (2018) 471-482. [25] D.M. Yang, Y. Gao, H.J. Liu, C.Q. Sun, Thermal shock resistance of bimodal

structured thermal barrier coatings by atmospheric plasma spraying using nanostructured partially stabilized zirconia, Surf. Coat. Technol. 315 (2017) 9-16. [26] H. Chen, C. Ding, Nanostructured zirconia coating prepared by atmospheric plasma spraying, Surf. Coat. Technol. 150 (2002) 31-36. [27] F.F. Zhou, Y. Wang, L. Wang, Z.Y. Cui, Z.G. Zhang, High temperature oxidation and insulation behavior of plasma-sprayed nanostructured thermal barrier coatings, J. Alloys Compd. 704 (2017) 614-623. [28] H. Jamali, R. Mozafarinia, R. Shoja-Razavi, R. Ahmadi-Pidani, Comparision of hot corrosion behavior of nanostructrued and convensional YSZ thermal barrier coatings exposure to molten vanadium pentoxide and sodium sulfate, J. Eur. Ceram.

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[29] H. Jamali, R. Mozafarinia, R. Shoja-Razavi, R. Ahmadi-Pidani, M.R. Loghman-

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[30] F.F. Zhou, Y. Wang, Z.Y. Cui, L. Wang, J.F. Gou, Q.W. Zhang, C.H. Wang, Thermal cycling behavior of nanostructured 8YSZ,SZ/8YSZ and 8CSZ/8YSZ thermal barrier

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coatings fabricated by atmospheric plasma spraying, Ceram. Int. 43 (2017) 4102-4111. [31] R.S. Lima, A. Kucuk, C.C. Berndt, Bimodal distribution of mechanical properties (2002) 224-232.

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on plasma sprayed nanostructured partially stabilized zirconia, Mater. Sci. Eng. A 327 [32] G.R. Li, G.J. Yang, C.X. Li, C.J. Li, A comprehensive mechanism for the sintering

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of plasma-sprayed nanostructured thermal barrier coatings, Ceram. Int. 43 (2017) 96009615.

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Synthesis and characterization of nanostructured t′-YSZ spherical feedstocks for atmospheric plasma spraying, J. Alloy. Compd. 740 (2018) 610-616.

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[34] R. Ghasemi, R. Shoja-Razavi, R. Mozafarinia, H. Jamali, Comparison of microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings, Ceram. Int. 39 (2013) 8805-8813.

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Int. 32 (2006) 263-270. [37] S. Nath, I. Manna, J.D. Majumdar, Nanomechanical behavior of yttria stabilized zirconia (YSZ) based thermal barrier coating, Ceram. Int. 41 (2015) 5247-5256. [38] L.H. Gao, H.B. Guo, L.L. Wei, C.Y. Li, S.K. Gong, H.B. Xu, Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition, Ceram. Int. 41 (2015) 8305-8311. [39] Y. Zhao, Y. Gao, Deposition of nanostructured YSZ coating from spray-dried particles with no heat treatment, Appl. Surf. Sci. 346 (2015) 406-414. [40] C. Lamuta, G. Di Girolamo, L. Pagnotta, Microstructural, mechanical and tribological properties of nanostructured YSZ coatings produced with different APS

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[41] C.K. Lin, C.C. Berndt, Statistical analysis of microhardness variations in thermal spray coatings, J. Mater. Sci. 30 (1995) 111-117.

[42] G. Di Girolamo, F. Marra, C. Blasi, E. Serra, T. Valente, Microstructure,

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mechanical properties and thermal shock resistance of plasma sprayed nanostructured zirconia coatings, Ceram. Int. 37 (2011) 2711-2717.

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[44] M. Mutter, G. Mauer, R. Mücke, O. Guillon, R. Vaßen, Correlation of splat morphologies with porosity and residual stress in plasma-sprayed YSZ coatings, Surf. Coat. Technol. 318 (2017) 157-169.

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[45] D.P. Zhou, J. Malzbender, Y.J. Sohn, O. Guillon, R. Vaßen, Sintering behavior of columnar thermal barrier coatings deposited by axial suspension plasma spraying (SPS), J. Eur. Ceram. Soc. 39 (2019) 482-490. of

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[46] B. Lv, R. Mücke, X.L. Fan, T.J. Wang, O. Guillon, R. Vaßen, Sintering resistance advanced

plasma-sprayed

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microstructures, J. Eur. Ceram. Soc. 38 (2018) 5092-5100. [47] R.S. Lima, B.R. Marple, Thermal spray coatings engineered from nanostructured ceramic agglomerated powders for structural, thermal barrier and biomedical applications: a review, J. Therm. Spray Technol.16 (2007) 40-63. [48] L. Pawlowski, The Science and Engineering of Thermal Spray Coating, Second ed., John Wiley & Sons, New York, 2008. [49] A. Hjorhhede, A. Nylund, Adhesion testing of thermally sprayed and laser deposited coatings, Surf. Coat. Technol. 28 (2004) 208-218.

[50] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Development and implementation of plasma sprayed nanostructured ceramic coatings, Surf. Coat. Technol. 147 (2001) 48-54. Fig.1. SEM images of (a) NiCoCrAlYCe powders, (b) nanostructured 8YSZ feedstocks, (c) high magnification image from granulated powders in (b), and (d) conventional 8YSZ powders. The inset in (c) is the STEM image of nanostructured 8YSZ feedstocks and the inset in (d) is high magnification SEM image of conventional 8YSZ feedstocks. Fig.2 XRD patterns of NiCoCrAlYCe powders and corresponding coatings. Fig.3 Surface morphologies of (a) nanostructured 8YSZ coatings, (b) unmelted

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particles of (a) with high magnification and (c) conventional 8YSZ coatings.

Fig.4 Cross-sectional morphologies of (a) nanostructured 8YSZ coatings and (c) conventional 8YSZ coatings, (b) and (d) are magnified images corresponding to (a) and

(c) in the ceramic layer. The inset in (b) is the high magnification SEM image of

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unmelted particles

Fig.5 STEM images of (a) unmelted nanostructured powders, (b) the nanostructured (d) conventional 8YSZ coatings.

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equiaxed grains and (c) fine columnar grains in splats of nanostructured 8YSZ coatings,

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Fig.6 3D morphologies of (a) nanostructured 8YSZ and (b) conventional 8YSZ coatings measured by the profiler.

Fig.7 XRD patterns of feedstocks and coatings of (a,b,c) nanostructured 8YSZ and

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(d,e,f) conventional 8YSZ. Fig.8 Schematic illustration of the formation of nanostructured t´-8YSZ coatings with bi-modal microstructure. Fig.9 Load-displacement curves of (a) nanostructured 8YSZ and (b) conventional

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8YSZ coatings.

Fig.10 Weibull plots of elastic modulus and nanohardness on cross-section for (a)

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nanostructured 8YSZ and (b) conventional 8YSZ coatings. Fig.11 Indentation morphologies of (a) nanostructured and (b) conventional 8YSZ coatings (5kgf). Fig.12 The macro fracture photos of (a) nanostructured and (b) conventional 8YSZ coatings after tensile testing. Fig.13 Bonding strength of nanostructured and conventional 8YSZ coatings. Fig.14 The fracture microstructure of (a) nanostructured and (b) conventional 8YSZ coatings after tensile failure.

(b)

(c)

(d)

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Figure

30μm

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50nm

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Fig.1. SEM images of (a) NiCoCrAlYCe powders, (b) nanostructured 8YSZ feedstocks, (c) high magnification image from granulated powders in (b), and (d) conventional 8YSZ powders. The inset in (c) is the STEM image of nanostructured 8YSZ feedstocks and the inset in (d) is high magnification SEM image of conventional 8YSZ feedstocks.

Fig.2 XRD patterns of NiCoCrAlYCe powders and corresponding coatings.

(a)

Lamellae

(b)

Unmelted particles

Nano-zone

(c)

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Lamellae

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Cracks

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Fig.3 Surface morphologies of (a) nanostructured 8YSZ coatings, (b) unmelted

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particles of (a) with high magnification and (c) conventional 8YSZ coatings

(b)

(a)

Unmelted nanoparticle

Nanostructured 8YSZ coating

Superalloy

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NiCoCrAlYCe coating Microcolumnar grain

(d) (d)

(c)

Interlamellar crack

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Globular pore

Conventional 8YSZ coating

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NiCoCrAlYCe coating

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Intrasplat crack

Globular pores

Superalloy

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Fig.4 Cross-sectional morphologies of (a) nanostructured 8YSZ coatings and (c) conventional 8YSZ coatings, (b) and (d) are magnified images corresponding to (a) and (c) in the ceramic layer. The inset in (b) is the high magnification SEM image of unmelted particles.

(b)

(a) Nano-grain

(b)

20nm

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Fig.5 STEM images of (a) unmelted nanostructured powders, (b) the nanostructured equiaxed grains and (c) fine columnar grains in splats of nanostructured 8YSZ coatings, (d) conventional 8YSZ coatings.

Fig.6 3D morphologies of (a) nanostructured 8YSZ and (b) conventional 8YSZ

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(b)

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coatings measured by the profiler.

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Fig.7 XRD patterns of feedstocks and coatings of (a,b,c) nanostructured 8YSZ and (d,e,f) conventional 8YSZ.

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Fig.8 Schematic illustration of the formation of nanostructured t´-8YSZ coatings with bi-modal microstructure. (b)

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Fig.9 Load-displacement curves of (a) nanostructured 8YSZ and (b) conventional 8YSZ coatings.

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Fig.10 Weibull plots of elastic modulus and nanohardness data for (a) nanostructured

8YSZ and (b) conventional 8YSZ coatings. (b)

(a)

Unmelted particle Induced crack Induced crack

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Fig.11 Indentation morphologies of (a) nanostructured and (b) conventional 8YSZ coatings (5kgf).

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Fig.12 The macro fracture photos of (a) nanostructured and (b) conventional 8YSZ coatings after tensile testing.

Fig.13 Bonding strength of nanostructured and conventional 8YSZ coatings.

(a)

(b ) Nano-zones

Cracks

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Fig.14 The fracture microstructure of (a) nanostructured and (b) conventional 8YSZ coatings after tensile failure. Table 1. HVOF-sprayed parameters for NiCoCrAlYCe coatings 1850

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Oxygen flow rate (SCFH) Aviation kerosene (GPH)

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Spray distance (mm)

5

Feeding rate (g/min)

70

Parameters

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Nanostructured 8YSZ

Conventional 8YSZ

Current (A)

550

600

Voltage (V)

62

65

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Carry gas N2 flow rate (SCFH)

Ar flow rate (slpm)

35

35

H2 flow rate (slpm)

10

12

Spray distance (mm)

120

120

Feeding rate (g/min)

25

25

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Table 2. APS parameters for 8YSZ coatings

Table 3. Surface roughness of nanostructured and conventional 8YSZ coatings Coatings

Surface roughness Ra (μm)

Nanostructured 8YSZ

6.953±0.416

Conventional 8YSZ

9.243±0.439

Table 4. Summary of results obtained from Fig.10 Nanostructured 8YSZ

Conventional 8YSZ

mE1

6.81

N.A.

E range (GPa)

86.217-115.31

N.A.

mE2 (GPa)

4.55

7.88

E range (GPa)

119.58-197.78

112.93-200.54

mH1

8.44

N.A.

H range (GPa)

3.9944-5.2476

N.A.

mH2

3.23

3.92

H range (GPa)

6.3919-13.603

5.8539-16.689

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