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Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings
Journal Pre-proof Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings
Fazhang Lu, Wenzhi Huang, Haitao Liu PII:
S0257-8972(19)31280-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125290
Reference:
SCT 125290
To appear in:
Surface & Coatings Technology
Received date:
14 June 2019
Revised date:
10 December 2019
Accepted date:
23 December 2019
Please cite this article as: F. Lu, W. Huang and H. Liu, Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings, Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2019.125290
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© 2018 Published by Elsevier.
Journal Pre-proof Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings Fazhang Lu, Wenzhi Huang, Haitao Liu Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, 109 De Ya Rd, Changsha 410073, China *
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Corresponding authors: Email: [email protected] (W.Z. Huang),
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[email protected] (H.T. Liu).
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Tel No: +86 731 84573176, fax: +86 731 84576578 Abstract
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The effects of Al2O3 content and spraying power on bonding strength and thermal shock
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resistance of plasma-sprayed 8YSZ-Al2O3 composite coatings with thickness of ~900μm were investigated. Thermal shock resistance decreased obviously with increasing the Al2O3
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content and test temperature, while bonding strength showed an increasing trend. Relatively, the effect of spraying power on thermal shock resistance and bonding strength was small.
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Moreover, the Young's modulus of coating surface was increased from 99.07GPa to 134.59GPa with increasing Al2O3 content from 10wt% to 40wt%, while the corresponding thermal expansion coefficient (CTE) decreased from 9.68×10-6/oC to 7.29×10-6/oC. Thus, residual stresses on the surface of composite coatings changed from tensile (+13.3MPa) to compressive (-189.0MPa), which would cause greater stresses gradient as well as strain energy in the direction of coating thickness. During thermal shock testing, thermal stress increased sharply with increasing the test temperature and Al2O3 content. Meanwhile, introduction of Al2O3 resulted in denser microstructure, which was detrimental for stress releasing. According to the results, the horizontal crack propagation in the interface area caused by thermal stress and volume contraction caused by amorphous phase recrystallization were considered to be the main factors for thermal shock failure. Keywords: Thick 8YSZ-Al2O3 composite coatings; Thermal shock resistance; Phase
Journal Pre-proof evolution; Residual stress; Failure analysis 1. Introduction 8YSZ with low thermal conductivity and relatively high coefficient of thermal expansion (CTE) has been widely used as the material of thermal barrier coatings (TBCs). However, some shortcomings such as oxygen permeability, low fracture toughness (0.55 MPa·m1/2) and poor thermal corrosion resistance affect its comprehensive performance and
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further application [1,2]. Meanwhile, Al2O3 coatings are commonly used in anti-wear and
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anti-corrosion applications as high strength, low density and chemical inertness [3,4]. It is
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found that the properties of 8YSZ and Al2O3 are complementary to each other. The properties
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of 8YSZ may be well regulated by adding proper amount of Al2O3. Recently, plasma-sprayed
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8YSZ-Al2O3 composite coatings with thin thickness have attracted increasing attention of many researchers [517]. Some optimizations and explorations are gradually developed in
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the composite system. Tarasi et al. [59] demonstrate that the pseudo-eutectic of
460μm
deposited by
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(8YSZ-60wt%Al2O3) composite coating with thickness
solution-precursor plasma spraying (SPPS) exhibits better thermal shock resistance than that of 8YSZ at 1080oC. Based on these studies, amorphous phase is formed as the high quenching rate during SPPS process, which plays an important role in microstructure, mechanical properties and durability of composite coating. In addition, quenching with liquid nitrogen-cooled substrates during atmosphere plasma spraying (APS) process lead to the coexistence of a mixture of amorphous, metastable and stable phases in the pseudo-eutectic freestanding coatings with thickness of 600-700μm. After heat treatment at 1400oC, metastable-to-stable phase transition, crystallization of amorphous phase as well as the
Journal Pre-proof reduction of porosity would occur in the coatings, which promotes an increase in hardness and wear resistance [10,11]. It is discovered that Al2O3 addition and annealing treatment could regulate the porosity, Young's modulus and hardness of plasma-sprayed 8YSZ-Al2O3 composite coatings with thickness of 400μm [12]. Meanwhile, the fracture toughness increases from 1.93 MPa·m1/2 at 10wt% alumina content to a maximum of 2.31 MPa·m1/2 at 40wt% alumina and then decreases to 2.09 MPa·m1/2 at 55wt% alumina for the as-sprayed
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coatings. Furthermore, nanostructured 8YSZ-13wt%Al2O3 composite coating with thickness
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of 280-350μm is successfully deposited by APS. The addition of nano-Al2O3 could
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effectively inhibit grain growth of zirconia phase and reduce the interface stress between
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ceramic coat and thermal growth oxide layer (TGO). As a result, thermal cyclic lifetime of
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the coating (953 cycles) is higher than that of traditional 8YSZ coating (785 cycles) at 1100oC [1315]. Moreover, by comparing with 8YSZ coating, 8YSZ-40wt%Al2O3 composite
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coating with thickness of 500μm deposited by APS exhibits better resistance in thermal
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oxidation and corrosion test due to their denser structure [16,17]. According to these studies, introduction of Al2O3 can regulate phase composition, microstructure and mechanical properties of the 8YSZ-Al2O3 composite coatings, further enhancing their thermal stability, oxidation and corrosion resistance. However, as the thermal conductivity of Al2O3 (1400K, bulk, 5.8 w/(m·K)) is higher than 8YSZ (1273K, sprayed, 2.1 w/(m·K)), introduction of Al2O3 inevitably leads to the decrease in thermal insulation [18]. Under the same condition of coating composition, increasing the ceramic thickness to produce thick TBCs can be an approached way [19]. Note in the literature that the thermal insulation of traditional zirconia-based TBCs increases significantly with increasing coating
Journal Pre-proof thickness from 500μm to 1.8mm [20]. By the same token, the substrate temperature is decreased by 4~6oC for every 25.4μm increasing in ceramic thickness [21,22]. Therefore, thick 8YSZ-Al2O3 composite coating might be deposited to further improve its thermal insulation. However, increasing coating thickness normally results in significant stress gradients in the direction of ceramic thickness as well as poor thermal shock resistance [2325]. Few studies carried out on the investigation regarding thermal shock resistance of
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thick 8YSZ-Al2O3 composite coatings with thickness of ~900μm via APS process.
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In the work presented herein, 8YSZ-Al2O3 composite coatings with thickness of
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~900μm were deposited on the nickel based superalloy substrate by APS, and the influences
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of the test temperature and Al2O3 content on thermal shock resistance and coating failure was
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studied. Typical failure analysis of composite coatings was performed based on phase composition, microstructure and stress evolution before and after thermal chock cycles.
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2. Materials and methods
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2.1. Feedstock and sample preparation Commercial 8YSZ powders (SY133, Beijing Sangyao, China) with particle dimensions of 15-45μm and Al2O3 powders (SY111, Beijing Sangyao, China) with particle dimensions of 22-40μm were employed as feedstock. To obtain the composite powders with good flowability, the deionized water, arabic gum and ammonium citrate tribasic were added into the powders mixture of 8YSZ and Al2O3 to fabricate slurry. Subsequently, the water-based slurry was ball-milled for 72h at 360rpm rotation speed with zirconia balls to mix these oxide powders, and then they were spray-dried to produce spherical composite powders. 8YSZ-Al2O3 composite powders with Al2O3 content of 10wt%, 20wt%, 30wt% and 40wt%
Journal Pre-proof are referred to as powders ZAP10, ZAP20, ZAP30 and ZAP40. The slurry parameters and spray drying parameters are presented in Table 1. According to ISO 4490-2014 and ISO 3953-2011, the flowability and actual tap density of composite powders were obtained, respectively. Nickel based superalloy (Ni3Al) plates of 40×20×4mm3 and disks of Ø25×4mm3 grit blasted with alumina particles were used as the substrates. Bond coat and ceramic coat were
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deposited via APS process (APS-3000, Beijing Aeronautical Manufacturing Technology
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Research Institute, China). The thicknesses of bond coat and ceramic coat were designed to
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be ~75μm and ~900μm, respectively. Bond coat was deposited at 38kW by using commercial
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CoNiCrAlY powders (KF-309, Beijing). Ceramic coats were deposited at different spraying
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powers (34, 38 and 42kW) to optimize spraying parameters. The deposition conditions are presented in Table 2. Composite coatings deposited using powders ZAP10, ZAP20, ZAP30
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and ZAP40 are referred to as coatings ZAC10, ZAC20, ZAC30 and ZAC40. Freestanding
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coatings without bond coat were obtained by depositing composite powders onto the graphite substrates at 42kW and then removed coating from the substrate. 2.2. Characteristics of composite coatings Phase composition of feedstock powders and as-deposited coatings were identified by XRD (D8 ADVANCE, Bruker, Germany) measurements using Cu Kα radiation (λ=1.541Å) in a 2θ range of 20-80° at a scanning speed of 6°min-1. The X-ray source is operated at an accelerating voltage of 40kV and current of 40mA. Microstructural characterization of feedstock powders and as-deposited coatings were performed by scanning electron microscope (Quanta 200, FEI, USA) coupled with
Journal Pre-proof energy-dispersive X-ray spectroscopy (EDS). The cross-section was received by cold-mounted in epoxy resin, then sectioned with a low velocity diamond saw and polished with SiC paper and diamond paste. The 2D micro-porosity of as-deposited coatings was evaluated by using Image J analysis software. More than five pictures randomly taken from the polished cross-sections were averaged to evaluate the porosity. The density of as-deposited coating was measured and calculated by Archimedes' principle, and three
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measurements were performed to determine the average value for each freestanding coating.
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The standard load and unload method of Nano-indentation test are used to obtain the
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Young's modulus of composite coatings. The measurement was carried out on the surface of
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as-deposited coatings by the aid of Nano-indentation (G200, Agilent, USA) with a Berkovich
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indenter. A maximum depth limit of 1000 nm was required due to the porous and inhomogeneous nature of ceramic coatings. The surface approach velocity and holding time
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was set at 10nm/s and 10s, respectively [26,27]. According to the Oliver and Pharr methods,
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the reduced modulus can be expressed as: Er
S 2 hc
24.5
(1)
Where hc is the contact depth; the elastic contact stiffness S ( S
dp ) can be obtained from dh
the slope of the initial portion of the unloading curve; β closely relates to the indenter geometry (β=1.034 for a Berkovich indenter); the contact depth hc can be determined from the load-depth curve, it can be written as:
hc hmax
Pmax S
(2)
Where ε is the geometric constant associated with the indenter types (ε=0.75 for a Berkovich
Journal Pre-proof indenter); the peak load (Pmax), the maximum indentation depth (hmax) can be obtained from the load-depth curves. Therefore, Young's modulus can be calculated by Eq (3):
1 1 2 1 i2 Er E Ei
(3)
Where υ and υi are Poisson's ratio of the ceramic coat and the indenter, respectively; the elastic constants of the diamond indenter are Ei=1141GPa and υi=0.07; Poisson's ratio (υ) of
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as-deposited coatings is calculated based on rule of mixtures (ROM). In addition, due to the
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dimension of indentation is small, many voids and defects of as-deposited coatings may lead to more diffuse measurement results. So the Weibull statistical analysis was used to dispose
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the results [12]. The Weibull distribution function is given as follow: (4)
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1 ln ln m ln E m ln E0 1 w
Where E is the measured value of Young's modulus; m and E0 are the Weibull modulus and
i 0.5 , where i is the i-th rank and n is the total number of data points. There is a linear n
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w
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characteristic value, respectively. The measured value is sorted in ascending order and letting
1 relationship between ln ln and ln E , m and E0 can be easily obtained from the 1 w
fitting Weibull plot. The data scatter of measured value can be reflected by the Weibull modulus. The CTE of substrate and freestanding coatings with dimension of 25×5×2mm3 was measured in air from 27oC to 1100oC using a dilatometer (DIL 402 C, NETZSCH, Germany) at a heating rate of 5oC/min. Fractional change in length, ΔL/L as a function of temperature was measured and the CTE was measured from the slope of the curve.
Journal Pre-proof Residual stresses on the surface of freestanding and supported coatings was measured by X-ray stress analyzer (LXRD, Proto, Canada) based on Bragg's law [28,29]:
n 2d sin
(5)
Where n is the order of reflection; d is the crystal plane spacing; and θ is Bragg's diffraction angle. The existence of residual stress leads to the difference of the crystal plane spacing between the coating and feedstock powder. The strain εψ in the direction of incident angle ψ
d d d0 cot 0 0 d d0
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associated with the diffraction peak displacement:
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can be obtained by the relative change of the diffraction crystal plane spacing, which is
(6)
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Where θ0 is Bragg’s angle at the diffraction peak in the feedstock powder (assuming no
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residual stress); θψ is Bragg’s angle with residual stress. The residual stresses on the surface
E 2 cot 0 2 1 180 sin 2
(7)
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r
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of as-deposited coatings at the corresponding ψ direction can be calculated by Eq (7):
Where E approximatively takes the measured value above; υ is also calculated based on ROM. There is a linear relationship between 2θψ and sin2ψ, the residual stresses on the surface of as-deposited coatings can be obtained by measuring the displacement of diffraction lines under different directions. In the present work, diffraction measurements using Cr Kα radiation (λ=2.291Å) were taken at five ψ values (ψ=-30o, -15o, 0o, 15o and 30o). The reflection (213) of non-transformable tetragonal phase (t′-ZrO2) at about 2θ=151.8° was used as the diffraction plane for lattice plane spacing measurement in residual stress analysis. 2.3. Properties test
Journal Pre-proof The bonding strength of the supported coatings with dimensions of Ø25×4mm3 was examined using a pull-off tester (Electronic Universal Materials Tester, CSC-1101, China) at a stretching rate of 1mm/min. Commercially available adhesive (FM-1000) was used to bond the specimens. To reduce the influence of random errors, the bonding strength was taken as the average value of three measurements. Thermal shock tests were conducted in an automated bottom-drop furnace (RSQ06,
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Changsha, China) at different temperatures (900, 1000 and 1100oC), and thermal shock cycles
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were used to characterize the thermal shock resistance of as-deposited coatings. The
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supported coatings with dimensions of 40×20×4mm3 were hanged on the center of a
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heat-resistant brick which was installed on a vertical elevator. When the elevator was lowered,
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the specimens were cooled by air. In each cycle, the specimens were held isothermally for 5 min and then taken out to cool down in air for 5 min. Thermal shock tests were repeated until
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at least 5% of the coating surface area was lost, and the number of cycles to failure obtained
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by averaging three measurements was defined as thermal shock lifetime. 3. Results and discussion
3.1. Phase and microstructure analysis Typical XRD patterns of feedstock powders and composite coatings are compared in Fig. 1. The diffraction peaks at about 2θ=30.17o, 35.01o, 50.22o, 59.78o, 62.71o and 74.12o are indexed to the (101), (110), (112), (211), (202) and (220) reflections of t′-ZrO2, while the diffraction peaks located at about 2θ=28.3o and 31.7o are related to the (-111) and (111) reflections of monoclinic phase (m-ZrO2). The diffraction peaks of Al2O3 powder at about 2θ=25.71o, 43.46o and 57.55o are related to the (012), (113) and (116) reflections of
Journal Pre-proof rhombohedral phase (α-Al2O3). By comparison, the relative diffraction peak intensity of α-Al2O3 increases gradually with increasing Al2O3 content, while that of t′-ZrO2 is decreasing. Thus, the phase composition of composite powders is t′-ZrO2, m-ZrO2 and α-Al2O3. Fig. 2 shows that as-synthesized powders are spherical particles with diameter of ~50μm after spray drying. In addition, as compared in Fig. 3, with increasing Al2O3 content from 10wt% to 40wt%, the flowability and actual tap density of composite powders decreases from
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43.3s/50g to 47.4s/50g and from 1.73g/cm3 to 1.61g/cm3, respectively. Thus, composite
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powders with uniform dimensions, proper density and good flowability are suitable for APS
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[30].
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After coating deposition, phase composition of composite coatings can be confirmed in
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Fig. 1(b). The diffraction peaks at about 2θ=30.24o, 35.21o, 50.46o, 60.03o, 62.88o and 74.26o are related to t'-ZrO2. Weak diffraction peaks at about 2θ=25.71o and 43.46o related to
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α-Al2O3 are only presented in coatings ZAC30 and ZAC40. The single weak peak about
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2θ=45.87o related to the (400) reflection of cubic phase (γ-Al2O3) suggested that small amounts of γ-Al2O3 is possible existed in composite coatings [31]. Furthermore, as shown in the magnified view in Fig. 1(b), the bulge peak of t'-ZrO2 at about 2θ=30.24 widens and decreases gradually with increasing Al2O3 content, implying the existence of amorphous phase [59]. Part of the melting Al2O3 and 8YSZ splats have no time to recrystallize as such high quenching rate (106K/s) during APS process, thus resulting in the formation of amorphous phase and bits of γ-Al2O3. No other phase in composite coatings is formed indicating the interaction between Al2O3 and 8YSZ is not obvious [3133]. Typical surface and cross-sectional morphologies of composite coatings are compared in
Journal Pre-proof Fig. 4 and Fig. 5, respectively. As shown in Fig. 4, many micro-sized pores and splats in different melting states are presented on the rough surface of coatings. The content of fully melted splats increases gradually with increasing Al2O3 content. Al2O3 is easier to fully melt during APS because of the melting point of Al2O3 (2040oC) is lower than that of 8YSZ (2700oC). The fully melted Al2O3 splats with higher thermal conductivity are beneficial to the uniform dissipation of heat energy. In addition, it could help to maintain the 8YSZ in the
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molted state for a longer time period due to the specific heat capacity of Al2O3 is higher than
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8YSZ [34,35]. According to the cross-sectional morphologies (Fig. 5), the thicknesses of
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bond coat and ceramic coat are obtained to be 70-80μm and 870-880μm, respectively. The
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fine and larger pores are randomly distributed in the cross-section of coatings. Consider that
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the formation of fine pores could be ascribed to the voids within spray-dried powders, whereas the large ones are resulted from the imperfect overlapping due to the high-energy
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impacts from incoming splats [36,37]. In addition, high magnification cross-sectional
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morphologies and point element content analysis are displayed in Fig. 6 to further confirm the microstructure and element distribution of composite coatings. Some micro-sized white regions in back-scattering images as marked in Fig. 6(b-d) become more obvious with increasing Al2O3 content. EDS is used to distinguish the element content in white and the other regions. Fig. 6(e) and (f) exhibit the element content of point 1 and point 2 in Fig. 6(d), respectively. Observe that element of point 1 in white regions mainly consists of zirconium (Zr), while aluminium (Al) content of point 2 increases obviously. Thus, the white regions may correspond to un-melted or semi-melted 8YSZ splats, while the rest gray area is the homogeneous composite matrix of 8YSZ and Al2O3 splats. According to the study, Al2O3
Journal Pre-proof splats are segregated as grains between 8YSZ grains and as particles within 8YSZ grain boundaries in the matrix [12,38]. However, many un-melted and semi-melted 8YSZ splats are surrounded by fully melted 8YSZ-Al2O3 matrix. The influence of Al2O3 content on the density and porosity of composite coatings are compared in Fig. 7. Note that the density goes down from 4.93g/cm3 to 4.20g/cm3 and the porosity decreases from 20.32% to 12.38% with increasing Al2O3 content from 10wt% to
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40wt%, respectively. The decrease in the density of coatings results from the density of Al2O3
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(4.0g/cm3) is much lower than that of 8YSZ (6.1g/cm3). Increasing Al2O3 content brings
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better melting state and more content of fully melted splats. The fully melted splats could fill
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the voids and defects and decrease the porosity of composite coatings [12,39].
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3.2. Influences on bonding strength and thermal shock resistance of composite coatings The bonding strength is an important macro-mechanical property for TBCs. Fig. 8 shows
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that typical tensile failure occurs at the bond coat/ceramic coat interface, indicating the
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interface area is the most unstable of composite coatings [40]. Fig. 9 shows the bonding strengths of composite coatings deposited at different spraying powers. The bonding strengths of coating ZAC10 deposited at 34kW, 38kW and 42kW are measured to be 14.9MPa, 15.7MPa and 14.4MPa, respectively. Relatively, coatings ZAC10 and ZAC20 deposited at 38kW have the largest bonding strength of 15.7MPa and 17.1MPa respectively, while the largest bonding strength of 19.1MPa and 19.7MPa for coatings ZAC30 and ZAC40 can be obtained by spraying at 34kW. All the coatings have the smallest bonding strengths by spraying at 42kW. Considering the existence of error bars, these changes are relatively subtle within the applied spraying powers range. However, lower spraying power (34kW and 38kW)
Journal Pre-proof tends to produce greater bonding strengths. According to the studies, higher spraying power not only brings higher flame temperature and better melting state, but also causes interface defects and oxidation of bond coat. Thus, the contact area between ceramic coat and bond coat is decreased, which could have a detrimental influence on bonding strength [40,41]. Meanwhile, with increasing Al2O3 content from 10wt% to 40wt%, the bonding strengths of composite coatings deposited at 42kW are increased from 14.4MPa to 17.8MPa. The same
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increasing tendency is also presented in composite coatings deposited at 34kW and 38kW.
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The bonding strength is dominated by the combining intensity between different spraying
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splats as well as the residual stress in the substrate-coating system [42]. The melting state of
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composite coatings is improved by adding more Al2O3, thus causing larger contact area
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between ceramic coat and bond coat and a better interface bonding. Furthermore, the residual
below.
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stress is also an important factor affecting the bonding strength, which will be discussed
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Thermal shock resistance is a key factor affecting the durability of thick TBCs. Fig. 10 shows the typical macro images after thermal shock cycles at 900oC. Note that the spallation appears at coating edge firstly and then propagates gradually during thermal shock cycles. When the spallation area is large enough, thermal shock failure of composite coatings will happen. Fig. 11 shows the thermal shock lifetimes of composite coatings at 900oC, 1000oC and 1100oC. Coating ZAC10 deposited at 42kW has the best thermal shock resistance at different test temperatures. The influence of spraying power on thermal shock lifetimes for coating ZAC10 is the most obvious. As the Al2O3 content beyond 10wt%, moderate spraying power (38kW) is more conductive to improve thermal shock resistance, while the effect of
Journal Pre-proof spraying power is also in decline. Correspondingly, deposition in higher spraying powers (38kW and 42kW) tends to cause the defects and cracks in ceramic coatings, which is useful for releasing the stress intensity during thermal shock cycles. Overall, the effect of spraying power on thermal shock resistance is not prominent [43,44]. In addition, the influence of Al2O3 content on thermal shock resistance is consistent at different spraying powers. As shown in Fig. 11(a), when thermal shock test at 900oC, the
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thermal shock lifetimes of coatings ZAC10 and ZAC20 deposited at 42kW are 379 and 308
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cycles, respectively. However, the thermal shock lifetimes of the coatings ZAC30 and ZAC40
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decrease sharply to 105 and 18 cycles, respectively. When thermal test at 1000oC (Fig. 11(b)),
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thermal shock lifetimes of coatings ZAC10, ZAC20 and ZAC30 deposited at 42kW decrease
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rapidly from 379 to 90 cycles, 308 to 62 cycles and 105 to 26 cycles, respectively. At the same time, the thermal shock lifetimes of coating ZAC40 decrease from 18 to 11 cycles.
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Nevertheless, when the test temperature arrives at 1100oC (Fig. 11(c)), the thermal shock
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lifetimes of coatings ZAC10 and ZAC20 deposited at 42kW decrease sharply to 18 and 12 cycles, while that of coatings ZAC30 and ZAC40 fall to 8 and 6 cycles, respectively. It is concluded that thermal shock resistance of composite coatings decreases obviously with increasing the Al2O3 content and test temperature [45]. Excessive Al2O3 (30wt% and 40wt%) has a detrimental influence on thermal shock resistance. Remarkably, increasing in bonding strength does not normally result in better thermal shock resistance. The durability of composite coatings is dominated by many factors, such as phase composition, microstructure and mechanical properties, while spraying power only has a fine-tuning effect on bonding strength and thermal shock resistance. The typical thermal
Journal Pre-proof shock failure analysis is performed on composite coatings deposited at 42kW as follow. 3.3. Thermal shock failure analysis of composite coatings Typical XRD patterns of composite coatings deposited at 42kW before and after thermal shock cycles are compared in Fig. 12. Note that no new phase is formed after thermal shock cycles at different test temperatures. However, as shown in the magnified view, the maximum intensity peak for t'-ZrO2 at about 2θ=30.24 becomes shaper and stronger with increasing test
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temperature, indicating the crystallinity of composite coatings is improved. The increasing of
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crystallinity with test temperature is related to the recrystallization of amorphous phase
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[3133]. The recrystallization occurs between 900oC and 1000oC accompanied with a volume
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contraction, which could have a detrimental effect on their thermal shock resistance [46,47].
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Fig. 13 shows the cross-section morphology of composite coatings deposited at 42kW after 900oC thermal shock cycles. Macroscopic spallation resulted from large horizontal crack
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propagation occurs at ceramic coat side, which is near and parallel to the bond coat/ceramic
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coat interface [48,49]. Many cracks and defects caused by thermal shock test randomly distributed far away from the rupture, which can release strain energy and toughen coating to some extent [50,51]. To identify the element distribution of failure section, Fig. 14 shows the corresponding elemental mapping of coating ZAC20. Note that the Al element is evenly distributed within ceramic coat and bond coat. No Al aggregation related to TGO is observed, indicating that no significant TGO layer is formed at the bond coat/ceramic coat interface during such short exposure cycles [52,53]. Introduction of Al2O3 could be conductive to slow down the dissipation of oxygen [17]. The stress intensity as well as strain energy could be the dominant mode of thermal shock failure [54,55].
Journal Pre-proof Generally, residual stresses in composite coatings consists of two factors after spraying process: quenching stress (σq) resulted from the contraction of individual splats and cooling stress (σc) derived from the CTE between the coatings and substrate as they cool together after deposition. The quenching stress and cooling stress can be calculated by Eqs (8) and (9), respectively [56]:
q E c
c T
m T s
(8)
Ec c s T0 Ts c
(9)
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of
Ec tc 1 2 Es t s
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Where Ec and Es are Young's modulus of the coatings and substrate; υc is Poisson's ratio of the
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coatings; αc and αs are CTE of the coatings and substrate; Tm, Ts and T0 are lamella melting
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temperature, deposition temperature and substrate temperature; tc and ts are the thickness of the coatings and substrate, respectively. Es, αs, Ts, T0, tc, ts are deemed to be constant at the
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same spraying power in this work. Residual stress of composite coating is related to Young's
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modulus (Ec) and CTE (αc) from the equations, which will further discuss in this paper. Fig. 15 shows the corresponding pictures of Young's modulus measurements. As shown in Fig. 15(a), the dimension of Nano-indentation is in a few microns along with the triangular indentation shape. The selected indentation area is relatively flat. Fig. 15(b) shows that the maximum load increases continuously at about the same indentation depth with increasing Al2O3 content. Note from Fig. 15(c) that the Young's modulus on the surface of coatings increases from 99.07GPa to 134.59GPa with increasing Al2O3 content from 10wt% to 40wt%. The variation is consistent with the other available work [12]. Accordingly, the Weibull plot about data scatter of Young's modulus is shown in Fig. 16. The Weibull modulus as shown in
Journal Pre-proof Table 3 obtained from the slope of fitting Weibull plot increases from 11.78 to 16.76 with increasing Al2O3 content from 10wt% to 40wt%, reflecting the improvement of homogeneity of Young's modulus. Fig. 17 shows that thermal expansion in the length direction (a) and CTE of the coatings (b) are obvious smaller than that of substrate from room temperature to 1100oC. As shown in Table 4, with increasing Al2O3 content from 10wt% to 40wt%, the average CTE decreases
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from 9.68×10-6/oC to 7.29×10-6/oC for composite coatings, which is close to the other study
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[18]. Remarkably, the obvious contract point in the thermal expansion and CTE curve of
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composite coatings indicates the existence of thermal contraction between 900oC and 1000oC,
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which could be caused by crystallization of amorphous phase [46,47].
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Fig. 18 shows the residual stresses on the surface of freestanding and corresponding supported coatings. With increasing Al2O3 content from 10wt% to 40wt%, residual stresses of
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freestanding coatings (mainly includes quenching stresses, the cooling stress is neglected)
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changes from tensile (+60.2MPa) to compressive (-42.1MPa). By contrast, residual stresses of supported coatings changes from +13.3MPa to -189.0MPa, which are the superposition of quenching stresses and cooling stresses. Thermal mismatch stress (cooling stress) can be determined based on the difference of residual stresses in corresponding freestanding and supported coatings [26,57]. In this study, the quenching stresses are considered to be identical in corresponding freestanding and supported coatings. Thus, the calculated cooling stresses increase from -46.9MPa to -146.9MPa as shown in Fig. 18. The quenching stresses normally consist of the thermal mismatch as well as elastic mismatch stresses between 8YSZ and Al2O3 splats [12,38]. Increasing Al2O3 content could form more composite matrix around the
Journal Pre-proof 8YSZ particles which causes greater quenching compressive stresses. Meanwhile, cooling compressive stress caused by thermal mismatch between the substrate and coatings increases obviously with increasing Al2O3 content and plays a decisive role in this system. Consequently, introduction of Al2O3 leads to the increase in Young's modulus and the decrease in CTE of composite coatings, thus bringing increasing surface residual compressive stresses. Meanwhile, thick coatings are easier to accumulate residual stresses as well as strain
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energy in the direction of ceramic thickness [2325]. The critical strain energy release rate
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(fracture toughness) of coating cross-section would decrease when the ceramic coat is
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extruded by excessive compressive stresses. Thus, composite coatings could have small
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fracture toughness when Al2O3 content is too much, especially in the bond coat/ceramic coat
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interface [58-60]. However, the bonding strength measured in this work is improved with increasing Al2O3 content, indicating the residual compressive stress may result in an increase
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in fracture toughness. This requires further verification in the future.
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In addition, thermal stress (σR) caused by thermal expansion mismatch between the coatings and substrate should be considered as the main factor for coating failure during thermal shock testing, which can be presented by the following equation [56]:
R
T Ec 1 c
(10)
Where Δα is the CTE difference between the substrate and coating; ΔT is the temperature difference; Ec and υc are the Young's modulus and Poisson's ratio of composite coatings; respectively. According to Eq (10), due to the difference of CTE and Young's modulus, composite coatings experience greater thermal stresses during thermal shock cycles with
Journal Pre-proof increasing the test temperature and Al2O3 content, thus resulting in a significant increase of thermal stress intensity as well as strain energy. Meanwhile, loose microstructure with more defects and pores is helpful for stress release [50,51]. The more content of fully melted splats and porosity could improve the ability of releasing thermal stress. Composite coatings become denser with increasing Al2O3 content, so it is not conductive for releasing the stress during thermal shock cycles.
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Overall, when the thermal stress intensity at the crack tip exceeds its fracture toughness,
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the formation of larger crack would occur in the coating [61]. On the one hand, the crack
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propagation within ceramic coat could not cause the spallation or delamination, but act as a
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barrier to release stress [40]. On the other hand, the cracks near the bond coat/ceramic coat
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interface would suffer from greater stresses intensity, thus resulting in drastic crack propagation and cross-linking to form many larger cracks and spallation. Therefore, the
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horizontal crack propagation caused by thermal stress in the interface area is the main cause
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of thermal shock failure [55,62,63]. Coatings ZAC30 and ZAC40 suffer from greater thermal stresses intensity than coatings ZAC10 and ZAC20 during thermal shock testing as the larger Young's modulus and CTE, thus also causing the decrease of their thermal shock resistance. The poor thermal shock resistance of all the coatings at 1100oC is because the thermal stress intensity is obvious larger than coating fracture toughness. It could be concluded that the thermal stress intensity during thermal shock testing and its fracture toughness play a decisive role in the thermal shock failure of composite coatings. 4. Conclusions In this paper, thermal shock resistance of thick 8YSZ-Al2O3 composite coatings
Journal Pre-proof decreased sharply with increasing the Al2O3 content and test temperature. The main conclusions are summarized as follows: (1)
Increasing Al2O3 content made the coating melt better and denser, resulting in a larger contact area between the coatings and substrate as well as a better bonding strength. However, it was detrimental for strain energy releasing.
(2)
Introduction of Al2O3 led to the increase in Young's modulus and the decrease in CTE
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of composite coatings, thus resulting in greater surface residual compressive stresses.
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A significant stress gradient would be generated in the direction of coating thickness,
Thermal stresses increased significantly with increasing the Al2O3 content and test
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(3)
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which may have a detrimental influence on thermal shock resistance.
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temperature. Based on the results, thermal shock failure of composite coatings was mainly dependent on the horizontal crack propagation caused by thermal stress
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intensity, intrinsic residual stress in the interface area and the volume contraction
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caused by amorphous phase recrystallization. Acknowledgments
This work was financially supported by the project supported by the Natural Science Foundation of Hunan Province (Grant No. 2019JJ40337). References [1]
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Table 1: Powder agglomeration parameters. Spraying drying parameter 50-52wt%
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8YSZ-Al2O3
0.5-0.8wt%
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Defoamer:
2.0-2.5wt%
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Binder: Arabic gum
Entry temperature (oC)
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Solid particles:
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Slurry parameter
Ammonium citrate tribasic
240-250
Chamber air temperature (oC)
160
Exit temperature (oC)
115-120
Feeding speed (g/min)
60-65
Rotational speed of nozzle (r/min)
18000
Atomizing air flow rate (m3/h)
25
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Table 2: Plasma spraying conditions for bond coat and ceramic coat.
Arc current (A)
Ceramic coat
550
550
68
76
69 62
46
36
H2 flow rate (slpm)
9.5
13 10.6 8
Powder feeding rate (g/min)
18
20
Spray distance (mm)
100
120
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Ar flow rate (slpm)
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Arc voltage (V)
Bond coat
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Parameter
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ZAC10
m
11.78
ZAC30
ZAC40
12.04
13.14
16.76
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ZAC20
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Table 3: Weibull modulus of Young's modulus of composite coatings.
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CTE(10-6/oC)
16.5
ZAC10
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ZAC20
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Ni3Al
9.68
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Table 4: Average CTE of the substrate and composite coatings.
8.77
ZAC30
ZAC40
8.07
7.29
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Figure Captions
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Fig. 1. Typical XRD patterns of (a) feedstock powders and (b) corresponding composite
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coatings deposited at 42kW.
(d) ZAP40.
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Fig. 2. Typical morphologies of composite powders: (a) ZAP10, (b) ZAP20, (c) ZAP30 and
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Fig. 3. The curve of the flowability and actual tap density of composite powders. Fig. 4. Typical surface morphologies of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 5. Typical cross-sectional morphologies of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 6. Typical cross-sectional morphologies with a high magnification of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30, (d) ZAC40 and the element content analysis of (e) point 1 and (f) point 2. Fig. 7. The curve of density and porosity of composite coatings deposited at 42kW.
Journal Pre-proof Fig. 8. Typical photographs of tensile failure section of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 9. Bonding strengths of composite coatings deposited at different spraying powers. Fig. 10. Typical photographs of composite coatings deposited at 42kW after 900oC thermal shock cycles: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 11. Thermal shock lifetime of composite coatings deposited at different spraying powers
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during thermal shock test at 900oC (a), 1000oC (b) and 1100oC (c).
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Fig. 12. Typical XRD patterns of composite coatings deposited at 42kW after thermal shock
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cycles at different temperatures: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40.
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Fig. 13. Cross-sectional micrographs of composite coatings deposited at 42kW after 900oC
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thermal shock cycles: (a) ZAC10, (b) ZAC20, (c) ZAC30, (d) ZAC40. Fig. 14. Typical cross-sectional failure micrograph of coating ZAC20 deposited at 42kW (a),
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and the corresponding elemental mapping.
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Fig. 15. (a) Typical indentation morphology, (b) typical load and unload curve of nano-indentation test and (c) Young's modulus on the surface of composite coatings. Fig. 16. Weibull plot of Young's modulus of composite coatings deposited at 42kW. Fig. 17. Variation of thermal expansion (a) and CTE (b) with increasing test temperature of the substrate and composite coatings deposited at 42kW. Fig. 18. Residual stress of freestanding coatings and the corresponding supported coatings deposited at 42kW.
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Journal Pre-proof Declaration of competing 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Author Contributions section Fazhang Lu: Investigation, Resources, Data Curation, Writing - Original Draft Wenzhi Huang: Conceptualization, Methodology, Formal analysis, Writing - Review & Editing, Visualization, Funding acquisition
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Haitao Liu: Conceptualization, Methodology, Supervision, Project administration
Journal Pre-proof Highlights 1. Thermal shock resistance decreased obviously with increasing test temperature and Al2O3 content. 2. The volume contraction caused by amorphous phase recrystallization would be bad for durability. 3. The addition of Al2O3 resulted in significant residual compressive stress and thermal
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stress.
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4. The horizontal cracks propagation caused by thermal stress intensity in the interface area
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was the main factor for coating failure.
Fazhang Lu, Wenzhi Huang, Haitao Liu PII:
S0257-8972(19)31280-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125290
Reference:
SCT 125290
To appear in:
Surface & Coatings Technology
Received date:
14 June 2019
Revised date:
10 December 2019
Accepted date:
23 December 2019
Please cite this article as: F. Lu, W. Huang and H. Liu, Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings, Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2019.125290
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© 2018 Published by Elsevier.
Journal Pre-proof Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings Fazhang Lu, Wenzhi Huang, Haitao Liu Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, 109 De Ya Rd, Changsha 410073, China *
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Corresponding authors: Email: [email protected] (W.Z. Huang),
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[email protected] (H.T. Liu).
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Tel No: +86 731 84573176, fax: +86 731 84576578 Abstract
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The effects of Al2O3 content and spraying power on bonding strength and thermal shock
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resistance of plasma-sprayed 8YSZ-Al2O3 composite coatings with thickness of ~900μm were investigated. Thermal shock resistance decreased obviously with increasing the Al2O3
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content and test temperature, while bonding strength showed an increasing trend. Relatively, the effect of spraying power on thermal shock resistance and bonding strength was small.
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Moreover, the Young's modulus of coating surface was increased from 99.07GPa to 134.59GPa with increasing Al2O3 content from 10wt% to 40wt%, while the corresponding thermal expansion coefficient (CTE) decreased from 9.68×10-6/oC to 7.29×10-6/oC. Thus, residual stresses on the surface of composite coatings changed from tensile (+13.3MPa) to compressive (-189.0MPa), which would cause greater stresses gradient as well as strain energy in the direction of coating thickness. During thermal shock testing, thermal stress increased sharply with increasing the test temperature and Al2O3 content. Meanwhile, introduction of Al2O3 resulted in denser microstructure, which was detrimental for stress releasing. According to the results, the horizontal crack propagation in the interface area caused by thermal stress and volume contraction caused by amorphous phase recrystallization were considered to be the main factors for thermal shock failure. Keywords: Thick 8YSZ-Al2O3 composite coatings; Thermal shock resistance; Phase
Journal Pre-proof evolution; Residual stress; Failure analysis 1. Introduction 8YSZ with low thermal conductivity and relatively high coefficient of thermal expansion (CTE) has been widely used as the material of thermal barrier coatings (TBCs). However, some shortcomings such as oxygen permeability, low fracture toughness (0.55 MPa·m1/2) and poor thermal corrosion resistance affect its comprehensive performance and
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further application [1,2]. Meanwhile, Al2O3 coatings are commonly used in anti-wear and
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anti-corrosion applications as high strength, low density and chemical inertness [3,4]. It is
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found that the properties of 8YSZ and Al2O3 are complementary to each other. The properties
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of 8YSZ may be well regulated by adding proper amount of Al2O3. Recently, plasma-sprayed
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8YSZ-Al2O3 composite coatings with thin thickness have attracted increasing attention of many researchers [517]. Some optimizations and explorations are gradually developed in
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the composite system. Tarasi et al. [59] demonstrate that the pseudo-eutectic of
460μm
deposited by
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(8YSZ-60wt%Al2O3) composite coating with thickness
solution-precursor plasma spraying (SPPS) exhibits better thermal shock resistance than that of 8YSZ at 1080oC. Based on these studies, amorphous phase is formed as the high quenching rate during SPPS process, which plays an important role in microstructure, mechanical properties and durability of composite coating. In addition, quenching with liquid nitrogen-cooled substrates during atmosphere plasma spraying (APS) process lead to the coexistence of a mixture of amorphous, metastable and stable phases in the pseudo-eutectic freestanding coatings with thickness of 600-700μm. After heat treatment at 1400oC, metastable-to-stable phase transition, crystallization of amorphous phase as well as the
Journal Pre-proof reduction of porosity would occur in the coatings, which promotes an increase in hardness and wear resistance [10,11]. It is discovered that Al2O3 addition and annealing treatment could regulate the porosity, Young's modulus and hardness of plasma-sprayed 8YSZ-Al2O3 composite coatings with thickness of 400μm [12]. Meanwhile, the fracture toughness increases from 1.93 MPa·m1/2 at 10wt% alumina content to a maximum of 2.31 MPa·m1/2 at 40wt% alumina and then decreases to 2.09 MPa·m1/2 at 55wt% alumina for the as-sprayed
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coatings. Furthermore, nanostructured 8YSZ-13wt%Al2O3 composite coating with thickness
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of 280-350μm is successfully deposited by APS. The addition of nano-Al2O3 could
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effectively inhibit grain growth of zirconia phase and reduce the interface stress between
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ceramic coat and thermal growth oxide layer (TGO). As a result, thermal cyclic lifetime of
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the coating (953 cycles) is higher than that of traditional 8YSZ coating (785 cycles) at 1100oC [1315]. Moreover, by comparing with 8YSZ coating, 8YSZ-40wt%Al2O3 composite
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coating with thickness of 500μm deposited by APS exhibits better resistance in thermal
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oxidation and corrosion test due to their denser structure [16,17]. According to these studies, introduction of Al2O3 can regulate phase composition, microstructure and mechanical properties of the 8YSZ-Al2O3 composite coatings, further enhancing their thermal stability, oxidation and corrosion resistance. However, as the thermal conductivity of Al2O3 (1400K, bulk, 5.8 w/(m·K)) is higher than 8YSZ (1273K, sprayed, 2.1 w/(m·K)), introduction of Al2O3 inevitably leads to the decrease in thermal insulation [18]. Under the same condition of coating composition, increasing the ceramic thickness to produce thick TBCs can be an approached way [19]. Note in the literature that the thermal insulation of traditional zirconia-based TBCs increases significantly with increasing coating
Journal Pre-proof thickness from 500μm to 1.8mm [20]. By the same token, the substrate temperature is decreased by 4~6oC for every 25.4μm increasing in ceramic thickness [21,22]. Therefore, thick 8YSZ-Al2O3 composite coating might be deposited to further improve its thermal insulation. However, increasing coating thickness normally results in significant stress gradients in the direction of ceramic thickness as well as poor thermal shock resistance [2325]. Few studies carried out on the investigation regarding thermal shock resistance of
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thick 8YSZ-Al2O3 composite coatings with thickness of ~900μm via APS process.
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In the work presented herein, 8YSZ-Al2O3 composite coatings with thickness of
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~900μm were deposited on the nickel based superalloy substrate by APS, and the influences
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of the test temperature and Al2O3 content on thermal shock resistance and coating failure was
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studied. Typical failure analysis of composite coatings was performed based on phase composition, microstructure and stress evolution before and after thermal chock cycles.
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2. Materials and methods
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2.1. Feedstock and sample preparation Commercial 8YSZ powders (SY133, Beijing Sangyao, China) with particle dimensions of 15-45μm and Al2O3 powders (SY111, Beijing Sangyao, China) with particle dimensions of 22-40μm were employed as feedstock. To obtain the composite powders with good flowability, the deionized water, arabic gum and ammonium citrate tribasic were added into the powders mixture of 8YSZ and Al2O3 to fabricate slurry. Subsequently, the water-based slurry was ball-milled for 72h at 360rpm rotation speed with zirconia balls to mix these oxide powders, and then they were spray-dried to produce spherical composite powders. 8YSZ-Al2O3 composite powders with Al2O3 content of 10wt%, 20wt%, 30wt% and 40wt%
Journal Pre-proof are referred to as powders ZAP10, ZAP20, ZAP30 and ZAP40. The slurry parameters and spray drying parameters are presented in Table 1. According to ISO 4490-2014 and ISO 3953-2011, the flowability and actual tap density of composite powders were obtained, respectively. Nickel based superalloy (Ni3Al) plates of 40×20×4mm3 and disks of Ø25×4mm3 grit blasted with alumina particles were used as the substrates. Bond coat and ceramic coat were
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deposited via APS process (APS-3000, Beijing Aeronautical Manufacturing Technology
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Research Institute, China). The thicknesses of bond coat and ceramic coat were designed to
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be ~75μm and ~900μm, respectively. Bond coat was deposited at 38kW by using commercial
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CoNiCrAlY powders (KF-309, Beijing). Ceramic coats were deposited at different spraying
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powers (34, 38 and 42kW) to optimize spraying parameters. The deposition conditions are presented in Table 2. Composite coatings deposited using powders ZAP10, ZAP20, ZAP30
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and ZAP40 are referred to as coatings ZAC10, ZAC20, ZAC30 and ZAC40. Freestanding
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coatings without bond coat were obtained by depositing composite powders onto the graphite substrates at 42kW and then removed coating from the substrate. 2.2. Characteristics of composite coatings Phase composition of feedstock powders and as-deposited coatings were identified by XRD (D8 ADVANCE, Bruker, Germany) measurements using Cu Kα radiation (λ=1.541Å) in a 2θ range of 20-80° at a scanning speed of 6°min-1. The X-ray source is operated at an accelerating voltage of 40kV and current of 40mA. Microstructural characterization of feedstock powders and as-deposited coatings were performed by scanning electron microscope (Quanta 200, FEI, USA) coupled with
Journal Pre-proof energy-dispersive X-ray spectroscopy (EDS). The cross-section was received by cold-mounted in epoxy resin, then sectioned with a low velocity diamond saw and polished with SiC paper and diamond paste. The 2D micro-porosity of as-deposited coatings was evaluated by using Image J analysis software. More than five pictures randomly taken from the polished cross-sections were averaged to evaluate the porosity. The density of as-deposited coating was measured and calculated by Archimedes' principle, and three
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measurements were performed to determine the average value for each freestanding coating.
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The standard load and unload method of Nano-indentation test are used to obtain the
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Young's modulus of composite coatings. The measurement was carried out on the surface of
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as-deposited coatings by the aid of Nano-indentation (G200, Agilent, USA) with a Berkovich
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indenter. A maximum depth limit of 1000 nm was required due to the porous and inhomogeneous nature of ceramic coatings. The surface approach velocity and holding time
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was set at 10nm/s and 10s, respectively [26,27]. According to the Oliver and Pharr methods,
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the reduced modulus can be expressed as: Er
S 2 hc
24.5
(1)
Where hc is the contact depth; the elastic contact stiffness S ( S
dp ) can be obtained from dh
the slope of the initial portion of the unloading curve; β closely relates to the indenter geometry (β=1.034 for a Berkovich indenter); the contact depth hc can be determined from the load-depth curve, it can be written as:
hc hmax
Pmax S
(2)
Where ε is the geometric constant associated with the indenter types (ε=0.75 for a Berkovich
Journal Pre-proof indenter); the peak load (Pmax), the maximum indentation depth (hmax) can be obtained from the load-depth curves. Therefore, Young's modulus can be calculated by Eq (3):
1 1 2 1 i2 Er E Ei
(3)
Where υ and υi are Poisson's ratio of the ceramic coat and the indenter, respectively; the elastic constants of the diamond indenter are Ei=1141GPa and υi=0.07; Poisson's ratio (υ) of
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as-deposited coatings is calculated based on rule of mixtures (ROM). In addition, due to the
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dimension of indentation is small, many voids and defects of as-deposited coatings may lead to more diffuse measurement results. So the Weibull statistical analysis was used to dispose
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the results [12]. The Weibull distribution function is given as follow: (4)
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1 ln ln m ln E m ln E0 1 w
Where E is the measured value of Young's modulus; m and E0 are the Weibull modulus and
i 0.5 , where i is the i-th rank and n is the total number of data points. There is a linear n
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w
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characteristic value, respectively. The measured value is sorted in ascending order and letting
1 relationship between ln ln and ln E , m and E0 can be easily obtained from the 1 w
fitting Weibull plot. The data scatter of measured value can be reflected by the Weibull modulus. The CTE of substrate and freestanding coatings with dimension of 25×5×2mm3 was measured in air from 27oC to 1100oC using a dilatometer (DIL 402 C, NETZSCH, Germany) at a heating rate of 5oC/min. Fractional change in length, ΔL/L as a function of temperature was measured and the CTE was measured from the slope of the curve.
Journal Pre-proof Residual stresses on the surface of freestanding and supported coatings was measured by X-ray stress analyzer (LXRD, Proto, Canada) based on Bragg's law [28,29]:
n 2d sin
(5)
Where n is the order of reflection; d is the crystal plane spacing; and θ is Bragg's diffraction angle. The existence of residual stress leads to the difference of the crystal plane spacing between the coating and feedstock powder. The strain εψ in the direction of incident angle ψ
d d d0 cot 0 0 d d0
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associated with the diffraction peak displacement:
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can be obtained by the relative change of the diffraction crystal plane spacing, which is
(6)
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Where θ0 is Bragg’s angle at the diffraction peak in the feedstock powder (assuming no
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residual stress); θψ is Bragg’s angle with residual stress. The residual stresses on the surface
E 2 cot 0 2 1 180 sin 2
(7)
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r
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of as-deposited coatings at the corresponding ψ direction can be calculated by Eq (7):
Where E approximatively takes the measured value above; υ is also calculated based on ROM. There is a linear relationship between 2θψ and sin2ψ, the residual stresses on the surface of as-deposited coatings can be obtained by measuring the displacement of diffraction lines under different directions. In the present work, diffraction measurements using Cr Kα radiation (λ=2.291Å) were taken at five ψ values (ψ=-30o, -15o, 0o, 15o and 30o). The reflection (213) of non-transformable tetragonal phase (t′-ZrO2) at about 2θ=151.8° was used as the diffraction plane for lattice plane spacing measurement in residual stress analysis. 2.3. Properties test
Journal Pre-proof The bonding strength of the supported coatings with dimensions of Ø25×4mm3 was examined using a pull-off tester (Electronic Universal Materials Tester, CSC-1101, China) at a stretching rate of 1mm/min. Commercially available adhesive (FM-1000) was used to bond the specimens. To reduce the influence of random errors, the bonding strength was taken as the average value of three measurements. Thermal shock tests were conducted in an automated bottom-drop furnace (RSQ06,
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Changsha, China) at different temperatures (900, 1000 and 1100oC), and thermal shock cycles
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were used to characterize the thermal shock resistance of as-deposited coatings. The
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supported coatings with dimensions of 40×20×4mm3 were hanged on the center of a
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heat-resistant brick which was installed on a vertical elevator. When the elevator was lowered,
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the specimens were cooled by air. In each cycle, the specimens were held isothermally for 5 min and then taken out to cool down in air for 5 min. Thermal shock tests were repeated until
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at least 5% of the coating surface area was lost, and the number of cycles to failure obtained
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by averaging three measurements was defined as thermal shock lifetime. 3. Results and discussion
3.1. Phase and microstructure analysis Typical XRD patterns of feedstock powders and composite coatings are compared in Fig. 1. The diffraction peaks at about 2θ=30.17o, 35.01o, 50.22o, 59.78o, 62.71o and 74.12o are indexed to the (101), (110), (112), (211), (202) and (220) reflections of t′-ZrO2, while the diffraction peaks located at about 2θ=28.3o and 31.7o are related to the (-111) and (111) reflections of monoclinic phase (m-ZrO2). The diffraction peaks of Al2O3 powder at about 2θ=25.71o, 43.46o and 57.55o are related to the (012), (113) and (116) reflections of
Journal Pre-proof rhombohedral phase (α-Al2O3). By comparison, the relative diffraction peak intensity of α-Al2O3 increases gradually with increasing Al2O3 content, while that of t′-ZrO2 is decreasing. Thus, the phase composition of composite powders is t′-ZrO2, m-ZrO2 and α-Al2O3. Fig. 2 shows that as-synthesized powders are spherical particles with diameter of ~50μm after spray drying. In addition, as compared in Fig. 3, with increasing Al2O3 content from 10wt% to 40wt%, the flowability and actual tap density of composite powders decreases from
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43.3s/50g to 47.4s/50g and from 1.73g/cm3 to 1.61g/cm3, respectively. Thus, composite
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powders with uniform dimensions, proper density and good flowability are suitable for APS
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[30].
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After coating deposition, phase composition of composite coatings can be confirmed in
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Fig. 1(b). The diffraction peaks at about 2θ=30.24o, 35.21o, 50.46o, 60.03o, 62.88o and 74.26o are related to t'-ZrO2. Weak diffraction peaks at about 2θ=25.71o and 43.46o related to
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α-Al2O3 are only presented in coatings ZAC30 and ZAC40. The single weak peak about
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2θ=45.87o related to the (400) reflection of cubic phase (γ-Al2O3) suggested that small amounts of γ-Al2O3 is possible existed in composite coatings [31]. Furthermore, as shown in the magnified view in Fig. 1(b), the bulge peak of t'-ZrO2 at about 2θ=30.24 widens and decreases gradually with increasing Al2O3 content, implying the existence of amorphous phase [59]. Part of the melting Al2O3 and 8YSZ splats have no time to recrystallize as such high quenching rate (106K/s) during APS process, thus resulting in the formation of amorphous phase and bits of γ-Al2O3. No other phase in composite coatings is formed indicating the interaction between Al2O3 and 8YSZ is not obvious [3133]. Typical surface and cross-sectional morphologies of composite coatings are compared in
Journal Pre-proof Fig. 4 and Fig. 5, respectively. As shown in Fig. 4, many micro-sized pores and splats in different melting states are presented on the rough surface of coatings. The content of fully melted splats increases gradually with increasing Al2O3 content. Al2O3 is easier to fully melt during APS because of the melting point of Al2O3 (2040oC) is lower than that of 8YSZ (2700oC). The fully melted Al2O3 splats with higher thermal conductivity are beneficial to the uniform dissipation of heat energy. In addition, it could help to maintain the 8YSZ in the
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molted state for a longer time period due to the specific heat capacity of Al2O3 is higher than
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8YSZ [34,35]. According to the cross-sectional morphologies (Fig. 5), the thicknesses of
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bond coat and ceramic coat are obtained to be 70-80μm and 870-880μm, respectively. The
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fine and larger pores are randomly distributed in the cross-section of coatings. Consider that
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the formation of fine pores could be ascribed to the voids within spray-dried powders, whereas the large ones are resulted from the imperfect overlapping due to the high-energy
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impacts from incoming splats [36,37]. In addition, high magnification cross-sectional
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morphologies and point element content analysis are displayed in Fig. 6 to further confirm the microstructure and element distribution of composite coatings. Some micro-sized white regions in back-scattering images as marked in Fig. 6(b-d) become more obvious with increasing Al2O3 content. EDS is used to distinguish the element content in white and the other regions. Fig. 6(e) and (f) exhibit the element content of point 1 and point 2 in Fig. 6(d), respectively. Observe that element of point 1 in white regions mainly consists of zirconium (Zr), while aluminium (Al) content of point 2 increases obviously. Thus, the white regions may correspond to un-melted or semi-melted 8YSZ splats, while the rest gray area is the homogeneous composite matrix of 8YSZ and Al2O3 splats. According to the study, Al2O3
Journal Pre-proof splats are segregated as grains between 8YSZ grains and as particles within 8YSZ grain boundaries in the matrix [12,38]. However, many un-melted and semi-melted 8YSZ splats are surrounded by fully melted 8YSZ-Al2O3 matrix. The influence of Al2O3 content on the density and porosity of composite coatings are compared in Fig. 7. Note that the density goes down from 4.93g/cm3 to 4.20g/cm3 and the porosity decreases from 20.32% to 12.38% with increasing Al2O3 content from 10wt% to
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40wt%, respectively. The decrease in the density of coatings results from the density of Al2O3
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(4.0g/cm3) is much lower than that of 8YSZ (6.1g/cm3). Increasing Al2O3 content brings
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better melting state and more content of fully melted splats. The fully melted splats could fill
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the voids and defects and decrease the porosity of composite coatings [12,39].
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3.2. Influences on bonding strength and thermal shock resistance of composite coatings The bonding strength is an important macro-mechanical property for TBCs. Fig. 8 shows
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that typical tensile failure occurs at the bond coat/ceramic coat interface, indicating the
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interface area is the most unstable of composite coatings [40]. Fig. 9 shows the bonding strengths of composite coatings deposited at different spraying powers. The bonding strengths of coating ZAC10 deposited at 34kW, 38kW and 42kW are measured to be 14.9MPa, 15.7MPa and 14.4MPa, respectively. Relatively, coatings ZAC10 and ZAC20 deposited at 38kW have the largest bonding strength of 15.7MPa and 17.1MPa respectively, while the largest bonding strength of 19.1MPa and 19.7MPa for coatings ZAC30 and ZAC40 can be obtained by spraying at 34kW. All the coatings have the smallest bonding strengths by spraying at 42kW. Considering the existence of error bars, these changes are relatively subtle within the applied spraying powers range. However, lower spraying power (34kW and 38kW)
Journal Pre-proof tends to produce greater bonding strengths. According to the studies, higher spraying power not only brings higher flame temperature and better melting state, but also causes interface defects and oxidation of bond coat. Thus, the contact area between ceramic coat and bond coat is decreased, which could have a detrimental influence on bonding strength [40,41]. Meanwhile, with increasing Al2O3 content from 10wt% to 40wt%, the bonding strengths of composite coatings deposited at 42kW are increased from 14.4MPa to 17.8MPa. The same
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increasing tendency is also presented in composite coatings deposited at 34kW and 38kW.
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The bonding strength is dominated by the combining intensity between different spraying
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splats as well as the residual stress in the substrate-coating system [42]. The melting state of
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composite coatings is improved by adding more Al2O3, thus causing larger contact area
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between ceramic coat and bond coat and a better interface bonding. Furthermore, the residual
below.
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stress is also an important factor affecting the bonding strength, which will be discussed
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Thermal shock resistance is a key factor affecting the durability of thick TBCs. Fig. 10 shows the typical macro images after thermal shock cycles at 900oC. Note that the spallation appears at coating edge firstly and then propagates gradually during thermal shock cycles. When the spallation area is large enough, thermal shock failure of composite coatings will happen. Fig. 11 shows the thermal shock lifetimes of composite coatings at 900oC, 1000oC and 1100oC. Coating ZAC10 deposited at 42kW has the best thermal shock resistance at different test temperatures. The influence of spraying power on thermal shock lifetimes for coating ZAC10 is the most obvious. As the Al2O3 content beyond 10wt%, moderate spraying power (38kW) is more conductive to improve thermal shock resistance, while the effect of
Journal Pre-proof spraying power is also in decline. Correspondingly, deposition in higher spraying powers (38kW and 42kW) tends to cause the defects and cracks in ceramic coatings, which is useful for releasing the stress intensity during thermal shock cycles. Overall, the effect of spraying power on thermal shock resistance is not prominent [43,44]. In addition, the influence of Al2O3 content on thermal shock resistance is consistent at different spraying powers. As shown in Fig. 11(a), when thermal shock test at 900oC, the
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thermal shock lifetimes of coatings ZAC10 and ZAC20 deposited at 42kW are 379 and 308
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cycles, respectively. However, the thermal shock lifetimes of the coatings ZAC30 and ZAC40
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decrease sharply to 105 and 18 cycles, respectively. When thermal test at 1000oC (Fig. 11(b)),
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thermal shock lifetimes of coatings ZAC10, ZAC20 and ZAC30 deposited at 42kW decrease
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rapidly from 379 to 90 cycles, 308 to 62 cycles and 105 to 26 cycles, respectively. At the same time, the thermal shock lifetimes of coating ZAC40 decrease from 18 to 11 cycles.
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Nevertheless, when the test temperature arrives at 1100oC (Fig. 11(c)), the thermal shock
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lifetimes of coatings ZAC10 and ZAC20 deposited at 42kW decrease sharply to 18 and 12 cycles, while that of coatings ZAC30 and ZAC40 fall to 8 and 6 cycles, respectively. It is concluded that thermal shock resistance of composite coatings decreases obviously with increasing the Al2O3 content and test temperature [45]. Excessive Al2O3 (30wt% and 40wt%) has a detrimental influence on thermal shock resistance. Remarkably, increasing in bonding strength does not normally result in better thermal shock resistance. The durability of composite coatings is dominated by many factors, such as phase composition, microstructure and mechanical properties, while spraying power only has a fine-tuning effect on bonding strength and thermal shock resistance. The typical thermal
Journal Pre-proof shock failure analysis is performed on composite coatings deposited at 42kW as follow. 3.3. Thermal shock failure analysis of composite coatings Typical XRD patterns of composite coatings deposited at 42kW before and after thermal shock cycles are compared in Fig. 12. Note that no new phase is formed after thermal shock cycles at different test temperatures. However, as shown in the magnified view, the maximum intensity peak for t'-ZrO2 at about 2θ=30.24 becomes shaper and stronger with increasing test
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temperature, indicating the crystallinity of composite coatings is improved. The increasing of
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crystallinity with test temperature is related to the recrystallization of amorphous phase
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[3133]. The recrystallization occurs between 900oC and 1000oC accompanied with a volume
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contraction, which could have a detrimental effect on their thermal shock resistance [46,47].
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Fig. 13 shows the cross-section morphology of composite coatings deposited at 42kW after 900oC thermal shock cycles. Macroscopic spallation resulted from large horizontal crack
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propagation occurs at ceramic coat side, which is near and parallel to the bond coat/ceramic
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coat interface [48,49]. Many cracks and defects caused by thermal shock test randomly distributed far away from the rupture, which can release strain energy and toughen coating to some extent [50,51]. To identify the element distribution of failure section, Fig. 14 shows the corresponding elemental mapping of coating ZAC20. Note that the Al element is evenly distributed within ceramic coat and bond coat. No Al aggregation related to TGO is observed, indicating that no significant TGO layer is formed at the bond coat/ceramic coat interface during such short exposure cycles [52,53]. Introduction of Al2O3 could be conductive to slow down the dissipation of oxygen [17]. The stress intensity as well as strain energy could be the dominant mode of thermal shock failure [54,55].
Journal Pre-proof Generally, residual stresses in composite coatings consists of two factors after spraying process: quenching stress (σq) resulted from the contraction of individual splats and cooling stress (σc) derived from the CTE between the coatings and substrate as they cool together after deposition. The quenching stress and cooling stress can be calculated by Eqs (8) and (9), respectively [56]:
q E c
c T
m T s
(8)
Ec c s T0 Ts c
(9)
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of
Ec tc 1 2 Es t s
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Where Ec and Es are Young's modulus of the coatings and substrate; υc is Poisson's ratio of the
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coatings; αc and αs are CTE of the coatings and substrate; Tm, Ts and T0 are lamella melting
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temperature, deposition temperature and substrate temperature; tc and ts are the thickness of the coatings and substrate, respectively. Es, αs, Ts, T0, tc, ts are deemed to be constant at the
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same spraying power in this work. Residual stress of composite coating is related to Young's
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modulus (Ec) and CTE (αc) from the equations, which will further discuss in this paper. Fig. 15 shows the corresponding pictures of Young's modulus measurements. As shown in Fig. 15(a), the dimension of Nano-indentation is in a few microns along with the triangular indentation shape. The selected indentation area is relatively flat. Fig. 15(b) shows that the maximum load increases continuously at about the same indentation depth with increasing Al2O3 content. Note from Fig. 15(c) that the Young's modulus on the surface of coatings increases from 99.07GPa to 134.59GPa with increasing Al2O3 content from 10wt% to 40wt%. The variation is consistent with the other available work [12]. Accordingly, the Weibull plot about data scatter of Young's modulus is shown in Fig. 16. The Weibull modulus as shown in
Journal Pre-proof Table 3 obtained from the slope of fitting Weibull plot increases from 11.78 to 16.76 with increasing Al2O3 content from 10wt% to 40wt%, reflecting the improvement of homogeneity of Young's modulus. Fig. 17 shows that thermal expansion in the length direction (a) and CTE of the coatings (b) are obvious smaller than that of substrate from room temperature to 1100oC. As shown in Table 4, with increasing Al2O3 content from 10wt% to 40wt%, the average CTE decreases
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from 9.68×10-6/oC to 7.29×10-6/oC for composite coatings, which is close to the other study
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[18]. Remarkably, the obvious contract point in the thermal expansion and CTE curve of
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composite coatings indicates the existence of thermal contraction between 900oC and 1000oC,
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which could be caused by crystallization of amorphous phase [46,47].
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Fig. 18 shows the residual stresses on the surface of freestanding and corresponding supported coatings. With increasing Al2O3 content from 10wt% to 40wt%, residual stresses of
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freestanding coatings (mainly includes quenching stresses, the cooling stress is neglected)
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changes from tensile (+60.2MPa) to compressive (-42.1MPa). By contrast, residual stresses of supported coatings changes from +13.3MPa to -189.0MPa, which are the superposition of quenching stresses and cooling stresses. Thermal mismatch stress (cooling stress) can be determined based on the difference of residual stresses in corresponding freestanding and supported coatings [26,57]. In this study, the quenching stresses are considered to be identical in corresponding freestanding and supported coatings. Thus, the calculated cooling stresses increase from -46.9MPa to -146.9MPa as shown in Fig. 18. The quenching stresses normally consist of the thermal mismatch as well as elastic mismatch stresses between 8YSZ and Al2O3 splats [12,38]. Increasing Al2O3 content could form more composite matrix around the
Journal Pre-proof 8YSZ particles which causes greater quenching compressive stresses. Meanwhile, cooling compressive stress caused by thermal mismatch between the substrate and coatings increases obviously with increasing Al2O3 content and plays a decisive role in this system. Consequently, introduction of Al2O3 leads to the increase in Young's modulus and the decrease in CTE of composite coatings, thus bringing increasing surface residual compressive stresses. Meanwhile, thick coatings are easier to accumulate residual stresses as well as strain
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energy in the direction of ceramic thickness [2325]. The critical strain energy release rate
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(fracture toughness) of coating cross-section would decrease when the ceramic coat is
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extruded by excessive compressive stresses. Thus, composite coatings could have small
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fracture toughness when Al2O3 content is too much, especially in the bond coat/ceramic coat
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interface [58-60]. However, the bonding strength measured in this work is improved with increasing Al2O3 content, indicating the residual compressive stress may result in an increase
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in fracture toughness. This requires further verification in the future.
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In addition, thermal stress (σR) caused by thermal expansion mismatch between the coatings and substrate should be considered as the main factor for coating failure during thermal shock testing, which can be presented by the following equation [56]:
R
T Ec 1 c
(10)
Where Δα is the CTE difference between the substrate and coating; ΔT is the temperature difference; Ec and υc are the Young's modulus and Poisson's ratio of composite coatings; respectively. According to Eq (10), due to the difference of CTE and Young's modulus, composite coatings experience greater thermal stresses during thermal shock cycles with
Journal Pre-proof increasing the test temperature and Al2O3 content, thus resulting in a significant increase of thermal stress intensity as well as strain energy. Meanwhile, loose microstructure with more defects and pores is helpful for stress release [50,51]. The more content of fully melted splats and porosity could improve the ability of releasing thermal stress. Composite coatings become denser with increasing Al2O3 content, so it is not conductive for releasing the stress during thermal shock cycles.
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Overall, when the thermal stress intensity at the crack tip exceeds its fracture toughness,
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the formation of larger crack would occur in the coating [61]. On the one hand, the crack
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propagation within ceramic coat could not cause the spallation or delamination, but act as a
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barrier to release stress [40]. On the other hand, the cracks near the bond coat/ceramic coat
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interface would suffer from greater stresses intensity, thus resulting in drastic crack propagation and cross-linking to form many larger cracks and spallation. Therefore, the
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horizontal crack propagation caused by thermal stress in the interface area is the main cause
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of thermal shock failure [55,62,63]. Coatings ZAC30 and ZAC40 suffer from greater thermal stresses intensity than coatings ZAC10 and ZAC20 during thermal shock testing as the larger Young's modulus and CTE, thus also causing the decrease of their thermal shock resistance. The poor thermal shock resistance of all the coatings at 1100oC is because the thermal stress intensity is obvious larger than coating fracture toughness. It could be concluded that the thermal stress intensity during thermal shock testing and its fracture toughness play a decisive role in the thermal shock failure of composite coatings. 4. Conclusions In this paper, thermal shock resistance of thick 8YSZ-Al2O3 composite coatings
Journal Pre-proof decreased sharply with increasing the Al2O3 content and test temperature. The main conclusions are summarized as follows: (1)
Increasing Al2O3 content made the coating melt better and denser, resulting in a larger contact area between the coatings and substrate as well as a better bonding strength. However, it was detrimental for strain energy releasing.
(2)
Introduction of Al2O3 led to the increase in Young's modulus and the decrease in CTE
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of composite coatings, thus resulting in greater surface residual compressive stresses.
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A significant stress gradient would be generated in the direction of coating thickness,
Thermal stresses increased significantly with increasing the Al2O3 content and test
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(3)
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which may have a detrimental influence on thermal shock resistance.
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temperature. Based on the results, thermal shock failure of composite coatings was mainly dependent on the horizontal crack propagation caused by thermal stress
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intensity, intrinsic residual stress in the interface area and the volume contraction
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caused by amorphous phase recrystallization. Acknowledgments
This work was financially supported by the project supported by the Natural Science Foundation of Hunan Province (Grant No. 2019JJ40337). References [1]
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Table 1: Powder agglomeration parameters. Spraying drying parameter 50-52wt%
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8YSZ-Al2O3
0.5-0.8wt%
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Defoamer:
2.0-2.5wt%
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Binder: Arabic gum
Entry temperature (oC)
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Solid particles:
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Slurry parameter
Ammonium citrate tribasic
240-250
Chamber air temperature (oC)
160
Exit temperature (oC)
115-120
Feeding speed (g/min)
60-65
Rotational speed of nozzle (r/min)
18000
Atomizing air flow rate (m3/h)
25
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Table 2: Plasma spraying conditions for bond coat and ceramic coat.
Arc current (A)
Ceramic coat
550
550
68
76
69 62
46
36
H2 flow rate (slpm)
9.5
13 10.6 8
Powder feeding rate (g/min)
18
20
Spray distance (mm)
100
120
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Ar flow rate (slpm)
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Arc voltage (V)
Bond coat
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Parameter
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ZAC10
m
11.78
ZAC30
ZAC40
12.04
13.14
16.76
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ZAC20
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Table 3: Weibull modulus of Young's modulus of composite coatings.
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CTE(10-6/oC)
16.5
ZAC10
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ZAC20
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Ni3Al
9.68
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Table 4: Average CTE of the substrate and composite coatings.
8.77
ZAC30
ZAC40
8.07
7.29
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Figure Captions
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Fig. 1. Typical XRD patterns of (a) feedstock powders and (b) corresponding composite
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coatings deposited at 42kW.
(d) ZAP40.
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Fig. 2. Typical morphologies of composite powders: (a) ZAP10, (b) ZAP20, (c) ZAP30 and
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Fig. 3. The curve of the flowability and actual tap density of composite powders. Fig. 4. Typical surface morphologies of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 5. Typical cross-sectional morphologies of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 6. Typical cross-sectional morphologies with a high magnification of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30, (d) ZAC40 and the element content analysis of (e) point 1 and (f) point 2. Fig. 7. The curve of density and porosity of composite coatings deposited at 42kW.
Journal Pre-proof Fig. 8. Typical photographs of tensile failure section of composite coatings deposited at 42kW: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 9. Bonding strengths of composite coatings deposited at different spraying powers. Fig. 10. Typical photographs of composite coatings deposited at 42kW after 900oC thermal shock cycles: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40. Fig. 11. Thermal shock lifetime of composite coatings deposited at different spraying powers
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during thermal shock test at 900oC (a), 1000oC (b) and 1100oC (c).
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Fig. 12. Typical XRD patterns of composite coatings deposited at 42kW after thermal shock
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cycles at different temperatures: (a) ZAC10, (b) ZAC20, (c) ZAC30 and (d) ZAC40.
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Fig. 13. Cross-sectional micrographs of composite coatings deposited at 42kW after 900oC
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thermal shock cycles: (a) ZAC10, (b) ZAC20, (c) ZAC30, (d) ZAC40. Fig. 14. Typical cross-sectional failure micrograph of coating ZAC20 deposited at 42kW (a),
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and the corresponding elemental mapping.
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Fig. 15. (a) Typical indentation morphology, (b) typical load and unload curve of nano-indentation test and (c) Young's modulus on the surface of composite coatings. Fig. 16. Weibull plot of Young's modulus of composite coatings deposited at 42kW. Fig. 17. Variation of thermal expansion (a) and CTE (b) with increasing test temperature of the substrate and composite coatings deposited at 42kW. Fig. 18. Residual stress of freestanding coatings and the corresponding supported coatings deposited at 42kW.
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Journal Pre-proof Declaration of competing 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Author Contributions section Fazhang Lu: Investigation, Resources, Data Curation, Writing - Original Draft Wenzhi Huang: Conceptualization, Methodology, Formal analysis, Writing - Review & Editing, Visualization, Funding acquisition
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Haitao Liu: Conceptualization, Methodology, Supervision, Project administration
Journal Pre-proof Highlights 1. Thermal shock resistance decreased obviously with increasing test temperature and Al2O3 content. 2. The volume contraction caused by amorphous phase recrystallization would be bad for durability. 3. The addition of Al2O3 resulted in significant residual compressive stress and thermal
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stress.
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4. The horizontal cracks propagation caused by thermal stress intensity in the interface area
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was the main factor for coating failure.