Isothermal oxidation behavior and kinetics of thermal barrier coatings produced by cold gas dynamic spray technique

Accepted Manuscript Isothermal oxidation behavior and kinetics of thermal barrier coatings produced by cold gas dynamic spray technique

Abdullah Cahit Karaoglanli, Ahmet Turk, Ismail Ozdemir PII: DOI: Reference:

S0257-8972(16)31309-3 doi: 10.1016/j.surfcoat.2016.12.021 SCT 21878

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

29 March 2016 1 December 2016 5 December 2016

Please cite this article as: Abdullah Cahit Karaoglanli, Ahmet Turk, Ismail Ozdemir , Isothermal oxidation behavior and kinetics of thermal barrier coatings produced by cold gas dynamic spray technique. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2016), doi: 10.1016/ j.surfcoat.2016.12.021

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Isothermal Oxidation Behavior and Kinetics of Thermal Barrier Coatings Produced by Cold Gas Dynamic Spray Technique Abdullah Cahit Karaoglanli 1

1,*

2

, Ahmet Turk , Ismail Ozdemir

3

Departments of Metallurgical and Materials Engineering, Faculty of Engineering, Bartin University, 74100, Bartin, Turkey, [email protected], [email protected] Departments of Materials Engineering, Faculty of Engineering, Celal Bayar University, 45140,

PT

2

Manisa, Turkey, [email protected], [email protected]

Departments of Mechanical Engineering, Faculty of Engineering, Izmir Katip Celebi University,

RI

3

SC

35620, Izmir, Turkey, [email protected]

Abstract:

NU

The cold gas dynamic spray (CGDS) method was employed to deposit the CoNiCrAlY bond coats of thermal barrier coating (TBC) system. The oxidation behavior of the coatings were

MA

investigated under isothermal oxidation at 1000 °C, 1100 °C and 1200 °C for 8, 24, 50 and 100 h. Recent studies on TBCs have concentrated on the CGDS process and its properties under working conditions. The motivation of this study is to investigate the oxidation

D

behavior of TBCs produced using CGDS technique under service conditions and to determine

PT E

the oxidation growth kinetics of thermally grown oxide (TGO). The results show that the isothermal degradation of coatings was considerably influenced by microstructure of coating,

CE

interfacial oxide growth rate, oxidation temperature and time.

Keywords: Cold gas dynamic spraying (CGDS); Thermal barrier coatings (TBCs);

AC

Oxidation; Thermally grown oxide (TGO); Oxidation rate 1. Introduction

Gas turbine engines are used to convert burning products to kinetic energy to obtain propulsion in the aerospace industry as a power generation facility [1]. Thermal Barrier Coatings (TBCs) are usually preferred to reduce the heat transfer and decrease the thermal loads of gas turbine components such as turbine blades, vanes and combustors [2-4]. The structure of state-of-the art TBCs which are used for gas turbines includes 4 layers. The first layer is nickel-based super alloy substrate to take advantage of high creep strength, superior mechanical and chemical properties. The second layer or bond coat is MCrAlY alloy which

ACCEPTED MANUSCRIPT provides better adhesion between substrate and top coat as well as oxidation and corrosion resistance. MCrAlY (M= Co, Ni or CoNi/NiCo) alloys formed protective alumina layer are generally used as bond coat layer [3-4]. The third layer is thermally grown oxide (TGO), however, this layer occurs inherently during the coating process and grows with oxidation. The last and the most important part which is a ceramic top coating for thermal insulation [46].

PT

In TBCs, generally used bond coatings are diffusion aluminides or MCrAlY coatings. When they compared with each other, MCrAlY alloys have more resistant to oxidation and

RI

corrosion under service conditions [7-8]. Especially, CoNiCrAlY have more optimum

SC

features against both oxidation and hot corrosion than NiCrAlY, NiCoCrAlY or CoCrAlY coatings. YSZ is a state of art material used for ceramic TBCs, due to its low thermal

NU

conductivity, high coefficient of thermal expansion (CTE) and thermal stability in high temperature. YSZ is widely used due to its fracture toughness and CTE properties [9-10].

MA

TBC systems are affected negatively by high temperature oxidation and hot corrosion failure mechanisms. As a result of these mechanisms, some formations such as thermal expansion

D

mismatch, excessive growth of TGO, bond coat rumpling, Al depletion of metallic bond coat, delamination and spallation of ceramic top coat layer were observed on these TBC systems [6,

PT E

11-13]. Understanding of failure mechanism in the TBCs is a key factor for increasing the coating durability and reliability [14].

CE

TBCs are commonly applied onto the gas turbine components such as blades and vanes to produce ceramic top coatings for thermal insulation using techniques such asAtmospheric

AC

Plasma Spray (APS), Electron Beam-Physical Vapour Deposition (EB-PVD), Suspension Plasma Spraying (SPS) and/or Solution Precursor Plasma Spraying (SPPS) [15-16]. Performances of APS are much faster and cheaper compared to EB-PVD, whereas coating life is relatively shorter [17-18]. In the production of bond layer composing of MCrAlY alloy in TBCs, thermal spray methods such as Atmospheric Plasma Spray (APS) and High Velocity Oxy-Fuel (HVOF) spraying are used [19-20]. Metallic bond coatings produced by APS method have high oxide content due to the atmospheric conditions during deposition. On the other hand, in HVOF technique, oxidation occurs at lower rates mainly on powder particles, which results from free oxygen on combustion gas [21]. Due to high oxygen affinity of especially aluminum and yttria elements in bond coatings, the produced coatings have high

ACCEPTED MANUSCRIPT oxide content as a result of rapidly developing oxidation process during the thermal spray coating process [22]. Another application, developed as an alternative to thermal spray coating methods and commonly used in recent years, is the application of Cold Gas Dynamic Spray (CGDS) process for bond coatings [23-24]. In this method, coating is formed through plastic deformation upon impact of spray particles, temperature of which is much lower than the melting point. Besides its convenience for production of coating with distinct type and

PT

structure, the method comes into prominence due to its superior properties such as keeping original powder composition and microstructure properties and obtaining oxide-free, dense

SC

RI

coating structure [25-27].

There is limited number of studies in the literature in which the microstructural analyses as

NU

well as TGO formation and growth behaviours of TBCs with CGDS sprayed bond coat are thoroughly investigated following the isothermal oxidation tests conducted using different temperatures and oxidation periods. Therefore, indepth evaluation and investigation of the

MA

microstructural changes, TGO formation and growth kinetics, and interfacial oxide growth rate of the TBC system deposited on Inconel 718 substrate with CGDS method, was

PT E

D

performed in the present study.

2. Experimental

CE

2.1. Materials and methods

AC

Nickel-based superalloy Inconel 718 plates were used as substrate material. The diameter and thickness of the substrates were respectively 25 mm and 4 mm. Commercial CoNiCrAlY bond coat (Sulzer-Metco USA, Amdry 9951, 5-37 μm) and ZrO2–8 wt.% Y2O3 top coat (GTV Germany, -45+20 μm) powders were deposited on grit-blasted superalloy substrates. The chemical composition of CoNiCrAlY powder is shown in Table 1. Plasma Giken CGDS system and a GTV F6 APS system were used as spraying systems. The thickness of the ceramic top coat was 300 μm, and that of the bond coats was 100 μm. All spraying parameters are shown in the Table 2. Oxidation tests of the TBC samples were conducted in a high temperature furnace with air atmosphere at temperatures ranging from 1000 °C, 1100 °C and 1200 °C for 8, 24, 50 and

ACCEPTED MANUSCRIPT 100h. The TGO thickness values were measured from the SEM micrographs as a function of oxidation time. During TGO layer thickness calculations which were performed using the measurements conducted on SEM microstructure images under 2500 magnification, measurements were conducted on four different microstructure images per specimen, and readings varying between 10 and 20 were obtained for each microstructure. All conducted measurements were carried out on the regions in which TGO layer maintained its continuousness and integrity. Measurements of TGO layer, which lays parallel to the interface

PT

of bond/top coating, were conducted on growth direction, i.e. from the point perpendicular to

RI

bond coat surface.

SC

As-sprayed and oxidized coating specimens for metallographic examination were first cut perpendicular to the coating surface with a low-speed saw, and then polished with grinding

NU

and polishing systems. A detailed observation of the microstructure of the oxide scale was undertaken by scanning electron microscopy (Tescan, MAIA3 XN, Czech Republic). A detailed analysis of the phase and composition was carried out by the X-ray diffraction (XRD)

MA

technique (Rigaku SmartLab X-Ray, Japan) and energy dispersive X-ray spectroscopy (EDS), respectively. Surface roughness (Ra) of uncoated substrate, metallic bond coat and ceramic

D

top coat were measured by a roughness tester (Mitutoyo SJ-310, Japan). Cut off length 0.8 mm was used during the measurements. Microhardness tester (Qness GmbH Q10, Austria) is

PT E

used for determining the hardness values of coatings. Hardness test was carried out with 25 gf load with 15 s durations for coatings. The porosities were evaluated by calculating the area

software Lucia.

CE

fraction of the pores against that of cross-section of analyzed coatings with image analysis

AC

3. Results and Discussion

3.1. Microstructure of TBCs

The initial microstructure of the TBC structure is presented in Fig.1. The CGDS bond coating exhibits a low level of porosity, discontinuity opening and crack content. They generally exhibit a dense structure. Porosities were observed at the regions near the ceramic top coating. This is due to the deformation and subsequent cohesion of the particles in the lower layers by means of their own energy and the following particles energy as a phenomenon arising from the depositing characteristic of the process, while on the top layers cohesion is provided only through impact energy of the impacting particle. Porosity measurements for the CGDS bond

ACCEPTED MANUSCRIPT coating resulted in an average porosity of 1.5 ± 1%. Porosity measurements of CoNiCrAlY bond and YSZ top coats were performed on 10 images taken for each coating layer from 5 samples, on which the microstructure of matrix and porosity structures were defined. Ceramic top coat structure which was produced by APS technique, has voids, oxide contents and porosities heterogeneously distributed throughout the coating thickness of the top coat. YSZ ceramic top coatings have a lamellar structure which originates typical microstructure

PT

feature of plasma spraying [28-29]. Porosity content is found to vary from 6 to 8% in ceramic thermal barrier coating. These microstructural features are a consequence of the coating

RI

deposition processes [26, 28-29].

SC

In Fig. 2, the interface microstructures obtained after oxidation periods of 8, 24, 50 and 100 h at 1000 °C, are shown. The CoNiCrAlY bond coat, produced with CGDS technique, consists

NU

of γ-(Ni,Co) matrix and β-(Co,Ni)Al precipitate phases. β-(Co,Ni)Al precipitate and γ-(Ni,Co) matrix structures can be clearly observed. The Al-rich β-precipitates exhibited an oxidation-

MA

dependent formation and it diminished as a result of time dependent decrease in the Al concentration. The thickness of the TGO formation increased with time. After the oxidation process, the oxygen concentration increased on the ceramic top coating and bond coating

PT E

D

interface leading to the formation of Al2O3 layer at the interface. Structural integrity of TGO layer and the change in Al2O3 concentration were analyzed by conducting energy dispersive X-Ray spectroscopy (EDX) analyses along the bond-top coat

CE

interface of the TGO layer. As a result of the conducted analyses, the TGO structure was observed to be present along the coating interface, and the Al2O3 formation was found to have

AC

undergone no major structural change in the initial stages of the oxidation, in general. The interface Al concentration decreased with time, thus the Al2O3 layer lost its uniformity. The interface SEM microstructure of the TBC system having CGDS bond coat, after the oxidation period of 100 h at 1000 °C and distribution of the elements in this area is shown in Fig. 3. As seen from the microstructure and elemental distribution, the TGO oxide structure forming at the interface of bond and top coatings, have Al and O contents. Trace amount of mixed oxide consisting of a combination of CoO, NiO, Cr2O3, (Co, Ni) (Cr, Al)2O4 spinels are also observed at the interface. The amount of Al-rich β-precipitate reduced due to the formation of Al2O3 depending on the increasing oxidation.

ACCEPTED MANUSCRIPT

The microstructural analyses carried out after the oxidation periods of 8, 24, 50 and 100 h at 1100 °C and the interface SEM microstructure after oxidation period of 100 h at 1100 °C as well as the elemental distribution in this region, are shown in Fig. 4 and Fig. 5. After the oxidation tests, the TGO formation was found to occur at the interface and exhibit a time-

PT

dependent growth. The Al-rich β-precipitate formation occured distinctly at the initial stages of oxidation, whereas as a result of the increasing oxidation period β-precipitates decreased

SC

RI

with reduced amount of Al and β-phase depletion areas formed.

Especially after the 50 h oxidation period at 1100 °C, β-precipitates were found to be

NU

decreasing starting from the TGO surface and depletion areas were observed. As seen from the microstructue and elemental distribution in Fig. 5, the TGO structure forming at the interface of bond and top coats generally consists of Al2O3 and a trace of mixed oxide

MA

structures. The microstructural analyses carried out after the oxidation of 8, 24, 50 and 100 h at 1200 °C and the interface SEM microstructure after 100 h oxidation at 1200°C as well as

D

the distribution of the elements in this region is shown in Fig. 6 and Fig. 7.

PT E

TGO growth was found to be at higher rates as compared to the oxidation periods at 1000 °C and 1100 °C oxidation processes. The β-precipitate formation on the CoNiCrAlY bond coat consisting of β-(Co, Ni)Al precipitate and γ-(Ni, Co) matrix structure exhibited a time

CE

dependent decrease. The distance of the forming depletion areas to the bond coat as from the TGO surface are higher than those observed at 1000 °C and 1100 °C temperatures, since the

AC

Al diffusion at 1200°C is higher which in turn leads to a faster depletion of Al-rich βprecipitate. As seen from the microstructure and elemental distribution in Fig. 7, the TGO formation consists of Al2O3 and Ni and Cr based spinels. The densities of Al and O elements are observed to be high in the TGO region, and mixed oxide formations are present in the regions other than Al2O3 region. After the oxidation tests, carried out under 1000 °C, 1100 °C and 1200 °C temperatures, the characteristics of oxide formations at the bond coat-top coat interface of TBC systems showed similarities, whereas their structural composition changed under elevated temperatures. Aforementioned temperatures were chosen for the oxidation tests in an attempt to analyze the

ACCEPTED MANUSCRIPT rapid structural alterations that the TBCs undergo under high temperature service conditions. Especially at 1200 °C, the depletion of Al content and the increase in oxide layer thickness occurred at remarkably higher levels when compared to the other temperatures. XRD patterns of the as-sprayed ceramic top coatings and oxidized TBCs at 1000 °C, 1100 °C and 1200 °C for 100 h are shown in Fig 8. It shows that both consists of tetragonal ZrO2 which means there is no any phase transformation at the end of the oxidation tests. This is related to stability of metable tetragonal (t1) ZrO2 which can be stable up to 1173 °C. In fact,

PT

formation of t+cubic (c) ZrO2 phases should be detected at 1200 °C but t+c-ZrO2 was not

RI

dedected on XRD graph.

SC

3.2. Growth kinetics of the TGO layer

TGO thickness values of produced TBCs were obtained as a result of oxidation studies carried

NU

out at different temperature and time periods. The regions in which TGO layer has lost its integrity within uniform structure and mixed oxide parts such as local and fast growing spinel,

MA

chromate developed in the form of ripples, are not included in measurements. Similarly, the oxide coatings which were subject to discontinuous growth at a rate that disables measuring, were also not included. TGO layer thicknesses of TBCs, obtained as a result of 8, 24, 50 and

D

100 h oxidation tests at 1000 °C, 1100 °C and 1200 °C temperatures, are given in Table 3.

PT E

Thickness-time variation graphs for TGO layers, formed as a result of oxidation processes, is given in Fig. 9.

CE

When the kinetic behavior of bond coats are examined, there exists three different stages of TGO growth specified as transition stage, parabolic and steady state and chemical stage [30-

AC

31]. When long-term oxidation behavior of alloys with MCrAlY content are investigated, it is shown in various researches that the growth of TGO layer exhibits a parabolic and (steadystate) growth [23, 30, 32]. As a result of this study, it was observed that CGDS coatings retain their parabolic and stable state growth form. This parabolic growth generally develops in relation with diffusion of anions (oxygen) and cations (Al-Cr etc) [33].

Growth rate of an oxide layer, which has no cracks and has not lost the contact surface with underlying alloy and which keeps developing till being exposed to spalling, depends on the diffusion rate of the atoms passing through the layer [34-36]. Considering that diffusion rate

ACCEPTED MANUSCRIPT varies with thickness and the thickness varies with time, the relation, given in Equation 1, is found between thickness growth rate and time [37].

The oxide growth follows the parabolic rate law, given by:

PT

(1)

This acquired calculation is equation of a parabolic curve and accounts for a valid growth for

RI

a continuous TGO layer [15, 38]. Which phase will be formed during TGO growth, is closely

SC

related to microstructure of bond coat. Since formation rates of different phases also differ, TGO growth form varies with microstructure. This is the reason why different growth kinetics

NU

are obtained as a consequence of application of the same bond coating using different methods. In case the growth of TGO layer is parabolic, the formula in Equation 1 is

(2)

D

MA

differentiated with respect to time and these results in the formula given in Equation 2[37].

In this formula, h represents TGO thickness, kp represents parabolic velocity constant and t

PT E

represents time, thus the slope of (h2 – t) graph gives parabolic velocity constant. Velocity constants of TBC systems tested using this method at 1000 °C, 1100 °C and 1200 °C, and obtained for TGO thicknesses are given in Table 4. The Arrhenius plot gives an activation

CE

energy for TGO formation for the TBC with the CGDS sprayed bond coat. kp coefficient given in Table 4, can be expressed with an Arhenius equation depending on temperature and

AC

activation energy [37]. Accordingly, the equation of kp coefficient is shown at Equation 3.

(3)

While ko value is constant and independent from temperature for thickening, Q parameter denotes the activation energy which is effective on TGO growth, where R denotes the gas constant, and T denotes temperature. The relation in Equation 4 is obtained as natural logarithm of above-mentioned equation is applied [37].

ACCEPTED MANUSCRIPT (4)

According to this equation, when -ln(kp) – (1/T) graph is drawn for TGO layer, straight linear lines are obtained on the graph and their equation are defined by Equation 5. (5)

PT

In here, while y is“-ln(kp)”, m, i.e. slope is “Q/R”; meaning that if “Q/R”,as the slope of generated lines, is multiplied by “R’’, i.e. gas constant, the production will be the activation

RI

energy of the mechanism which is effective on TGO formation. “-ln(kp)” – “(1/T)” graph of TGO that develop on bond coat as well as slope of generated line in TBC, is given in Fig. 10

SC

based on above-mentioned approach. Note that, the kp values used in the graph are given in terms of µm2/sn instead of µm2/has an attempt to facilitate obtaining the activation energy in

NU

terms of joule. Activation energy value defined using slopes of the lines on the graph. Within these temperature regimes, the activation energy was determined to be 83 kJ/mol for the

MA

coating produced with CGDS method. These correspond to the activation energy required for the diffusion of O2 in Al2O3, indicating that the TGO growth is kinetically limited by O2 and

D

Al2O3 diffusion.

PT E

This result obtained without any surface treatment or other processes like use of nanostructured powders. Low activation energy demonstrates superiority of CGDS technique compared to other production techniques. In CGDS bond coated TBCs, dense structure, low

CE

porosity and defects, provide richer β-NiAl phases and hence, lower Gibbs free energy requires at initial oxidation stage for α-Al2O3 formation [39-40]. The activation energy was

AC

also calculated previously from an Arrhenius plot of TGO scale thickness vs. exposure time by Mercier et al., Yuan et al., Sridharan et al., Chen et al., Soboyejo et al., Madhwal et al. and Nash et al. approximately 46,87 kJ/mol, 130 kJ/mol, 139 kJ/mol, 188 kJ/mol, 437 kJ/mol, 446 kJ/mol, 208,5 kJ/mol and 129 kJ/mol respectively [28,38,41-44]. According to literature studies, this result shows the superiority of CGDS compared to other coating techniques. 4. Conclusions In the present study, the isothermal oxidation behavior and kinetics of cold gas dynamic sprayed TBCs have been conducted and evaluated at three reference temperatures 1000 °C,

ACCEPTED MANUSCRIPT 1100 °C and 1200 °C from 8 h to 100 h. The salient conclusions arising from this work are as follows: 1. The microstructure of YSZ ceramic top coating consists of porosities and microcracks which enable the formation of a strain tolerant coating, while allowing the diffusion of oxygen and enabling oxidation of CoNiCrAlY bond coat. On the other hand, CoNiCrAlY bond coat structure was found to have a dense structure in general, whereas it was found to have a porosity content at the regions near the top coating surface.

PT

2. TGO formation and oxidation behavior in TBC was considerably influenced by the microstructure of coating. Depending on the increasing oxidation temperature and time, TGO

RI

layer was found to form at the interface of bond and top coatings, and increase with increasing

SC

time period.

3. In the bond coat structures, microstructural analyses of which were carried out after the

NU

oxidation tests, formation of Al-rich β-(Co,Ni)Al precipitates was observed, while depletion of β precipitate structures were observed to be depleting with increasing oxidation effect of Al concentration in the bond coat. The distance of forming depletion areas to the bond coat as

MA

from the TGO surface increased depending on the increasing oxidation. 4. At the initial stages of the oxidation, the TGO layer was found to be uniform in the form of

D

Al2O3. After the oxidation tests with increasing time periods, as a result of depletion of Al in the coating structure and the reaction of oxygen with other components in the coating

PT E

structure, the phases belonging to mixed oxide structures such as (Co,Ni,Cr) and NiO were observed in TBC system.

5. The TGO growth kinetics is parabolic in the temperature range between 1000 OC and 1200 O

CE

C. The rate of TGO growth is higher at higher temperatures due to higher reaction rates and

Al depletion. Especially at 1200 °C, the depletion of Al content and the increase in oxide

AC

layer thickness occurred at remarkably higher levels when compared to the other temperatures.

6. The oxidation kinetics studies on TBCs were performed carried out after isothermal oxidation tests. The TGO thicknesses for all applied temperatures and time periods were measured, the growth coefficients of TGO structures were calculated in accordance with the temperatures, and the activation energy required for the development of TGO layers were found based on the variation of these coefficients depending on temperature. The measured activation energy correspond to growth kinetics that is controlled by the diffusion. The activation energy of TBC was found to be 83 kJ/mol.

ACCEPTED MANUSCRIPT 7. XRD analysis shows that both consists of tetragonal ZrO2 which means there is no any phase transformation at the end of the oxidation tests.

Further investigations are ongoing to fully understand and determine in which level the elemental diffusion affecting formation and growth of TGO structure changes with

PT

composition and microstructural properties of bond and top coating structures.

RI

Acknowledgements

SC

This investigation was financially supported by The Scientific and Technological Research Council of Turkey (TUBITAK, 111M265). The authors would like to thank P l asm a Gi ken

NU

C o. for C oNi C rAl Y col d spra y coat i n g deposi t i on. The authors also gratefully acknowledges the Chemnitz University of Technology, Institute of Materials Science and

AC

CE

PT E

D

MA

Engineering Department for their helpful technical support.

PT

ACCEPTED MANUSCRIPT

SC

RI

References

[1] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, John Wiley &

NU

Sons, New York, NY, ISBN 978-0-471-49049-4 (2008).

[2] J.J. Tang, Y. Bai, J.C. Zhang, K. Liu, X.Y. Liu, P. Zhang, Y. Wang, L. Zhang, G.Y. Liang, Y. Gao, J.F. Yang, J Alloy Compd. 688 (2016) 729–741.

MA

[3] A.C. Karaoglanli, B. Demirel, A. Turk, R. Varol, K.M. Doleker, Appl Surf Sci. 354 (2015) Part B, 314-322.

D

[4] P. Sokolowski, L. Pawlowski, D. Dietrich, T. Lampke, D. Jech, J. Therm. Spray Technol. 25 (1) (2016) 94-104.

PT E

[5] Y.Z. Liu, S.J. Zheng, Y.L. Zhu, H. Wei, X.L. Ma, J Eur Ceram Soc. 36 (7) 2016 1765– 1774.

[6] S. Nath, I. Manna, J. D. Majumdar, Corros. Sci. 88 (2014) 10-22.

CE

[7] J.R. Davis, Nickel, Cobalt and Their Alloys, ASM Specialty Handbook, USA, ISBN: 087170-685-7 (2000).

AC

[8] P. Richer, Development of Conventional and Nanocrystalline Bond Coats by Cold Gas Dynamic Spraying for Aerospace Thermal Barrier Coatings, PhD Thesis, University of Ottowas, Ottowa Ontario Canada, 1-227, 2010. [9] P. Song, Influence of Material and Testing Parameters on the Lifetime of TBC Systems with MCrAlY and NiPtAl Bondcoats, PhD Thesis, Forschungszentrum Julich, 1-126, 2011. [10] G. Pulci, J. Tirillò, F. Marra, F. Sarasini, A. Bellucci, T. Valente, C. Bartuli, Surf. Coat. Technol. 268 (2015) 198-204. [11] A.C. Karaoglanli, Sci. Advanc. Mater. (2015) 173-177. [12] R. Xu, X.L. Fan, W.X. Zhang, T.J. Wang, Surf. Coat. Technol. 253 (2014) 139-47.

ACCEPTED MANUSCRIPT [13] W.R. Chen, X. Wu, B.R. Marple, D.R. Nagy, P.C. Patnaik, Surf. Coat. Technol. 202 (2008) 2677-2683. [14] K.W. Schlichting, N.P. Padture, E.H. Jordan, M. Gell, Mat. Sci. Eng. A-Struct. 342 (2003) 120-130. [15] A.G. Evans, D. R. Mumma, J. W. Hutchinson, G. H. Meierc, F. S. Pettit, Prog. Mater. Sci. 46 (5) (2001) 505-553. [16] M. Zhai, D. Li, Y. Zhao, X. Zhong, F. Shao, H. Zhao, C. Liu, S. Tao, Ceram Int. 42 (10)

PT

(2016) 12172-12179.

[17] S. Sridharana, L. Xiea, E. H. Jordan, M. Gella, K.S. Murphy, Mat. Sci. Eng. A-Struct.

RI

393 (1-2) (2005) 51-62.

SC

[18] S.W. Myoung, S.S. Lee, H.S. Kim, M.S. Kim, Y.G. Jung, S.I. Jung, T.K. Woo, U. Paik, Surf. Coat. Technol. 215 (2013) 46–51.

NU

[19] V.D.L. Claudio, L.C. Jacques, L. Haowen, L. Kaspar, A.A. Khaled, A. Lallit, Acta. Mater.71 (2014) 306-318.

[20] M.N. Baiga, F.A. Khalida, F.N. Khana, K. Rehman, Ceram. Int. 40 (2014) 4853-4868.

MA

[21] Y. Li, C.J. Li, Q.Z Zhang, G.Y Yang, C.X. Li, J. Therm. Spray Technol. 19 (1-2) (2010) 168-177.

D

[22] P. Song, D. Naumenko, R. Vassen, L. Singheiser, W.J. Quadakkers, Surf. Coat. Technol. 221 (2013) 207-213.

PT E

[23] Q. Zhang, C.J. Li, C.X. Li, G.J. Yang, S.C. Lui, Surf. Coat. Technol. 202 (2008) 33783384.

4 (2008) 495-516.

CE

[24] E. Irissou, J.G. Legoux, A.N. Ryabinin, B. Jodoin, C. Moreau, J. Therm. Spray Technol.

[25] A. Bonadei, T. Marrocco, Surf. Coat. Technol. 242 (2014) 200–206.

3962-3974.

AC

[26] P. Richer, M. Yandouzi, L. Beauvais, B. Jodoin, Surf. Coat. Technol. 204 (24) (2010)

[27] Y. Li, C.J. Li, G.J. Yang, L.K. Xing, Surf. Coat. Technol. 205 (2010) 2225-2233. [28] F.H. Yuan, Z.X. Chen, Z.W. Huang, Z.G. Wang, S.J. Zhu, Corros Sci. 50 (2008) 16081617. [29] Z.G. Liu, W.H Zhang, J.H Ouyang, Y. Zhou, J Alloy Compd. 647 (2015) 438-444. [30] K. Ma, J.M. Schoenung, Surf. Coat. Technol. 205 (2011) 5178-5185. [31] R. Eriksson, PhD thesis, High-temperature degradation of plasma sprayed thermal barrier coating systems, Linköping University, April 2011, ISBN 978-91-7393-165-6, Linköping Sweden, (2011) 1-109.

ACCEPTED MANUSCRIPT [32] W.R. Chen, R. Archer, X. Huang, B.R. Marple, J. Therm. Spray Technol. 17(5-6) (2008) 858-864. [33] P. Puetz, X. Huang, Q. Yang, Z. Tang, J. Therm. Spray Technol. 20 (3) (2011) 621-629. [34] D. Young, High temperature oxidation and corrosion of metals, Elsevier Corrosion Series, ISBN 978-0-08-044587-8, 2008. [35] L. Ajdelsztajn, F. Tang, G.E. Kim, V. Provenzano, J.M. Schoenung, J. Therm. Spray Technol. 14 (2003) 23-30.

PT

[36] D. Toma, W. Brandl, U. Koster, KOSTER U., Surf. Coat. Technol. 120-121 (1999) 8-15. [37] S. Bose, High Temperature Coatings, Butterworth-Heinemann, Elsevier, ISBN-13: 978-

RI

0-7506-8252-7 (2007).

SC

[38] M. Madhwal, E.H. Jordan, M. Gell, Mat. Sci. Eng. A-Struct.384 (2004) 151-161. [39] S. Ahmadian, E.H. Jordan, Surf. Coat. Technol. 244 (2014) 109-116.

NU

[40] S. Nath, I. Manna, J.D. Majumdar, Corros. Sci. 88 (2014) 10-22. [41] D. Mercier, C. Kaplin, G. Goodall, G. Kim, M. Brochu, Surf. Coat. Technol. 205 (2010) 2546-2553.

MA

[43] S. Sridharan, L.D. Xie, E.H. Jordan, M. Gell, Surf. Coat. Technol. 179 (2004) 286–296. [43] W.R. Chen, X. Wu, B.R. Marple, P.C. Patnaik, Surf. Coat. Technol. 197 (2005) 109-115.

D

[44] W.O. Soboyejo, P. Mensah, R. Diwan, J. Crowe, S. Akwaboa, Mat. Sci. Eng. A-Struct.

AC

CE

PT E

528 (2011) 2223–2230.

ACCEPTED MANUSCRIPT Figure Captions Fig. 1: Cross-section microstructure of the TBC. Fig. 2: The SEM microstructures of TBC specimens with CGDS CoNiCrAlY bond and APS YSZ top coating after oxidation at 1000 °C: a. 8 h, b. 24 h, c. 50 h and d. 100 h. Fig. 3: Elemental analysis image of the TBCs with CGDS bond coat after oxidation period of 100 h at 1000 °C.

PT

Fig. 4: The SEM microstructures of TBC specimens with CGDS CoNiCrAlY bondand APS YSZ top coating after oxidation at 1100 °C: a. 8 h, b. 24 h, c. 50 h and d. 100 h.

RI

Fig. 5: Elemental analysis image of the TBCs with CGDS bond coating after 100 h oxidation at 1100 °C.

SC

Fig. 6: The SEM microstructures of TBC specimens with CGDS CoNiCrAlY bond and APS YSZ top coating after oxidation at 1200 °C: a. 8 h, b. 24 h, c. 50 h and d. 100 h.

NU

Fig. 7: The elemental analysis image of TBCs with CGDS bond coat after 100 h oxidation at 1200 °C.

1100 °C and 1200 °C for 100 h.

MA

Fig. 8: XRD patterns of the as-sprayed ceramic top coatings and oxidized TBCs at 1000 °C,

Fig. 9: Experimentally determined average TGO thickness of TBCs as a function of oxidation

D

time at 1000 oC, 1100 oC and 1200 oC.

AC

CE

PT E

Fig. 10: -ln (kp) – (1/T) graph obtained with relation to TGO for CGDS TBC system.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 1

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 2

Fig. 3

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 4

Fig. 5

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 6

Fig. 7

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

Fig. 8

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig. 9

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Fig. 10

ACCEPTED MANUSCRIPT Table I: Chemical composition of CoNiCrAlY powder

Nominal composition of respective elements (mass %) Co Ni Cr Al 8.0 38 32.5 21

AC

CE

PT E

D

MA

NU

SC

RI

PT

Powder Composition CoNiCrAlY

Y 0.5

ACCEPTED MANUSCRIPT Table II: Spray parameters for metallic bond and ceramic top coat powder deposition

CGDS-CoNiCrAlY Bond Coatings Gas temperature

Sprey pressure 3.0 MPa

600 °C

Gun speed

Stand-off distance

Helium

15 mm APS-YSZ Top Coatings Electrical power 40 kW

Hydrogen flow rate

Powder feed rate

13 slpm

25 g/min

44 slpm

Stand-off distance 90 mm

AC

CE

PT E

D

MA

NU

SC

630 A

Argon flow rate

PT

Arc Current

RI

20 mm/sec

Working gas

ACCEPTED MANUSCRIPT Table III: TGO layer thickness values (µm) of TBCs after the oxidation tests at 1000 °C, 1100 °C and 1200 °C for 8, 24, 50 and 100 h

Time (h)

Temperature 1100 °C

1200 °C

1,61 1,91 2,24 2,79

2,11 2,62 3,02 3,66

2,41 3,01 3,62 4,58

AC

CE

PT E

D

MA

NU

SC

RI

PT

8 24 50 100

1000 °C

ACCEPTED MANUSCRIPT Table IV: Velocity constant values acquired with respect to temperature

Temperature

Velocity constant (μm2/h)

1000 OC

0,0555

1100 OC

0,0938

O

0,1624

AC

CE

PT E

D

MA

NU

SC

RI

PT

1200 C

ACCEPTED MANUSCRIPT Highlights ► CoNiCrAlY and YSZ (ZrO2+Y2O3) powders were used for coating materials. ► TBCs were subjected to isothermal oxidation tests.

PT

► Mechanism for the detrimental effect of TGO on coating oxidation was revealed. ► The TGO growth kinetics is parabolic in the temperature range between 1000 OC and 1200 O

SC

RI

C.

► The rate of TGO growth is higher at higher temperatures due to higher reaction rates and

AC

CE

PT E

D

MA

NU

Al depletion.