NASA Technical Reports Server (NTRS) 20050080708: Nondestructive Evaluation of Thermal Barrier Coatings by Mid-infrared Reflectance

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NONDESTRLCTIVE EVALUATION OF IERMAI- BARRIER COATINGS RY MID
INFRARED REHLECTANCT. IMAGING

Jefliey . ride, Chaules M. Spc
NASA Glenn Reseurch Contce
97000 Brooks Ie.
Cleveland, OH 44135,

ler, ane James A. Nesbitt

Ricaaed #. Main
Cleveland Sate University
Cleveland, OF

ABSTRACT

The applicstion of rsid-inftured retleutemee (MIR) imaging w suite darmage in shermal
banier coatiags (TTHCs} bas been extended liv a previcusly domoustrated seee-averaged
speetrascopie sualysin inal Io ‘aecorae a practical imaging hwo that provides the spatie: resolution
nesdad to quickly identi Ioealized regions af TBC damage ny visual inspection. lltumication
‘onfes ard image ealleetian prrcedures were developed ‘o prexluce illumingion-nomnalized
Nail reflectance images after sablraction of he baesground thera entissiom, ML
seflectance innges were collected witl a bandpass Flier couered at 6 wavelengls of 4 microns,
‘which stovided the optimum balaace berween gyal sensitivity 0 buried cracks and coaficg
ercson, but vith a dexirable insensitivity to'TIBC stateviag und absorplion tam ambien gases,
[samples une prosented ofthe application nf VIR reflectance imaging te mocitor daruge
progcession in phasma-sprayed Ht sturia-stilized 2itoonia [8YS7) TACs subjected 09 either
fucace cye‘iny wr alumina pacicls jet erosion. Those results shor thal MIR reMevlanos imaging
vem rekably track the progressive of buried de‘amnotion crnoks produced hy thermal cycling and
tem alse be used to determine when ang lacal sectiex of the TAC: hns eredod ean an
saweplable limit, Modeling of Uae effects of busied cracks an erosion om reflsetance will be
presented lo show the dependence n! damage sensiivty tr TIC thickness,

INTRODUCIION
Reliable diagcwntic taals are needed to ldeat’y the kiesrion and sever-y of deyredstion in

thermal Purtier coatings C118C an lurhine cine com-genents so sto provide adequa
‘warning of w1en damage is approshing performance. of safety-Crvalening tbresaolds. Mteviows
‘work has dewimstraced that ould-intarcd (MIR ee‘lectance speccrossopy exhibits a use‘ul

‘ity to both TRC crasion ard delarsination progression, and has the particular advantage
Dover many other diaynnate methods ir. exhibicing a sensitivity ty the early stages of deDond
progression chat enables euly warning of subsequem spallation,! MIR seflectance speutroscopy
Takes advanlage of the macitourn inunaparency of TBC in the 3-5 ym wavelewath eegien 2
slowing mush greater pocotstion iow the TIC thar wt visible waveleagths ts slewet che
progress ofthe buried TBC delamination crack uetwork ths, uldicatcly prostuces THC flue,
Haviever, as previously impiementsd," MIR refleranoe speclniseomy was unsuitable Kar
practica. TTS health monizoriog because Creve measursineats averayed over large atens (20.6
smc: diameter prevhuting the Getsevon of highly “ecalized damage, In additin, mens:
were resiicred to smell luhoralery specks that could he inne eto the spectrametse

Fre propint or ei af» paper nerd er proscaoma
Canarees. Borsss ionges ay ae made beer eon
fellvaton, ie cvare sulci wth aut rat
"bo cad or"eproduscd whoa he penriaion a Re hr.
snclesure, As presosree in this paper, “hese limavians aave been ovecvome by applying MIR,
ref.gctanes 25 an imaging wehnique thac provides praclivl, easily inlenpreted eellh mewslering
Fool using visu inspection wrth the spatial solution to dently Inculized damage,

EXPERIMENTAL PROCLDLRIS
MIR Reflocaanee Lmaging

All images were obtained with a Paoenis Undign Systems, Suma Mera) MIR camer
with a DX» cooled 320256 indium sntimorice (aS) focal slang curay. A nasrows 4.0 ym
‘wavelength tandpass fer (CWTIM 0.15 umy was incusted on the satura because this
wavelength affors good damage senstcivity while heing suiSieieally distant om potcatially
inferfar'ng OH and Cb sbso-peions at 3 ani 4.25 ue, respectively. ‘The camera was pusilioned
120 men oon the speciman. MIR illomiion was proviced by a SiC IR emirot opciated at SU
Whe illuninstion iow. Ure source eas collimated with a 63.5 mn diameter off-axis parabolic
tnitvor thar Lltininated the specimens svittout shadavsing hy the source o= less ol intensity wth
Gistamee This “raporact to nore that while rae aan’ pasabolic mirnin provides collimated,
reprodueih=ilumrington, the illumination intensity is nex-uniform: dhe intensity of the bean is
vigher where il ellects from areas a he mi:vor eloscr to the souroe. In order to improve signal
st nis eack iuage repress au everays nf 80D Rarase expire 30 Haste, Adlon y, a
detector Lmepyation time of 4.5 ms was ermplyed lor wack ruams to mudeaicy signs) Level

A sequence of image processing staps wns performes to produsy 2 tue MIR rsilestanes
image. These slops are necessary because the untreated Image, Jarman Includes hah ercund
‘hormal radiation emitted by tae apceiaten as well as llocainacion snd pixel-lv-pivel sersilivity

nang lB 3) SES FILLS IDR

VF Laan FD w

¢ vad vars the image coordinates, Sy) iste ind.vidua piel sensitivity, Jay isthe
yeslinmdependen Feld-aFview Mumication intensin, Rewind) iste sporimta refer
Ain Fam the thermal radian enti by he saeco, Th thumal radiation rea i
fq, 1 isebrinareé hy sober ths image obtsined with ts llcnination of. 22")
‘39 ftom the image ahtsined wi he iharinien on

a
SG SMES IR eal)

A Tuli sorreotion is then mavis ta romave the dependence on illeeninatian anu pixelun-pixe!
stance image. f

soclspi collected “rm
702, that fills tee
field af vow, his navurslizauion produces un image where "he ive. intensity is equal to tke
speeienen sefloeranc

an Ge

Wea Roe,
Specimen Preparation

The TBC specimens were pregarod by stmospherie ploonca-spraying atop cost of Sut
‘ula-stabilzee zlvonia (8¥SZ) onie 2S, mm diameter Rene NS superalloy dikes that
precoaied with &- 120 win-thick NiCTATY hard coat by low pressure plasma-spraying. Selected,
samples were 4:3 produced for whieh the hand eval. seus upplied to onky half of the sebsteae top
surlive helore the YSZ 103 cacr depesiton, La addition, irestanding plasmna-spravod S¥SZ
IUS-8YS7) TBCS swore produced by sprayig cama secrificial carbo substrates Uhal were Later
hhcat emt in gi at 860°C us buen off Cae carbon. Al ss-tepasived TBCs were heat treuled for 3
minal RIKYC ca clieninateresidusl oxygen defisioncis thet praducs a grayish oélonaton: in the
xpeayed spevinems,

‘Furnace eveting tests were pertormed ina tebe urna in nr, with eae evels consisting
of a 5-mir hole ut 1 163°C Ipllowsé by a LSemin coolieg peried to ~#20°C. ADer initial
ineemgted funaaee eyelicg tats esublished TBC failure (bucked o¢ spall ures exceeding,
D086 of total TBC anea) ocearring al~200 eycles, Fume cyeling of thuse specimens wus hulls
ua. diene! stopping points, specifically al $0, 100, and 150 oycles (2, , mal af oyelic I
MU reflectance imayes ware collected for *hose specimens, und SEM esumination of prepared
TBC cross-sections was perirced. Controlled erosion was pelformed using w Kuvaker
(Bohemia, NY) K¥37U0 At: Jet Brosion Tester with a S-mamrdiamerer wovle using 5i.m-
diameter unira powder impinging oa the specimen ul un smgle of 30° oF noua,

BSULIS
The basis for using MIR rellectunce ia monitor TBC erosioa isthe decrease in MT
reflectincs thal acconppanies seduction in IIRC highness, Th> ability to diseeimigate TRC
thicstess is sivn clearly in MIR ref ectance iuuagey (ul images obtained wth 2m-
‘wavelength bandgess er} of w wide range of 2 A TBC Ihickaestss ranging Sem 86 Us
813 am (Hig, L. The VOR teflectanes mages in Fig. 1 are shnwn in Lai Formats that display the
spevinen seffectanes obtained thraugh image processing (Equations 1-2) either ina conventional
gragnanls image ora 3-D intensity plo. Taking aelvantage of the spatial resobutinn provided by
MIR re‘leet:nee imaging, che demonstrasal thickness depoudenss of MIR weflectance ci be
scitived tn idemily highly localized ornded regions. Mg 2 displays MIR reflectance inayen al
150 am th’ck IBC specimens subjested to 1 and 2 mis of lagelized ehumina jet tosion. The
ceraded -egions aze evidect by their lower rlfeotsnce dhe to the reduced TBC shiekness caused,
by Li enision.

Gum = 140 um

Reflectance

¢ (, Thickness dependence of MIEK retlectames inaages uf freestanding SY SZ TBCS,
(Top) Grayscale reflectance image. (Rutinm) 3-D Reflectance intensity plot.
gure 2. MIR reflectince images of 150-umthick TB
eft) and 2 min (righO oC Weal alumina particle jel eresion

“VER refleccance imaging onn alsa he used ta mocitor the THC delamieation. Ta
demonstrate the spabial sesilling oF Ere imaging apimvach aud to determize the range of
tHhecknesses ever Which the proatess of LBC eclamiaatioa could 2c distinguishoe, WLR
reflectance images were nequited from fieestavd:ng TBCs that ac been backside-coeted ovcr
Jha Jer hack surfaces (hal pucked} Th 2-pthick NEAT hy sgtler depos! lion fa mmmig an
avbevent scbotote (he wilaeked Wall mibnies a delanicated TRC). Because mo het teat
vas applied vo produce “delamination”, avy differences in MIR reflectance could be atcibated to
the presence or absence of the adlerent backside NiAl coating, Fig, 3 shows 2 sequeace cf 73,
125, and 295 usm thick bal-backed ECs that show a significantly lower scflestace over the
‘well-delined hell-backed seerian for Drs wo Thinner TCS, with the roflechimee diferenee
Uscreasing with increased TISC thickwess until the winteas is only barely discersuble at 295 js
‘diskuess These :osuls indioare MIX roflocrauce should bs capable of disccening '| BC
elon ranion wife good spatial selution, but with dsemcssiag coutcas: as tac TBC thickness
Fig. 4 demanstiaces that MIR re‘Teolance finaging ean ely meni the ual
clelamination progression associated with furnace cycling af TC-camed subsceates Fig. 4
compares MLR reflectmnee images of :75-tam thie TEC-coated specimens afr (, 30, 100, 151,
and 300 france eyslos ivisibls buckling was observed at 200 cycles}. Ihre isa contiaveus
‘ncreune imrelleviance al 4-17 soteeiength Thor 0,56 10 0.75 That is wssocialce wilh Zaure
pegerssion. whale theo a2 a aoticeable retlectance cillerences uruer vibe light illuminati,
SEM inspection Fig $} of the crassssections of the same fumace-oyelcdl specianoas confisned a
TRC fils progression similar to that observed i provious smdics." the fomsrion and erowt=
of's delamination erzck nctwork alg the bottom of the TBC, mostly above the therma‘ly grows
sil (TGO) Cul graves on bsp ay the Bane eos, Backsealar electra dussetion provided
sapercn contrast far erase

rod Us seennulary elovlri: deeetion, Fig, 5 shows thal
‘crack nerwode atows by engihening and widening oF indi viduel cracks as wll ay inervssing
‘rack intarconuectedness. While TGO geosmth alse acones, very lite TGO growth was observed
sder 50 eyeles Guveange TGO thickness was 4.6, 45, and $.6 jun thick st SU, :0D, and 150
‘spelen respective

Because delanvization jragression prceeded unilarely user the surtaces the re
‘veled specimens imaged in Hg, %. another set of spceimens was prepared whose the boat coat
seas appli lo omy hall of the substrate top surszee before the SYSZ vop oat éepostion
Tecinise TIVs thal noel ex Fil ack

rier an with akigher deyree a
‘1SCs with bond soars! these test spesiniens were imaged tp demonstrate sparialy resalved
‘oniteving 0 localized delamination. Fig, 6 shows MI teflectance imsges of ore 175-yuncthich
BYS7-couteé spevime air Dy 10 30 furnace cycies. T

Fuad wt while the righ ald

sefloctance (corsistont with Fig. 4), the en-boa-cval hel shows fol a yreuler increase in
overall reflectance along with localized bright seajons that prow feom the edge Hal eventually
cvolve ino visible edgc-initiared delamization

Reflectance

&
\ 2

with bacleside hnll-eonted with NiAl to mimis uilherent substrate, Niall backside enating is
on Jett half of eaeh image. (Top) Grayxeale refleetamee image. (otto) 3-D Reflectance
iameusity plot.

Oeyeles Svyeles “1D eyeles 180 cycles 200 eyeles
R056 R=0.65 R068 Rett W075

ro, MIR refloetance grayseale images of 17S-potethick YSZ. TEC-coated
specimens afler 0, 50, 100, 150, and 200 furnace eyeles. Avernge reflectance for eacl,
specie és also indicated, Vinible TRC lnekling wax abvervad ut 200 eyeles,
Figure, Bucksculter electron images of cross-sections of 175-yam-thick BY8Z THC:
spreimens after 8, $0, 100, and $0 furnace eyeles.

Heycles 10 cycles 2 eyeles 3H cycles

Figure 6. MIR refloetaneo grayscale images of the same 17S-pim-thick BYSZ TRC-couted
spesimen aficr 0, 10, 20, and 30 furnace cycles, Bond coat was applied only to left half of
inven hefure SYSZ. Lop coat depusition,
DISCUSSION
The abilzy of MIR redleetance imaging us-nep -¥gisns af TRC erosion er peril
<dclarsination rlies both on the dcp pencteatioa of MIDE wavelenahs into the TRC ns well as the
intran-prosluciry mechunisms prodcced by ehaages in BC thickness or the inlauecivn of
leaied eracks. In crder to predic! the range af TBC chicknesses ovor wikich rodnetions in TEC
thickness odelemunstion could be discerned from MAIR reflects. gur-fhix Kalbolka-Mcuk
epgroximmion was applied tothe cxtrcme cases ol'a ampletely aulkerent and q com gletely
deluchesl TRC. The deuched TBC was mnedelod simply by a uniform air gap “tween the TAC
‘nd ike substrate. Jn this mode, the substrate (hamd cont) was assigned a diffuse reflectance of
45.43, and calenlatons peedioted dhe itinduction wf an uit yap pwoduces a very high diffuse
reilectence of 0.81 at the TCaic enp incerZace due ta large sompsinent of ole internal
reilection wigig from the large indes of reeiaction change netass ‘hat tutestane (Irom 21.49
1 10). The suutlring amd absoeplien excEicicars for the plasma-spesyed BYSZ TICs aca
thot a um were determined to 2¢ 359 ond 0.035 mr, xcspoccively (caleulaed feo:
sishistance #8. thicsness -neeurrements Fem a yeries wf treostmuding TEs), The
predicted effoct of introducing the air gap or. the reliectanee at 4 pen eacsiength #84 Timesior al
TRC ihickneys is plored in Fp. 7, This aot has important implicabius forthe range of THC
thickeses wciere MIR reilgctance will be useful Sex monitoring exosion of delaminatia. Tiss,
rellectance curve for the altucked TTC shoves cose sousrity to changes in TBC thickaes a
TBO thickness irsreases. Secozuly, ue dickens (which procuees the contrast in reflectance
betivook am adaerent and completely detucied TIBC devreages with inarcusing’ LBC thickness,
snd itean he samel-ided that MIR zetlectanice wil not he sell for either erosion or delaminetion
monitoring fiv THs thicker thar $00 jeu, ‘Two expeimen‘al aoinls (al Mand 200 eve
representing allavzed za delncheé, wespostively} anve beca adced to Fig. 7, illusiratiry gensd
gioement wit tae prediction, These eatculolions igners dhe poreutal cist of TGO growth on
Telleclenee: ho-ever, the expected parabolic TCO growth res ly ra deercasing growed zate,
end the TGO g-owlh mocesurensoans indicate idle growth beycnd 56-zycles. Therevbre, the TOO
‘gvret would anly have a significant sffeer on the incicase in sellec"ance nbserved during the
sSatly stages of furnace eycling. It should also he noted that chese resalts ate for plasma-spreyed
TTRCS ond chat MIR redleetance 2nay prove seul up much greeter thisknesses for election
Frere. paysieal vapor deposited (EB
cvellivierts
Jn suninary, MIR reflevloswe ming bas been develoged along with he image
processing zequited to remove the eras radiation amiteed By the specimen and fhactuations
‘due te nonuniform iLuminction aed pivel-bo-pixel sensitivity variation, Fhis imaging apprseeh
provides « practical, cosy interpretad health monitgring fool using visnal inspectian withthe
spatial resolution 16 identify Localized darnaze Frama erescm sl the ability to prodict tae onset
and Incation of TTC deaupiustion, MIK reflectance irsaginy shoves excellenl po‘catic. a8 4
sliggnostic Gol fur Investigating TBC failaee puogiossion ia the lalsrauory, and al shams
poteatial for belveen-llight engine irspzctians for TBC erosion aac! eadly-stage delarninalia in
‘viroumtonts whe-e sufere Juul des nt oecur

IND} coatings, which fave mach Inwsor seacse:ng
i

Attached TEC
= = TBC with underlying crack
W Allached TBC measured
© Delaminated TEC measured

Reflectance
eeeee

° 200 400 800 800 1900
TBC Thickness (pm)

Figure 7. Predicted effoet uPintroduction of erack (gap) st plusmaa-sprayed 8YS7,
TBCisubstrate ioterfuce on refleetanes at 4.9 um wavelength 23 9 function of TBC
thickness, Measured reflectance values are als» ineluded for a TRC specimen after 0 cycles
(attached) and 200 cyeles (delaminared).

ACKNOWLEDGMENTS

"he authors wish to-thanic Geoige Ligsler, Sundy Leivsler. and Gury Kostyak for eaaring
oposition, Richer! Moncry foe crosiou testing, Chuck Grvet for Fumace eyeling, and Like
artes for wsislanee in developing ths imzge acquisition saftiar,

REFERENCES

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ermal Beever Coatings hy Mi-Infiared Reflectance,
a

leatth Vinitaring oF
erm, Big. Sel. Pra, 24[4], S116
8.

“LL Eldtidge, C.M. Spuctler, KW. Sree, al LR, Murkbum. “Indaved Radiative
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711. DeMasi-Marein, XD. Shefisy and §. Bose, "Mecharis af Degradation ancl
File alasnaeposited Thermal havi Coasiag ASME J. Eg Cas Turki
owes, 112. $21-26 (1990

7. Makine, T, Kuntome, L. Sab, aud 1. Kista, “Thermal Kaiaion Properties of
Ceramic Miler,” Ff trator dpm os. 13/4, 33-50 (1985