Damage Evaluation of Thermal Barrier Coatings Subjected to a High-VelocityImpingement of a Solid Sphere under Room and High Temperature Conditions+1

Kiyohiro Ito+2 and Masayuki Arai

Department of Mechanical Engineering, Faculty of Engineering, Tokyo University of Science, Tokyo 125-8585, Japan

Thermal barrier coatings (TBCs) applied to turbine blades in jet engines or gas turbines are at risk of high-velocity impingement fromdifferent foreign objects, which can cause severe damage from delamination of the TBC. In this study, a high-velocity impingement test systemis developed under high-temperature conditions to understand the delamination mechanism of TBCs under actual operation conditions. In thissystem, a spherical impactor with a diameter of 1­8mm can be impinged onto a TBC specimen at different temperatures up to 900°C. High-velocity impingement tests were conducted on atmospheric plasma-sprayed TBC specimens with 8mass% yttria-stabilized zirconia topcoat (TC)under room temperature (RT) and 900°C conditions. The results indicate that a hemispherical indentation was formed at 900°C, which indicatesplastic deformation of the TC, unlike the brittle deformation observed at RT. In addition, vertical and interfacial cracks formed under both RTand 900°C conditions. Cross-sectional observation revealed that the formation process of the interfacial crack at 900°C was different from that atRT. In particular, the interfacial crack tended to become significantly longer at more than 170m/s at 900°C.[doi:10.2320/matertrans.MT-T2021001]

(Received May 11, 2021; Accepted September 24, 2021; Published November 5, 2021)

Keywords: thermal barrier coatings, foreign object damage, high-velocity impingement, delamination, crack

1. Introduction

Thermal barrier coatings (TBCs) are widely applied tothe surfaces of turbine blades in jet engines and gas turbinesin thermal power plants to protect the metallic substrate fromhigh-temperature combustion gas flow.1) TBCs typicallyconsist of two layers; one is a ceramic top coating (TC)with low thermal conductivity, and the other is a metallicbond coating (BC) for suppression of the mismatch ofthermal expansion coefficients between the TC andsubstrate.2) The TC layer is typically deposited by atmospher-ic plasma spraying (APS) or electron beam-physical vapordeposition (EB-PVD). The former is employed for land-based gas turbines; the latter is employed for jet engines.

TBCs are exposed to high-temperature environmentsduring operation. Delamination of the TC layer is inducedby aging degradations such as oxidation of the BC layer.Numerous researchers have attempted to improve thedelamination resistance of the TC layer. Katayanagi et al.proposed an improvement method for the delaminationresistance of the TC layer. This method positively promotesthe internal oxidation of the BC by adding a small amount ofCeO2 or ZrO2 to the CoNiCrAlY matrix, which is typicallyused for a BC.3) Ito et al. reported that the interfacial fracturetoughness can be improved by removing unmelted particlesand controlling the TC/BC interfacial roughness by polishingand grit-blasting the BC surface.4)

In addition to such aging degradations, sudden damagecaused by the impingement of foreign objects must beaddressed. When foreign objects such as sand, volcanic ash,and tiny metallic fragments are ingested into a jet engineor gas turbine, the high-velocity impingements of the foreign

objects onto the surface of the turbine blade can lead tocracks or delamination of the TC layer. This type of damageis called foreign object damage (FOD), and is particularlyproblematic in jet engines.5,6) FOD can cause delaminationof the TC layer, which can lead to severe damage to the jetengine or gas turbine. Therefore, it is necessary to clarify thedamage mechanism of TBCs caused by FOD.

Research regarding the damage mechanism of TBCscaused by FOD is being actively conducted globally.7­9)

Chen et al. evaluated the damage mechanism of TBCsformed by EB-PVD based on the burner rig test.10) In thistest, alumina powder with a diameter of 50­560 µm wasinjected into a high-temperature combustion gas frame andimpinged onto the surface of a TBC specimen heated at1232°C in the velocity range of 10­170m/s. In addition,the interfacial crack length formed at the TC/BC interfacewas estimated based on the energy release rate evaluatedfrom the stress distribution obtained by finite elementanalysis (FEA). The results indicated that the estimatedinterfacial crack length was virtually consistent with thatobserved in the experiment. However, it was difficult toreveal the causality, i.e., rigorously associating the individualimpingement of particles with the resultant damages in thiskind of experiment where numerous small particles impingeonto a specimen.

Choi et al. investigated the damage mechanism of TBCsformed by EB-PVD based on a high-velocity impingementtest with a solid sphere.11) In this test, a high-carbonchromium steel ball (JIS code: SUJ2) with a diameter of1.6mm was impinged onto a TBC specimen at 150­300m/s.Subsequently, the interfacial crack length formed at theTC/BC interface resulting from the impingement test wasmeasured. In addition, they proposed an equation for theestimation of the interfacial crack length based on the energyconservation during the impingement process and reportedthat the interfacial crack length estimated by the proposedequation was in reasonable agreement with that evaluatedby the impingement test. Unlike an experiment based on

+1This Paper was Originally Published in Japanese in the J. Japan ThermalSpray Society 58(1) (2021) 4­10. The abstract and captions of Figs. 1­8,10 and 11 were slightly modified from the original paper.

+2Corresponding author, E-mail: kiyohiro@rs.sus.ac.jp, Present address:Department of Mechanical and Electrical Engineering, Faculty ofEngineering, Suwa University of Science, Chino 391-0292, Japan

Materials Transactions, Vol. 62, No. 12 (2021) pp. 1703 to 1709©2021 Japan Thermal Spray Society

numerous small particle impingements, the impingement ofa projectile can be definitively associated with the resultantdamage in the high-velocity impingement test using a solidsphere. Therefore, it is a powerful method for clarifying thedamage mechanism of TBCs caused by FOD. The majorityof impingement tests have been conducted at room temper-ature (RT). However, it is important to evaluate the damagecaused under high-temperature environments that are closerto the actual environment in gas turbines.

Our research group has developed a high-velocityimpingement testing system where a solid sphere can beimpinged onto a TBC specimen at RT.12) Using this system,the damage mechanism of TBCs and metallic substratescaused by FOD has been investigated.13,14) In this study, toclarify the damage mechanism of TBCs caused by FODunder a high-temperature environment, a new high-velocityimpingement testing system (H-SPITS) wherein a solidsphere can be impinged at environmental temperatures upto 900°C is developed. The influences of temperature andimpingement velocity on the deformation and damagebehaviors of TBCs are investigated through a high-velocityimpingement test conducted at RT and 900°C. In addition,the problem of interfacial crack formation is discussed bysimulating the impingement of a solid sphere on a TBCspecimen using FEA.

2. Outline of Developed H-SPITS

A schematic of the H-SPITS is displayed in Fig. 1. In thissystem, a solid sphere (projectile) with a diameter of 1­8mmis accelerated through a stainless steel pipe by high-pressurenitrogen (N2) or helium (He) gas, followed by impingementonto the surface of a TBC specimen placed in a vacuumchamber. A cylindrical ceramic fiber heater (VC401E06A,Sakaguchi E.H VOC Corp., Japan) is installed in the vacuumchamber. A quartz glass pipe is placed in the heater. Acylindrical-shaped TBC specimen with a marginally smallerdiameter than the inner diameter of the quartz glass pipecan be used. It is inserted into the quartz glass pipe fromthe rear side of the vacuum chamber. The temperature ofthe specimen can be controlled to a target value using aK-type sheathed thermocouple (1HKX325, CHINO Corpo-ration, Japan) and desktop-type temperature control device(DSSP23, SHIMADEN Co., Ltd., Japan). As indicated inFig. 1, the specimen is not located in the center of the heaterbut marginally in front of it to prevent damage to the heaterby a rebounded projectile. In addition, the thermocouplecontacts the rear surface of the specimen. Therefore, it should

be noted that a small temperature gradient is generated inthe specimen. In fact, for the high-temperature test, the rearsurface of the specimen was heated to approximately 950°Cto maintain the front surface temperature at 900°C.

The vacuum chamber in the H-SPITS is displayed inFig. 2. The pressure in the vacuum chamber can bemoderated by an oil-sealed rotary vacuum pump (G50DA,ULVAC KIKO, Inc., Japan) up to approximately 500 Pa.Thus, the oxidation of the specimen can be suppressed evenunder maximum temperature conditions. Moreover, the lowair resistance contributes to the enhancement of theimpingement velocity of the projectile. The impingementvelocity of the projectile is measured using two continuouswave semiconductor lasers (LDU33, SIGMAKOKI CO.,LTD., Japan) placed at a 60mm interval. The time gapbetween these two lasers obscured by a projectile is detectedusing an oscilloscope (DCS-7506, TEXIO TECHNOLOGYCORPORATION, Japan).

The specifications of the H-SPITS are summarized inTable 1. The maximum impingement velocity depends onthe size of the projectile. A maximum impingement velocityof 700m/s can be achieved with a projectile of 1mmdiameter, while that is restricted to 450m/s with a projectileof 3mm diameter.

3. Damage Evaluation of TBC via H-SPITS

3.1 Materials and experimental procedureNi-based superalloy Hastelloy-X with a cylindrical shape

¤23.5 © 21mm was used as the substrate material. Thesurface of the substrate was roughened by grit-blasting

Fig. 1 Schematic of H-SPITS.

Fig. 2 Vacuum chamber in H-SPITS.

K. Ito and M. Arai1704

treatment. A CoNiClAlY alloy (AMDRY9954, OerlikonMetco Japan Ltd., Japan) was deposited on the roughenedsurface of the substrate as a BC layer using the high-velocityoxygen fuel technique (UnicoatLF/JP5000 gun, OerlikonMetco Japan Ltd., Japan) and 0.1mm thickness. Subse-quently, 8mass% Y2O3­ZrO2 (8YSZ, METCO 204NS,Oerlikon Metco Japan Ltd., Japan) was deposited on theBC layer by APS (Unicoat/F4 gun, Oerlikon Metco JapanLtd., Japan) up to a thickness of 1mm. This specimen wasdenoted as the TBC specimen. SUJ2 balls with a diameterof 3mm (Ohashikokyu Co., Ltd., Japan) were employed asprojectiles.

A high-velocity impingement test was conducted at RT and900°C using the H-SPITS. In the case of the impingementtest at 900°C, the pressure in the vacuum chamber wasreduced to approximately 500 Pa using a vacuum pump afterinserting the TBC specimen into the chamber. Subsequently,the TBC specimen was heated to 900°C at 10°C/min andmaintained at 900°C for 10min. After the impingement test,the TBC specimen was naturally cooled in the chamber untilthe temperature decreased to less than 150°C. The impinge-ment velocity of the projectile was controlled in the range of100­300m/s by the acceleration gas type and gas pressure.After the test, the indentation morphology and indentationdepth formed on the surface of the TBC specimen wereevaluated using a laser microscope (VK-X150, KEYENCECORPORATION, Japan). In addition, the surface and cross-section of the specimens were observed using a scanningelectron microscope (SEM, JCM-6000, JEOL Ltd., Japan) toevaluate the vertical cracks and interfacial crack lengths.

3.2 Experimental results and discussionsFigure 3 displays laser microscope images of the surface

of the TBC specimens subjected to an impingement of anSUJ2 ball at 271m/s under RT and 900°C. The brightness ofthe figure represents the height from the reference plane. Thereference plane was set as the average height of four points,being sufficiently distant from the indentation. Focusing onthe result for RT, the indentation had an irregular shapewith severe asperity; it can be confirmed that cracks anddelamination occurred around the indentation. Conversely,at 900°C, a neat hemispherical indentation was formed, andno large cracks or delamination were observed around theindentation. Thus, it was found that TC demonstrated brittledeformation and damage behavior at RT, whereas it indicatedhigh plastic deformation ability under high-temperature, such

as the 900°C environment. This tendency was irrespective ofthe impingement velocity. Watanabe et al. conducted a ballindentation test on 7YSZ-TBC formed by EB-PVD at high-temperature.15) It was reported that remarkable deformationand densification of the columnar structure of the TBCaround the indentation occurred at 1137°C. In our results,plastic deformation occurred at a relatively low temperatureof 900°C. In the future, the mechanical properties of YSZformed by APS at high temperatures should be evaluated.

Figure 4 displays the relationship between the impinge-ment velocity and indentation depth measured using a lasermicroscope. It can be observed that the indentation depthmonotonously increased as the impingement velocity increas-ed at both temperatures. However, this tendency wassignificantly different depending on the temperature. At arelatively high impingement velocity®more than 200m/s®the indentation became deeper at RT than at 900°C. Thisis attributed to the formation of deep valleys locally relatedto the cracks and delamination inside and around theindentation, as indicated in Fig. 3(a). Conversely, at arelatively low impingement velocity, less than 150m/s, the

Table 1 Specification of H-SPITS.

Fig. 3 Laser microscope images of surface of TBC specimens subjectedto impingement of SUJ2 ball at 271m/s under different temperatureenvironments: (a) RT and (b) 900°C.

Fig. 4 Relationship between impingement velocity and indentation depthmeasured using laser microscope.

Damage Evaluation of Thermal Barrier Coatings Subjected to a High-Velocity Impingement of a Solid Sphere 1705

indentation became deeper at 900°C. In this velocity range,large cracks and delamination did not appear, even at RT.Therefore, a deeper indentation was formed at 900°C, wherethe TC could be plastically deformed owing to thermalsoftening.

Figure 5 displays SEM images of the surface of the TBCspecimen subjected to the impingement of a SUJ2 ball at271m/s under RT and 900°C. These figures show thatnumerous cracks were radially formed by the indentationtoward the surroundings at both temperatures. Hereafter,these cracks are called vertical cracks. The relationshipbetween the impingement velocity and number of verticalcracks and the relationship between the impingement velocityand maximum length of the vertical cracks are displayed inFigs. 6 and 7, respectively. From these graphs, it can beobserved that the number of vertical cracks and maximumlength of the vertical cracks increased as the impingementvelocity increased, although these fluctuated only marginally.Conversely, no clear influence of temperature was observedon these tendencies. The formation of such vertical crackscan be qualitatively explained by the press-fitting problem ofthe cylinder. That is, in the impingement process of the SUJ2ball on the TC surface, compressive stress was generated inthe TC in the radial direction. Conversely, tensile stress wasgenerated in the TC in the circumferential direction. Thistensile stress resulted in the formation of the vertical cracks.

Figure 8 displays cross-sectional SEM images of the TBCspecimens subjected to the impingement of an SUJ2 ballunder different conditions. The results of low, medium, and

high impingement velocities within the experimental rangeare provided in Fig. 8(a) and (b), (c) and (d), and (e) and (f ),respectively. From the result of the low impingementvelocity, a cone crack propagating diagonally from the centerof the TC immediately below the indentation toward theTC/BC interface was formed at RT. This cone crack wasdeflected in a direction parallel to the interface near theinterface. In addition, an interfacial crack was formed atthe TC/BC interface immediately below the indentation.Conversely, no cone cracks were observed at 900°C, andonly an interfacial crack was formed immediately belowthe indentation. At medium impingement velocities, conecracks propagated along the interface at RT, and lateral crackspropagated horizontally from the bottom of the indentation.At 900°C, it can be confirmed that an interfacial crack formedimmediately below the indentation propagated in the TC,and other interfacial cracks formed at the TC/BC interfaceaway from the indentation. In the case of high impingementvelocities, at RT, interfacial cracks originating from conecracks further developed along the interface. In addition,numerous cracks occurred in the TC immediately below theindentation, inducing serious damage to the TC. It can beconfirmed that the interfacial cracks developed further at900°C. These observations indicate that the formation

Fig. 5 SEM images of surface of TBC specimens subjected to impinge-ment of SUJ2 ball at 271m/s under different temperature conditions:(a) RT and (b) 900°C.

Fig. 6 Relationship between impingement velocity and number of verticalcracks radially formed in vicinity of indentation.

Fig. 7 Relationship between impingement velocity and maximum lengthof vertical crack radially formed in vicinity of indentation.

K. Ito and M. Arai1706

process of interfacial cracks and cracks inside the TC due tothe impingement of a solid sphere is significantly different atRT and 900°C.

Figure 9 displays the relationship between the impinge-ment velocity and length of the interfacial crack at RT and900°C. In this study, the horizontal distance between thecrack tips, indicated in Fig. 8, was defined as the lengthof the interfacial crack. From the graph, the length of theinterfacial crack tended to increase with the impingement

velocity at both temperatures. Although there is no cleardifference related to the temperature, it can be confirmed thatthe length of the interfacial crack tended to increase sharplyat 170m/s and 900°C. As mentioned above, at less than170m/s, only an interfacial crack was formed, and it grewimmediately below the indentation, whereas at more than170m/s, other interfacial cracks were formed at the interfaceaway from the indentation. The formation of these interfacialcracks led to the sharp increase, as confirmed in Fig. 9.

4. Simulation of Impingement Process Based on FEA

4.1 Analysis model and conditionsTo clarify the factors resulting in the interfacial cracks

observed in the impingement test, the impingement processof a solid sphere onto a TBC specimen was simulated viaFEA. The commercial FEA code Marc (Ver. 2020, MSCSoftware Corporation, USA) was used for the analysis.Figure 10 displays the schematic of the axisymmetric FEAmodel of a TBC specimen subjected to high-velocityimpingement by a spherical projectile. The sizes of theprojectile and TBC specimen were set to be the same as thosein the experiment. The projectile was an elastic body, TC andBC were elastic-perfectly plastic bodies, and the substratewas an elastoplastic body obeying the Johnson­Cook (JC)flow stress model.16) Table 2 lists the mechanical propertiesof the projectile, TC, BC, and substrate used for the analysis.

Fig. 8 Cross-sectional SEM images of TBC specimens subjected to impingement of SUJ2 ball under different conditions: (a) 118m/s atRT, (b) 110m/s at 900°C, (c) 182m/s at RT, (d) 196m/s at 900°C, (e) 271m/s at RT, and (f ) 271m/s at 900°C.

Fig. 9 Relationship between impingement velocity and length of interfacialcrack.

Damage Evaluation of Thermal Barrier Coatings Subjected to a High-Velocity Impingement of a Solid Sphere 1707

The material constants of the JC model for the substrate arelisted in Table 3. The yield stress of the TC was determinedsuch that the indentation depth obtained by this analysiscorresponded to that obtained from the impingement testat 900°C. It was assumed that the projectile was SUJ2, BCwas CoNiCrAlY, and the substrate was Hastelloy-X. Themechanical properties of the BC and substrate were citedfrom the literature.17,18) The friction and thermal conductionat the contact interface between the projectile and TC werenot considered. The effect of heat generation due to plasticdeformation was also neglected. The minimum element sizewas set at 25 µm. The area near the indentation was

automatically remeshed at every increment during thesimulation to prevent excessive deformation of the elements.The impingement velocity was fixed at 200m/s. The stressdistribution in the z-direction near the TC/BC interface,which induces delamination of the TC, was evaluated.

4.2 Analysis resultsFigure 11 displays the contour plot of the z-direction stress

·zz in the TBC specimen during the impingement processobtained by FEA. Only tensile stress is indicated as a contourplot; the entire compressive stress region is indicated inwhite. As indicated in Fig. 11(a), when the projectileachieved the maximum penetration depth, compressive stresswas generated in the TC, BC, and substrate immediatelybelow the indentation. In addition, tensile stresses of severaltens of MPa were generated at the TC/BC interface awayfrom the indentation. However, as displayed in Fig. 11(b),a tensile stress exceeding 100MPa was instantaneouslygenerated at the TC/BC interface immediately below theindentation at the moment the projectile bounced off. Inaddition, a tensile stress exceeding 300MPa was generatednear the indentation edge. As indicated in Fig. 11(c), thestress at the interface immediately below the indentationbecame compressive again after the projectile bounced off.Conversely, it can be observed that a high-tensile stress wasgenerated at the TC/BC interface away from the indentation.This tensile stress was greater than that observed when theprojectile achieved the maximum penetration depth.

From these results, it can be considered that the interfacialcrack immediately below the indentation observed in Fig. 8was formed by the instantaneous tensile stress immediatelyafter the projectile bounced off. The instantaneous tensilestress was not confirmed by the impingement analysis of theprojectile on the TC monolayer specimen. This indicates thatthe difference in the mechanical properties of each layerresults in tensile stress. However, the generation mechanismof tensile stress is not well understood. Therefore, we willexperimentally investigate the mechanism and establish atheoretical model as a future work. On the other hand, thetensile stress generated during the projectile penetration orafter the projectile bounced off led to the formation andgrowth of other interfacial cracks formed at the interfaceaway from the indentation.

As a complementary analysis, a thermal stress analysis wasconducted using a TBC model with a precrack introduced atthe TC/BC interface. It was confirmed that the energy releaserate at the crack tip after the model was cooled from 900°Cto RT was approximately 0.2N/m. The interfacial fracturetoughness at the TC/BC interface in general TBCs isapproximately 1.0MPa·m0.5.19) The critical energy release

Fig. 10 Schematic of axisymmetric FEA model of TBC specimensubjected to high-velocity impingement of spherical projectile.

Table 2 Mechanical properties of TC, BC, and substrate used for analysis.

Table 3 Material constants of JC model of substrate used for analysis.

Fig. 11 Contour plot of ·zz in TBC specimen during impingement process; (a) at maximum penetration depth, (b) at moment projectilebounced off, and (c) after projectile bounced off.

K. Ito and M. Arai1708

rate becomes approximately 15N/m after converting theinterfacial fracture toughness to the critical energy release.This indicates that the possibility of interfacial crack growthduring cooling was significantly low.

5. Conclusion

To clarify the damage mechanism of TBCs subjected tohigh-velocity impingement by a foreign object under a high-temperature environment, we developed an original H-SPITS. Using this system, an SUJ2 ball with a diameterof 3mm was impinged onto a TBC specimen in the velocityrange of 100­300m/s at RT and 900°C. The surface andcross-section of the TBC were observed to evaluate thedamaged conditions. We also performed an FEA toinvestigate the factors of crack formation. The conclusionsare summarized as follows:(1) The results of the impingement test confirmed that the

deformation behavior of the TC significantly dependson the temperature. TC exhibited brittle deformationbehavior at RT, whereas ductile deformation behaviorwas observed at 900°C.

(2) At both temperatures, vertical cracks formed radiallyfrom the indentation toward the surroundings. Thenumber of vertical cracks and their maximum lengthincreased with the impingement velocity; however,the influence of temperature on these values was notconfirmed.

(3) Cross-sectional observations revealed that interfacialcracks were formed at the TC/BC interface in allvelocity ranges, irrespective of the temperature. Inaddition, the process of interfacial crack formationdiffered significantly depending on the temperature. Itwas also confirmed that the length of the interfacialcrack increased with the impingement velocity andincreased sharply at greater than 170m/s at 900°C.

(4) From the FEA, it was confirmed that a relatively largetensile stress was instantaneously generated at the TC/BC interface immediately after the projectile bouncedoff. In addition, tensile stress was also generated at theTC/BC interface away from the indentation during the

penetration and bouncing process of the projectile. Itwas concluded that these tensile stresses led to theformation of interfacial cracks.

Acknowledgments

This study was supported by a research grant from theJapan Thermal Spray Society in 2019. We would like toexpress our gratitude to TOCALO Co., Ltd. for constructingthe TBC specimens.

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