The effect of sol-gel Al2O3 interlayer on oxidation behaviour of TBC system

Surface & Coatings Technology 350 (2018) 469–479

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The effect of sol-gel Al2O3 interlayer on oxidation behaviour of TBC system ⁎

T

H. Abdeldaim , N. El Mahallawy Design and Production Engineering, Faculty of Engineering, Ain Shams University, Abasia, Cairo, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermal barrier coating Al2O3 intermediate layer Sol-gel Thermally grown oxide layer Residual stresses Triple layer model

Surface engineering plays an essential role in enhancing high pressure turbines performance, which work under severe thermal and mechanical conditions. Thermal Barrier Coating (TBC) system is economically demanded for efficient, environmental friendly and extended gas turbine life. The recent study intends to extend TBC life, which exposed to cyclic loading conditions. An intermediate α-Alumina layer was deposited by sol-gel on APSCoNiCrAlY bond layer surface, which was deposited on Ni-superalloy substrate. APS-YSZ was deposited as ceramic top coat on the top of alumina layer. The thermal behavior of the modified TBC system was compared with the standard one after 40, 80 and 160 thermal cycles at 1150 °C using optical microscopy, Scanning Electron Microscopy (SEM) equipped by Energy Dispersive X-ray analysis (EDS), X-ray Diffraction (XRD) and Raman spectroscopy. The effect of the intermediate layer on the residual stresses generated in ceramic layer during operation was studied. The oxidation kinetics were evaluated by measuring cracks lengths within ceramic layer and measuring Thermally Grown Oxide (TGO) layer thickness. Experimental evidence was gathered showing that the presence of the interlayer Al2O3 layer has potential to reduce cracks lengths and TGO thickness by suppressing the formation of detrimental oxides. The Al2O3 layer acts as a barrier for oxygen diffusion. This effect improves the oxidation resistance of metallic CoNiCrAlY Bond, hence increases TBC lifetime.

1. Introduction Thermal barrier coating (TBC) is extensively used as protective, corrosion and heat resistant coating on hot sectors parts of power plants and aircraft turbines. [1]. The commercial TBC system consists of a Ni and/or Co-based superalloy as substrate, MCrAlY, where M = Co and/ or Ni, metallic bond coat and Yttria Stabilized or partially stabilized Zirconia (YSZ) ceramic top coat [2, 3]. The bond coat provides hot corrosion and oxidation resistance to substrate, beside enhancing top coat/substrate adhesion, while the ceramic top coat, usually YSZ-Y2O3 mass content 7–8%, provides thermal insulation, due to its low thermal conductivity [4]. At elevated temperature service, oxygen breaks through the interconnected pores within the ceramic top coat, causing bond coat oxidation. Inevitably, Thermally Grown Oxide (TGO) layer is formed at the top coat/bond coat interface [5]. MCrAlY bond coat composition is tailored to develop α-Al2O3 scales, which acts as a barrier for oxygen diffusion [6]. α-Al2O3 has low growth rate and provides a good adhesion between the top and bond layers. α-Al2O3 possesses, also, a low oxygen diffusivity because it has a closed packed hexagonal structure of oxygen anions with two kinds or octahedral gaps filled with Al cations, which protects the bond coat against further oxidation [7]. Hence, a

⁎

sufficient Aluminum reservoir should be guaranteed to form and grow a thermodynamically protective and stable TGO layer. The optimum Al content to guarantee α-Al2O3 formation varies between 4 and 18 wt% of bond coat composition [8]. After Al depletion of bond coat, fast growing Ni–Co–Cr-rich TGO oxides would start to form, like Cr2O3, NiO, and (Ni, Co) (Cr, Al)2O4 (Spinel phases) [9, 10]. It is known that, the TGO has a key role in TBC spallation failure. The failure may occur due to many reasons; i) volumetric expansion accompanied with TGO growth, which causes high compressive residual stresses within top coat [11]; ii) high thermal stresses due to the mismatch in coefficient of thermal expansion between the adjacent layers during thermal cycling service [9]. When the internal stresses exceed the critical crack propagation stress level of top coat, cracking, spallation and failure of TBC occurs [12]. Hence, controlling the TGO growth rate is a key to enhance the durability of TBC system and prolong its lifetime. Recently, wide research is focused on improving oxidation behavior of the bond coat and controlling the formation of TGO using different production methods like, applying aluminizing treatment on bond coat surface [13, 14], pre-oxidation process of bond coat before top coat deposition [15], bond coat surface shot peening [16] and microwave top coat surface densification [17]. Q. Zhang [18] used High Velocity

Corresponding author. E-mail address: [email protected] (H. Abdeldaim).

https://doi.org/10.1016/j.surfcoat.2018.07.035 Received 27 November 2017; Received in revised form 14 June 2018; Accepted 10 July 2018 Available online 11 July 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 350 (2018) 469–479

H. Abdeldaim, N. El Mahallawy

Oxygen Fuel (HVOF) technique to produce bond coat with high density and low porosity to get better resistance against oxidation. G. Pulci [19] examined the use of rare earth alloying elements like Re, Hf and Ta to improve the oxidation and corrosion resistance without dropping the creep resistance of the bond coat. In last few years, a novel technique was introduced to modify the oxidation resistance of MCrAlY by depositing a thin Alumina interlayer between the bond coat and the top coat before top coat deposition [20–22]. The deposited alumina acts as oxygen barrier to limit TGO growth and avoid Aluminum depletion within the bond coat, thus potentially enhance TBC system durability. Several researchers [14, 15] used Al2O3 as a protective layer using several techniques of coating. Heating was used in most of the introduced techniques to develop the alumina layer. These techniques produce coatings with initial oxide layer, depending on the Al content within bond coat, which accelerates its depletion [9]. After the depletion of Al in bond coat, other non-protective scales will start to form [10]. Moreover, the pre-formed phases of the TGO are considered a critical factor influencing the life of TBC system. The transformation from the metastable γ and θ-Alumina phases to the stable α-Alumina phase has a profound effect on TGO/top coat interface integrity upon thermal cycles. This phase transformation adds additional residual stresses due to the associated volumetric change [11]. A suggestion of a pre-formation of a uniform α-Alumina scale before the deposition of topcoat could improve the durability of TBC systems; hence, to produce intermediate coats with high oxidation resistance, an alternative coating technique should be considered. Sol-Gel, on the other hand, is a material deposition process whereby a coating is developed by dipping the substrate within a pre-prepared solution to form the required coating layer by chemical reaction. Owing to the controlled deposition atmosphere, coatings are usually produced with less substrate oxidation. In the present paper, thermal cycling oxidation behavior of Air Plasma Sprayed (APS)-TBC protected with α-Alumina sol-gel intermediate protective layer on APS-MCrAlY bond coat was evaluated and compared with the conventional unprotected TBC system. The effect of the alumina interlayer was studied in terms of microstructure, TGO growth, and crack propagation in the TBC system before and after 40, 80 and 120 thermal cycles at 1150 °C using optical light microscopy, Digital microscopy, Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray spectroscopy (EDS), X-Ray Diffraction analysis (XRD) and Raman spectroscopy. The stress conducted due to thermal cycling within ceramic top coat was evaluated using Raman spectroscopy.

Table 2 Plasma spray deposition parameters for bond coat and top coat layers. Parameters

APS CoNiCrAlY bond coat

APS YSZ top coat

Arc current (A) Electrical power (kW) Argon flow rate (slpm) Hydrogen flow rate (slpm) Powder feed (g/min) Spraying distance (mm)

600 40 65 14 30 140

630 40 44 13 25 90

Aero engines, Munich, Germany- was used as substrate. The chemical composition at.% of as-received substrate is shown in Table 1. The CoNiCrAlY bond coat layer was sprayed from commercial feedstock powder Co-32Ni-21Cr-8Al-0.5Y wt% with particles nominal size 20–53 μm (catalog number: 60.95, GTV GmbH, Luckenbach, Germany) using APS on the substrate with aimed thickness 150 μm. The ZrO2–8 wt % Y2O3 powder (catalog number 40.23.1, GTV GmbH, Luckenbach, Germany) with nominal powder size of 30-85 μm was used to deposit the ceramic top coat using APS with aimed thickness 200 μm. The thicknesses of top and bond coats were selected to minimize residual stresses and shrinkage forces due to deposition process [11, 12]. The plasma spray deposition parameters for both bond coat and top coat are shown in Table 2. The applied APS parameters were standard parameters optimized to gain low porosity coatings from the applied feedstock materials and were provided by the plasma torch supplier. Both bond coat and top coat were deposited using F6 Air plasma torch (TU Chemnitz, Germany)

2.1.2. Modified TBC system In modified TBC, α-Alumina intermediate layer was deposited using sol-gel technique on bond coat raw surface prior to top coat deposition as described by Chengbin Jing [23] with target thickness < 1 μm. For preparation of the sol gel, 50 ml of deionized water was heated in glass beaker to 80 °C using magnetic stirrer with hot plate, then, 5 ml Aluminum sec-butoxide (Product Nr. A13044, Alfa Aesar GmbH, Karlsruhe, Germany) was added drop by drop to the water with continuous stirring. It was observed that the temperature of the mixture rose to 90 °C due to the occurred hydrolysis process. After that, 15 ml of Ethyl acetoacetate, EAcAc +99% C6H10O3 (Product Nr. A12544, Alfa Aesar GmbH, Karlsruhe, Germany) was added to the solution with continuous stirring. After 2 min, 3 ml of nitric acid was added to the solution. Meanwhile, the solution became transparent. The pH measured using MP512 precision pH meter and was found to be about 3. Then the solution is ready for dipping process. Alumina film was deposited onto the bond coat via dip-coating method. The substrate was immersed into the sol reservoir and soaked for 5 min while the solution is stirred. Then, the substrate was withdrawn from the solution using self-prepared withdrawal machine with withdrawal rate 50 mm/min. The coated samples were dried for two days at room temperature to form the alumina gel film. After that, the samples were sintered at 1100 °C for 4 h in vacuum chamber (working pressure ≈ 0.005 mbar) with heating and cooling rates ± 0.5 °C/min. The sintering process was done to ensure complete transformation of the alumina layer into Alpha phase. After that, the ceramic top coat was sprayed on the top of the alumina layer with the same conditions as in commercial system (Set 1). Table 3 shows the different conditions for the two sets of prepared samples.

2. Experimental work 2.1. Specimen preparation In recent study, two different sets of samples were prepared. The first set was the standard commercial reference TBC system which comprises of Inconel 600 substrate, CoNiCrAlY bond coat and YSZ top coat. The second set was the modified TBC system with the protective alumina layer which comprises of Inconel 600, CoNiCrAlY, Sol-gel protective alumina and YSZ. Three samples were prepared for each condition. 2.1.1. Commercial TBC system Samples 30 × 10 mm with 5 mm thickness of Inconel 600(DIN2.4816, UNS N06600) Ni-based superalloy -supplied form MTU

Table 1 The chemical composition at.% of the as-received Ni-based superalloy substrate Inconel 600 (DIN 2.4816, UNS N06600). Element

C

Si

Mn

Cu

S

Cr

Fe

Ni

P

Al

Ti

Chemical composition at.%

0.05–0.10

> 0.5

> 1.0

> 0.5

> 0.015

14–17

6–10

< 72

> 0.02

> 0.3

> 0.3

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Table 3 Different conditions of the prepared samples. Set group

Sample num.

Superalloy substrate

Bond coat

Al2O3 overlay layer conditions

Top coat

Set 1

1–0

Inconel 600

APS CoNiCrAlY

No overlay layer

Set 2

2–1 2–2

Inconel 600

APS CoNiCrAlY

1 dip 2 dips

APS 8% YSZ APS 8% YSZ

2.2. Testing and characterization To simulate high temperature exposure inside a turbine, samples were exposed to thermal cycling oxidation test by rapid heating of the samples to 1150 ± 10 °C in programmable front-loading furnace (Arnold Schroder N 11/H) followed by a dwell time for 30 min at 1150 °C under atmospheric pressure in static air, then quenching using forced air for 2 min to 150–200 °C. Heating and cooling rates were ± 400 °C/min. Thermal cycling parameters were selected to obtain failure within TGO and surrounded area [21]. After that, samples cross sections were prepared for as-sprayed and oxidized samples, using standard metallographic techniques. The specimens were studied after 40, 80 and 160 cycles. The samples were examined before and after thermal cycling using Olympus GX51optical light microscope, JXA-8100, JEOL Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) and Panalytical XPert PRO PW 3040/60 X-ray Diffraction (XRD). Raman spectra were collected from YSZ layer using He: Ne laser source with wave length 633 nm using Renishaw Raman spectroscopy. At each sample, forty spectra were collected along 40 μm interface and averaged. The cracks lengths generated within top coat and TGO layer thickness were measured from SEM images using Fijiimage J image analyzer. Thirty measurements were made randomly for each coating along the entire length of the oxidized samples.

Fig. 1. SEM microstructure cross section of as-sprayed a) Commercial TBC system showing cracks and pores within ceramic topcoat and oxides within bond coat, b) modified TBC system showing the deposited Alumina layer (pointed by the arrows) which generated after 1 dip.

3. Results and discussion 3.1. Microstructural characterization

0.31 ± 0.0186 μm after two dips. The roughness of the alumina layer after sintering Ra was 4.4 μm. It is obvious that there is no significant effect of the presence of intermediate layer on bond coat surface roughness, due to the small thickness of the deposited alumina layer.

3.1.1. As sprayed samples The microstructure of the as-deposited set 1 and set 2 specimens are shown in Fig. 1. The ceramic YSZ layer of the commercial TBC system has a lamellar structure and full of micro-cracks and porosities, which is the typical feature of APS technique as shown in Fig. 1a. The bond coat contains oxides and scales, due to the heat associated with APS deposition technique. The thicknesses of top and bond layers were found about 200 ± 20 μm and 150 ± 10 μm, respectively. The top surface of the bond coat has high irregularity, rough and uneven surface due to the splashing of liquid droplets and partially-molten particles during conventional APS deposition. The roughness Ra of the bond coat surface measured using TR100 surface roughness tester was 4.6 μm. The adhesion between top coat and bond coat depends mainly on the mechanical interlocking due to interface roughness. No observable formed TGO was detected at the interface between bond coat and bond coat for as-sprayed samples. For the as-sprayed modified TBC samples, a nearly continuous thin alumina layer was observed at the interface between top coat and bond coat layers as shown in Fig. 1b. The morphological features of top coat and bond coat are the same as in set 1 samples. The deposited Alumina layer exhibits a good contact and mechanical interlocking with both zirconia and CoNiCrAlY layers without any evidence of damage due to top coat deposition. The average thickness of the developed alumina layer (Teq) was 0.23 ± 0.0138 μm after one dip and

3.1.2. Thermal cycled samples The micrographs collected in Fig. 2 show cross-sectional microstructure evolution of commercial and modified TBC samples after 40, 80 and 160 thermal cycles. After 40 cycles, the standard TBC microstructure, Fig. 2a, indicates formation of a thin oxide layer at top/bond coat interface with micro voids at area around TGO. On the other hand, for the modified TBC system microstructure, no discontinuity or recognizable interface could be evidenced between TGO and deposited Alumina as shown in Fig. 2b. The alumina layer is still perfectly adherent to both top and coat layers and it is indistinguishable from the TGO. After 80 thermal cycles, several gray mixed oxide phases are observed within the TGO, in commercial TBC samples, Fig. 2c. The EDS on set 1 samples confirmed that the oxide layer contains excessive amounts of Cr, Co, Ni and Y. While, The TGO layer in the modified TBC system, as shown in Fig. 2d, was found more uniform and compact compared with the standard system. No spinal clusters were found. The EDS on set 2 samples found that the oxide layer contains only Al and O. Moreover, no discontinuity or recognizable interface could be evidenced between generated TGO and deposited Alumina layer in modified TBC system, 471

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Fig. 2. SEM cross section of samples after thermal cycling test for a) standard TBC after 40 cycles b) modified TBC after 40 cycles c) Standard TBC after 80 cycles with EDS analysis d) modified TBC after 80 cycles with EDS analysis e) Standard TBC after 160 cycles and f) modified TBC after 160 cycles.

3.2. Phase constituents

After 120 thermal cycles, mixed oxides have extensively formed within commercial TBC samples, as shown in Fig. 2e, in addition to an evident spallation of ceramic top coat and fragmentation of bond coat. The TGO layer in the conventional TBC system became obviously dispersive, and some voids appeared in the bond coat along the interface. Horizontal cracks parallel to the interface were observed in the ceramic top coat, while, the modified TBC system still retains the original compactness between the layers as shown in Fig. 2f with less oxide layer thickness.

3.2.1. As sprayed samples XRD analysis was applied to investigate the formed phases of the asdeposited layers. The top surface of the ceramic top coat of the two prepared samples sets was examined just after deposition. Fig. 3a confirms formation of tetragonal phase of YSZ with no detected monoclinic brittle phase prior thermal cycling. For the second group of samples, XRD analysis was applied on the top surface of the deposited alumina layer after sintering before top coat deposition. to detect the formed alumina layer phase. The analysis, Fig. 3b, confirmed that the protective alumina layer is totally transformed into α-Alumina after sintering, in addition to, some traces of bond coat phases were also

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Fig. 3. XRD patterns for a) YSZ ceramic top coat after deposition, b) Deposited-alumina layer over the bond coat using Sol-gel coating after sintering at 1100 °C prior top coat deposition.

of mixed oxides during thermal cycling. The major change between commercial and modified TBC concerning bond coat microstructure is the phase transformation. The commercial system BC phase transformed into γ’-Ni3Al solid solution with associated β-NiAl dissolution. This is due to Al depletion of the bond coat. Hence, the alumina interlayer has a significant effect in slowing aluminum consumption from the bond coat.

detected due to the low thickness of the alumina layer. 3.2.2. Thermal cycled samples After thermal cycling test, XRD analysis was applied to detect the phases formed after 120 thermal oxidation cycles. The analysis was done on top surface of TGO after removing the ceramic top coat. The analysis on oxidized commercial TBC, Fig. 4a, detected different mixed oxide phases like, spinel phases (Ni, Co) (Cr, Al)2O4, rhombohedral chromia Cr2O3, few traces of detrimental alumina phases, as well γ’Ni3Al phase of bond coat base material. No peaks of α-Alumina were detected. The formation of these clusters was extensively reported and investigated in the literature [24, 25]. These mixed oxides are normally considered a cause of TBC weakness and spallation. The formation of mixed oxides owes to the high porosity of top coat, which allows oxygen permeability during thermal cycling. Presence of oxygen atoms at the interface with concentration higher than the critical concentration needed for formation of bond coat metal non-detrimental oxides, allows non-selective oxidation happening and (Cr,Al)2O3 + (Co,Ni) (Cr,Al)2O4 + NiO will be formed at interface. XRD pattern for the modified TBC system is shown in Fig. 4b. The analysis detected intense peaks of α-Alumina, in addition to, β-bond coat base material. No spinel phases were detected. That confirms that deposited alumina layer acts as barrier for oxygen diffusion from top coat to bond coat. The concentration of the oxygen at interface became lower, which permits formation of α-Alumina scales and delays formation of other detrimental oxides and spinels. The alumina layer provided protection for TBC, because of its ability to suppress formation

3.3. Crack length, TGO thickness and oxidation kinetics model To study the cracks generated within ceramic layer and investigate the relation between cracks lengths and TGO thickness, thirty micrographs were taken from each sample. Fiji image-J was used to measure cracks lengths for micrographs at 100×. Fig. 5 shows an example of crack length and TGO thickness measuring method. Fig. 6 shows average and maximum horizontal crack length generated within ceramic layer for both sets of samples. It is obvious that the cracks propagate faster in commercial TBC samples than in modified TBC samples. The maximum horizontal crack length decreased by 35–40% and the average horizontal crack length decreased by about 20% with presence of sol-gel alumina protective layer. The minimum obtained crack length corresponds to an alumina layer with 0.23 μm thickness. Average TGO thickness (δeq) was measured after cycling using Fiji image-J. Thirty micrographs were taken from each sample at 400×. The entire oxide layer after cycling is considered TGO due to increase in thickness compared to deposited alumina layer and difficulty in differentiating between generated oxide layer, especially alumina scales,

Fig. 4. XRD patterns for a) Generated TGO layer for commercial TBC system after 160 thermal cycles b) generated TGO layer for modified TBC system after 160 thermal cycles. 473

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Fig. 6. a: Maximum Crack lengths generated within ceramic top coat after 160 thermal cycles for both groups of samples, b: average cracks lengths generated within ceramic top coat after thermal cycling for both groups of samples.

Fig. 5. Example of using image-J for photo analysis after 160 thermal cycles to determine a) the crack length within the ceramic top coat. B) The generated TGO thickness at the interface between top coat and bond coat.

amax = k δneq

Fig. 8 shows the relation between maximum crack length and average TGO thickness (δeq) of both sets samples, which illustrates that crack length increases with increasing TGO thickness, regardless of microstructure of TGO. The value of growth exponent (n) is determined to be ≈0.74 when growth constant (k) ≈ 150 for commercial TBC. On the other side, the growth exponent (n) is determined to be ≈1.4 when growth constant (k) ≈ 43 for modified TBC. The growth exponent of 0.74 for the first set indicates that the diffusion happened of an element within the scale itself, while a growth exponent of 1.4 for the second set indicates that growth is dictated by oxygen diffusion through the scale layer and the oxidation is in its earlier stages [6, 7, 9]. The dense αalumina scale serves as an effective cation diffusion barrier, thereby preventing the diffusion and subsequent oxidation of other elements during service. The information generated from this relation can be used to establish a model which permits a prediction of system behavior under operational conditions and minimizing incidents of TBC premature failure.

and the deposited alumina layer. The TGO equivalent thickness (δeq) was calculated (Eq. (1)) by dividing the TGO area by the TGO length. TGO area was measured, as shown in Fig. 5b, by drawing a polygon along TGO. TGO length was measured by drawing a line passing throw the center of the TGO layer. This method was used due to heterogeneous TGO growth and the rough top coat/bond coat interface. Fig. 7 shows the TGO average thickness after 160 thermal cycles. It is obvious that the intermediate layer influences TGO thickness. Presence of the alumina layer decreased thickness of TGO by 15%–20% compared to reference TBC conditions. The minimum obtained TGO thickness, also, corresponds to alumina layer with 0.23 μm thick.

δeq =

∑ (cross sectional TGO area) ∑ (cross sectional length TC/BC interface)

(2)

(1)

Experimental data evidenced that the Al2O3 layer enhanced CoNiCrAlY coatings resistivity and produced a lower TGO thickness compared to standard system. The statistical results indicated that the initial Al2O3 scale effectively decreased the oxidation rate of TBC system from 0.077 ± 0.0094 μm h−1 for the standard coating to 0.061 ± 0.007 μm h−1 for the modified coating. Regression analysis confirmed by several researchers [16, 22] indicates that there is relationship between mass gain or change oxide scales thickness (δeq) and the maximum crack length in the TBC, which expressed by the power law shown in Eq. (2), where k is growth constant and n is growth exponent. The growth exponent (n) provides information about the oxide growth mechanism since the overall rate of reaction is dictated by slowest reaction.

3.4. Elemental mapping and stress evolution Raman spectroscopy was used to generate cross-sectional elemental distribution mapping of samples prior and after thermal cycling. The maps of samples prior to thermal cycling are collected in Fig. 9. For the commercial TBC, Fig. 9a, Monoclinic YSZ is distinguished for ceramic top coat, while Al2O3 is randomly distributed within bond coat starting from the interface with top coat. Little amounts of chromia and spinel phases are detected at interface. For the modified TBC, Fig. 9b, YSZ is also distinguished for ceramic top coat, while Al2O3 is randomly 474

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Fig. 7. Average TGO thickness for commercial and modified TBC systems after 160 thermal cycles.

Fig. 8. Relation between TGO average thickness and maximum crack length.

of ZrO2 [26]. It is known that the Raman peaks shift of the toward higher wave number means an increase in compressive in-plane residual stresses and, in inverse order, the shift toward lower wave number means increase in tensile in-plane residual stress. The peak shift is attributed to an increase in vibrational amplitude of the atoms under stresses, whereas the peak sharpening is attributed to decrease in oxygen vacancies [27]. The peak around 640 cm−1 is usually selected for the stress analysis of ZrO2. Fig. 12 shows the Raman spectra obtained for ceramic YSZ TC in the two sets of samples after thermal cycling. The residual stresses generated within the coating layers is studied from the shift of spectra peaks (ΔΩ), where ΔΩ = ωs − ωo. ωo is the peak position of the reference state and ωs the peak position of the studied stressed state. Teixeira [28] obtained a linear relationship between residual stresses and Raman peak shift in the APS-ZrO2-8Y2O3 coatings with PSC equal to 220 cm−1/MPa, which means each cm−1 spectra peak shift corresponds to about 220 MPa residual in-plane stress [28]. The shift of Raman spectrum peaks (ΔΩ) of the peak at 640 cm−1 and the generated residual stresses after thermal cycling corresponding to Teixeira relation are shown in Table 4. The residual stresses are compressive for both commercial and modified TBC. For commercial TBC, the value of the residual stresses reached −1018.6 MPa after 160 thermal cycles. The value for the residual stresses dropped to

distributed within bond coat with a continuous layer at interface. Little amounts of chromia and spinel phases are also detected at the interface. The cross-sectional Raman mapping of samples after 160 cycles are collected in Fig. 10. For the commercial TBC, Fig. 10a, there is no substantial changes for ceramic monoclinic YSZ. No alumina phases are detected at interface which means aluminum depletion of bond coat around the interface. Massive amounts of chromia and spinel phases are detected at interface. For the modified TBC, as shown in Fig. 10b, no substantial changes are observed for ceramic YSZ. A thin Al2O3 layer is detected at interface. Below this layer, there is an aluminum depleted zone, underneath, Al2O3, is randomly distributed through bond layer. Less amounts of chromia and spinel phases are detected between Al2O3 layer and top coat. The failure of the TBC system is always defined by spallation of ceramic top coat, hence studying the generated residual stresses within the ceramic layer is a vital process to model the oxidation behavior and predict the failure of the system. Raman spectroscopy spectrums are used to evaluate residual stresses generated within ceramic top based on the measurement of the Raman Piezo-Spectroscopic Coefficient (PSC) coat after cycling assuming that top coat layer is under a plane stress state due to the low thickness of TBC layers. Fig. 11 shows typical Raman spectrum for non-stressed YSZ in region 100 cm−1 to 800 cm−1 with the six well identified peaks for monoclinic and tetragonal phases 475

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Fig. 9. Cross sectional Raman Spectroscopy mapping of as-sprayed TBC for YSZ, Al2O3, Cr2O3 and Spinel phases for a) commercial standard TBC system, b) modified TBC system.

−565.4 MPa in modified TBC. It is obvious that the growth of stresses in commercial TBC is higher than in modified TBC. The compressive residual stress arises in the coating during thermal cycling owing to mismatch of coefficient of thermal expansion between YSZ and bond coat, high-temperature creep, interface oxide and coating sintering. The generated mixed oxides grow rapidly and thus lead to high local volumetric expansion and causes high level of local stresses within coating layers upon heating and cooling. Stress relieving could lead to spallation at interface. The formation of uniform and compact TGO layer in modified TBC was helpful to mitigate residual stress in the YSZ coat by

about 45%.

3.5. Thermal stresses mathematical model By implementing cartesian coordinates, the in-plane thermal in the triple layer TBC system could be expressed by Hook's law (Eq. (3)), where σx, σy are the generated stresses in two perpendicular axes, E is young's modules, ν is Poisson's ratio, α is Coefficient of Thermal Expansion (CTE), ΔT is temperature change. 476

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Fig. 10. Cross sectional Raman Spectroscopy mapping of TBC after thermal cycling for YSZ, Al2O3, Cr2O3 and Spinel phases for a) commercial standard TBC system, b) modified TBC system.

εx =

1 (σx − νσy ) + α∆T E

εcr = Kσ mt

By substituting Eq. (3) into Eqs. (4) and (5) and solving for ε, the following equation (Eq. (6)) is obtained.

Due to the symmetry of the TBC sample, stress in X-direction σx and stress in Y-direction σy are equal, so that, by taking into consideration the thermal stresses and creep loads simultaneous, the stress of each layer could be expressed as shown in Eq. (4), where εcr is the creep strain. The creep strain of each layer can be calculated using Eq. (5), where K and m are material dependent constant factors. The properties used for the TBC system at 1150 °C are shown in Table 5.

σ=

E [ε − α∆T + εcr ] 1−ν

(5)

(3)

(

)

E

⎡∑i = p, q, b, s 1 −iν αi Ki ⎤ ∆T − i ⎦ ε= ⎣ Ei ∑i = p, q, b, s 1 − ν Ki

(

i

)

Ei ε 1 − νi cr i

(6)

By assuming quasi-static condition, ε could be defined by neglecting the creep effect. Hence the stress could be defined as shown in Eq. (7).

(4) 477

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Fig. 11. Typical Raman Spectrum for reference YSZ Ceramic TC with denotation of each peak [25]. Table 5 Properties of TBC layers materials at 1150° [3]. Property

Top coat

TGO

Bond coat

Substrate

α (×10−6/°C) E (GPa) Poisson's ratio Creep factor(K) Creep exponent(m)

10.1 56 0.11 2.25e-12 1

9.3 325 0.24 2.2e-12 1

17.3 120 0.31 7.3e-9 2.45

17 135 0.33 1.8e-8 3

The stresses in the ceramic top coat are calculated according to Eq. (7). The maximum stress for commercial TBC has reached 836.6 MPa, while for modified TBC has reached 698.6 MPa. The difference between the modelled and the practical values of stresses may be due to the neglected effects while constructing the model like, creep stress, ceramic top layer sintering effect and crack stress relieving. 4. Conclusions The present work investigated the influence of α-alumina protective intermediate layer deposited by sol-gel on the behavior of TBC system under thermal cycling test. The system comprised of APS-YSZ 8% ceramic top coat and APS-CoNi32Cr21Al8Y0.5 bond coat. The major findings on the oxidation behavior, TGO growth and crack nucleation and growth can be summarized as follows:

Fig. 12. Raman Spectrum for YSZ Ceramic TC after thermal Cycling for standard TBC Set (1) and modified TBC set (2).

Table 4 Raman peak shift and residual stresses in YSZ ceramic TC after 160 thermal cycles. Sample group

Sample number

Peak shift (cm−1)

State of stresses (MPa)

1 2

1–0 2–2

−4.63 −2.89

−1018.6 −635.8

σ=

E [ε − α∆T ] 1−ν

1) Alumina layer deposited by sol-gel resulted in an intermediate protective layer with a good adhesion to ceramic top coat and metallic bond coat due to the mechanical interlocking. 2) The alumina intermediate layer had a significant effect on the TBC system thermal behavior by suppressing generation of detrimental mixed oxides and formation of protective alumina layer compared to the reference system leading to an improved TBC durability. 3) The maximum horizontal crack length decreased by 35–40% with presence of sol-gel alumina protective layer and the average

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Surface & Coatings Technology 350 (2018) 469–479

H. Abdeldaim, N. El Mahallawy

horizontal crack length decreased by about 20%. 4) Presence of the alumina layer decreased thickness of TGO by 15%–20% compared to reference TBC conditions. 5) The results indicated that the initial Al2O3 scale effectively decreased the oxidation rate of TBC system from 0.077 ± 0.0094 μm h−1 for the standard coating to 0.061 ± 0.007 μm h−1 for the modified coating. 6) A power law relationship between the maximum crack length in the TBC and TGO thickness likely exists, which may be useful for lifetime prediction for APS-TBCs 7) Presence of the alumina layer decreased generated residual stresses after thermal cycling by 45% compared to reference TBC conditions.

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Acknowledgment

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The present project is supported by the Egyptian Science and Technology Development Fund (STDF) within a GESP project with ID 5319. The authors would like to acknowledge the institute of TUChemnitz for characterization and thermal cycling test. Special acknowledgments are due to Eng. I. E. Ali, Dr. T. Grund, and Prof. T. Lampke.

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