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Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings
Surface & Coatings Technology 201 (2006) 3942 – 3946 www.elsevier.com/locate/surfcoat
Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings M.D. Chambers ⁎, David R. Clarke Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106-5050, United States Available online 25 September 2006
Abstract The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188–189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the 5D0 → 7F2 emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected. © 2006 Elsevier B.V. All rights reserved. Keywords: Raman scattering spectroscopy; Photoluminescence spectroscopy; Electron-beam PVD; Europium YSZ; Luminescence lifetime; Thermal aging; Crystal structure evolution
1. Introduction The use of luminescence to measure temperature dates back to the early 1950's [3]; the various methods of temperature determination as well as techniques and materials were reviewed in detail by Allison and Gillies [4]. One method is to compare the measured decay in luminescence intensity, after pulsed excitation, to a previously established calibration curve describing the dependence of lifetime on temperature [3]. The usual implementation of luminescence thermometry involves the use of a phosphor material with known luminescence characteristics either painted onto a component or incorporated into it as a composite. In thermal barrier coating systems, where extrinsic sensors present the risk of premature failure, the use of luminescent ions doped into the crystal structure of the thermal barrier coating itself can alleviate this difficulty, allowing thermometry with only a slight change in the composition of the system and no change its physical configuration. Such solid solution doping also ensures thermodynamic compatibility and ⁎ Corresponding author. E-mail address: [email protected] (M.D. Chambers). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.011
hence long-term stability at high temperatures. Commonly used high-temperature phosphors, such as rare-earth doped yttrium aluminum garnet, yttrium oxide and oxysulphides, are not chemically compatible with zirconia or pyrochlore thermal barrier coatings and consequently will react at high temperatures, degrading both the phosphor and the coating. The vast majority of current thermal barrier coatings, whether deposited by electron beam evaporation or plasma-spraying, are yttria-stabilized zirconia in the metastable tetragonal-prime structure. After extended exposure at very high temperatures, this metastable phase is known to evolve into a mixture of tetragonal and cubic phases. On cooling, the tetragonal phase can transform to the monoclinic phase. This transformation, though rarely seen in service parts, is considered undesirable since it can be accompanied by microcracking and mechanical degradation of the coating. Additionally, though the monoclinic phase can revert to tetragonal at high temperatures the monoclinic transformation is not completely reversible, as evidenced by the much more rapid return to monoclinic on subsequent cooling. To address the question as to whether long-term, high temperature exposure affects the luminescence spectra and
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
3943
luminescence lifetimes from Eu-doped YSZ coatings, a system previously [1,2] studied for thermometry, electron-beam evaporated coatings have been aged to produce various concentrations of monoclinic zirconia. 2. Experimental details The coatings studied were prepared by electron beam (EB– PVD) evaporation under standard deposition conditions onto alumina substrates. The alumina substrates were chosen rather than the usual superalloy so that the coatings could be heated to 1425 °C for the aging studies. A novel method was used to make the Eu-doped coatings. It involved repeated infiltration of a standard YSZ ingot (7 a/o YO1.5) with europium nitrate solutions and calcination at 900 °C until approximately 1 atomic percent of EuO1.5 was achieved. Coatings of these materials were deposited in two stages; a 10 μm layer of Eu-doped YSZ was first deposited followed by a thick layer of undoped YSZ
Fig. 2. Monoclinic concentration versus low temperature (150 °C) annealing time for a coating already aged at 1425 °C for 195 h. The fit is an AMJK relationship.
for a total coating thickness of approximately 150 μm; this structure was chosen so that the coatings could also be used to demonstrate through-thickness temperature measurement capability. The coatings were aged at 1425 °C in air for the durations of 20 min, 90 min, 10 h, 75 h, or 195 h to promote the decomposition of as-deposited tetragonal-prime phase to tetragonal and cubic. They were then aged at 150 °C, again in air, to accelerate the martensitic transformation of tetragonal to monoclinic [5]. Raman spectroscopy was used to evaluate the crystal structure and monitor the transformation from tetragonal-prime structure of the as-deposited coating to the monoclinic phase. The fraction of monoclinic phase was quantified from the area, I, of the Raman peaks characteristic of the monoclinic phase at 182 and 191 Rcm− 1and the tetragonal peaks at 148 and 263 Rcm− 1 [6]; (Fig. 1). The empirical relationship used was: fm ¼
Im182 þ Im191 148 263 0:97ðIt It Þ þ Im182
þ Im191
ð1Þ
where fm is the fraction of monoclinic phase and the subscripts m and t refer to monoclinic and tetragonal, respectively. The as-deposited coatings were tetragonal and remained so even after 195 h at 1425 °C. However, subsequent low temperature aging at 150 °C in air, caused partial transformation to the monoclinic phase. In fact, the extent of transformation was found to depend on both the length of high-temperature aging as well as on the time of low temperature aging. Coatings aged for 75 h and 195 h at 1425 °C slowly transformed with time at 150 °C with the kinetics following the well known Avrami– Johnson–Mehl–Kolmogorov (AJMK) relation: Fig. 1. Raman spectra of coatings both as deposited and after a two stage heat treatment (195 h at 1425 °C, 304 h at 150 °C). The peaks characteristic of tetragonal and monoclinic zirconia are indicated. After heat treatment the fraction of monoclinic has risen to about 50%.
ftYm ðtÞ ¼ ftYm;l ½1−expðkt n Þ
ð2Þ
where the fraction transformed at long times is dependent on the high temperature treatment, that is, the amount of material that
3944
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
resolved intensity, including either one or two exponential terms depending on the profile: −t −t −t þ I2 exp ð3; 4Þ ; I ¼ I1 exp I ¼ I1 exp s s1 s2 where I is intensity, t is time, and τ is the lifetime. The effect of disorder on the lifetime was matched with a stretched exponential function when necessary [7]: 1=C −t I ¼ I1 exp ð5Þ s1 Fig. 3. Schematic of the experimental setup for making pulsed laser lifetime measurements.
had been transformed from tetragonal-prime to tetragonal during the high-temperature exposure (Fig. 2). High resolution luminescence spectra of the coatings were taken with a Jobin–Yvon spectrometer (514.5 nm excitation); lower resolution spectra with 532 nm excitation were taken to verify similarity. Lifetime measurements were made using pulsed 532 nm lasers. For luminescence measurements at low and moderate temperatures, a Nd:YVO4 frequency-doubled laser (532 nm) was used and the pulse duration and periodicity were controlled using a function generator, and the pulse duration was varied to rule out saturation and rise-time effects. The response of this particular system limited measurements to lifetimes of greater than 5 μs. To measure the faster decays characteristic of higher temperatures, a frequency-doubled Nd: YAG laser with a pulse length of ∼10 ns was used. The photostimulated luminescence from the coating was collected, filtered, detected with a photomultiplier and the signal recorded on a digital oscilloscope (Fig. 3). The luminescence decay lifetime was determined by the fit of an exponential function to the time-
where Γ, the disorder parameter, was restricted to values less than 1.05. The maximum temperature of our lifetime measurements was limited by the time response of our detection system. 3. Effect on aging and phase transformation on the luminescence spectrum Fig. 4 shows the luminescence spectrum of the as-deposited coatings. The peak of greatest intensity, and the one used for the lifetime measurements, is associated with the 5D0 to 7F2 [8] transition and has a wavelength of 606 nm. The effects of aging and transformation are shown in Fig. 5. After high temperature heat treatment, the luminescence peaks sharpen. This is attributed to a decrease in the local atomic disorder within the zirconia, an observation consistent with the sharpening of the Raman peaks reported earlier [5]. An additional peak appears as a shoulder at ∼ 615 nm. This is attributed to a change in symmetry around the Eu3+ ions, usually associated in other compounds with breaking of the local symmetry around the luminescent ion; based on the singlet nature of the 5D0 to 7F0 peak throughout, the Eu+3 is believed to occupy only one kind of crystallographic site [9]. There is also a very slight shift in wavelength of the 5D0 to 7F2 peak as the zirconia transforms from tetragonal to monoclinic
Fig. 4. Luminescence spectrum of the as-deposited Eu-doped 7YSZ coating together with Eu+3 energy diagram (after Dieke [11]) showing excitation and subsequent de-excitation from the 5D0 level.
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
Fig. 5. Evolution of luminescent spectrum with aging; transitions are labeled, with higher wavelength transitions belonging to the 7F2 and 7F3 states. The 7F1 triplet sharpens upon aging, and the 7F2 emission at 615 nm, seen as a shoulder on the 606 nm peak, increases in intensity.
(Fig. 6). The origin of this shift to longer wavelengths is not fully understood but is believed to be due to the volume expansion of the unit cell as tetragonal zirconia transforms to monoclinic, reducing the mean magnitude of the crystal field felt by the electrons in the Eu+3 ions. 4. Effect of transformation on luminescence decay The luminescence decay at low measurement temperatures is characterized by a single exponential whose value is indepen-
Fig. 6. Decay profile of the 606 nm peak in the as-deposited condition at low and intermediate temperatures after 532 nm excitation. While the fundamental decay is invariant throughout the low and intermediate temperature regimes, there is a faster decay also observed in the intermediate temperature regime which can in some cases convolute the fundamental.
3945
Fig. 7. Comparison between an as deposited coating and a 50% monoclinic coating of the luminescence decay profile of the 606 nm peak on similar semilog scales after 532 nm excitation.
dent of both temperature and monoclinic concentration. At intermediate temperatures a double exponential is needed to fit the decay data (Fig. 6). The two decay time constants τ reflect differences in the underlying decay mechanisms. At still higher temperatures, the decay again becomes a single exponential. The origin of these different temperature regimes will be discussed in a future contribution. In the high temperature regime the decay rate is sensitive to both temperature and the presence of the monoclinic phase (Fig. 7) suggesting that it can, nevertheless, be used for thermometry purposes. The presence of the monoclinic phase shifts the onset of thermal quenching to somewhat lower temperatures but otherwise leaves the temperature dependence of the luminescence
Fig. 8. Temperature vs. time constant of the 606 nm peak from decay curves after 532 nm excitation. The two extreme cases, 0% monoclinic and 50% monoclinic, are shown.
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M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
decay of Eu:7YSZ unchanged (Fig. 8). The curve through the data corresponds to the multi-phonon model for luminescence lifetime applied to zirconia where the multi-phonon non-radiative relaxation process is in competition with the radiative process: 1 1 1 ¼ þ s sR sMPR
ð6Þ
where τR is the radiative rate and τMPR is non-radiative rate described by " sMPR ðT Þ ¼
s0MPR
Jx −1 1 þ exp kT
−1 #−DE=Jx
ð7Þ
Here ω is the phonon frequency and ΔE is equal to the energy of the 5D0 to 7F2 transition, about 2.05 eV; thus, the exponent is equal to the number of phonons required to account for the energy of relaxation. In this model, the temperature dependence is attributed to non-radiative transfer of excited state energy to lattice phonons whose population is given by phonon statistics [10]. The values of ω and τ0MPR have been allowed to vary in the fitting. The fits shown in Fig. 8 yield phonon energies of 597 Rcm− 1 and 604 Rcm− 1 for the data from the as-deposited and the highly transformed coatings, respectively, corresponding nearly with the highest energy phonon modes at elevated temperatures. 5. Discussion and conclusions The results presented here indicate that the luminescence from Eu-doped YSZ coatings is remarkably insensitive to long term aging and transformation from the tetragonal to the monoclinic phase. While there are systematic changes over long times at high temperature and transformation, these are sufficiently small that they do not affect the collection and detection of the luminescence signal. For instance, the observed shift in wavelength of the 5D0 to 7 F2 transition covers only a 0.2 nm range, even after transformation of half of the coating to the monoclinic phase, and is much smaller than both the width of the luminescence peaks
themselves and the width of the interference filter used to detect the luminescence in the decay measurements. Based on the relative phase-insensitivity of the immediate environment around the Eu+3 ions in YSZ [12], this is not surprising. To place these observations into perspective, the high temperatures used (1425 °C) are considerably higher than current TBCs are exposed to in service where coatings also experience little or no transformation to monoclinic. The changes in wavelength and decay lifetimes, though detectable, are small and not expected to offer a better means of non-destructive monitoring the aging and degradation of YSZ coatings in service than other techniques, such as Raman microscopy. The error that phase degradation would induce in decay lifetime thermometry, even for the extreme case of a coating that is 50% monoclinic, will result in only a small overestimate of temperature. In particular, at temperatures above about 1000 °C, where the monoclinic phase has transformed into tetragonal, the difference in measured lifetimes is within the experimental scatter. Acknowledgments The authors are grateful to Molly Gentleman of the University of California at Santa Barbara for her invaluable discussions and to Dr. Ken Murphy of Howmet Corporation for depositing the coatings studied in this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
K.L. Choy, et al., Surf. Eng. 16 (6) (2000) 496. M.M. Gentleman, D.R. Clarke, Surf. Coat. Technol. 188–189 (2004) 93. L.C. Bradley, Rev. Sci. Instrum. 24 (1953) 219. S.W. Allison, G.T. Gillies, Rev. Sci. Instrum. 68 (1997) 2615. V. Lughi, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1287. D.R. Clarke, F. Adar, J. Am. Ceram. Soc. 65 (6) (1982) 284. J. Klafter, M.F. Shlesinger, Proc. Natl. Acad. Sci. U. S. A. 83 (1986) 848. G. Blasse, et al., J. Phys. Chem. Solids 27 (10) (1966) 1587. G. Blasse, A. Bril, Philips Res. Rep. 21 (5) (1966) 368. L.A. Riseberg, H.W. Moos, Phys. Rev. 174 (2) (1968) 429. G.H. Dieke, H.M. Crosswhite, Appl. Opt. 2 (7) (1963) 675. Ping Li, I-Wei Chen, J. Am. Ceram. Soc. 77 (1) (1994) 118.
Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings M.D. Chambers ⁎, David R. Clarke Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106-5050, United States Available online 25 September 2006
Abstract The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188–189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the 5D0 → 7F2 emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected. © 2006 Elsevier B.V. All rights reserved. Keywords: Raman scattering spectroscopy; Photoluminescence spectroscopy; Electron-beam PVD; Europium YSZ; Luminescence lifetime; Thermal aging; Crystal structure evolution
1. Introduction The use of luminescence to measure temperature dates back to the early 1950's [3]; the various methods of temperature determination as well as techniques and materials were reviewed in detail by Allison and Gillies [4]. One method is to compare the measured decay in luminescence intensity, after pulsed excitation, to a previously established calibration curve describing the dependence of lifetime on temperature [3]. The usual implementation of luminescence thermometry involves the use of a phosphor material with known luminescence characteristics either painted onto a component or incorporated into it as a composite. In thermal barrier coating systems, where extrinsic sensors present the risk of premature failure, the use of luminescent ions doped into the crystal structure of the thermal barrier coating itself can alleviate this difficulty, allowing thermometry with only a slight change in the composition of the system and no change its physical configuration. Such solid solution doping also ensures thermodynamic compatibility and ⁎ Corresponding author. E-mail address: [email protected] (M.D. Chambers). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.011
hence long-term stability at high temperatures. Commonly used high-temperature phosphors, such as rare-earth doped yttrium aluminum garnet, yttrium oxide and oxysulphides, are not chemically compatible with zirconia or pyrochlore thermal barrier coatings and consequently will react at high temperatures, degrading both the phosphor and the coating. The vast majority of current thermal barrier coatings, whether deposited by electron beam evaporation or plasma-spraying, are yttria-stabilized zirconia in the metastable tetragonal-prime structure. After extended exposure at very high temperatures, this metastable phase is known to evolve into a mixture of tetragonal and cubic phases. On cooling, the tetragonal phase can transform to the monoclinic phase. This transformation, though rarely seen in service parts, is considered undesirable since it can be accompanied by microcracking and mechanical degradation of the coating. Additionally, though the monoclinic phase can revert to tetragonal at high temperatures the monoclinic transformation is not completely reversible, as evidenced by the much more rapid return to monoclinic on subsequent cooling. To address the question as to whether long-term, high temperature exposure affects the luminescence spectra and
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
3943
luminescence lifetimes from Eu-doped YSZ coatings, a system previously [1,2] studied for thermometry, electron-beam evaporated coatings have been aged to produce various concentrations of monoclinic zirconia. 2. Experimental details The coatings studied were prepared by electron beam (EB– PVD) evaporation under standard deposition conditions onto alumina substrates. The alumina substrates were chosen rather than the usual superalloy so that the coatings could be heated to 1425 °C for the aging studies. A novel method was used to make the Eu-doped coatings. It involved repeated infiltration of a standard YSZ ingot (7 a/o YO1.5) with europium nitrate solutions and calcination at 900 °C until approximately 1 atomic percent of EuO1.5 was achieved. Coatings of these materials were deposited in two stages; a 10 μm layer of Eu-doped YSZ was first deposited followed by a thick layer of undoped YSZ
Fig. 2. Monoclinic concentration versus low temperature (150 °C) annealing time for a coating already aged at 1425 °C for 195 h. The fit is an AMJK relationship.
for a total coating thickness of approximately 150 μm; this structure was chosen so that the coatings could also be used to demonstrate through-thickness temperature measurement capability. The coatings were aged at 1425 °C in air for the durations of 20 min, 90 min, 10 h, 75 h, or 195 h to promote the decomposition of as-deposited tetragonal-prime phase to tetragonal and cubic. They were then aged at 150 °C, again in air, to accelerate the martensitic transformation of tetragonal to monoclinic [5]. Raman spectroscopy was used to evaluate the crystal structure and monitor the transformation from tetragonal-prime structure of the as-deposited coating to the monoclinic phase. The fraction of monoclinic phase was quantified from the area, I, of the Raman peaks characteristic of the monoclinic phase at 182 and 191 Rcm− 1and the tetragonal peaks at 148 and 263 Rcm− 1 [6]; (Fig. 1). The empirical relationship used was: fm ¼
Im182 þ Im191 148 263 0:97ðIt It Þ þ Im182
þ Im191
ð1Þ
where fm is the fraction of monoclinic phase and the subscripts m and t refer to monoclinic and tetragonal, respectively. The as-deposited coatings were tetragonal and remained so even after 195 h at 1425 °C. However, subsequent low temperature aging at 150 °C in air, caused partial transformation to the monoclinic phase. In fact, the extent of transformation was found to depend on both the length of high-temperature aging as well as on the time of low temperature aging. Coatings aged for 75 h and 195 h at 1425 °C slowly transformed with time at 150 °C with the kinetics following the well known Avrami– Johnson–Mehl–Kolmogorov (AJMK) relation: Fig. 1. Raman spectra of coatings both as deposited and after a two stage heat treatment (195 h at 1425 °C, 304 h at 150 °C). The peaks characteristic of tetragonal and monoclinic zirconia are indicated. After heat treatment the fraction of monoclinic has risen to about 50%.
ftYm ðtÞ ¼ ftYm;l ½1−expðkt n Þ
ð2Þ
where the fraction transformed at long times is dependent on the high temperature treatment, that is, the amount of material that
3944
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
resolved intensity, including either one or two exponential terms depending on the profile: −t −t −t þ I2 exp ð3; 4Þ ; I ¼ I1 exp I ¼ I1 exp s s1 s2 where I is intensity, t is time, and τ is the lifetime. The effect of disorder on the lifetime was matched with a stretched exponential function when necessary [7]: 1=C −t I ¼ I1 exp ð5Þ s1 Fig. 3. Schematic of the experimental setup for making pulsed laser lifetime measurements.
had been transformed from tetragonal-prime to tetragonal during the high-temperature exposure (Fig. 2). High resolution luminescence spectra of the coatings were taken with a Jobin–Yvon spectrometer (514.5 nm excitation); lower resolution spectra with 532 nm excitation were taken to verify similarity. Lifetime measurements were made using pulsed 532 nm lasers. For luminescence measurements at low and moderate temperatures, a Nd:YVO4 frequency-doubled laser (532 nm) was used and the pulse duration and periodicity were controlled using a function generator, and the pulse duration was varied to rule out saturation and rise-time effects. The response of this particular system limited measurements to lifetimes of greater than 5 μs. To measure the faster decays characteristic of higher temperatures, a frequency-doubled Nd: YAG laser with a pulse length of ∼10 ns was used. The photostimulated luminescence from the coating was collected, filtered, detected with a photomultiplier and the signal recorded on a digital oscilloscope (Fig. 3). The luminescence decay lifetime was determined by the fit of an exponential function to the time-
where Γ, the disorder parameter, was restricted to values less than 1.05. The maximum temperature of our lifetime measurements was limited by the time response of our detection system. 3. Effect on aging and phase transformation on the luminescence spectrum Fig. 4 shows the luminescence spectrum of the as-deposited coatings. The peak of greatest intensity, and the one used for the lifetime measurements, is associated with the 5D0 to 7F2 [8] transition and has a wavelength of 606 nm. The effects of aging and transformation are shown in Fig. 5. After high temperature heat treatment, the luminescence peaks sharpen. This is attributed to a decrease in the local atomic disorder within the zirconia, an observation consistent with the sharpening of the Raman peaks reported earlier [5]. An additional peak appears as a shoulder at ∼ 615 nm. This is attributed to a change in symmetry around the Eu3+ ions, usually associated in other compounds with breaking of the local symmetry around the luminescent ion; based on the singlet nature of the 5D0 to 7F0 peak throughout, the Eu+3 is believed to occupy only one kind of crystallographic site [9]. There is also a very slight shift in wavelength of the 5D0 to 7F2 peak as the zirconia transforms from tetragonal to monoclinic
Fig. 4. Luminescence spectrum of the as-deposited Eu-doped 7YSZ coating together with Eu+3 energy diagram (after Dieke [11]) showing excitation and subsequent de-excitation from the 5D0 level.
M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
Fig. 5. Evolution of luminescent spectrum with aging; transitions are labeled, with higher wavelength transitions belonging to the 7F2 and 7F3 states. The 7F1 triplet sharpens upon aging, and the 7F2 emission at 615 nm, seen as a shoulder on the 606 nm peak, increases in intensity.
(Fig. 6). The origin of this shift to longer wavelengths is not fully understood but is believed to be due to the volume expansion of the unit cell as tetragonal zirconia transforms to monoclinic, reducing the mean magnitude of the crystal field felt by the electrons in the Eu+3 ions. 4. Effect of transformation on luminescence decay The luminescence decay at low measurement temperatures is characterized by a single exponential whose value is indepen-
Fig. 6. Decay profile of the 606 nm peak in the as-deposited condition at low and intermediate temperatures after 532 nm excitation. While the fundamental decay is invariant throughout the low and intermediate temperature regimes, there is a faster decay also observed in the intermediate temperature regime which can in some cases convolute the fundamental.
3945
Fig. 7. Comparison between an as deposited coating and a 50% monoclinic coating of the luminescence decay profile of the 606 nm peak on similar semilog scales after 532 nm excitation.
dent of both temperature and monoclinic concentration. At intermediate temperatures a double exponential is needed to fit the decay data (Fig. 6). The two decay time constants τ reflect differences in the underlying decay mechanisms. At still higher temperatures, the decay again becomes a single exponential. The origin of these different temperature regimes will be discussed in a future contribution. In the high temperature regime the decay rate is sensitive to both temperature and the presence of the monoclinic phase (Fig. 7) suggesting that it can, nevertheless, be used for thermometry purposes. The presence of the monoclinic phase shifts the onset of thermal quenching to somewhat lower temperatures but otherwise leaves the temperature dependence of the luminescence
Fig. 8. Temperature vs. time constant of the 606 nm peak from decay curves after 532 nm excitation. The two extreme cases, 0% monoclinic and 50% monoclinic, are shown.
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M.D. Chambers, D.R. Clarke / Surface & Coatings Technology 201 (2006) 3942–3946
decay of Eu:7YSZ unchanged (Fig. 8). The curve through the data corresponds to the multi-phonon model for luminescence lifetime applied to zirconia where the multi-phonon non-radiative relaxation process is in competition with the radiative process: 1 1 1 ¼ þ s sR sMPR
ð6Þ
where τR is the radiative rate and τMPR is non-radiative rate described by " sMPR ðT Þ ¼
s0MPR
Jx −1 1 þ exp kT
−1 #−DE=Jx
ð7Þ
Here ω is the phonon frequency and ΔE is equal to the energy of the 5D0 to 7F2 transition, about 2.05 eV; thus, the exponent is equal to the number of phonons required to account for the energy of relaxation. In this model, the temperature dependence is attributed to non-radiative transfer of excited state energy to lattice phonons whose population is given by phonon statistics [10]. The values of ω and τ0MPR have been allowed to vary in the fitting. The fits shown in Fig. 8 yield phonon energies of 597 Rcm− 1 and 604 Rcm− 1 for the data from the as-deposited and the highly transformed coatings, respectively, corresponding nearly with the highest energy phonon modes at elevated temperatures. 5. Discussion and conclusions The results presented here indicate that the luminescence from Eu-doped YSZ coatings is remarkably insensitive to long term aging and transformation from the tetragonal to the monoclinic phase. While there are systematic changes over long times at high temperature and transformation, these are sufficiently small that they do not affect the collection and detection of the luminescence signal. For instance, the observed shift in wavelength of the 5D0 to 7 F2 transition covers only a 0.2 nm range, even after transformation of half of the coating to the monoclinic phase, and is much smaller than both the width of the luminescence peaks
themselves and the width of the interference filter used to detect the luminescence in the decay measurements. Based on the relative phase-insensitivity of the immediate environment around the Eu+3 ions in YSZ [12], this is not surprising. To place these observations into perspective, the high temperatures used (1425 °C) are considerably higher than current TBCs are exposed to in service where coatings also experience little or no transformation to monoclinic. The changes in wavelength and decay lifetimes, though detectable, are small and not expected to offer a better means of non-destructive monitoring the aging and degradation of YSZ coatings in service than other techniques, such as Raman microscopy. The error that phase degradation would induce in decay lifetime thermometry, even for the extreme case of a coating that is 50% monoclinic, will result in only a small overestimate of temperature. In particular, at temperatures above about 1000 °C, where the monoclinic phase has transformed into tetragonal, the difference in measured lifetimes is within the experimental scatter. Acknowledgments The authors are grateful to Molly Gentleman of the University of California at Santa Barbara for her invaluable discussions and to Dr. Ken Murphy of Howmet Corporation for depositing the coatings studied in this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
K.L. Choy, et al., Surf. Eng. 16 (6) (2000) 496. M.M. Gentleman, D.R. Clarke, Surf. Coat. Technol. 188–189 (2004) 93. L.C. Bradley, Rev. Sci. Instrum. 24 (1953) 219. S.W. Allison, G.T. Gillies, Rev. Sci. Instrum. 68 (1997) 2615. V. Lughi, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1287. D.R. Clarke, F. Adar, J. Am. Ceram. Soc. 65 (6) (1982) 284. J. Klafter, M.F. Shlesinger, Proc. Natl. Acad. Sci. U. S. A. 83 (1986) 848. G. Blasse, et al., J. Phys. Chem. Solids 27 (10) (1966) 1587. G. Blasse, A. Bril, Philips Res. Rep. 21 (5) (1966) 368. L.A. Riseberg, H.W. Moos, Phys. Rev. 174 (2) (1968) 429. G.H. Dieke, H.M. Crosswhite, Appl. Opt. 2 (7) (1963) 675. Ping Li, I-Wei Chen, J. Am. Ceram. Soc. 77 (1) (1994) 118.