Al2O3 + YSZ multilayered thermal barrier coatings

Surface & Coatings Technology 258 (2014) 804–813

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Processing and characterization of CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ multilayered thermal barrier coatings Mehmet Mumtaz Dokur, Gultekin Goller ⁎ Department of Metallurgical and Materials Engineering, Istanbul Technical University, 34469 Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 17 April 2014 Accepted in revised form 24 July 2014 Available online 4 August 2014 Keywords: CYSZ Al2O3 Thermal spray Thermal conductivity Thermal cycle Bonding strength

a b s t r a c t CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ multilayer ceramic coatings were produced in 4, 8 and 12 layered with a total thickness of ~400 μm by high-velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) processes. Microstructure, thermal and mechanical properties were evaluated. The thickness of metallic bonding layer and total thickness of ceramic top layer were measured as 100 ± 10 μm and 400 ± 20 μm, respectively. Porosity levels of multilayer coatings were observed between 6 and 13%. Thermal conductivity values of the CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ coatings were in the range of 0.99 to 1.50 W/mK. The bonding strength of as-sprayed coatings was increased from 5.4 to 10.1 MPa for CYSZ/Al2O3, and 8.7 to 11.5 MPa for CYSZ/Al2O3 + YSZ coatings with the increasing number of layers. Existence of phase transformation (γ → α) for Al2O3 coating was observed after the thermal cycle test. The results indicated that the thermal conductivity and thermal cycle strength of CYSZ/Al2O3 + YSZ TBCs were higher than CYSZ/Al2O3 based thermal barrier coatings. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Thermal barrier coatings (TBCs), usually consist of metallic bond and ceramic top coats, are often applied to hot components in gas turbine engines to improve the efficiency [1]. The bond coat, in the form of MCrAlY (M = Ni, Co or a combination of both), increases coating adherence and reduces the coefficient of thermal expansion (CTE) mismatch between the top coat and the substrate. The ceramic top coat reduces surface temperatures of the underlying components in the gas turbine engines. These components are exposed to temperatures above the melting point of superalloys. The use of TBCs can result in a temperature reduction as much as 170 °C at the substrate surface [2]. A gas turbine system contains three main components consisting of compressor, combustion chamber and turbine. The maximum temperatures measured during the service of these components are; 320 °C for compressor, 1300 °C for combustor, and 1050 °C for turbine. Although, generally Ni-based superalloys (IN738, HAST X etc.) are used for combustor and turbine components, different steel alloys such as AISI309 and AISI 316L are also being used for the same components [3]. There are several ceramic materials (Al2O3, 7–8YSZ, CeO2 + YSZ etc.) used as TBC coating [4]. Yttria-stabilized zirconia (YSZ) is the most widely used ceramic top ⁎ Corresponding author at: Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469 Istanbul, Turkey. Tel.: + 90 212 2856891; fax: +90 212 2853427. E-mail address: [email protected] (G. Goller).

http://dx.doi.org/10.1016/j.surfcoat.2014.07.077 0257-8972/© 2014 Elsevier B.V. All rights reserved.

coat. It possesses low thermal conductivity (2.2 W/mK [5]) and high CTE (10.7 × 10−6 K−1 [4]). Ceria–yttria stabilized zirconia (CYSZ) has higher CTE (13 × 10−6 K−1) than YSZ [6], and Al2O3 has very high hardness and chemical inertness. However, it is not possible to use it as a single coating for the top coat, because the sprayed coating of alumina contains unstable phases such as γ-Al2O3. γ-Al2O3 transforms into αAl2O3, accompanied by a significant volume change (~15%) during thermal cycling [7,8]. In order to reduce the thermal conductivity and residual stress on coatings, multilayer or functionally graded (FG) thermal barrier coatings are introduced as a solution. Multilayer TBCs, which can be referred as composite coatings, consist of two or more coating layers having different functions. In this system, heat resistance can be increased by a second material which has a lower thermal conductivity than the other one. Su et al. studied the influence of layering Al2O3 and YSZ on the thermal conductivity and phase evaluation of small particleplasma sprayed (SPPS) coatings [5]. In [5], multilayer ceramic top coatings were prepared in 4, 20 and 40 layers with a total thickness of 200–380 μm. In another study [9], An et al. produced multilayer coatings using Al2O3 and YSZ powders in 2, 8 and 260 layered coatings by electron beam physical vapor deposition (EB-PVD). Mechanism of adhesion can be described as a sprayed deposit onto a roughened surface by the mechanism of particle interlocking [10]. There is no research in literature relating to thermal and mechanical properties of multilayer coatings containing CYSZ, Al2O3 and YSZ. The purpose of this research is to investigate the microstructure, thermal and mechanical properties and

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phase stability between CYSZ/Al2O3 (Type I) and CYSZ/Al2O3 + YSZ (Type II) multilayer coatings produced in 4, 8 and 12 layered by highvelocity oxy-fuel (HVOF) and air plasma spraying (APS) processes.

2. Materials and methods Ceria and yttria stabilized zirconia (CYSZ, Metco 205NS: ZrO2 , 24CeO2, 2.5Y2O3, an average particle size range of − 90 + 16 μm), aluminum oxide (Al2O3, Metco 105NS: α-Al2O3, particle size range of − 45 + 15 μm) and yttria stabilized zirconia (YSZ, Metco 204BNS: ZrO2 8Y2O3, particle size range of − 75 + 45 μm) were used as starting powders. Multilayer ceramic coatings were produced in 4, 8 and 12 layered (Fig. 1). TBC consisting of CYSZ and Al 2 O 3 coatings (CYSZ/Al 2 O 3) was specified as Type I. YSZ was mixed in weight percentage (1:1) with Al 2O 3 for producing Type II TBC. YSZ and Al 2O 3 powders were weighed and mixed, and then subjected to ball milling for 24 h in ethanol, using zirconia balls. The resulting mixtures were subsequently dried in air. Stainless steel (C 0.08%, Cr 18%, Fe 68%, Mn 2%, Ni 11%, Si 1%) and NiCoCrAlY powder (Ni 23Co 20Cr 8.5Al 4Ta 0.6Y, Amdry 9951, SulzerMetco, − 37 μm) were selected as substrate and the bond coat layer for both coatings. Substrates with a diameter of 25 mm and 2 mm in thickness were subjected to cleaning and grit blasting process. NiCoCrAlY powder was sprayed onto the prepared surface in a total thickness of 100 ± 10 μm by HVOF process (2700 DJHE DJ, SulzerMetco). Coating of powders was sprayed to bond coating layer by APS system (9MBM, SulzerMetco). Total thickness of each top layer was about 100, 50 and 33 μm for 4, 8 and 12 multilayer coatings, respectively. Spraying parameters of HVOF and APS processes are presented in Table 1. Production methods and coating types of thermal barrier coatings are given in Table 2. Microstructural characterization was carried out by field emission gun scanning electron microscope (JSM 7000 F, JEOL). Porosity measurements were also performed using backscattered electron microscope images with image analysis software (Image J). The thermal conductivity measurements were performed with a laser flash system at room temperature in argon atmosphere. The specimens were in disc form (15.8 mm in diameter). The coating

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Table 1 APS and HVOF spraying parameters of ceramic top coats and NiCoCrAlY. Parameters

CYSZ

Al2O3

Al2O3 + YSZ

NiCoCrAlY

Hydrogen flow rate (l/min) Argon flow rate (l/min) Amper (A) Voltage (V) Oxygen flow rate (SLPM) Air flow rate (SLPM) Propane flow rate (SLPM) Spray distance (mm) Spray angle to surface (°) Powder feed (lb/h)

42 18 500 65 – – – 75 90 6.5

40 12 400 60 – – – 75 90 5.5

40 12 400 60 – – – 75 90 5.5

– – – – 140 385 90 250 90 28

is separated from substrate by the treatment of NaOH solution and subjected to measurement. There are several methods to evaluate thermal cycle properties [11–15]. In these researches, test procedure is based on heating up and cooling down. The main difference between these methods is only in the types of heating and cooling sources. CO2 laser system with a power of 3 kW was used for the heating source. Samples were heated up to 1150 °C (± 50) and held about 60 s and then quenched to room temperature in 60 s. The surface temperature of the sample was measured by an optical pyrometer. The diameter of laser spot was 1.2 mm. By defocusing the spot, the effective laser area was widened to 25 mm in diameter. Bonding strength of measurements was performed according to the ASTM-C633 standard. The coated and uncoated surfaces of the substrate were glued to an apparatus (a cylinder 25.4 mm in diameter, 25.4 mm long) that was just grit blasted and then tested in a universal testing machine. A high performance epoxy adhesive (3 M Bison Epoxy) was used to join the two apparatuses. The bonding strength was the maximum tensile strength measured with an INSTRON 1195 tester at a crosshead speed of 1 mm/min. The bonding strength value is the average result of the three measurements. The bonding strength of values is obtained by calculating the relationship between load and area when the failure occurs on the sample. The fracture region showed

Fig. 1. Schematic view of 4, 8 and 12 layered (a) CYSZ/Al2O3, and (b) CYSZ/Al2O3 + YSZ coatings in thickness of 400 μm.

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operating with CuKα (k = 0.154186 nm) radiation at 30 kV and 15 mA, (2θ) was between 20 and 80°, with a step width of 0.02°.

Table 2 Production methods and coating types of TBCs. Sample code

Coating type

CA04 CA08 CA12 CAY04 CAY08 CAY12

CYSZ/Al2O3 CYSZ/Al2O3 CYSZ/Al2O3 CYSZ/Al2O3 + YSZ CYSZ/Al2O3 + YSZ CYSZ/Al2O3 + YSZ

Type

Type I

Type II

Number of layers

Production method of bond coat

Production method of ceramic top coats

4 8 12 4 8 12

HVOF

APS

cohesive and adhesive types of coatings. Fracture types were evaluated with cross-sectional scanning electron microscopy images of samples, the ratio of fracture types was also calculated and the bonding strength of thermally cycled samples was also determined. The phase analyses of the powders, as-sprayed coatings and thermal cycled samples were performed by using an X-ray diffractometer (Rigaku Miniflex),

3. Results and discussion 3.1. Microstructural characterization Microstructures of Type I and Type II prepared in 4, 8 and 12 layer TBCs are given in Fig. 2. Multilayer TBCs consisted of two or more coating layers. It is observed that microstructural images of layers showed the characteristic views for multilayer systems as reported in previous studies (Fig. 2) [5,9]. Ceramic top coat consisted of CYSZ and Al2O3 which are seen as light and dark colored, respectively for Type I coating (Fig. 2(a)–(c)). On the other hand, YSZ is clearly visible in the Al2O3 layer on Type II coating (Fig. 2(d)), and CYSZ layer is more porous compared to Al2O3 layer. However, good adhesion was achieved between the bond coat and the substrate as well as between the bond coat and ceramic top coat consisting of multilayers. The average thicknesses were approximately 100 ± 10 and 400 ± 20 μm for the bond coat and the top coat, respectively.

Fig. 2. Cross-section images of the CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ coatings in 4, 8 and 12 layered (a) CA04, (b) CA08, (c) CA12, (d) CAY04, (e) CAY08, and (f) CAY12.

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ceramic top layers prepared in 4, 8 and 12 layers were 13, 11 and 7% for Type I and 10, 8 and 6% for Type II coatings, respectively. Some amount of porosity was visible in CYSZ layer, and porosity level decreased with increasing number of layers on Type I and Type II coatings. Similar situations are reported in literature for different coatings as well, and the porosity levels were in the range of 5 and 12% [18,19]. Decreasing of porosity is mainly due to decreasing thickness of each CYSZ layer on 8 and 12 layer coatings. 3.3. Thermal conductivity

Fig. 3. Porosity cracks and parallel cracks at splat boundaries on the CYSZ layer produced by APS process.

The results of the thermal conductivity measurements of Type I and Type II coatings are given in Table 3. As seen from the table, thermal conductivity values of Type I and Type II changed between 0.99 and 1.50 W/mK. Type I with 4 layers had the same thermal conductivity value with Type II, having identical number of layers. However, lower thermal conductivity values were obtained for Type II having higher number of layers than Type I. Thermal conductivity value of Type I showed an increase with the increasing number of layers. Such a phenomenon could be explained with the decreasing thickness of a layer caused by decreasing porosities on the CYSZ layer mentioned above. However, the same behavior was not observed for Type II coatings. Thermal conductivity values did not show any change with increasing number of layers. The reason for this was the addition YSZ to the Al2O3 powder on Type II coatings. In particular, an increase in the number of layers in Type I coating causes a decrease in the amount of porosity, which in turn, increases the thermal conductivity. On the other hand, because of the addition of YSZ to alumina, the thermal conductivity values of Type II did not result in a change as the number of layers was increased. In addition, the thermal conductivity values of Type I of coating were significantly increased accordingly to Type I. Cao et al. reported the thermal conductivity values of CYSZ, Al2O3 and YSZ on the review of ceramic materials for thermal barrier coatings as 2.77, 5.8 and 2.12 W/mK, respectively [4]. In addition, Su et al. reported the thermal conductivity values of multilayer coatings consisting of Al2O3 and YSZ in 4 and 40 layered as ~ 2.5 and ~3.0 W/mK at room temperature, respectively [5]. In contrast to this result, lower thermal conductivity values were achieved for Type I and Type II multilayer coatings produced in this study. 3.4. Thermal cycle test

Fig. 4. SEM image of dense difference between CYSZ (light) and Al2O3 (dark) layers.

3.2. Porosity measurements APS coatings had significant amounts of microstructural defects such as cracks, both parallel (at splats boundaries) and normal to the metal/ ceramic interface, and also porosities [16]. CYSZ layer produced by APS had such defects as shown in Fig. 3. Existence of defects could help to reduce thermal conductivity of the TBCs because of mitigating stresses mainly due to the thermal expansion mismatch between the metal and ceramic [17]. Al2O3 and Al2O3 + YSZ layers are dense in the structure and they have less porosity than CYSZ as shown in Fig. 4. Porosity levels of

Type I and II coatings were exposed to very fast heating and cooling rates during thermal cycle test. The strength values were determined repeating these heating and cooling steps. In general, macrocracks or pull outs occur on the surfaces of the TBCs after a certain number of cycles. Thermal cycle life of the coatings is determined as the cycle number at which the beginning of cracking is observed. 500 cycles were applied to Type I and Type II samples during tests. The surface images of the thermally cycled samples are given in Fig. 5 and any cracks or pull outs were not observed on the surfaces of Type I and Type II coatings. Samples were prepared for metallographic characterization. The cross-sectional image of Type I with 4 layers (CA04) is shown in Fig. 6(a). Vertical cracks are clearly visible on the fourth layer of the ceramic top coat, Al2O3. Partial separation is also observed on the first

Table 3 Thermal conductivity results of the first and the second type coatings and bonding strength results of CYSZ/Al2O3 and CYSZ/Al2O3 + YSZ (C: Cohesive, A: Adhesive). Sample code

Thermal conductivity (W/mK)

Bonding strength (MPa)

Fracture type

CA04 CA08 CA12 CAY04 CAY08 CAY12

1.07 1.28 1.50 1.07 0.99 1.00

5.4 8.7 10.1 8.7 9.6 11.5

25% C–75% A 93% C–7% A 60% C–40% A 100% A 97% C–3% A 70% C–30% A

± ± ± ± ± ±

0.5 0.2 0.7 0.1 0.7 1.7

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Fig. 5. The images of the samples after thermal cycle test.

CYSZ layer on the CA04 sample. Separations became more significant for the samples of CA08 (Fig. 6(b) and (c)) and CA12 (Fig. 6(d) and (e)). Previous studies [11,20] showed that the major factor causing degradation of thermal barrier coatings was thermal stresses. The thermal expansion coefficient difference between the substrate and coating induces stresses at high temperatures [21–23]. The typical thermal expansion coefficient of stainless steel, NiCoCrAlY bond coat, CYSZ and Al2O3 is reported as 17.3 × 10− 6, 17.5 × 10− 6, 13 × 10−6 and 9.6 × 10− 6 K− 1, respectively [4,8,24,25]. In this study, the substrate and bond coat were stainless steel and NiCoCrAlY. The thermal expansion coefficient of bond coat was almost near to that of the substrate because of having the same chemical compositions. The ceramic top coat materials consisted of CYSZ and Al2O3. Interface stresses upon cooling the thermal barrier coating were expected due to the differences on CTE values of these three materials; bond coat, CYSZ and Al2O3. These stresses can lead to the formation of cracks. Then, spallation occurred at the interfaces of the layer with the increasing and/or enlarging of the cracks. Therefore, thermal stresses due to thermal expansion coefficient mismatch between the layers were the major factor for failure on Type I coatings. On the other hand, γ-Al2O3 transforms into α-Al2O3 at high temperatures, above 1100 °C, accompanied by a significant volume change of

Fig. 6. The cross-section images of the thermal cycled samples (a) CA04, (b) CA08—edge, (c) CA08—center, (d) CA12—edge and (e) CA12—center.

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approximately 15% [6,7]. This situation leads to cracking of alumina. As shown in Fig. 6(d), a crack occurred on the alumina layer mainly due to the volume change during transformation. However, cross-sectional images (Fig. 7) of Type II coatings showed that there are no cracks or separations between the layers or within of each layer. Adding YSZ powder to alumina could have a preventing effect of volume change on the alumina during the thermal cycle test and reduce thermal

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expansion coefficient mismatch between the layers. Many studies on thermal cycle of functionally graded materials and multilayer coatings have been performed in recent years. Han et al. reported that cracks occurred after 200 and 46 cycles where thermal cycling test conducted at 1250 °C with a 15 second heating and water quenching on multilayer coatings consisting of CYSZ and NiCoCrAlY, respectively [12]. In another study, Pidani et al. performed thermal cycle test at 950 °C with 5 min heating and water quenching for as-sprayed and laser-glazed CYSZ coatings on the furnace. They have reported the average number of failure cycles for as-sprayed and laser glazed coating as 253 and 309, respectively [8]. In contrast to these results, any deformation was not observed on Type II thermal barrier coatings at the end of 500 cycles. The bond coat temperature in gas turbine engines generally exceeds 950 °C. Oxygen penetration begins through the ceramic top coat at this temperature. Oxygen reacts with Al inside the bond coat, resulting in the formation of a third layer; thermally grown oxide (TGO) between the bond coat and the ceramic top coat. The TGO layer in the form of α-Al2O3 has a thickness of approximately 1–10 μm and TGO's growth is slow and uniform [15]. TGO layers were seen for all types of coatings in Fig. 8. Their thickness was changed from 0.5 μm to 1.5 μm. EDS analysis was performed for all coatings. The results are similar for all types of coatings. The composition of bond coat, TGO and top coat has been done for CA04, and given in Fig. 9 and Table 4. Weight concentration of aluminum was increased from 9.7% on the bond coat to 21.6% on the TGO layer. In addition, concentration of nickel was decreased from 43.3% on the bond coat to 23.7% on the TGO layer. The decrease is mainly due to diffusion of Al and Ni elements on the bond coat layer. 3.5. Bonding strength

Fig. 7. The cross-section images of the thermal cycled samples (a) CAY04, (b) CAY08 and (c) CAY12.

The results of bonding strength values and fracture type for Type I and Type II coatings prepared in 4, 8 and 12 layers are given in Table 3. Three parallel samples were tested. Bonding strength in multilayer coatings is stronger than two layered coatings because of the interfaces. The thermal stresses can be decreased significantly by using graded layers in FGM coatings; therefore, the bond strength can be improved significantly [26]. In our study, bonding strength values of samples are increased with increasing number of layers for both types. Bonding strength values are increased from 5.4 to 10.1 MPa for Type I and 8.7 to 11.5 MPa for Type II coatings with increasing number of layers from 4 to 12, respectively. In Ref. [27], two layered and functionally graded thermal barrier coatings were produced using YSZ and NiCoCrAlY powders by APS process bonding strength of two layered and FGM thermal barrier coatings were 9.3 and 17.8 MPa, respectively [27]. Lime et al. reported that bonding strength of FGM thermal barrier coating produced by using YSZ and NiCoCrAlY powders was measured as 13.5 MPa and 19.4 MPa for 300 μm and 500 μm thickness values, respectively. Bonding strength value of NiCoCrAlY with a thickness of 300 μm was also reported as 36 MPa [28]. In another study [29], thermal barrier coating was produced using NiCoCrAlY as a bond coat and YSZ as ceramic top coat powders by APS process. Bonding strength of coating was reported between 12 and 28 MPa. Compared to the results stated in literature (Table 5), bonding strength values are comparable for Type II coatings. Fracture surfaces of coatings are shown in Fig. 10. Fracture has occurred through the first layer of the ceramic top coat and bond coat. It shows that sufficient amount of bonding between ceramic top coats and bond coat layer could not be maintained compared to each ceramic top coat layer. Adhesion strength measurement was also applied to 300 and 500 thermally cycled CAY12 (12 layered CYSZ/Al2O3 + YSZ coatings) samples. Strength of coating was expected to decrease due to the thermal stresses on materials during thermal cycle test [30]. Adhesion strength of as-sprayed CAY12 was 11.5 MPa as shown in Table 3. After thermal

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Fig. 8. Microstructure images of TGO layer belonging to Type I and Type II coatings.

cycle test consisting of 300 and 500 cycles, adhesion strength values decreased to 10.2 and 7.8 MPa, respectively. Fracture surfaces of the thermally cycled samples are shown in Fig. 11. Fracture occurred between ceramic top coat and bond coat because of weak bonding between these layers. Cohesive fracture type was shown due to some parts of ceramic top coat adhered to bond coat.

3.6. Phase analysis Phase analysis was performed on the surface of the coatings. The top layers were Al2O3 and Al2O3 + YSZ for Type I and Type II coatings, respectively. The XRD patterns of Type I and Type II coatings in 4, 8 and 12 layers for as-sprayed and thermally cycled samples were given in Fig. 12(a) and (b), respectively. α-Al2O3, as starting powders, had a rhombohedral crystal structure. Phase transformation occurred from α-Al2O3 to γ-Al2O3 after the APS process. The temperature of the samples was about 1150 °C during the thermal cycling test. γ-Al2O3 sintered and transformed to α-Al2O3 at this temperature. YSZ had tetragonal and monoclinic phases. Due to the disappearing monoclinic phases after the APS process, the YSZ was fully tetragonal structure.

4. Conclusions CYSZ/Al2O3 (Type I) and CYSZ/Al2O3 + YSZ (Type II) multilayer ceramic coatings were produced using HVOF and APS processes in 4, 8 and 12 layers. • Good adhesion was achieved between the bond coat and the substrate as well as between the bond coat and ceramic top coat consisting of multilayers. • Al2O3 and Al2O3 + YSZ layers are dense in the structure and they have less porosity than CYSZ. • The thermal conductivity values of Type I and Type II were changed between 0.99 and 1.50 W/mK. • Thermal conductivity of Type I showed an increase with increasing number of layers. Thermal conductivity values were 1.07, 1.28 and 1.50 W/mK for 4, 8 and 12 layered coatings, respectively. • Thermal conductivity of Type II coatings did not show any change with increasing number of layers. Values were 1.07, 0.99 and 1.00 W/mK for 4, 8 and 12 layered coatings, respectively. • Cracks or pull outs did not occur on the surface of both multilayer TBCs after thermal cycle test. • The major factor of cracks or partial spallation is the thermal stresses due to CTE mismatch between the layers. Also, phase transformation

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Fig. 9. EDS analysis and microstructure of coating belonging to CA04.

Table 4 Concentration of elements on bond coat and the TGO layer (in wt%). Element Bond coat layer (AMDRY 997)

Bond coat layer (as-sprayed)

Bond coat layer (thermally cycled)

TGO

Ni Co Cr Al O Y Ta

43.16 24.19 20.17 8.11 – – 4.27

43.38 22.85 19.87 9.69 – – 4.20

23.77 18.15 19.27 21.63 17.17 – –

43.9 23.0 20.0 8.5 – 0.6 4.0

of alumina at high temperatures causes cracks on Al2O3 layer of Type I coatings. • The reason why there are no cracks or separations on Type II is the addition of YSZ in the structure. Adding YSZ powder to alumina prevents volume changes during the thermal cycle and reduces the difference CTE mismatch between the layers. • The thickness of α-Al2O3 based TGO layer was between 0.5 and 1.5 μm.

• The bonding strength was increased from 5.4 to 10.1 MPa for Type I and 8.7 to 11.5 MPa for Type II coatings with increasing number of layers. • Increment of bonding strength is about 46 and 24% for Type I and Type II coatings, respectively. • After thermal cycle test consisting of 300 and 500 cycles, bonding strength decreased to 10.2 and 7.8 MPa, respectively. Acknowledgments This work was supported by the Scientific and Technological Research Council of Turkey, 1002 Short Term Research and Development Funding Program with a project number of 111M430. The authors thank Prof. Dr. Yilmaz Taptik and Assoc. Prof. Dr. Ozgul Keles for their contributions on thermal spray processes. References [1] [2] [3] [4] [5] [6]

Table 5 Comparison of bonding strength values for Type I and Type II coatings with the other studies. Coatings

Coating thickness 300 μm

NiCoCrAlY + YSZ [26] NiCoCrAlY + YSZ (FGM) [26] NiCoCrAlY + YSZ (FGM) [27] NiCoCrAlY [27] NiCoCrAlY + YSZ [28] NiCoCrAlY + CYSZ/Al2O3 NiCoCrAlY + CYSZ/ Al2O3 + YSZ

400 μm

[8] [9]

500 μm

1 mm 9.3 MPa 17.8 MPa

13.5 MPa

[7]

19.4 MPa

[10] [11] [12] [13]

36 MPa 12–28 MPa 5.4– 10.1 MPa 8.7– 11.5 MPa

[14] [15] [16] [17]

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Fig. 10. Fracture surface images of as-sprayed coatings.

[18] J.R. Davis, Handbook of Thermal Spray Technology, Thermal Spray Society and ASM International, 2004. [19] J. Wu, H. Guo, L. Zhou, L. Wang, S. Gong, J. Therm. Spray Technol. 19 (2010) 1186–1194. [20] H.L. Tsai, P.C. Tsai, J. Mater. Eng. Perform. 4 (1995) 689–696. [21] A.N. Khan, J. Lu, J. Mater. Process. Technol. 209 (2009) 2508–2514. [22] Z. Zhang, J. Kameda, S. Sakurai, M. Sato, Metall. Mater. Trans. A36 (2005) 1841–1854. [23] H.L. Tsai, P.C. Tsai, Surf. Coat. Technol. 71 (1995) 53–59. [24] Y. Liu, C. Persson, S. Melin, J. Wigren, J. Mater. Sci. Technol. 14 (2005) 258–263. [25] Y. Liu, C. Persson, S. Melin, J. Mater. Sci. Technol. 13 (2004) 377–380. [26] Y.W. Gu, K.A. Khor, Y.Q. Fu, Y. Wang, Surf. Coat. Technol. 96 (1997) 305–312. [27] Z.L. Dong, K.A. Khor, Y.W. Gu, Surf. Coat. Technol. 114 (1999) 181–186. [28] C.R.C. Lima, R.E. Trevisan, J. Therm. Spray Technol. 6 (1997) 199–204. [29] R. Ghasemi, R. Shoja-Razavi, M. Mozafarinia, H. Jamali, Ceram. Int. 39 (2013) 8805–8813. [30] M. Gell, E. Jordan, K. Vaidyanathan, K. McCarron, B. Barber, Y. Sohn, V.K. Tolpygo, Surf. Coat. Technol. 120–121 (1999) 53–60.

Fig. 11. Fracture surfaces of thermally cycled samples (a) 300, and (b) 500 cycles.

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Fig. 12. The XRD patterns of powders, as-sprayed and thermal cycled samples of (a) CYSZ/Al2O3, and (b) CYSZ/Al2O3 + YSZ coatings.

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