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Metallographic techniques for evaluation of ThermalBarrier Coatings produced by Electron BeamPhysical Vapor Deposition

Matthew Kellya,⁎, Jogender Singha, Judith Toddb, Steven Copleya, Douglas Wolfea

aApplied Research Laboratory, The Pennsylvania State University, 115 MRI building, University Park, PA 16802, United StatesbEngineering Sciences and Mechanics Department, The Pennsylvania State University, University Park, Pa, 16802, United States

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +1 814 574 1317E-mail address: mjk268@psu.edu (M. Kelly)

1044-5803/$ – see front matter. Published bydoi:10.1016/j.matchar.2007.07.010

A B S T R A C T

Article history:Received 15 November 2006Received in revised form 5 July 2007Accepted 8 July 2007

Thermal Barrier Coatings (TBC) produced by Electron Beam Physical Vapor Deposition(EB-PVD) are primarily applied to critical hot section turbine components. EB-PVD TBCfor turbine applications exhibit a complicated structure of porous ceramic columnsseparated by voids that offers mechanical compliance. Currently there are no standardevaluation methods for evaluating EB-PVD TBC structure quantitatively. This paperproposes a metallographic method for preparing samples and evaluating techniques toquantitatively measure structure.TBC samples were produced and evaluated with the proposed metallographic techniqueand digital image analysis for columnar grain size and relative intercolumnar porosity.Incorporation of the proposed evaluation technique will increase knowledge of the relationbetween processing parameters and material properties by incorporating a structurallink. Application of this evaluation method will directly benefit areas of quality control,microstructural model development, and reduced development time for process scaling.

Published by Elsevier Inc.

Keywords:Thermal Barrier CoatingsTBCMicrostructureEB-PVDGrain sizePorosityImage analysisMetallography

1. Introduction

Modern hot section turbine components are compositesystems comprised of a metallic structural component,metallic bond coat, Thermally Grown Oxide (TGO) of thebond coat, and a ceramic Thermal Barrier Coating (TBC) [1].Rationale for the layered system design is to reduce thermalconductivity, increase adhesion of the ceramic top layer, byreducing sharp changes in thermal expansivity mismatch,improve oxidation resistance, and reduce the diffusion rateof corrosive products to the structural members [1,2]. TBCproduced by either Electron Beam Physical Vapor Deposition(EB-PVD), or thermal spray processes, have been used toprotect metallic components in the hot sections of a turbinesuccessfully for more than two decades [1,3]. TBC produced bythe EB-PVD method are typically described as columnar [4–6]

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in nature and offer high lateral mechanical compliance due tohigh aspect ratio voids between columns [7], whereas similarcoatings produced by thermal spray techniques offer lowerthermal conductivity and lateral compliance due to lamellarformation and discontinuous porosity [8–10]. Performance ofeither coating for a particular engineering application isinevitably determined by the coating microstructure dictatedby the coating method and process parameters. Understand-ing the relationships between process parameters for acoating system, coating microstructure, and performance isessential in the design and development of future turbinecomponent systems.

Standard microstructure evaluation techniques for coat-ings produced by EB-PVD combine qualitative surface mor-phology evaluation withmetallographic techniques applied tocoating cross section. Outward surfaces of coated components

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produced by the EB-PVD method are visually compared withhistoric Structural Zone Models (SMZ) [5,11,12] to determinemorphology qualitatively. Qualitative evaluation offers aquick and economical method for rough refining of processparameters. However, this method of evaluation leaves muchto be desired on information about grain and void structure.It has been shown that slight variations in the amount ofporosity, porosity type, average pore size, grain size, or grainsize distribution may have a dramatic effect on mechanical,optical, thermal, and electrical properties of engineeredmaterials [13–15]. For TBC produced by EB-PVD, this indeedis the case. Reductions in thermal conductivity have beenachieved through the increase of intra-columnar porosity,termed “feathery” porosity [4,16]. Thermal cyclic coating life-time is increased when inter-columnar sintering is reduced,due to increased volume fraction of inter-columnar “ribbon-like” porosity [17]. Corrosion is also a key factor in coating lifetime, and is affected by the amount and size of “pipe-like”porosity incorporated into the coating [18]. With many argu-ments on microstructural relations cited in the literature,quantitative shape analysis of features and microstructuralmeasurements in the plane of interest are rarely reported.

Quantitative metallographic techniques for evaluatingmicrostructure of TBC have been developed and implementedwith success for coatings produced by thermal spray process-es [19,20]. Using polished surfaces containing the normalthickness (standard coating cross section) is an acceptabletechnique for evaluating microstructure of coatings producedby thermal spray techniques because the “splat-like” mor-phology is also normal to the substrate and well representedin that plane. Porosity, splat thickness, and splat diameter caneasily be identified and studies have been conducted furtheridentifying the crack network associated with thermalsprayed coatings [21]. Application of these techniques to TBCproduced by EB-PVD does not yield such successful results.While standard cross section surfaces are indeed useful indetermining thickness and surface roughness, microstruc-tural evaluation is difficult due to orientation of the columnpore network. Columns and intercolumnar voids are orien-tated such that the length is in the plane of polishing. Becauseof the high aspect ratio, during polishing of standard crosssections, columns have the tendency to “pull out” leavingadditional voids that would increase the measured porosityvalue of that plane. Additionally, high aspect ratio poresorientedwith the length in the plane of evaluation are difficultto measure due to the depth of view of most microscopytechniques. Difficulties in preparation and evaluation of stan-dard cross sections have delayed the wide spread use ofquantitative evaluation of EB-PVD TBC.

2. Experimental

2.1. Sample Production

Test samples were generated to demonstrate a typical micro-structure revealed by this method. The substrate material wasnickel alloy 625 cut into 1.9 cm diameter buttons 0.476 cmthick. The buttons were polished smooth using successivesteps of wet grinding with silicon carbide paper to an 800 grit

finish. The nickel substrates were “heat tinted” in air at 700 °Cfor 15 min to remove surface organic compounds and form anoxide layer. The surface and the side of each test specimenwere grit blasted in a Unihone brand grit blaster using highpurity 400 micron-size aluminum oxide particles to removesurface oxide and produce the desired surface finish. Thedistance from the edge of the nozzle to the surface of thesamples was approximately 38 cm with a pressure of 200 kPa.The angle of the nozzle with respect to the sample surfacewas45° to minimize the amount of embedded Al2O3 particlesincorporated into the substrate surface. The grit blast time oneach sample varied between 10–15 s and was performed untila uniform matte finish was obtained. Grit blasted substrateswere ultrasonically cleaned in acetone for 20 min, rinsed withmethanol, ultrasonically clean again in methanol for 20 min,and dried with nitrogen gas.

The prepared substrates were tack welded to strips ofstainless steel foil that were tack welded to an 8.64 cmdiameter mandrel. The mandrel was loaded into an industrialprototype Sciaky EB-PVD unit consisting of six EB-guns and athree-ingot continuous feeding system described in previouspapers [22,23]. The deposition chamber and gas feed lineswere evacuated to a pressure of 10−3 Pa. Two electron beamswere scanned in a raster pattern on the graphite heaterassembly to bring the heating surfaces to ∼1200 °C. After20 min of chamber heating, samples were positioned 28 cmabove the center of 4.93 cm diameter 7 wt.% Yttria StabilizedZirconia (7YSZ) ingot (lot number 1297726) provided by TransTech Inc. of Adamstown, MD. Samples were then rotated at12 RPM and allowed to soak at an average temperature of1020°C for 20 min. During the soak period oxygen wasintroduced near the samples at 100 sccm to grow a thinuniform oxide layer (TGO). Following the TGO period, sampleswere exposed to an established 7YSZ vapor at an averagechamber pressure of 1.7 ⁎10−3 Torr. The evaporation rate wasestablished at 5.7 grams per minute resulting in a depositionrate of 2 μmperminute. Depositionwas continued for a periodof 90 min, producing a 177±4 micron-thick 7YSZ coating.Samples were left to cool under vacuum and 200 sccm oxygenflow for 10 min before venting to atmospheric conditions.Coatings exhibited typical faceted columnar morphologiesdesirable for TBC, as shown in Fig. 1.

2.2. Sample Sectioning Mounting

Samples to be evaluated by the following method may besectioned to size using a standard diamondwafering saw suchthat the saw wheel rotates into the ceramic before thesubstrate to reduce spalling and coating damage. Use ofwhole-test coupons for mounting is advisable. Sectioned orwhole samples for evaluation should be infiltrated by metal-lographic epoxy exhibiting; chemical stability to alcohol-basedsolvents, low-viscosity, vacuum stability, and low-expansivitychange during hardening. Ideally samples should be placed(coating side up) in an infiltration container and preferablybrought to vacuum before epoxy is injected. It is also accept-able to immerse samples face up in epoxy and then evacuatethe chamber for 2 to 10 min. Samples should then be trans-ferred (coating side down) to the desired container with a flatbottom and a sacrificial metal shim for curing. Sacrificial

Fig. 1 –Deposited TBC surface, displaying classic faceted surface (A), and columnar morphology (B) of fractured cross section.

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shims are used to negate loss of sample during initial roughgrinding and can be used for taper mounting described byVander Voort [24]. Typical shims are of 440-grade stainlesssteel between 50 to 200 μmthick. Cure of epoxy under pressuremay be beneficial (as described by ASM handbook [20]) forthermal sprayed coatings. The effects for EB-PVD TBC have yetto be quantitatively evaluated.

2.3. Transcolumnar Polishing Procedure

Fully cured mounts are then removed and ready for polishing.The polishing procedure for fully-automatic and semi-auto-matic polishing equipment is listed in Table 1. Manualmethods for grinding and polishing are acceptable and pre-sented in this work as follows. Manual polishing should beconducted with the same polishing cloths and similar forcesas listed in Table 1, but requires fixtures for holding samples

Table 1 – Transcolumnar polishing procedure for 8YSZ on fully

# Surface Coolant /lubricant Abrand t

1c Paper Water 320 gr2 Paper Water 600 gr3 Paper Water 800 gr4 Paper Water 1200 g5e No/low-nap synthetic clothf Alcohol based extending fluid 3 μm6 No/low-nap synthetic clothf Alcohol based extending fluid 1 μm7g Dense full nap chem. cloth Distilled Water Colloi

0.02–0

a American National Standards Institute (ANSI) designation for abrasiveb Polishing pressure given in Pa and PSI values of force should be calcula1in2*1 PSI=1 lbf].c This step is to be repeated after cleaning until metal shim is completeld Complementary rotation: polishing surface and sample rotate in the sae This step may need to be repeated after cleaning to remove all scratchf Use of hard nylon cloth with fine grid spacing was used for presented rg Cloth is initially soaked with distilled water and colloidal silica solutionh Last 20 s of this step requires a distilled water wash to prevent colloidai Contrary rotation: polishing surface and sample rotate in opposite direc

to prevent detrimental bevel and tilting. Use of high-nappolishing cloths (as used in the final colloidal silica step)should be limited or avoided due to preferential removal ofepoxy between columns leading to poor column edge reten-tion and increased amounts of pullout.

2.4. Imaging Procedure

Due to electromagnetic properties of Yttria Stabilized Zirconia,imaging for quantitative analysis on phase percentage andcolumn structure should be conducted with electron micros-copy images. Optical microscopy can be conducted for opaquematerials or translucent materials with the aid of reactivecoatings. Use of reactive platinum coatings applied by PVD (asdescribed by Brindley and Leonhardt [19]) is an acceptablemethod for reducing the optical transmission of polished YSZcolumns when electron microscopy is prohibitive, or porosity

-automatic or semi-automatic polishing equipment

asive sizeype, ANSIa

Time,s

Pressureb Surfacespeed,RPM

Relativerotation

kPa PSI

it SiC 20–30 55–69 8–10 150–300 Complementaryd

it SiC 20–30 55–69 8–10 150–300 Complementaryit SiC 20–30 55–69 8–10 150–300 Complementaryrit SiC 20–30 55–69 8–10 150–300 Complementarydiamond 120 97–117 14–17 150 Complementarydiamond 120 97–117 14–17 150 Complementarydal silica.06 μm Al2O3

80h 41–55 6–8 150 Contrary i

grit size.ted from actual mount surface area [1 mm2*1 kPa=0.1 N,

y removed.me direction.es N3 μm.esults.prior to use.l silica from drying on polished surface.tions.

Fig. 2 –Taper mounted TBC sample showing orientation forimaging. Position “a” is the onset of column tips exposedfrom metallographic preparation and position “b” is thepolished substrate.

Fig. 3 –Cross sectional preparation of tapermounted samplesused for calculation of tilt and relative image position.

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distinction is required. Reactive coating provides a methodfor distinction of open porosity using optical microscopy.Polished surfaces, to be evaluated by Scanning ElectronMicroscopy (SEM), should be coated with a thin conductivefilm to reduce charging effects of non-conductive metallo-graphic mounts. Backscattered Electron Detection is recom-mended due to the increased contrast discrimination betweencoating and epoxy filled voids. Digital images should becaptured to utilize optimum contrast for image analysis andprevent saturation of grain structure as discussed elsewhere[25].

Imageswere taken such that image positionwas correlatedwith physical location in column thickness. The techniqueemployed in this research for position correlation, whenstereological techniques were not employed, is to imagesequential lines parallel with the primary tilt axis, recordingplanar position for each image to be used for later depthverification. The primary tilt axis is orthogonal to both surfacenormal n and the shortest line from a to b, where a is polishedregion of column tips and b is the region of polished substrate,as shown in Fig. 2. Position along the line ab, where the firstcolumns and the onset of first growth are recorded for eachsample. The position for each image was recorded for laterdepth determination. The required number of images per lineshould meet or exceed standard statistical requirements forgrain size analysis for a single sample [26,27]. Magnificationfor each set of images was determined to negate errors due toresolution of minimum features evaluated (provided guardframes are implemented [26]). The same procedure was alsofollowed to determine column size (if columns are multi-grained).

For the analysis presented, sequential images were takenat planes of equal distance from the TGO. The number ofimages taken at each plane was such that an estimatedminimumof 1000 grains could be analyzed. Magnification wasdetermined during imaging so that average grains occupied anestimated minimum area of 500 square pixels. The number ofgrains analyzed may be reduced, provided that the samplesshow reasonable grain size uniformity in planes of evaluation,

and small changes in variance due to additional evaluation[27]. Resolution should be increased when evaluating shapeand size for features with large aspect ratios, similar to“ribbon-like” porosity (due to possible pixilation errors ofthin features).

2.5. Verification of Depth

The position of evaluation is very important for understandingthe relation of microstructural changes with respect toposition in coating thickness. Measured features of columnsand porosity may change greatly throughout the thickness forcoatings produced by EB-PVD. Precise knowledge of theposition, with respect to the thickness of the coating, wheretrans-columnar images were taken, is therefore essential inunderstanding the entire columnar growth process. Whenstereological evaluation techniques are employed, the posi-tion of an image, with respect to the onset of growth, can bedetermined by producing a cross section of the sample afterthe final plane of evaluation has been imaged. Where positionof the final evaluation plane is measured, from the final crosssectional thickness, position of sequential evaluation planesis calculated based on known material removal for each suc-cessivemetallographic preparation step, and the total numberof material removal steps between final evaluation plane andplane of interest.

Position of images produced by the taper mount sectionswas determined similar to that of standard stereologicalmetallography where position was determined from crosssectional evaluation and simple geometric calculations. Onceimaging was completed, samples were sectioned through theline ab (shown in Fig. 3) and mounted (cut face down) fortypical cross sectional evaluation. Angle θ is calculated usingthe following formula, h ¼ tan�1 ac

ab

� �where ab is the distance

between images where columns were first exposed and thesubstrate was reveled and ac is the measured coating thick-ness from cross sectional evaluation sectioned through lineab. The position of the image of interest with respect to TGOlayer, is calculated using ti=sin(θ) ⁎D where, ti is the distancefrom the TGO to the evaluated image plane. D is the distancebetween the image at point b and the image of interest, asmeasured by the digital imaging procedure and angle thetacalculated above. Taper angles are typically less than 5° toreduce feature distortion.

Fig. 4 –TBCmicrostructural features selected with digital image analysis and highlighted. Intercolumnar porosity shown at leftin red, separated grains shown at right with a pseudo-color overlay.

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2.6. Image Analysis

Quantitative evaluation of images presented was conductedonly for column grain size and intercolumnar porosity as anexample of typical results of interest. Evaluation was con-ducted using standard commercial image analysis software.For work presented in this paper, Clemex Vision PE by ClemexTechnologies Inc. (Longueuil, Canada) was used. Intercolum-

Fig. 5 –Backscattered Scanning Electron Images of the same samthickness. Images A, B, C, and D are at 150, 96, 64 and 11 μm res

nar porosity was selected as regions defined to have grey scalelevels below that of column and grain boundaries. Interco-lumnar porosity was represented as a percent of total imagearea. An example of regions defined as intercolumnar porosityis shown in Fig. 4. Column area was defined as regions in theimage with grey scale threshold values greater than that ofvoids and user determined grain boundaries. Grains wereseparated using built-in software functions that perform low

ple taken at different positions with respect to the coatingpectively from the TBC/TGO interface.

Fig. 6 –Quantitative image analysis results of (A) intercolumnar porosity and (B) grain size distribution with respect to positionin coating thickness. Results of (C) intercolumnar porosity and (D) grain size distribution with respect to position in coatingthickness a 380 μm thick film where mandrel diameter was decreased by 42%, rotation rate was decreased by 42%, samplesurface speed was decreased by 66%, and a decrease in sample deposition temperature of 5%. Grain size distribution pointslabeled D90, D50, and D10 correspond to the 90% cumulative grain size, median grain size, and 10% cumulative grain size,respectively.

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level operations which extend incompletely-formed bound-aries to separate grains. Grain size was then calculated fromthe measured area of each separated grain and reported interms of the diameter of a circle with equal area. A pseudo-color overlay of the separated and measured grains is shownin Fig. 4.

3. Results and Discussion

Yttria Stabilized Zirconia coatings produced (as described)were prepared using the presented metallographic prepara-tion method. They were also evaluated for column grain sizeand relative porosity. Images of coating microstructure at dif-ferent positions (with respect to thickness) for a single sampleare shown in Fig. 5. Comparing microstructure of the differentimages shown in Fig. 5, it is clear qualitatively, differences ofgrain size and descriptive values of porosity exist. To illustratethe changes quantitatively in structure, with respect to thick-ness for the shown coating, digital image analysis wasconducted using commercial image analysis software. Mea-surements were conducted for relative intercolumnar porosity

percent and columnar grain size distribution at each selectedposition with respect to thickness. Results of both interco-lumnar porosity and columnar grain size distributions arepresented in Fig. 6A and B respectively. As expected changesin intercolumnar porosity and grain size were observed quan-titatively. To illustrate microstructural differences, observedin coatings produced by different processing parameters,quantitative results for relative intercolumnar porosity per-cent and columnar grain size distribution are shown in Fig. 6Cand D (respectively), for a thick coating produced in the samesystem under similar conditions. Differences in process pa-rameters are as follows: 42% decrease in mandrel diameter,42% decrease in rotation rate, 66% decrease in sample surfacespeed, and 5% decrease in sample deposition temperature.

Further statistical analysis of each of the 27 grain size dis-tributions was conducted to determine if significant distribu-tion changes occurred during coating growth. All grain sizedistributions for the described deposition parameters werefound to have good correlation with normal Gaussian distribu-tionscalculated fromeachmeanandstandarddeviation.Valuesfor the square of the Pearson product moment correlation co-efficient ranged between0.983and0.998.Deviationofmeasured

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grain size distributions is attributed to a slight right-tailedskewness observed in all measured grain size distributions.

While only columnar grain size and relative porosity mea-surement from SEM images are presented, application ofadditional measurements is valid. Different descriptive mea-surements for feature size and shape are listed in ASMHandbook of Metallography and Microstructures [26]. Addi-tional measurements of some particular interest for TBCevaluation are as follows: relative area percent of porosity typeseparated by aspect ratio, count of grains per column, aspectratio of columns, interconnectivity of porosity, and ratio offeature perimeter to area. Because of the lack of quantitativeinformation, relations between particular features and coatingproperties are yet unknown. Additional structural parametersneed to be defined to accurately characterize TBC and developrelationship to performance.

4. Conclusion

The proposedmetallographic technique and evaluationmeth-od was developed to evaluate microstructure of poroustranslucent ceramic coatings produced by Physical VaporDeposition. It has been shown to offer significant advantagesover traditional evaluation techniques often employed forThermal Barrier Coatings. Surfaces prepared using thesetechniques were evaluated quantitatively for descriptive para-meters of grain size and intercolumnar porosity. The demon-strated evaluation provides new quantitative information ofmicrostructure through the thickness of the coating. Quanti-tative differences of coatings produced by varying deposi-tion parameters have also been shown. The accuracy of thismethod is similar to standard quantitative metallographicevaluation; limited by the quality of metallographic prepara-tion, operator error with image analysis, position determina-tion, and resolution of imaging device.

Overall this technique provides a needed tool in the largereffort to characterize structure of TBCs (produced by EB-PVD)for completing the link between system processing, materialstructure, and coating performance. This technique hasdemonstrated the ability to observe and measure micron-scale and larger structural features quantitatively. Additionalcrystallographic and sub-micron features play an importantrole in material performance, and will require additionaltechniques not yet investigated. Surfaces prepared as de-scribed could be evaluated by Orientation Imaging Microscopy(OIM) and Atomic Force Microscopy (AFM) for additional infor-mation on crystallographic structural orientation and sub-micron features. Combinations of techniques will be requiredto accurately describe the complex structure Physical VaporDeposited films, in an effort to better understand the relationsbetween processing parameters, structure, and performance.

Acknowledgement

This researchwas sponsored by the United States NavyManu-facturing Technology (ManTech) Program, Office of NavalResearch, under Navy Contract N00024-02-D-6604, Delivery

Order 0019. Any opinions, findings, conclusions, or recom-mendations expressed in this material are those of theauthors and do not necessarily reflect the views of the U.S.Navy.

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