Available online at https://www.doczj.com/doc/368933811.html,
ScienceDirect
Journal of the European Ceramic Society35(2015)
249–257
The application of Eu3+photoluminescence piezo-spectroscopy in the LaMgAl11O19/8YSZ:Eu double-ceramic-layer coating system
Sumei Zhao a,b,Zhimin Ren c,Yu Zhao d,Jiaying Xu a,b,Binglin Zou a,Yu Hui a,b,
Ling Zhu e,Xin Zhou a,b,Xueqiang Cao a,?
a State Key Laboratory of Rare Earth Resources Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun130022,China
b University of the Chinese Academy of Sciences,Beijing100049,China
c Beijing Power Machinery Institute,Beijing100074,China
d Shenyang Jianzhu University,Shenyang110168,China
e Department o
f Chemical and Biological Engineering,Changsha University of Science and Technology,Changsha410114,China
Received5May2014;received in revised form17July2014;accepted29July2014
Available online7September2014
Abstract
A non-destructive inspection technique based on the relationship between the stress and peak position of5D0→7F2transition of Eu3+ions in 8YSZ has been developed to measure the residual stress in TBCs.A LaMgAl11O19/8YSZ:Eu(LaMA/8YSZ:Eu)double-ceramic-layer coating was prepared by atmospheric plasma spraying.Thermal cycling test was conducted and the residual stress in the8YSZ:Eu layer was analyzed with the application of the Eu3+photoluminescence piezo-spectroscopy.There existed both compressive and tensile stresses in the8YSZ:Eu layer of the as-sprayed LaMA/8YSZ:Eu coating with an average tensile stress value of55MPa.As thermal cycling test going on,the compressive stress disappeared and the tensile stress increased gradually.In addition,the application of8YSZ:Eu luminescence sublayer could also be used to indicate the spallation and damage degree of LaMA/8YSZ:Eu coating.
Crown Copyright?2014Published by Elsevier Ltd.All rights reserved.
Keywords:Thermal barrier coating;Eu3+photoluminescence piezo-spectroscopy;Residual stress;Non-destructive technique
1.Introduction
Thermal barrier coatings(TBCs),which protect the substrate materials against oxidation and thermal corrosion,have been widely used in the hot section of gas turbine engines for the thermal protection to improve the fuel ef?ciency and prolong the lifetime of components.1–4The performance of TBCs is affected by thermal stresses generated by the temperature gradients in the TBC,thermal expansion mismatch between the ceramic and the metal,phase transformations,ceramic sintering,resid-ual stresses arising from the deposition process and corrosive ?Corresponding author at:State Key Laboratory of Rare Earth Resources Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun130022,China.Tel.:+8643185262285;
fax:+8643185262285.
E-mail address:xcao@https://www.doczj.com/doc/368933811.html,(X.Cao).and erosive attack.5,6TBCs have a tendency to spall and debond during thermal cycling under high temperature condition.It is believed that the spallation of the ceramic component in TBCs is a result of stresses generated in service.Residual stress is one of the important factors affecting the durability of TBCs during service.7–9Residual stresses develop in TBCs due to the thermal expansion mismatch between the different layers of the TBC sys-tem.Thermally grown oxide(TGO),forming along the irregular BC/TC interface at elevated temperatures,is considered to be the main cause to residual stress at the BC/TC interface.10,11The failure mechanism of TBC is normally related to the stress.12–16 Therefore,it is necessary and important to evaluate and predict the development of the stress in the TBCs during thermal cycles.
As TBCs are thin and brittle,the study of the residual stresses is a complex issue.There is a need for a non-destructive technique for the residual stress measurement. Several non-destructive techniques such as X-ray diffraction
https://www.doczj.com/doc/368933811.html,/10.1016/j.jeurceramsoc.2014.07.029
0955-2219/Crown Copyright?2014Published by Elsevier Ltd.All rights reserved.
250S.Zhao et al./Journal of the European Ceramic Society35(2015)249–257
(XRD)techniques,6,17neutron diffraction(ND),14,18micro-Raman techniques,19–21and the?nite element method (including the mathematical calculation),22ruby?uorescence spectroscopy(RFS)23–26have been applied to investigate the coating stresses.Among these non-destructive techniques,XRD technique is widely used to measure the residual stress in metal-lic and less frequently in ceramic materials.27The main limit of this method is the shallow penetration of the X-ray beam, which generally restricts the analysis to the surface regions of the sample(50-60?m outer layer of TBCs).16The problem of studying thick coating on metal components can be solved by using ND.However,the cost of this method is expensive.RFS is a non-destructive inspection technique for the measurement of residual stresses within the TGO layer consisting of a-Al2O3 with Cr3+solute.This technique which takes the advantage of the transparency of ceramic coating at visible wavelengths can provide local stress information of the TGO.However,it is dif-?cult to measure the stress in the ceramic coating.Therefore,it is highly demanded to develop a new method to overcome the limitation of these techniques in order to monitor the residual stress in the ceramic layer of TBCs.
Concepts for luminescence sensing of TBCs have been proposed by Gentleman et al.28Luminescence sublayers (europium-doped or terbium-doped sublayers)have been used to self-indicate the delamination or the spallation in TBCs.29–32 Recently,rare-earth oxides have been introduced into ceramic as a temperature sensor indicator.33–39The emission spectrum of Eu3+has been used to monitor the phase structure of ZrO2.40 Eu3+ion is one of the most typical luminescent rare-earth which provides a very intense and sharp luminescence spectrum when doped into the host material.The peak position of lumi-nescence spectrum of Eu3+is sensitive to pressure,41similar to the luminescence spectrum of Cr3+in the?-Al2O3,because the energy level can be changed by the crystal parameters of the host material.The topcoat material widely used today is yttria partially stabilized zirconia(YSZ),especially8YSZ due to the low conductivity and high thermal expansion coef?cient.42,43 Luminescent layer can be introduced into TBCs by low-level doping of the TBC itself without introducing a completely distinct phosphor layer that might detrimentally affect TBC performance.44
A new approach based on the relationship between stress and the position of the main peak of5D0→7F2transition of Eu3+ ions in8YSZ has been developed as a non-destructive inspection technique to measure the residual stress in TBCs.45The Eu3+ luminescent layer was applied in this method to ensure that the detected position in TBCs can be well detected and controlled.
In our work,Eu3+photoluminescence piezo-spectroscopy was used to detect the residual stress of the inner8YSZ layer in LaMgAl11O19/8YSZ:Eu(LaMA/8YSZ:Eu)double-ceramic-layer coating system.The schematic description of LaMA/8YSZ:Eu coating with Eu3+photoluminescence piezo-spectroscopy layer is shown in https://www.doczj.com/doc/368933811.html,MA is an attractive material of TBCs,and the thermal cycling failure of LaMA/8YSZ coating was investigated which mainly focused on the microstructure evolutions,the crystal chemistry characteris-tics,the phase stability and so on.46,47In our work,the residual stress of the inner8YSZ layer in LaMA/8YSZ:Eu coating was
investigated.
2.The theory of Eu3+photoluminescence
piezo-spectroscopy
The luminescence of Eu3+ions is decided by intra-
con?guration f–f transition.Due to the shielding effect of the
5s25p6electrons,the4f energy levels of Eu3+are independent
of the crystals?eld.However,the probabilities of transition
between different energy levels are strongly dependent on the
crystals?eld because of the spectral selection rules.There
are several obvious emission peaks from the excited level of 5D0→7F J(J=0-6)for Eu3+ions.The transition of5D0→7F2 is allowed by an electric dipole mechanism and therefore its
intensity is signi?cantly in?uenced by the crystal?eld.The tran-
sition of5D0→7F1is magnetic-dipole driven and its intensity is not signi?cantly altered by perturbation of the crystalline?eld. The(5D0→7F2)/(5D0→7F1)emission ratio(i.e.the asymme-try ratio,R)can be used as an index to measure the site symmetry of Eu3+ions.40Except the peak intensity,the peak position of the5D0→7F2transition is also can be affected by the local envi-ronment of the Eu3+ions.In our work,the peak position shift of5D0→7F2transition was used to detect the residual stress in the LMA/8YSZ coating.The main peak of5D0→7F2transi-tion shifts linearly with pressure.The relationship between the peak position and pressure can be described by the following equation45:
E(p)=E(0)+αp(1) whereαis the shift coef?cient of6.82cm?1/GPa,E(p)is the peak position(cm?1)at pressure p(GPa),and E(0)is the peak position(cm?1)at ambient conditions.The equation also can be written as:
σ=(Eσ?E0)/α(2) where Eσis the measured peak position,the E0is the unstressed peak position(about16507.05cm?1at ambient conditions),and α=6.82cm?1/GPa.
3.Experimental
3.1.Synthesis of8YSZ:Eu and LaMA powder
8YSZ:Eu(Eu3+,1at%)powder was synthesized by a solid
state reaction method.8YSZ(Beijing General Research Insti-
tute of Mining and Metallurgy)and Eu2O3(99.99%,Shanghai
Yuelong New Materials Co.,Ltd)were selected as the starting
materials.Rare earth oxides powders(8YSZ,Eu2O3)were heat-
treated at1000?C for1h in air in the present study because rare
earth oxides are hygroscopic.These two powders were mixed
together in proper ratio.A deionized water-based suspension
of this mixed powder was ball-milled for24h using zirconia
balls.The slurry was then dried and heated at1600?C for24h
to obtain the8YSZ:Eu powder.The as-synthesized8YSZ:Eu
powder was ball milled together with Arabic gum,triammonium
S.Zhao et al./Journal of the European Ceramic Society35(2015)249–257
251
Fig.1.Schematic description of LaMA/8YSZ:Eu double-ceramic-layer TBCs with the Eu3+photoluminescence piezo-spectroscopy and the cross-sectional micro-graph of the corresponding as-sprayed LaMA/8YSZ:Eu coating.
citrate and deionized water for72h.The obtained slurry was then spray-dried(Jiangsu Yangguang Ganzao Co.,Ltd),lead-ing to the formation of8YSZ:Eu powder with free-?owing for plasma spraying.
LaMA was synthesized in the similar https://www.doczj.com/doc/368933811.html,2O3 (99.99%,Guangdong Chenghai Chemicals Co.,Ltd),?-Al2O3 (99.99%,Tangshan Huatai Functional Ceramic Materials Co., Ltd),and MgO(99.2%,Wuzi Zehui Chemicals Co.,Ltd)were selected as the starting materials.The ball-milled slurry were died and heated at1600?C for12h to obtain LaMA powder.
3.2.Preparation of coating
Coatings were atmospheric plasma-sprayed on the Ni-based superalloy substrate(30mm in diameter and3mm in thickness) with a Ni-23.7Co-20Cr-8.7Al-0.6Y-3.5Ta(wt%)bond coat (100?m in thickness),using the Sulzer Metco plasma spraying unit with a F4-MB gun.The cross-sectional microstructure of the as-sprayed LaMA/8YSZ:Eu coating is shown in Fig.1.The average thickness of LaMA coating is90±25?m and the aver-age thickness of the8YSZ:Eu layer is60±25?https://www.doczj.com/doc/368933811.html,MA/8YSZ coating was also prepared in this work.
3.3.Thermal cycling tests
The thermal cycling test was carried out on a burner-rig setting with a coal gas/oxygen?ame.During thermal cycling test,the coating surface was heated from room temperature to 1250±50?C for5min,followed by quenching to room temper-ature within2min by cooling air jet.The surface temperature was measured with a pyrometer operating at a wavelength of 8-13?m.The cycling process repeated again and again until the coating is failed.
3.4.Characterization
The phase structures of powders and coatings were analyzed by X-ray diffraction(XRD)(XRD,Bruker D8Advance)with Cu Karadiation at a scan rate of8?/min.
The coating samples were embedded in a transparent epoxy resin,and then sectioned with a low speed diamond saw and polished with diamond pastes down to1?m.The surface and cross-sectional morphology were characterized by scan-ning electron microscopy(SEM,XL-30ESEM FEG,Mico FEI Philips)equipped with energy dispersive X-ray spectrometer (EDS).
To observe luminescence from the exposed sublayer,eroded specimens were viewed under254nm UV illumination provided by an UV viewing lamp(ZF-1).The luminescence spectra of the8YSZ:Eu powder and coating were collected by Lab RAM HR800Raman system(Horiba Jobin Yvon,France).The focal length of the Raman system is800mm.In the Raman sys-tem,a532.19nm laser line was used as the excitation line. The spectral resolution was≤0.65cm?1(with1800g/mm grat-ing,×10objective magnitude).There are several excitation peaks at254nm,395nm,466nm and532nm in the excita-tion spectrum.31However,the optical band-gap of YSZ was reported to be～5eV,with band-gap absorption in the mid-UV and absorption in the mid-IR.28The excitation line used for the luminescence spectra measurement was532.19nm,where YSZ is translucent rather than the UV wavelength where YSZ is opaque.28Wavenumber in the range of1400-2600cm?1oper-ating in the room temperature was obtained.
4.Results and discussion
4.1.Thermal cycling behavior of LaMA/8YSZ:Eu coating
Surface photographs of LaMA/8YSZ:Eu coating as-sprayed and during thermal cycling under standard white illumination and UV(254nm)illumination are shown in Fig.2.The as-sprayed LaMA layer is luminescence transparent under254nm UV light illumination as can be seen in Fig.2(a’).In order to investagate the luminescence observed in Fig.2was cused by the Eu3+ions in inner8YSZ layer or the defects the LMA ceramic layer under UV illumination,both LaMA/8YSZ and LaMA/8YSZ:Eu double ceramic layer coating were prepared. The surface photographs of the as-sprayed LaMA/8YSZ and LaMA/8YSZ:Eu coating under UV illumination are shown in
252
S.Zhao et al./Journal of the European Ceramic Society 35(2015)
249–257
Fig.2.Surface photographs of LaMA/8YSZ:Eu coating under standard white light illumination and UV (254nm)illumination:as-sprayed (a and a ),after 2322thermal cycles (b and b ),and after 4202thermal cycles (c and c ).
the Fig.3.The left one is the LaMA/8YSZ coating and the right one is the LaMA/8YSZ:Eu coating.There is no red lumimes-cence observed from the LaMA/8YSZ coating.So,the red luminescence observed in Fig.2is from the inner 8YSZ:Eu layer.Besides,pure LaMA material did not show any luminescence under UV excitation.48–50Although LMA has a wide UV band,there is still red luminescence produced in the inner 8YSZ:Eu layer under UV illumination and pass through the LMA layer.This may be due to that the thickness of the LMA layer in this coating system is thin.In addition,LMA is in a good melt-ing condition during plasma spraying and the as-spryaed LMA layer is amorphous.A typical emission spectrum for excita-tion at 532.19nm for the as-sprayed 8YSZ:Eu coating can be
obtained as shown in Fig.4.Emission peaks corresponding to the 5D 0→7F 0,5D 0→7F 1,and 5D 0→7F 2transitions of Eu 3+can be seen in the emission spectra.According to the relation-ship between the peak position of 5D 0→7F 2and the pressure described in Section 2,the peak position measured in the emis-sion spectrum was used to calculate the residual stress of the 8YSZ:Eu coating.After 2300thermal cycles at 1250?C,a small area of spallation appeared at the beveled edge of the coating.It also can be seen that the luminescence transparent ability of the LaMA ceramic layer became weaker after thermal cycling test.In addition,the luminescence spot where the LaMA layer was spalled from the coating was much obvious than other area.This means that the application of 8YSZ:Eu luminescence sublayer
31
Fig.3.Surface photographs of the as-sprayed LaMA/8YSZ (left)and LaMA/8YSZ:Eu coatings (right)under UV (254nm)illumination.
S.Zhao et al./Journal of the European Ceramic Society35(2015)249–257
253
Fig.4.The typical emission spectrum for excitation at532.19nm for the as-sprayed LaMA/8YSZ:Eu coating.
also can be used to indicate the spallation and damage degree of
LaMA/8YSZ:Eu coating,which the top ceramic layer is lumi-
nescence transparent.After4202thermal cycles,the spallation
area enlarged and new spallation can be seen in the coating
surface.Furthermore,as the thermal cycling going on,the out
LaMA layer became less light transparent.This could be due to
the formation and propagate of horizontal cracks in the LaMA
top ceramic layer.Since the optical-signal attenuation occurs
when light travels from LaMA to others(air,n=1.0),the hori-
zontal cracks can greatly attenuate the optical signal.In addition,
LaMA is in a good melting condition during plasma spraying and
the as-sprayed coating is amorphous with a low porosity.How-
ever,after thermal cycling,lots of platelet-like grains formed
and more pores appeared.This phenomenon also aggravates the
optical-signal attenuation.
4.2.The measurement of the residual stress in8YSZ:Eu
layer
A quarter of coating surface was chosen to measure the resid-
ual stress distribution.The selected spots distribution was shown
in Fig.5.The peak positions of the emission peak corresponding
to the5D0→7F2transition were listed in Table1.The values listed in the table are Raman shift values.Some data in the table
are red and italic marked.The shape of emission spectra cor-
responding to these data is distorted compared with the typical
emission spectra.This phenomenon means that the lumines-
cence transparent ability of LaMA in these spots position is
weakened with thermal cycling test going on.This kind of dis-
torted emission spectra was shown in Fig.6(a).The peak position
in this kind of spectra cannot be read accurately.The distorted
emission spectra began to appear after500thermal cycles and
frequency increased with thermal cycling going on.Some blanks
can be seen in the table.The emission spectra corresponding to
the blank one are almost a slant curve as shown in Fig.6(b).
The Fig.5.The spots selected in a quarter of the coating surface for the luminescence measurement to obtain the residual stress.
peak position of5D0→7F2transition in the curve absolutely cannot be read.The slant curve began to appear after3145ther-mal cycles,and frequency increased with thermal cycling going on.The distortion of the emission spectra exciting at532.19nm coincided with the luminescence attenuation phenomenon under 254nm UV illumination with thermal cycling going on.
The residual stress distribution in the8YSZ:Eu layer of the as-sprayed LaMA/8YSZ:Eu coating was shown in Fig.7.The stress distribution is inhomogeneous.There are both compres-sive and tensile stress in the8YSZ:Eu layer of the as-sprayed LaMA/8YSZ:Eu coating.The stress varied from a tensile stress of1.017GPa to a compressive stress of604MPa,with an average value of55MPa tensile stress.The residual stress is in tendency that changed from the tensile in the center to compressive near the edge.As the shape of some emission spectrum measured from the coating with thermal cycling test going on is distorted, the residual stress calculated for the coating after thermal cycling is not very accurate.However,the tendency of the change of the stress still can be obtained.As thermal cycling test going on,the compressive stress disappeared and the tensile stress increased. The average stress value is increased gradually as the thermal cycling test going on.
Quenching stress in tensile is one of the important parts of the residual stress in the as-sprayed TBCs.22When the coating was sprayed on the cooler substrate and cooled from the processing temperature to substrate temperautre,the quenching stress was generated in the ceramic coating due to the rapid contraction of the sprayed splats.In the LaMA/8YSZ:Eu coating system, quenching stress in tensile in8YSZ:Eu layer generated due to the rapid contraction of the8YSZ:Eu sprayed splats.However, a residual stress in compressive could generated in8YSZ:Eu layer due to the rapid contraction of the LaMA sprayed splats.
254S.Zhao et al./Journal of the European Ceramic Society35(2015)249–257
Table1
The peak position of emission peak corresponding to the5D0→7F2transition of LaMA/8YSZ:Eu coating as-sprayed and during thermal cycling.
Raman
Shift(cm-1)As-spryaed
500
cycles
1022
cycles
2322
cycles
3145
cycles
4202
cycles
002289.722286.872286.872289.262289.26--b
012282.212286.872286.472300.002300.00-- b
022286.962293.242304.44a2297.212297.21-- b
032283.402289.262299.47 a2297.212297.212307.55 042283.792289.662286.872302.792302.792289.26
052283.002293.632290.852286.872286.87-- b
062283.402286.872305.97 a2300.80 a2290.85 a-- b
102284.192286.472299.602286.872318.30 a2299.60 112283.002286.872308.75 a2302.392300.40 a2301.19 122283.402286.472312.33 a2286.472308.362302.79
132283.402286.872289.262296.422296.02-- b
142278.662293.25 a2306.45 a2299.602310.34 a2310.04 a 152286.562290.852296.822293.632293.632301.19 a 202283.402290.452289.662319.99 a2318.30 a2306.76 a
212281.032287.672296.822294.03-- b-- b
222278.662290.452286.872288.062296.02 a2308.36 a 232284.982286.472289.262286.472305.69 a2295.26 242283.002286.872286.472290.852325.86 a2304.44 a 302281.422291.252294.032296.02 a2300.09 a2314.72
312282.212286.472292.042300.00-- b2299.60
322282.612292.442291.642298.01 a2300.71 a2306.37 332283.402289.262290.452293.632296.98 a2293.24
402283.002286.872286.072298.012298.84 a-- b
412283.002287.672290.052299.602291.642303.20 a
422284.192293.632296.022319.50 a2306.76-- b
502283.002286.872290.452296.98 a2301.592313.53 512278.662286.872293.242293.632286.072290.05 602284.192286.872297.212300.00 a2286.872293.63
a The emiss ion spectra s hape is d istorte d compared with the typic al emiss ion sp ectra o f the Eu3+ions;
b The emiss ion spectr a shape is almost a slant curv e and the peak position ca n not be read.
Average2283.162288.832294.442296.942300.252301.41
S.Zhao et al./Journal of the European Ceramic Society 35(2015)249–257
255
Fig.6.The distorted emission spectra (a)and the spectra which is almost a slant curve,(b)measured in the spots position where the luminescence transparent ability of LaMA top ceramic is
weak.
Fig.7.The residual stress distribution in the 8YSZ:Eu layer of the as-sprayed LaMA/8YSZ:Eu coating.
Thermal stress resulted from difference of the mismatch of thermal expansion coef?cients (TEC)between the coating and substrate when the samples cooled from deposition temperature to room temperature.Unlike the quenching stress in tensile,the thermal stress due to the different TEC may be either compressive or tensile.Stress associated with changes in volume at solid-state phase transformation is also composition of residual stress.The coating may have a phase transformation in the process of thermal spraying and thermal cycling.This kind of stress often could be ignored in the as-sprayed coating.Tensile stress generates and accumulates with thermal cycling test going on.During heating,8YSZ:Eu layer is in a com-pressive stress state.This stress causes deformation when the temperature is high.During cooling,as the deformed material has a tend of contraction,tensile stress formed.The tensile stress accumulates and is gradually increased with thermal cycling process going on.
During spectra measurement,the spectral resolution was ≤0.65cm ?1(with 1800g/mm dispersive grating,×10objec-tive magnitude,and 800mm focal length).The Lorentz curve method is appropriate for peak position determination consider-ing the noise,baseline and asymmetry of the peak.The top half of the peak was selected and ?tted.The error of the ?tting proce-dure is small compared with 0.65cm ?1in the spectral resolution and can be ignored.Such as the analysis of the spectrum in the center position of the as-sprayed coating surface,the value of R 2is 0.9987and the error of peak position is ±0.038cm ?1during ?tting procedure.In Eq.(2),E σis the measured peak position,the E 0is the unstressed peak position (16507.05cm ?1at ambient conditions),and α=6.82cm ?1/GPa.The error of the Eu 3+photoluminescence piezo-spectroscopy resulted from the peak position measurement is about 95MPa.
5.Conclusions
A new approach based on the relationship between stress and the main peak position of 5D 0→7F 2transition of Eu 3+ions in 8YSZ has been developed as a non-destructive inspection tech-nique to measure the residual stress in TBCs.A LaMA/8YSZ:Eu double-ceramic-layer coating has been prepared by APS and thermal cycling test was conducted for the coating.Eu 3+photo-luminescence piezo-spectroscopy was used to detect the residual stress of the inner 8YSZ layer in the LaMA/8YSZ coating system.There are both compressive and tensile stress in the 8YSZ:Eu layer of the as-sprayed LaMA/8YSZ:Eu coating with a average value of 55MPa in tensile stress.As thermal cycling test going on,the compressive stress disappeared and the tensile stress increased.The average stress value is increased gradually as the thermal cycling test going on.In addition,the applica-tion of 8YSZ:Eu luminescence sublayer also can be used to indicate the spallation and damage degree of LaMA/8YSZ:Eu coating.However,the luminescence transparent ability of the LaMA top ceramic layer is weakened with the thermal cycling test going on due to the formation of cracks and increase of the porosity.
256S.Zhao et al./Journal of the European Ceramic Society35(2015)249–257
Acknowledgements
This work was?nancially supported by the projects of the National Natural Science Foundation of China(21171160, 21001017)and Lotus Scholars Program of Hunan. References
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