Surface and Coatings Technology 124 (2000) 1–12www.elsevier.nl/locate/surfcoat

Phase transformation and bond coat oxidation behavior ofplasma-sprayed zirconia thermal barrier coating

C.H. Lee *, H.K. Kim, H.S. Choi, H.S. AhnDepartment of Metallurgical Engineering, Research Institute of Steel Processes and Applications, Hanyang University, 17 Haengdang-dong,

Seongdong-ku, Seoul, South Korea 133-791

Received 2 February 1999; accepted in revised form 19 September 1999

Abstract

ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 thermal barrier coatings were prepared using the air plasma spray process. Phasetransformation in the ceramic top coating, bond coat oxidation and thermal barrier properties were investigated to compareZrO2–CeO2–Y2O3 with ZrO2–Y2O3 at 1300°C under high temperature thermal cycles. In the as-sprayed condition, both coatingsshowed a 7~11% porosity fraction and typical lamellar structures formed by continuous wetting by liquid droplets. In theZrO2–CeO2–Y2O3 coating, the phase ratio of tetragonal to cubic phase was 75:25 and ZrO2–Y2O3 coating had a 100% non-transformable tetragonal phase. There was no monoclinic phase in either coating. However, the phase ratio of the coatings waschanged after 1300°C thermal cycles. In the ZrO2–CeO2–Y2O3 coating, the ratio of tetragonal to cubic was changed to 88:12 anda monoclinic phase was still not detected, but a 10~19% monoclinic phase had formed in the ZrO2–Y2O3 coating. The life of thecoatings was found to be strongly dependent on the temperature which the bond coat experienced during exposure to a peaktemperature of 1300°C. When the bond coat experienced a temperature higher than 1100°C, the useful life of both thermal barriercoatings was decreased drastically and this was related to the oxidation behavior of the bond coat. Al2O3 formed preferentiallyalong the bond and top coat interface and the other oxides such as NiO and Ni(Cr,Al )2O4 spinel, which were believed to decreasethe coating life by oxide growth stress, formed rapidly at the top coat side of the interface at temperature higher than 1100°C.The thermally shocked ZrO2–Y2O3 coating exhibited a non-linear thermal expansion curve which was probably due to a reversibletetragonal–monoclinic transformation. The ZrO2–CeO2–Y2O3 coating was found to have better thermal cycling behavior thanthe ZrO2–Y2O3 coating. The reason for this could be that there was no phase transformation from tetragonal to monoclinic phase,which induces volume expansion. The thermal expansion mismatch is smaller and the effect of oxide growth stress is relativelysmall because of better thermal insulation. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Bond coat oxidation; Phase transformation; Thermal barrier effect; Thermal expansion mismatch

1. Introduction Among ceramic coating methods, plasma spray hasbeen widely used because there is less limitation of size

Many parts (such as engines in automobiles, turbine and shape and the lower costs involved. A ceramicblades in power plants, jet engines in vessels and air- coating for heat resistance, oxidation resistance and theplanes, etc.) require improved surface function because thermal barrier effect is known as a thermal barrierof the requirement of energy reduction with high effi- coating (TBC ). TBCs can cause an increase in combus-ciency. Such parts need heat, corrosion and wear resis- tion temperature by decreasing the temperature of thetant surfaces and, among many materials, ceramics are substrate and thus promote the efficiency of the engineclaiming a growing interest for application in those and improve the useful life. So far, the industrial state-environments. Although ceramics have a brittleness of-the-art coating system for TBC has been a plasmaproblem and require a complicated fabrication process, sprayed ZrO2–(6~8 wt.%) Y2O3 ceramic layer over anthe application of ceramic coating on the surface of MCrAlY (MNNi, Co, etc.) bond coat layer [1]. Whenmechanical parts is expected to grow, not only presently the application for more severe environments such asbut also widely in the future. higher temperatures, corrosion and strains is considered,

the ZrO2–CeO2–Y2O3 coating currently looks to be* Corresponding author. promising due to its high temperature stability, good

0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0257-8972 ( 99 ) 00517-4

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corrosion resistance, high fracture toughness and better 2. Experimental proceduresthermal insulation [2]. However, published reports deal-ing with CeO2-containing TBC are limited in spite of its 2.1. Specimen preparation and coatingvarious merits. Therefore, the purpose of this work isto clarify the thermal properties (such as phase trans- The chemical compositions of the substrates shown

in Tables 1 and 2 summarize the characteristics of theformation, bond coat oxidation and thermal expansionmismatch) of ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 plasma starting powders investigated. As a substrate, Ni-based

superalloy (Inconel 600) specimens in the form of diskssprayed coatings.

Fig. 1. Schematic diagram of thermal barrier effect test.

Table 1Chemical composition of substrates (%)

Class Ni Cr Fe Mn C Si P S

Inconel 600 >72 14~17 6~10 <1 <0.15 – – –SS-41 – – Bal. 0.67 0.16 0.17 0.029 0.020

Table 2Characteristics of powders

Powder ZrO2–CeO2–Y2O3 ZrO2–Y2O3 NiCrAlYManufacturing method HOSP HOSP Gas atomizedChemical composition (wt.%) ZrO2–25CeO2–2.5Y2O3 ZrO2–8Y2O3 Ni22Cr10Al1.0YParticle shape Spheroidal Spheroidal SpheroidalParticle size range (mm) −90+16 −106+11 −106+56

Table 3Parameters for plasma spraying

Coating Ampere Voltage Gas Flow rate (SCFH) Feed rate Spray distancematerials (A) (V ) (g/min) (mm)

Primary gas (N2) Secondary gas (H2) Carrier gas (N2)

ZrO2–CeO2–Y2O3 500 76 75 20 12 45 100ZrO2–Y2O3 500 76 75 20 12 45 65NiCrAlY 500 74 100 20 12 60 125

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Fig. 2. The number of thermal cycles at failure vs. thickness of ceramictop coating.

(30 mm diameter by 3 mm thickness) for the thermalshock resistance test, and low carbon steel(50×50×5 mm) for the thermal expansion test wereused. NiCrAlY powder was used as bond coating mate-rial. For ceramic coating materials, ZrO2–CeO2–Y2O3and ZrO2–Y2O3 powder, which were all manufacturedby a homogeneous oven spherical powder method(HOSP), were prepared.

The substrate was ultrasonically cleaned, grit blastedand then placed in an air plasma spray (APS) system Fig. 4. Porosity fraction of ceramic top coating: (a) ZrO2–CeO2–for overlaying with the NiCrAlY bond coating (200 mm). Y2O3 coating; (b) ZrO2–Y2O3 coating.As-sprayed bond coating specimens were then sprayedwith ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 top coatings in

samples were ultrasonically cleaned and dried beforean APS N2/H2 system. The coating thickness was variedeach thermal expansion measurement.from 370 to 770 mm. The optimum spray parameters

used for coating application are summarized in Table 3.2.2. Tests of coating propertiesThe thickness of the thermal expansion specimens was

1.5 mm and they were sprayed onto low carbon steelThermal shock resistance of coatings was determinedwith 5 mm thickness. Samples 5 mm wide and 50 mm

by a thermal cycling test in air. The 20 min thermallong were cut from these coatings using a low speedcycle consisted of a 5 min increase from room temper-diamond saw and then the carbon steel substrate wasature to 1300°C, a 10 min hold at 1300°C, followed bydissolved using 50% nitric acid. The substrate freecooling to room temperature in 5 min. Failure of thetop coating was defined as visible cracking after thespecimens had been cooled to room temperature (all ofthe surface-cracked specimens showed lamination andspallation of the coating). The schematic diagram of thethermal barrier effect test is shown in Fig. 1. Twothermocouples were used as sensors — one was locatedin the 1300°C high temperature environment and theother was bonded on the substrate surface to testtemperature benefit (DT ). Both were linked to therecorder and monitored in real time. When these twotemperatures became stable, the data were recorded.

Thermal expansion measurements were carried outin air using an alumina pushrod dilatometer for thecoatings which were substrate free. A heating rate of10°C/min was used, and the heating and cooling cycle

Fig. 3. Thermal barrier effect vs. thickness of ceramic top coating. ranged between 100 and 1300°C. Values of fractional

4 C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

(a) (b)

(c) (d)

Fig. 5. Cross-sectional microstructures of as-sprayed and thermally shocked coating: (a) ZrO2–CeO2–Y2O3 coating (as-sprayed); (b) ZrO2–Y2O3coating (as-sprayed); (c) ZrO2–CeO2–Y2O3 coating (thermally shocked); (d) ZrO2–Y2O3 coating (thermally shocked).

length changes were obtained and then the thermal of particles. SEM and EDS were used to characterizethe coating microstructure and fracture surface. Porosityexpansion coefficients were calculated. All specimens

were sectioned with a diamond saw and epoxy mounted fraction and size were measured with an image analyzer.XRD was used for phase identification. In the quantita-and fine-polished to minimize the smearing and pull-out

Table 4Phase fraction in the top coating

Coating As-sprayed After thermal shock

Tetragonal Cubic Monoclinic Tetragonal Cubic Monoclinic

C1a 74 26 <1 88 12 <1C2 76 24 <1 89 11 <1C3 76 24 <1 88 12 <1C4 75 25 <1 87 13 <1C5 73 27 <1 88 12 <1Y1b 100 <1 <1 83 7 10Y2 100 <1 <1 84 3 13Y3 100 <1 <1 78 9 13Y4 100 <1 <1 76 5 19Y5 100 <1 <1 76 7 17

a C=ZrO2–CeO2–Y2O3.b Y=ZrO2–Y2O3

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Fig. 6. Cross-sectional SEM images and line scanning results of as-sprayed ZrO2–Y2O3 coatings.

tive phase analysis of zirconia, the method described by 3. Results and discussionToraya et al. was used to determine the relative propor-tions of the monoclinic, tetragonal and cubic phases 3.1. Thermal shock resistance[3,4]. Step-scanning was carried out over two ranges of2h values. The low angle range of 27~32° was for the The accumulated number of thermal cycles at failure

with variation in thickness of the ceramic top coat are(111) tetragonal and cubic peaks and the (111) and(111∞) monoclinic peaks. The high angle scan was in the summarized in Fig. 2 and the temperatures in the figure

indicate maximum surface temperature of the bond coatrange of 71~76°, for the (400) and (004) tetragonalpeaks and the (400) cubic peak. Note that deconvolution during exposure to the 1300°C thermal cycles. The

ZrO2–CeO2–Y2O3 coating exhibited higher thermalof overlapping peaks, such as the {400} tetragonal/cubicpeaks, was performed manually using an iterative shock resistance than the ZrO2–Y2O3 coating through-

out the thickness range and this is believed to resultmethod.

Table 5Phase fraction before and after thermal expansion tests

Coating As-sprayed After thermal shock

Tetragonal Cubic Monoclinic Tetragonal Cubic Monoclinic

Before Cc 75 25 <1 91 9 <1After C 75 25 <1 92 8 <1Before Yd 100 <1 <1 79 <1 21After Y 100 <1 <1 78 <1 22

c C=ZrO2–CeO2–Y2O3.d Y=ZrO2–Y2O3

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Fig. 7. Cross-sectional SEM images and line scanning results of thermally shocked ZrO2–Y2O3 coatings.

from three main effects. Firstly, there is little phase Fig. 3. DT increased as the thickness of the ceramic topcoating increased and the ZrO2–CeO2–Y2O3 coatingtransformation between the monoclinic phase and tetra-

gonal phase in the ZrO2–CeO2–Y2O3 coating. Secondly, exhibited a higher DT than the ZrO2–Y2O3 coatingthroughout the entire thickness range. Generally in thestress generated by bond coat oxidation is less in the

ZrO2–CeO2–Y2O3 coating due to better thermal insula- ceramic materials, the lattice vibration controls thermalconductivity by hindering the transportation of phononstion. Finally, the coefficient of thermal expansion is

larger in the ZrO2–CeO2–Y2O3 coating. Thermal shock [5], so thermal conductivity becomes lower when thematerial contains elements of heavy atomic weight orresistance increased as coating thickness increased in the

same composition due to a decrease of oxide growth large ion radius. In the case of the mixture, the largerthe difference between the atomic weights the lower thestress by an increase of thermal insulation. However,

the slope in Fig. 2, which indicates the degree of increase conductivity. Therefore, the ZrO2–CeO2–Y2O3 coatingincreased DT more than the ZrO2–Y2O3 coating becausein thermal shock resistance, decreased because residual

stress formed during the high temperature thermal Ce has a much heavier atomic weight and a bigger ionradius than Y.cycling and phase transformation between tetragonal

and monoclinic phase might be increased with coatingthickness. When the bond coat layer in the 3.3. Microstructure of the coatingZrO2–CeO2–Y2O3 coating experienced a temperaturelower than 1100°C, the number of thermal cycles at The porosity fraction of the ceramic top coating is

shown in Fig. 4. Both coatings showed a 7~11% poros-failure increased suddenly and this may be due to theoxidation behavior of the bond coating. ity fraction and this did not alter much before or after

the thermal cycles. However, after the high temperaturethermal cycle porosity size increased from 5~7 mm to3.2. Thermal barrier effect20~30 mm because of high temperature sintering. Cross-sectional microstructures of as-sprayed and thermallyThe dependence between thermal barrier effect (DT )

and thickness of the ceramic top coating is shown in shocked coatings are shown in Fig. 5. With regards to

7C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

the as-sprayed condition, both coatings showed typicallamellar structures formed by continuous wetting byliquid droplets and had numerous oxide stringers, whichprobably formed during APS. The presence of an oxidelayer at the bond coat–ceramic interface in the failedspecimens, which were not present in the as-sprayedcoatings, indicates that the bond coat experiences hightemperature oxidation during thermal cycling. Failedspecimens underwent delamination of the TBC ceramicalong the bond coat–ceramic interface. Failure suggeststhat during high temperature thermal cycling, thermallyactivated processes possibly produced various stresses,such as bond coat oxide growth stress and phase trans-formation induced stress caused by a volume differencebetween tetragonal and monoclinic phases, and they areconcentrated near the topcoat–bondcoat interface whichcause TBC to spall from the substrate.

3.4. Bond coat oxidation

Cross-sectional SEM images and line scanning resultsof as-sprayed and after 1300°C thermal cycled TBCsystem with ZrO2–Y2O3 top coatings are shown inFigs. 6 and 7. Concerning as-sprayed samples, Al andO are concentrated in the oxide stringers located betweenthe NiCrAlY splats, and Ni and Cr are uniformlydistributed throughout the bond coat. However, afterthermal cycling the bond coat contains a generally highlevel of O, much higher than that of the as-sprayedbond coat. Ni and Cr diffuse from the bond coat to theceramic top coat and Al is distributed mainly near thebond coat due to diffusion in the high temperatureenvironment. To identify the oxide phases formed byhigh temperature thermal cycling, XRD data of bondcoatings as a function of temperature and time areshown in Fig. 8. Concerning the as-sprayed sample, onlythe c-Ni(Cr) phase was detected but, after thermalcycling at 1000°C for 50 h, the Al2O3 phase was formedmainly because Al is preferentially oxidized due to itshigher affinity for O than the other constituents of thebond coat [6 ]. After thermal cycling at 1100°C for 15 h,

Fig. 8. XRD patterns of bond coatings as a function of temperatureNiO and Ni(Cr,Al )2O4 spinels were formed, althoughand time: (a) as-sprayed; (b) heat treament at 1000°C for 50 h; (c)

the period of thermal cycling was short. Although other heat treatment at 1100°C for 15 h.data for the longer exposure at 1100°C and shorterexposure at 1000°C are not shown here because of the

in Fig. 9. The Al2O3 phase, found mainly near the bondlimited space, NiO and Ni(Cr,Al )2O4 spinels (almost)coat part in the oxide layer, is of a columnar structuredid not form unless the temperature of the bond coatand the NiO and Ni(Cr,Al )2O4 spinels, found mainlywas higher than 1100°C. From these data, it can be seennear the ceramic part in the oxide layer, are of athat the oxidation of the bond coat increases drasticallypolyhedral structure. So, thermally shocked specimensas the temperature exceeds 1100°C. This agrees with thehave a residual zirconia near the NiO andthermal shock resistance data in Fig. 2, which show howNi(Cr,Al )2O4 spinels.the number of thermal cycles at failure changes with the

Figs. 6–9 indicate that the onset of the growth ofexperienced temperature of the bond coat surface. InNiO and Ni(Cr,Al )2O4 spinels are more damagingother words, the exposed temperature influenced thebecause they exhibit a very high growth rate whichlifetime of the thermal barrier coating more than therapidly increases the volume of the bond coat. Intime of exposure. The microstructure of oxides which

were formed during high temperature oxidation is shown addition, NiO is an outward growing oxide [7]. Thus,

8 C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

Fig. 9. Microstructure of oxides: (a) Al2O3; (b) Ni(Cr,Al )2O4; (c) NiO.

the increase in volume of the thermally grown oxide ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 coatings are shown inFigs. 10 and 11. In the case of the as-sprayed condition,layer, especially that associated with the growth of NiO

into the ceramic top coat, appears to have caused the monoclinic phase was not detected in either coating.However, after thermal cycling (111) and (111∞) mono-stresses in the ZrO2 which are superimposed on the

thermally induced stresses which occur during high clinic phases were found only in the ZrO2–Y2O3 coating.In the as-sprayed condition, a cubic phase was detectedtemperature thermal cycling.in the ZrO2–CeO2–Y2O3 coating but not in theZrO2–Y2O3 coating, and after thermal cycling the intensity3.5. Phase transformationof cubic peaks in the ZrO2–CeO2–Y2O3 coating decreaseda little. It is well known that the presence of the monoclinicX-ray analyses were performed to determine the

extent of phase transformation. XRD patterns of the phase can reduce the useful life of TBCs owing to the

9C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

Fig. 11. XRD patterns of ZrO2–Y2O3 coatings: (a) analysis result ofthe monoclinic-tetragonal/cubic content; (b) analysis result of thetetragonal-cubic content.

Fig. 10. XRD patterns of ZrO2–CeO2–Y2O3 coatings: (a) analysisresult of the monoclinic-tetragonal/cubic content; (b) analysis resultof the tetragonal-cubic content.

coating had 10~19% monoclinic phase. Phase diagramsof ZrO2–Y2O3 and ZrO2–CeO2 [8,9] can show the reasonwhy a diffusionless phase transformation such as thislarge volume increase of about 3~4% associated with the

tetragonal–monoclinic phase transformation. happened after 1300°C thermal cycling. Concerning theas-sprayed samples, both coatings showed non-equilib-Therefore, the ZrO2–CeO2–Y2O3 coating can exhibit a

better thermal shock resistance than the ZrO2–Y2O3 coat- rium structures such as non-transformable tetragonal orcubic phases due to rapid cooling rates during the plasmaing as shown in Fig. 2. The phase fraction in the top

coating is summarized in Tables 4 and 5. With regards to spray coating process. However, after 1300°C thermalcycling, the metastable non-transformable tetragonalthe as-sprayed condition, the ratio of tetragonal to cubic

was about 75:25 in the ZrO2–CeO2–Y2O3 coating, and phase decomposed into the equilibrium cubic and tetra-gonal phase by diffusion of the stabilizer such as CeO2the ZrO2–Y2O3 coating had a 100% non-transformable

tetragonal phase. However, with regards to the after- and Y2O3. There are two types of this tetragonal phase,such as a high stabilizer non-transformable tetragonalthermal shock, the ratio of tetragonal to cubic was about

88:12 in the ZrO2–CeO2–Y2O3 coating and a monoclinic phase and a low stabilizer transformable tetragonal phase.In cooling to room temperature, the high stabilizer cubicphase was still not detected. However, the ZrO2–Y2O3

10 C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

Fig. 12. Thermal expansion data of as-sprayed coatings: (a) thermal expansion of ZrO2–CeO2–Y2O3 coating; (b) thermal expansion ofZrO2–Y2O3 coating; (c) thermal expansion coefficient of ZrO2–CeO2–Y2O3; (d) thermal expansion coefficient of ZrO2–Y2O3..

phase may be retained or it may transform to a high in the thermally shocked coating is probably that ther-mal expansion experiments were conducted after thestabilizer tetragonal phase. The low stabilizer tetragonal

phase may transform to monoclinic. From the viewpoint disappearance of residual stress and high temperaturesintering. To investigate the thermal expansion mismatchof phase equilibrium, it can be concluded that CeO2

provides partial stabilization of ZrO2 over a wide range stress which influences the lifetime of the TBC, theaverage thermal expansion coefficient of both coatingsof compositions. So, the ZrO2–CeO2–Y2O3 coating needed

a higher temperature and more time to form a trans- were calculated and this is shown in Fig. 12. The averagethermal expansion coefficient of the ZrO2–CeO2–Y2O3formable tetragonal phase to transform into the mono-

clinic phase which induces volume expansion. coating (4.35×10−6°C−1) is higher than that of theZrO2–Y2O3 coating (3.29×10−6°C−1). This means thatthe thermal expansion mismatch stress is smaller when3.6. Thermal expansion propertiesa ZrO2–CeO2–Y2O3 coating is used.

Furthermore, the thermally shocked ZrO2–Y2O3The thermal expansion data of as-sprayed and ther-mally shocked coatings are shown in Figs. 12 and 13. coating exhibits non-linear behavior attributed to the

phase transformation between the tetragonal and mono-Concerning the as-sprayed samples, both coatings showsome shrinkage after thermal expansion experiments clinic phases as shown in Fig. 13 and this agrees well

with XRD data shown in Fig. 11. From these data andfrom 100 to 1300°C for the heating and cooling cycles.This shrinkage is probably due to the formation of published reports dealing with the effect of tetragonal

precipitate size [10–12], the following route can bemicrocracks during heating and the consequent releaseof spray-process-induced residual stresses and high tem- expected for the phase transformation:perature sintering. However, in the thermally shockedcondition it is observed that there is 0.1% elongation in (a) Retained tetragonal monoclinicon heating at 200°C,the ZrO2–CeO2–Y2O3 coating and 0.05% elongation inthe ZrO2–Y2O3 coating. The reason for this elongation (b) Monoclinic tetragonal on heating at 550°C.

11C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

Fig. 13. Thermal expansion data of thermally shocked coatings: (a) thermal expansion of ZrO2–CeO2–Y2O3 coating; (b) thermal expansion ofZrO2–Y2O3 coating; (c) thermal expansion coefficient of ZrO2–CeO2–Y2O3; (d) thermal expansion coefficient of ZrO2–Y2O3.

From this phase transformation, it is likely that the and consequently oxide growth rate and resulting stressdecreased.ZrO2–CeO2–Y2O3 coating has better thermal shock

resistance than the ZrO2–Y2O3 coating due to the small However, in the light of phase transformation andresidual stress during high temperature thermal cyclingvolume change in the ZrO2–CeO2–Y2O3 coating.

4. Summary

Based on the above observed properties of plasma-sprayed ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 coatingsexposed to 1300°C thermal cycling, a schematic diagramof the lifetime controlling factors for the ZrO2–Y2O3and ZrO2–CeO2–Y2O3 can be drawn (Fig. 14). Themain factors influencing the life of TBCs have beenfound to be bond coat oxidation behavior, thermalexpansion mismatch stress and phase transformationinduced stress as a function of coating thickness asshown in Fig. 14.

In the light of bond coat oxidation, which might bethe most important factor among others, as coatingthickness increased the lifetime of TBC also increased Fig. 14. Schematic diagram of the lifetime controlling factors in the

thermal barrier coating system.because bond coat surface temperature would be lower

12 C.H. Lee et al. / Surface and Coatings Technology 124 (2000) 1–12

[2] S. Sodeoka, M. Suzuki, T. Inoue, K. Ueno, S. Oki, Proc. NTSCsuch as thermal expansion mismatch stress, the resulting(1995) 295.concentrated stress increased as coating thickness

[3] H. Toraya, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. C-119increased. Therefore, there may be a critical thickness of (June, 1984).top coat in the 1300°C high temperature environment. [4] H. Toraya, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc.

September C-183 (Sept, 1984).The ZrO2–CeO2–Y2O3 coating was superior to the[5] M.B. Bever, Encyclopedia Mater. Sci. Eng. 4 (1999) 4916–4922.ZrO2–Y2O3 coating concerning all of these factors —[6] S. Stecura, Thin Solid Films 182 (1989) 121.

bond coat oxidation, phase transformation and thermal [7] R.D. Maier, C.M. Scheuermann, C.W. Andrews, J. Am. Ceram.expansion mismatch stress. Soc. Bull. 60 (1981) 555.

[8] P. Duran, M. Gonzalez, C. Moure, J.R. Jurado, C. Pascual,J. Mater. Sci. 25 (1990) 5001.

[9] H.G. Scott, J. Mater. Sci. 10 (1975) 1527.[10] S. Rangaswamy, H. Herman, S. Safai, Thin Solid Films 73

(1980) 43.References[11] H.K. Kim, H.S. Choi, C.H. Lee, J. Kor. Inst. Met. Mater. 36 (9)

(1998) 1451.[1] S.H. Lee, J.S. Yoo, J.Y. Kim, C.S. Kang, J. Korean Inst. Met. [12] J.R. van Valzah, H.E. Eaton, Surf. Coat. Technol. 46 (1991)

289–300.Mater. 35 (1997) 1744.