Surface & Coatings Technology 200 (2006) 5869–5876www.elsevier.com/locate/surfcoat

Effects of phase constituents/microstructure of thermally grown oxide on thefailure of EB-PVD thermal barrier coating with NiCoCrAlY bond coat

J. Liu, J.W. Byeon, Y.H. Sohn ⁎

Advanced Materials Processing and Analysis Center and Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando,FL 32816-2455, United States

Received 18 August 2005; accepted in revised form 31 August 2005Available online 25 October 2005

Abstract

A correlation among thermal cycling lifetime, bond coat surface preparation, phase constituents and microstructure of thermally grown oxide(TGO) was examined for electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) consisting of ZrO2–7 wt.% Y2O3

(YSZ) ceramic topcoat, NiCoCrAlY bond coats and CMSX-4 superalloy. Variation in the bond coat surface is characterized based on surfaceroughness modification and pre-oxidation (1100 °C at PO2

of 10−8 atm up to 4 h) carried out prior to YSZ deposition by EB-PVD. TBC specimenswith pre-oxidized bond coats exhibited longer lifetimes than those without pre-oxidation, especially for metallographically polished bond coats.Given a surface roughness of NiCoCrAlY bond coats, TBC lifetime was observed to increase with an increase in the amount of α-Al2O3 in theinitial thermally grown oxide (TGO) scale, which was detected by photostimulated luminescence (PL). Using focused ion beam in-situ lift outtechnique (FIB-INLO), site-specific preparation of thermally cycled TBC specimens for transmission electron microscopy (TEM) wassuccessfully carried out. In addition to the presence of α-Al2O3, a small particulate phase identified as cubic Y2O3 was observed within the TGOscale by electron diffraction analysis on TEM.© 2005 Elsevier B.V. All rights reserved.

Keywords: Thermal barrier coatings; Surface modification; Photoluminescence; Thermally grown oxide; Thermal cycling

1. Introduction

Thermal barrier coatings (TBCs) have helped to improve thehigh temperature durability and performance of hot sectioncomponents in advanced gas turbine engines [1–5]. Generally,TBC system consists of four layers: ceramic topcoat, thermallygrown oxide (TGO), metallic bond coat, and superalloysubstrate. Electron beam physical vapor deposition (EB-PVD)has provided TBCs with advantageous properties, such asimproved durability, smooth surface finish and good erosionresistance [6]. In EB-PVD TBCs, characteristics of bond coatsurface and the growth of TGO are considered to be the crucialfactor influencing the failure of TBCs [7–9]. The initial defectsand surface irregularities of the bond coats can give rise to in-plane and out-of-plane tensile stresses. TGO growth results in a

⁎ Corresponding author. Tel.: +1 407 882 1181; fax: +1 407 882 1462.E-mail address: ysohn@mail.ucf.edu (Y.H. Sohn).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.08.140

constrained volume expansion that leads to compressive“growth” stresses (<1 GPa) persisting at all temperatures. Uponcooling, the thermal expansion mismatch between the TGO andbond coat leads to a high compressive residual stress in the TGO[10–14] that provides the strain energy that can drive spallation,typically within and/or near the TGO scale.

Phase constituent of the TGO is considered to be a criticalfactor influencing the adhesion at the TGO/YSZ interface. Theformation of the metastable θ- and/or γ-Al2O3 and itsconversion to the stable α-Al2O3 in the TGO has been reportedto have a profound effect on the structural integrity of the TGOduring thermal cycles [15,16]. The polymorphic transformationof Al2O3 may affect the residual stress in the TGO due tovolumetric constraint and the nucleation of sub-critical cracks[17,18]. Thus, the formation of an “optimum” TGO that onlyconsists of α-Al2O3 prior to the deposition of topcoat may helpto improve the durability and reliability of TBCs.

In this study, TBCs, produced by the EB-PVD as a functionof NiCoCrAlY bond coat surface roughness, were examined to

Table 1Summary of bond coat surface roughness, Ra, the fracture surface characteristics and lifetime of TBCs as a function of bond coat surface preparation

Bond coat surfacepreparation

Pre-oxidation a Average bond coat surfaceroughness (Ra, nm)

Fracture path Percentage ofTGO on theexposed bondcoat %

Average lifetimeof TBCs (thermalcycles)

Average St. Dev.

As-sprayed b No 1321.3 601.6 YSZ/TGO and TGO/Bond coat 70.5 87.5Yes 1321.3 601.6 YSZ/TGO and TGO/Bond coat 58.9 101.0

Hand-polished c No 4.3 8.4 * * 0.0Yes 302.6 21.1 TGO/Bond coat 18.0 95.0

Barrel-finished d No 320.1 10.3 YSZ/TGO 98.9 25.0Yes 491.2 22.9 TGO/Bond coat 24.1 50.0

*No observable TGO. Fracture between the YSZ and bond coats prior to thermal cycling.a Heat treatment at PO2

≌10−8 atm and 1100 °C for up to 4 h to form continuous TGO layer.b Surface finish as low pressure plasma was sprayed.c Surface finish by metallographic polishing.d Media tumble treatment for 90 min.

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investigate the influence of bond coat surface modification andpre-oxidation heat treatment on their lifetime, microstructuraldevelopment and failure characteristics.

2. Experimental procedure

Twenty-six disk-shaped specimens of CMSX-4 (Ni–9.0 Co–6.5 Cr–6.5 Ta–6.0 W–5.6 Al–3.0 Re–1.0 Ti–0.6 Mo–0.1 Hf inwt.%) superalloys coated with low pressure plasma sprayed(LPPS) NiCoCrAlY (PWA276) bond coat and EB-PVD ZrO2–7 wt.% Y2O3 (YSZ) topcoat were employed in this study.Various surface preparation and heat treatment were carried outbefore the deposition of the top YSZ coatings as listed in Table1. The bond coat surface can be categorized based on processingtechnique: as-sprayed (as low-pressure plasma sprayed), hand-polished metallographically down to 0.25 μm, and barrel-finished (media tumble treatment for 90 min). Moreover, aftereach kind of the bond coat surface processing, selectedspecimens were pre-oxidized in an oxidizing environment(PO2

≌10−8 atm) at 1100 °C for up to 4 h.Prior to YSZ deposition, bond coat surface roughness and

phase constituents within the initial oxide layer were

Fig. 1. Thermal cycling lifetime of TBCs as a function of bond coat surface finish aspecimens tested for the lifetime determination.

examined by optical profilometry (OP) (WYKOTM NT 3300optical profilometry) and photostimulated luminescence spec-troscopy (PL) (Renishaw™ System 1000B Ramanscope™),respectively. From the bond coat surface roughness profiles,the average value of the roughness Ra was calculated using:

Ra ¼ 1n

Xn

i¼1

Ri ð1Þ

where Ri is the surface roughness of each lateral resolution(0.3 μm) with vertical resolution of 0.1–3.0 nm. Eachmeasurement contained 1000 spatial sampling (n).

After the topcoat deposition, changes in the phaseconstituents of the TGO scale were examined by usingphotostimulated luminescence (PL). Intensity ratio of α-Al2O3 to the total intensity, Iα/T was determined by:

Iα=T ¼ IRIR þ IN þ Im

ð2Þ

where IR, IN, and Im refer to the integrated luminescenceintensities of α-Al2O3, N-luminescence, and metastable aluminaluminescence, respectively.

nd pre-oxidation heat treatment. Values in parenthesis represent the number of

Fig. 3. Magnitude of compressive residual stress in the α-Al2O3 scale for TBCs,determined from PL measurements, as a function of thermal cycle. The bondcoats for these TBCs were not pre-oxidized.

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A furnace thermal cycling test, that consisted of 10-min heat-up to 1121 °C, 40-min hold at 1121 °C, followed by 10-minforced air-quench, was carried out for all TBC specimens.During the test, specimens were periodically withdrawn for PLmeasurements at room temperature to assess the residual stressand phase transformations within the TGO as a function ofthermal cycles. The specimens were considered to have failedwhen the YSZ spallation area was greater than 50%.Microstructure and phase constituents of the fracture surfaces(i.e., the top surface of the bond coat where the YSZ had spalled,and the bottom surface of the spalled YSZ) and cross-sectionswere analyzed by scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (XEDS). Focused ion beamin-situ lift out technique (FIB-INLO) [19] was employed for thepreparation of site-specific transmission electron microscopy(TEM) specimens with an emphasis on the TGO scale. TEM andscanning TEM (STEM) analysis included high angle annulardark field (HAADF) imaging, electron diffraction, and XEDS.

3. Results

Fig. 1 shows the average thermal cycling lifetime of TBCspecimens as a function of bond coat surface preparation. Thereported lifetimes are low for all TBCs when compared to thoseavailable in numerous literatures. In addition, without pre-oxidation, TBCs with hand-polished bond coats failed beforeany thermal cycling (i.e., 0 lifetime). The lifetime of all TBCsincreased after pre-oxidation. The most significant improve-ment in TBC lifetime was observed for specimens with hand-polished bond coats that were pre-oxidized prior to the EB-PVDof YSZ.

Table 1 reports the bond coat surface roughness values. Theroughest surface was observed for as-sprayed bond coat, whilethe smoothest surface was observed for hand-polished bondcoat. Fig. 2 presents the typical PL spectra from the TGO scale

Fig. 2. Typical PL spectra from the TGO scale developed on the TBCspecimens with barrel-finished bond coats with and without pre-oxidationand TBC specimen with hand-polished bond coat and pre-oxidation, showingN-luminescence.

developed on the TBC specimen with barrel-finished bondcoats with and without pre-oxidation. Similar spectra wereobtained for TBC specimens with as-sprayed bond coat. The N-peaks of typical PL spectrum from the TGO scale developed onthe TBC specimen with hand-polished bond coats with pre-oxidation was also shown in Fig. 2.

Figs. 3 and 4 present the magnitude of compressive residualstress in the α-Al2O3 scale as a function of thermal cycle forTBCs produced without and with pre-oxidation heat treatment,respectively. In Fig. 3, the magnitude of the initial compressiveresidual stress is higher for TBCs with as-sprayed bond coat,and lower for the TBCs with barrel-finished bond coat.However, during the thermal cycling test, the compressiveresidual stress in the α-Al2O3 scale increased rapidly for TBCswith barrel-finished bond coats after 5 cycles. In Fig. 4, themagnitude of the initial compressive residual stress is thehighest for TBCs with hand-polished bond coats, followed byas-sprayed and barrel-finished. During thermal cycling test, themagnitude of compressive residual stress in the α-Al2O3 scale

Fig. 4. Magnitude of compressive residual stress within the α-Al2O3 scale forTBCs, determined from PL measurements, with pre-oxidized bond coats as afunction of thermal cycle.

Fig. 5. Cross-sectional backscattered electron micrographs of failed TBC specimens with (a) hand-polished and pre-oxidized bond coat, showing Yand Hf-rich oxidesembedded in the TGO; (b) Cross-sectional backscattered electron micrograph of TBC specimen with as-sprayed and pre-oxidized bond coat; showing Yand Hf or Ni,Co and Cr-rich particles embedded in the TGO.

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of TBCs with hand-polished and as-sprayed bond coatsremained high compared to that of TBCs with barrel-finishedbond coats as shown in Fig. 4. The standard deviation of thecompressive residual stress during thermal cycling issignificantly larger for TBCs with barrel-finished bond coat.The initial rise in compressive residual stress after few thermalcycles, as shown in Figs. 3 and 4 can be attributed to the

Fig. 6. (a) HAADF images of Y2O3 embedded in the TGO (α-Al2O3) for failed TBCthe Y2O3/α-Al2O3 interface boundaries are very rich in Hf; (b) Diffraction pattern ofmarked as (c) in (a); (d) HAADF image of TGO of as-sprayed and pre-oxidized bo

coverage of TGO scale from discontinuous to continuous[20,21]. In Gell's studies, the initial oxide scale did not form acontinuous layer until after at least 10, 1-h thermal cycles at1121 °C.

Cross-sectional microstructure of TBCs with pre-oxidizedbond coats are shown in Fig. 5. Fig. 5(a) presents the cross-sectional microstructure of failed TBCs with hand-polished

specimen with hand-polished and pre-oxidized bond coats. Small precipitates atY2O3 (cubic) particles marked as (b) in (a); (c) Diffraction patterns of α-Al2O3

nd coat.

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bond coat. Particles rich in Yand Hf were frequently found in theTGO scale. Fig. 5(b) shows the cross-sectional microstructure offailed TBCs with as-sprayed bond coats. Particles rich in Yand Hf were also frequently found within the TGO, which ispenetrated into the bond coats (i.e., pegging). Oxides rich inNi, Co, and Cr were frequently found near the fractureinterfaces, which may have led to the final failure.

Fig. 6(a) shows a HAADF image obtained from STEM ofparticles embedded within the TGO layer parallel to the YSZ/TGO/bond-coat interfaces for hand-polished and pre-oxidizedbond coats. The TGO primarily consisted of α-Al2O3 asconfirmed by the diffraction pattern in Fig. 6(c). XEDS showsthat the particles embedded in α-Al2O3 are rich in Y or Hf. Y-rich particles were identified as cubic-Y2O3 by the diffractionpattern shown in Fig. 6(b). Hf concentration was observed to behigher at Al2O3 grain boundaries and Al2O3/Y2O3 interphaseboundaries. Specifically, grain boundaries of Al2O3 gave rise tobrighter contrast in HAADF images in Fig. 6 (a). For TBCs withas-sprayed and pre-oxidized bond coats, the TGO also primarilyconsisted of α-Al2O3. Y2O3 and Cr-rich particles embedded inα-Al2O3 TGO were also observed as shown in Fig. 6(d).

Backscattered electron images from the bottom surface ofthe spalled YSZ and the top surface of the bond coats where

Fig. 7. (a) Backscattered electron image of the bottom surface of spalled YSZ and (b)without pre-oxidation. The spallation has occurred after 20 cycles along the YSZ/TGOand (d) the top surface of the bond coat for TBC specimen with barrel-finished and prebond coat interface.

YSZ had spalled are presented in Fig. 7. For TBC specimenwith barrel-finished bond coat without any pre-oxidation heattreatment, the phase constituents on the fracture surfacessuggest that the spallation occurred primarily along theinterfaces between the YSZ and TGO as shown in Fig. 7(a)and (b). However, after pre-oxidation, the phase constituentsof the fracture paths changed, along with the increase inthermal cycling lifetime. For TBC specimens with barrel-finished bond coat surface with pre-oxidation heat treatment,the phase constituents on the fracture surfaces indicate thatthe spallation occurred primarily along the TGO/bond coatinterface as presented in Fig. 7(c) and (d).

Characteristics of fracture paths and lifetimes of TBCs as afunction of bond coat surface preparation are summarized inTable 1. For TBCs with hand-polished and barrel-finished bondcoats after pre-oxidation, more TGO remained on the bottomsurface of the spalled YSZ with increases in the thermal cyclinglifetime. The fracture path remained similar for TBCs with as-sprayed bond coat (i.e., both interfaces of YSZ/TGO and TGO/bond coat as well as within the TGO) after the pre-oxidationheat treatment: the improvement in the lifetime of the TBCswith as-sprayed bond coats was not significant after the pre-oxidation heat treatment.

the top surface of the bond coat for TBC specimen with barrel-finished bond coatinterface. (c) Backscattered electron image of the bottom surface of spalled YSZ-oxidation bond coat. The spallation has occurred after 75 cycles along the TGO/

Fig. 9. Correlation among the initial residual compressive stress within the α-Al2O3 scale, bond coat surface preparation and the thermal cycling lifetime for

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4. Discussion

Fig. 8 correlates the bond coat surface preparationtechnique, the average bond coat surface roughness (Ra), andthe average thermal cycling lifetime of TBCs. For TBCswithout pre-oxidation heat treatment, a longer TBC lifetimewas observed with rougher bond coats as presented in Fig. 8(a). However, such a simple relation does not exist for TBCsafter the pre-oxidation heat treatment as presented in Fig. 8(b). TBCs with hand-polished and pre-oxidized bond coatsyielded higher durability than TBCs with barrel-finished andpre-oxidized bond coat. Clearly, the surface roughness of thebond coat alone is not the only factor that affects the lifetimeof the TBCs.

For TBC specimens produced without any pre-oxidationheat treatment, Fig. 9(a) shows the correlation among the initialresidual stresses of the TGO scale (α-Al2O3), bond coat surfacepreparation and the thermal cycling lifetime. The magnitude ofinitial compressive residual stress in the α-Al2O3 is only slightlyhigher for TBCs with as-sprayed bond coats, which has thelonger lifetime. For TBCs with pre-oxidized bond coats, themagnitude of the initial compressive residual stress in the α-Al2O3 was similar for the TBCs with as-sprayed and barrel-finished bond coats. However, the lifetime of these two types of

Fig. 8. Correlation among the bond coat surface preparation method, the bondcoat surface roughness and the lifetime of EB-PVD TBCs with NiCoCrAlYbond coat: (a) without pre-oxidation heat treatment; and (b) with pre-oxidationheat treatment.

TBCs specimens: (a) without pre-oxidation heat treatment and (b) with pre-oxidized bond coats.

specimens had a great difference as shown in Fig. 9(b).Throughout thermal cycling, for TBCs without pre-oxidationheat treatment, the magnitude of compressive residual stresswithin the TGO was observed to be lower for the specimenswith as-sprayed bond coat. These TBCs had a longer lifetime.For TBC specimens with barrel-finished bond coat and a shorterlifetime, the magnitude of the compressive residual stress in theTGO was higher as shown in Fig. 3. For TBCs with pre-oxidized bond coats, the magnitude of compressive residualstress within the TGO remained higher during thermal cyclingfor the TBCs with longer lifetime as shown in Fig. 4. Clearly,the initial compressive residual stress within the TGO is not theonly factor that can be directly and simply related to the lifetimeof the TBCs.

A longer lifetime was obtained for specimens with higherrelative luminescence intensity from the equilibrium α-Al2O3 inthe initial TGO layer for a given surface preparation/roughnessas shown in Fig. 10. Phase transformation from the metastableto stable phase may be accompanied by the formation of voidsdue to volumetric constraints. For example, there is about 4.7%volume contraction for θ- to α-Al2O3 phase transformation [18].Thus, these phase transformations can create damages at theYSZ/TGO interface. However, controlled pre-oxidation heattreatment that can promote the formation of α-Al2O3 and stoppremature failure at the YSZ/TGO interface can increase thelifetime of the TBCs.

According to Czech's study [22], a continuous α-Al2O3

layer formed on the hand-polished surface after oxidation at950° and 1000 °C. In this study, N-luminescence from TGOscale was observed from the pre-oxidized TBC specimens with

Fig. 11. Correlation among the bond coat surface preparation method, initialresidual stress within the α-Al2O3 scale, and initial relative intensity ratio of theequilibrium α-Al2O3 for pre-oxidized TBC specimens.

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hand-polished bond coat as shown in Fig. 2, presumably due toa significant presence of Cr3+ in α-Al2O3. The formation ofCr2O3 has been reported to assist the nucleation of α-Al2O3

[18]. In addition, hand-polishing may introduce residualstresses near the bond coat surface, which can provideadditional strain energy that can aid the equilibrium phasenucleation and/or transformation. Moreover, the hand-polishedprocess can remove the absorbed contaminants that canadversely affect the phase nucleation/transformation. Thesecontaminants can also influence the integrity of TGO/bond coatinterface [23].

Initial phase constituents within the TGO, not only affectedthe lifetime of TBCs, but also influenced the initial com-pressive residual stress within the α-Al2O3. Before thermalcycling, the higher relative luminescence intensity from theequilibrium α-Al2O3 corresponded to the larger magnitude ofinitial compressive residual stress for TBCs with pre-oxidizedbond coats as shown in Fig. 11.

A significant amount of spinels were observed at fractureinterfaces as shown in Fig. 5. According to Lee's and Haynes'sstudies [24,25], they found that the fracture in the bond coatoxidation products often takes place in the region of spinels.Spinels are considered to be brittle material with low fracturetoughness that weakens the interface [26,27].

The difference in the thermal expansion coefficient of thebond coat and TGO is much larger than that of the TGO andYSZ.[28] Therefore, after a significant repeated thermal fatigue,fracture would prefer to propagate at the TGO/bond coat

Fig. 10. Correlation among the relative initial intensity ratio of α-Al2O3, bondcoat surface preparation, and the thermal cycling lifetime of TBCs specimens:TBCs with (a) as-sprayed bond coat; (b) hand-polished bond coat; (c) barrel-finished bond coat.

interface. For specimen with shorter lifetime, spallation occursat the interface of YSZ/TGO, due to a poor adhesion betweenthe YSZ and the TGO. Thus, TBCs with the longest durabilitycan be achieved by having the fracture paths restricted to theTGO/bond coat interface when an optimum TGO scale with agood adhesion between YSZ and TGO is established via pre-oxidation heat-treatment. Similar spallation results were foundby Mumm and Evans using EB-PVD TBCs [29]. Based ontheir research, after short time (<10 h) of isothermal exposureat 1100 °C, delamination occurs primarily within the TGOand YSZ. After longer exposures (<100 h), delaminationoccurs principally along the interface of TGO and bond coat.

According to Wright [30], reactive elements (RE) (e.g., Y)inhibit S segregation at the metal-oxide interface. Also, REsegregate to oxide grain boundaries, where they can sig-nificantly reduce the outward transport of Al, hence decreasethe rate of oxidation (now mostly by oxygen transport),drastically change the oxide morphology and contribute to theimproved scale adherence (reduced interfacial void formation)[23]. However, oversupply of the RE can be detrimental.Increased concentration of RE in the bond coat can causeinternal oxidation and formation of RE oxides [24] (e.g., HfO2,Y2O3 and YAG) within the TGO scale as shown in Fig. 6. Also,the formation of RE oxides can act as fast diffusion paths foroxygen and cause mechanical disruption if incorporated into theTGO scale. Extensive formation of Y2O3 and other RE oxidesmay be responsible for overall poor performance of lifetimeobserved in this study.

5. Conclusion

Effects of surface preparation methods for a NiCoCrAlYbond coat on the thermal cyclic lifetime and failure of an EB-PVD TBC were investigated in this study. Findings from thisinvestigation are listed below:

• A longer lifetime during 1-h thermal cycling at 1121 °C wasobserved for EB-PVD TBCs with as-sprayed NiCoCrAlYbond coats regardless of pre-oxidation and TBCs with hand-polished bond coats only after pre-oxidation.

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• Lifetime of EB-PVD TBCs, in general, was observed toincrease when a pre-oxidation heat treatment at 1100 °C withPO2

=10−8 atm was carried out prior to the YSZ deposition.With pre-oxidation heat treatment, relative photostimulatedluminescence intensity of the α-Al2O3 increased. Thus, theimprovement in TBC lifetime can be correlated with theincrease in luminescence intensity of α-Al2O3 in the TGOscale given a surface preparation/roughness. The lifetimeimprovement due to the pre-oxidation was particularlysignificant to TBCs with hand-polished NiCoCrAlY bondcoat.

• Spallation-fracture paths depended on the lifetime of TBCs.Premature spallation of TBCs occurs at the interface betweenthe YSZ and TGO. Longer durability can be achieved byrestricting the fracture paths to the TGO/bond coat interface.

• Small particulate phase observed through the TGO scale wasidentified as Y2O3 (cubic) by diffraction analysis on TEM.While the addition of Y in the NiCoCrAlY bond coat helpsthe adhesion of the TGO scale, excessive alloying can lead todeleterious effects.

Acknowledgements

The authors would like to thank Drs. F.S. Pettit and G.H.Meier at University of Pittsburgh for pre-oxidation heattreatment of TBC specimens, and Mr. K.S. Murphy at Howmetfor processing the TBC. Authors would also like to express theirgratitude to Drs. M. Gell and E.H. Jordan for financial supportof this study, which was carried out as a part of UniversityTurbine Systems Research (No. 01-01-SR091) awarded bySouth Carolina Institute for Energy Studies administered by Dr.R.A. Wenglarz.

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