- Email: [email protected]
Concepts for luminescence sensing of thermal barrier coatings
Surface & Coatings Technology 188–189 (2004) 93 – 100 www.elsevier.com/locate/surfcoat
Concepts for luminescence sensing of thermal barrier coatings M.M. Gentleman*, D.R. Clarke Materials Department, College of Engineering, University of California, Santa Barbara, CA 93106-5050, United States Available online 12 October 2004
Abstract The basis for a number of concepts for in-situ monitoring thermal barrier coatings using non-contact luminescence is described. Restrictions imposed by phase compatibility with the thermally grown oxide formed between the bond-coat and the coating, as well as with the thermal barrier coating material are described. This naturally leads to the selection of rare-earth ions incorporated into the crystal structure of the coating, whether YSZ or one of the rare-earth pyrochlores, as appropriate chromophores. A rare-earth sensor embedded in a YSZ coating prepared by electron-beam deposition is demonstrated as well as a brainbowQ sensor. D 2004 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coatings; Luminescence; Phase compatibility, Zirconia
1. Introduction There has been a growing use of ceramic materials to provide thermal insulation to metal blades and other hot sections in gas turbines in the aerospace and power generation industry [1]. The importance of these coatings, commonly referred to as thermal barrier coatings, is that they enable metal components to be used at surface temperatures very close to, or even above, their melting temperature. In turn, this allows turbines to be operated at higher temperatures and, hence, at higher efficiencies. One consequence of the growing reliance on coatings to increase the gas inlet temperature of engines is that the coatings have to be bprime reliantQ, meaning that they must not fail during the lifetime of the component. This places a premium on the ability to monitor the coating and its integrity. Furthermore, as the life of the coating is believed sensitively related to the temperature of the metal immediately beneath the coating, it is also desirable to be able to monitor this temperature insitu through the thermal barrier coating. This is especially important, as the metal-surface temperature will increase as the coating is eroded away. In this contribution, we describe luminescence methods for sensing the bhealthQ of thermal barrier coatings. The * Corresponding author. Tel.: +1 805 893 3559; fax: +1 805 893 8971. E-mail address: [email protected] (M.M. Gentleman). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.005
realization of these concepts will depend on the development of appropriate technologies for implementing them and the willingness to apply these ideas to the aggressive environment of operating turbines. Therefore, the intent of this contribution is simply to present, and demonstrate, some of the essential ideas for assessing the health of thermal barrier coatings. 2. The thermal barrier coating system Thermal barrier coatings are multilayer systems [2–4]. They consist of an overlay coating of the insulating ceramic material on top of a thin metallic bbond-coatQ layer formed on the superalloy component. During exposure at high temperatures in air (and in typical turbine combustion environments), a thin-layer of aluminum oxide forms at the interface between the bbond-coatQ and the thermal barrier coating. The composition of the bbond-coatQ is chosen to form aluminum oxide, commonly called the thermally grown oxide (TGO), upon oxidation since aluminum oxide has the slowest oxygen diffusion rate of any oxide and hence leads to the lowest oxidation rate. The most common material presently used as a thermal barrier coating is yttriastabilized zirconia (YSZ) but there is increasing use of the rare-earth zirconates, such as gadolinium zirconate, Gd2Zr2O7, (GZO), on account of their lower thermal
94
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
conductivity at high temperatures [4,5]. Pertinent to optical sensing applications, the coatings are typically deposited by either plasma-spraying (PS) or electron-beam physical vapor deposition (EB-PVD). The former results in the formation of bsplatQ boundaries parallel to the coating that tends to scatter light and decrease optical transmission. The EB-PVD coatings have a columnar microstructure, which tend to cause preferential channeling of light into and out of the coating and scattering of light parallel to the coating. Both methods of deposition also result in the coatings having microscopic porosity, a microstructural feature desirable for further decreasing the low thermal conductivity
of these coatings below that of even the bulk material. It is important to note that YSZ coatings have the metastable tetragonal prime crystal structure [4] and are quite distinct from the fully stabilized, cubic zirconia (bsynthetic diamondQ) used in applications such as optical windows and substrates for thin-film growth [6]. Their composition is also different, usually 7–8 w/o Y2O3 as distinct to the 20 w/o of the cubic single crystals. Both YSZ and GZO coatings are translucent in the visible due to entrapped porosity. The optical band-gap of YSZ is reported to be ~5 eV [7]. The optical absorption spectrum of GZO is unknown but is believed to be similar to
Fig. 1. The optical absorption and scattering cross-sections for a YSZ material (top), redrawn following Ref. [8]. Superimposed are the wavelengths of two of the lasers used to excite luminescence from our sensors. Bottom, superimposed blackbody radiation curves for the indicated temperatures. The shaded band indicates the wavelength window over which any temperature sensor must operate in the presence of thermal radiation. At shorter wavelengths, absorption and the optical band-gap (Eg) of YSZ limit both luminescence excitation and emission. At longer wavelengths, sensing will be limited by the ability to distinguish the luminescence signal above the thermal radiation.
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
YSZ and other complex oxides, with band-gap absorption in the mid-UV and absorption in the mid-IR. The optical absorption properties of both materials at elevated temperatures have not been reported. The presence of porosity within the coatings causes optical scattering, particularly in the plasma-sprayed coatings. The frequency dependence of the optical absorption and optical scattering for a typical YSZ material are shown in Fig. 1 (top) [8]. As will be demonstrated, the difference in absorption in the visible and the UV, illustrated in Fig. 1, provides an opportunity to use lasers of different frequency to probe different depths into YSZ coatings for a variety of sensor applications. Also pertinent to the issue of sensing is that the primary TBC failure modes are spallation and delamination of the coating [9]. In both modes, the coating separates from the component with the actual failure path being within the ceramic coating, typically close to the interface with the thermally grown oxide. (In large thermal gradients, the failure path can shift to different planes in the coating.) Failure thus causes a local reduction in the thickness of the TBC with the consequence that the temperature of the metal under the spalled region increases because of the lower thermal resistance afforded by the thinner coating.
3. Sensing concepts Several concepts for monitoring the bhealthQ of thermal barrier coatings can be envisaged. These range from the possibility of assessing damage and wear to in-situ measurement of temperatures. Broadly speaking the con-
95
cepts can be classified as being either bvisualizationQ sensors or bmeasurementQ sensors. Possibly the simplest bvisualizationQ sensor is as an inspection method at room temperature, for instance standard boroscope inspection during periodic maintenance, to assess whether the coating has locally spalled away. In cases where the spalled regions are very large, they can be seen visually. This is the basis for boroscope inspection today. However, when the spalled regions are smaller, the concept of a bred-lineQ sensor facilitates greater contrast. The concept is shown in Fig. 2 and consists of an inner layer of the coating doped with a chromophore adjacent to the bond-coat. Where the coating is intact, the doped region is buried and so can only be probed with a laser wavelength to which the coating is transparent. Where the coating has spalled away or has been eroded, the doped layer is exposed and can be illuminated with a laser in the UV as well as in the visible. An elaboration of the bred-lineQ sensor is what might be termed the brainbowQ sensor, shown schematically in Fig. 3, consisting of a series of layers, each with a different dopant that luminesces at a different frequency. Under optical excitation in the visible, characteristic luminescence from each of the layers can be collected. As the thermal barrier coating erodes away, successive doped layers will be removed and so the visible luminescence will characterize the remaining layers, enabling the remaining thickness of the coating to be assessed. Alternatively, using mid-UV excitation above the optical band gap, it should be possible to detect the outermost layer in the brainbowQ sensor and thereby directly monitor the progress of erosion.
Fig. 2. Schematic diagram of the use of a buried luminescence layer as a bred-lineQ sensor. The same configuration, with a temperature sensitive chromophore, can be used to monitor the temperature of the coating in contact with the underlying bond-coat metal.
96
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 3. Schematic diagram of the brainbowQ sensor, consisting of a multilayer of luminescent dopants. As individual doped layers are eroded, worn away or spalled, the spectrum changes with the characteristic luminescence from those disappearing as they are eroded away, as shown schematically.
Both sensor configurations also form the basis of temperature measurement using dopants whose luminescence characteristics are temperature-dependent. The bredlineQ sensor configuration then provides a means, in principle, of monitoring the temperature of the coating in contact with the bond-coat. More ambitious, but also highly desirable would be the measurement of the temperature gradient through the thickness of the coating by monitoring the luminescence simultaneously from layers at both the inner and outer portions of the coating. Highly desirable, also, would be the measurement of the temperature at the outer surface of the coating. There are two traditional luminescence methods of determining temperature [10]. One is to measure the ratio of the intensities of different spectral lines as this ratio can be temperature dependent. The other is by measurement of the luminescence lifetime. In both cases, calibration curves must be established first so as to determine the temperature from these spectral measurements. Both work at U.C. Santa Barbara by ourselves and by Eldridge at NASA Glenn have demonstrating using the luminescence lifetime method to determine temperature of YSZ and GZO materials and coatings.
4. Phase compatibility As a thermal barrier coating is in direct, physical contact with the thermally grown aluminum oxide, phase compatibility of the coating material with aluminum oxide places very severe restrictions on the selection of possible luminescent materials and their microstructural location within a coating system. Luminescent materials that are not thermodynamically compatible with the coating will react and decompose with inevitable degradation in lumines-
cence and also, possibly, coating life. For the two commonest thermal barrier coatings materials, YSZ and GZO, the pertinent phase compatible diagrams with aluminum oxide are shown in Fig. 4 drawn from the work of Leckie et al. [11] and Leckie and Levi [12]. The figure consists to two pseudo-ternary phase diagrams joined along their common Al2O3–ZrO2 pseudo-binary. Of the phases in the compatibility diagrams of Fig. 4, the three best known to be hosts for luminescence ions [13,14], yttria (Y2O3), yttrium aluminate (YAlO3) and yttrium aluminum garnet (YAG), are seen to be incompatible with both the thermally grown oxide, aluminum oxide and t-ZrO2 (YSZ). This indicates that none of these phases can be used as stable luminescence sensors in direct contact with the thermally grown oxide formed by oxidation of the bond-coat and YSZ. If the thermal barrier coating were to be made from the pyrochlore phase (indicated as Py in Fig. 4), then it would be possible to use the GdAlO3 phase but the pyrochlore phase itself is not compatible with alumina. The lack of stability in oxidizing atmospheres at high temperatures obviously precludes the use of other well-known phosphors, such as the oxy-sulphides. These phase compatibility restrictions indicate that the coating materials themselves must be used as the hosts for the luminescing chromophore. Fortunately, both YSZ and GZO can accommodate tri-valent rare-earth ions by partial substitution for the Y3+ and Gd3+ ions, respectively, because of their similar ionic size and valence [15]. Indeed, there a number of reports of the photoluminescence of a number of rare-earth ions in YSZ [16– 18]. It should be noted that the phase compatibility restrictions are less severe for sensing the outer portions of the coating since the luminescence sensing material does not have to be compatible with alumina. Then the
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
97
Fig. 4. Diagram representing the thermodynamic phase stability of alumina with zirconia containing yttria or gadolina. The symbols refer to the crystal structure of the compounds: P—perovskite, C—cubic, G—garnet, F—fluorite, Py—pyrochlore. (Redrawn after Leckie et al. [11] and Leckie and Levi [12]).
perovskite phases, such as YAG, could be deposited as a sensing layer on YSZ coatings.
5. Selection of luminescence chromophore dopants As mentioned above, the trivalent rare-earth ions have some limited solubility in both the YSZ and GZO phases. As they are also well known to be luminescent ions [13,14] and widely used in phosphors and displays, the trivalent rare-earth ions are the natural choice for the class of dopants for use in luminescence sensing of thermal barrier coatings. For room temperature sensor applications, such as the bredlineQ sensor, there are several choices of the rare-earth dopant. For the brainbowQ sensors, the dopants in the different layers must be chosen so that there is little overlap between the luminescence lines of the different dopants. Guidance in selecting potential rare-earth ions dopants for YSZ and GZO coatings is provided by the Dieke diagram (Fig. 5) tabulating the luminescence from different energy levels of the indicated rare-earth ions in lanthanum chloride. This diagram indicates that for room temperature sensor applications almost all the rare-earth ions can serve as potential luminescence dopants as they luminesce over the entire visible spectrum and into the infra-red. For sensing applications at temperature, the selection of appropriate dopants is more restrictive since the luminescence signal must be detectable against the black-body radiation from the surroundings and the hot gas as well as the coating itself. So, to be detectable the luminescence must be at wavelengths below the high-energy edge of the black-body curve. This is shown in Fig. 1 (bottom) for a number of representative black-body temperatures and indicates that ions that luminescence at wavelengths below ~650 nm are required to avoid being swamped by blackbody radiation.
To measure temperature, the choice of the rare-earth ions is still more restrictive, particularly if the luminescence lifetime method is used. As yet, little is known about the temperature dependence of the luminescence lifetime from different rare-earth ions in either YSZ or GZO. However, it is known, for instance, that the temperature dependence of luminescence lifetime from Cr ions is dependent on the host crystal structure and can vary over several hundred degrees [19].
6. Concentration effects Once an appropriate chromophore ion has been identified, it is essential to establish its useful range of concentration for the particular sensor application. For visualization sensing, the doping concentration that maximizes the luminescence signal is most desirable, subject to phase stability considerations. On the other hand, for in-situ temperature measurement, although maximizing the luminescence intensity is clearly desirable, it is important that the selected concentration is one for which the lifetime decay varies over the temperature range of interest. The concentration at which the luminescence intensity is a maximum is set by the phenomenon of concentration quenching. This is illustrated by the intensity of different luminescence lines from Sm-doped YSZ excited using an argon-ion laser at 514 nm (Fig. 6). The intensity data as a function of concentration, normalized to the maximum intensity in each spectrum, is plotted in Fig. 7. The curves through the data correspond to a fit using the Johnson– Williams equation [20]:
g¼
cð 1 cÞ z c þ A ð 1 cÞ
ð1Þ
98
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 5. Dieke diagram for the energy levels of some of the candidate rare-earth ions that are soluble within the crystal structure of both YSZ and GZO coatings. For reference, the energies of candidate excitation lasers are also indicated.
Notionally, the exponent z represents an effective coordination number of isolated, like ions around the excited ion and the parameter A represents a ratio of optical cross sections but, in practice, they are usually used as fitting parameters since neither is usually known a-priori. The
Fig. 6. Luminescence spectrum in the visible for YSZ doped with 2 m/o SmO1.5 and excited at 514 nm.
Fig. 7. Luminescence intensity of Sm3+ ions in YSZ as a function of concentration illustrating the phenomenon of concentration quenching. The different curves correspond to the different luminescence lines at the wavelengths labeled. The curves through the data correspond to Eq. (1). Excitation at room temperature with an argon-ion laser operating at 514 nm.
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 8. Luminescence intensity versus concentration of Er3+ in YSZ illustrating the absence of concentration quenching for the emission lines at 645 and 678 nm but quenching for the lines at 543 and 562 nm.
values of z and A vary for the different spectral transitions in Fig. 7 but are typically of the order of 30 and 0.006. Also, as indicated by the data in Fig. 7, the Sm-dopant concentration at which the luminescence intensity peaks depends on the particular spectral transition. Interestingly, there are dopants and combinations of excitation and luminescence wavelengths over which no apparent concentration quenching occurs. This unexpected behavior occurs, for example, for the 654 and 678 nm lines from Er-doped YSZ whereas the lines at 543 and 562 nm exhibit concentration quenching (Fig. 8). At this stage in the development of high-temperature luminescence sensing, insufficient data on the concentration dependence of the luminescence lifetime decay and its temperature dependence has been accumulated to be specific about the optimum range of chromophore concentration.
99
Fig. 9. Luminescence from a buried ~10 Am layer of Er-doped YSZ in an EB-PVD YSZ coating and excited with a 514-nm laser. The presence of the R-line luminescence from the TGO in the same spectra confirms that the excitation laser penetrated to the buried sensor layer. No luminescence is excited with the 248-nm laser as it cannot penetrate into the TBC.
the Er3+ was detected through the full thickness of the coating (Fig. 9). In contrast, using the illumination at 248 nm, no luminescence from the Er-doped layer was detectable since the absorption length at 248 nm is substantially smaller than the coating thickness. This
7. Demonstration of sensor concepts In this section, we illustrate the basic concepts described in the preceding sections. Two types of samples were used. One was a 8YSZ thermal barrier coating deposited by electron beam deposition at UCSB. This consisted of a thin (~10 Am) layer of Er-doped YSZ deposited on an oxidized metal alloy and then over-coated with ~120 Am of undoped YSZ coating to make a standard thickness YSZ coating. For this demonstration, the composition of the sensor layer was Zr0.9625Y0.07Er0.005O1.9625. The other was a series of ceramic multilayer of differently doped layers created by sequential deposition of doped powder and sintering at 1500 8C. Both doped YSZ and doped GZO multilayers were produced this way to simulate the brainbowQ coatings based on both materials systems. When the buried Er-doped sensor was illuminated using the 514-nm excitation, the characteristic luminescence from
Fig. 10. Demonstration of two prototype brainbow sensorsQ containing ErO1.5 and EuO1.5 one fabricated out of YSZ and the other out of GZO. Excitation at 514 nm.
100
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
straightforward demonstration illustrates the combined use of a buried luminescence layer and selection of excitation wavelengths to discriminate between the surface and innerlayers of a thermal barrier coating. Incidentally, the presence in the spectrum of the R-line luminescence from the thermally growth oxide confirms that the 514-nm excitation penetrates through the TBC substantiating that the Er luminescence is from next to the TGO. Fig. 10 is an example of the luminescence spectra from two prototypical brainbowQ sensors, one based on YSZ and the other on GZO, both excited at 514 nm, consisting of ErO1.5 and EuO1.5 doped layers. As indicated by the labeling, the spectral lines from the individual layers are distinct and discernable.
8. Summary The restrictions of phase compatibility with the thermally grown aluminum oxide and the TBC coating itself place severe constraints on the possible luminescence sensor materials and suggest that the best chromophores are likely to be those that are incorporated into the crystal structure of the thermal barrier coating. For both YSZ and the rare-earth pyrochlore coatings, rare-earth dopants can serve as chromophores and a number of concepts for bvisualizationQ and bquantitativeQ luminescence sensing are demonstrated based on the optical properties of these coating materials and the luminescence properties of the rare-earth dopants. For temperature sensing, the choice of rare-earth dopants is much more restrictive as will be described elsewhere.
Acknowledgements The authors are grateful to Rafael Leckie and Professor Carlos Levi for the use of the electron-beam deposition
system at UCSB in fabricating the Er-doped YSZ bred-lineQ buried sensor illustrated in this work. Support for Ms. Gentleman during the course of this work has been provided by the NSF IGERT program at UCSB on Optical Materials and by the ONR-MURI-TBC program, also at UCSB.
References [1] R.A. Miller, NASA Report CP 3312 17 (1995). [2] K.H. Stern, Metallurgical and Ceramic Protective Coatings, Chapman and Hall, 1996. [3] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Petit, Prog. Mater. Sci. 46 (2001) 505. [4] D.R. Clarke, C. Levi, Annu. Rev. Mater. Res. 33 (2003) 383. [5] Maloney, M., 2000. U.S. Patent 6,117,560. [6] A.H. Heuer, L.W. Hobbs (Eds.), Science and Technology of Zirconia, Am. Ceram. Soc., Columbus, Ohio, 1981. [7] D.C. Harris, Materials for Infrared Windows and Domes, SPIE Optical Engineering Press, 1999. [8] S. Wahiduzzaman, T. Morel, Final Report to Oak Ridge National Laboratory, # ORNL/sub/88-22042/2 available as DE92-041384 from NTIS. [9] R. Hillery (Ed.), NRC report, National Academy Press, 1996. [10] Z.Y. Zhang, K.T.V. Grattan, Fiber Optic Fluorescence Thermometry, Chapman and Hall, 1994. [11] R.M.R. Leckie, N.R. Rebollo, J.Y. Yang, C.G. Levi, Microstructure stability issues in emerging TBC materials, in: W.A. Kaysser (Ed.), Proceedings of the International Symposium on Thermal Barrier Coatings and Titanium Aluminides, DLR, Bonn, Germany, 2002. [12] R.M.R. Leckie, C.G. Levi, to be published. [13] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, New York, 1994. [14] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Oxford University Press, 1989. [15] R.D. Shannon, Acta Crystallogr., A 32 (1976) 751. [16] V. Sergo, D.R. Clarke, unpublished research (1994). [17] J.P. Feist, A.L. Heyes, K.L. Choy, J. Mei, J.R. Nicholls, Proc. Inst. Mech. Eng. 215 (2001) 333. [18] J. Eldridge, NASA Glenn, private communication (2003). [19] Compiled and shown as Fig. 2.10 in Ref. [10]. [20] P.D. Johnson, F.E. Williams, J. Chem. Phys. 18 (1950) 1477.
Concepts for luminescence sensing of thermal barrier coatings M.M. Gentleman*, D.R. Clarke Materials Department, College of Engineering, University of California, Santa Barbara, CA 93106-5050, United States Available online 12 October 2004
Abstract The basis for a number of concepts for in-situ monitoring thermal barrier coatings using non-contact luminescence is described. Restrictions imposed by phase compatibility with the thermally grown oxide formed between the bond-coat and the coating, as well as with the thermal barrier coating material are described. This naturally leads to the selection of rare-earth ions incorporated into the crystal structure of the coating, whether YSZ or one of the rare-earth pyrochlores, as appropriate chromophores. A rare-earth sensor embedded in a YSZ coating prepared by electron-beam deposition is demonstrated as well as a brainbowQ sensor. D 2004 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coatings; Luminescence; Phase compatibility, Zirconia
1. Introduction There has been a growing use of ceramic materials to provide thermal insulation to metal blades and other hot sections in gas turbines in the aerospace and power generation industry [1]. The importance of these coatings, commonly referred to as thermal barrier coatings, is that they enable metal components to be used at surface temperatures very close to, or even above, their melting temperature. In turn, this allows turbines to be operated at higher temperatures and, hence, at higher efficiencies. One consequence of the growing reliance on coatings to increase the gas inlet temperature of engines is that the coatings have to be bprime reliantQ, meaning that they must not fail during the lifetime of the component. This places a premium on the ability to monitor the coating and its integrity. Furthermore, as the life of the coating is believed sensitively related to the temperature of the metal immediately beneath the coating, it is also desirable to be able to monitor this temperature insitu through the thermal barrier coating. This is especially important, as the metal-surface temperature will increase as the coating is eroded away. In this contribution, we describe luminescence methods for sensing the bhealthQ of thermal barrier coatings. The * Corresponding author. Tel.: +1 805 893 3559; fax: +1 805 893 8971. E-mail address: [email protected] (M.M. Gentleman). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.005
realization of these concepts will depend on the development of appropriate technologies for implementing them and the willingness to apply these ideas to the aggressive environment of operating turbines. Therefore, the intent of this contribution is simply to present, and demonstrate, some of the essential ideas for assessing the health of thermal barrier coatings. 2. The thermal barrier coating system Thermal barrier coatings are multilayer systems [2–4]. They consist of an overlay coating of the insulating ceramic material on top of a thin metallic bbond-coatQ layer formed on the superalloy component. During exposure at high temperatures in air (and in typical turbine combustion environments), a thin-layer of aluminum oxide forms at the interface between the bbond-coatQ and the thermal barrier coating. The composition of the bbond-coatQ is chosen to form aluminum oxide, commonly called the thermally grown oxide (TGO), upon oxidation since aluminum oxide has the slowest oxygen diffusion rate of any oxide and hence leads to the lowest oxidation rate. The most common material presently used as a thermal barrier coating is yttriastabilized zirconia (YSZ) but there is increasing use of the rare-earth zirconates, such as gadolinium zirconate, Gd2Zr2O7, (GZO), on account of their lower thermal
94
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
conductivity at high temperatures [4,5]. Pertinent to optical sensing applications, the coatings are typically deposited by either plasma-spraying (PS) or electron-beam physical vapor deposition (EB-PVD). The former results in the formation of bsplatQ boundaries parallel to the coating that tends to scatter light and decrease optical transmission. The EB-PVD coatings have a columnar microstructure, which tend to cause preferential channeling of light into and out of the coating and scattering of light parallel to the coating. Both methods of deposition also result in the coatings having microscopic porosity, a microstructural feature desirable for further decreasing the low thermal conductivity
of these coatings below that of even the bulk material. It is important to note that YSZ coatings have the metastable tetragonal prime crystal structure [4] and are quite distinct from the fully stabilized, cubic zirconia (bsynthetic diamondQ) used in applications such as optical windows and substrates for thin-film growth [6]. Their composition is also different, usually 7–8 w/o Y2O3 as distinct to the 20 w/o of the cubic single crystals. Both YSZ and GZO coatings are translucent in the visible due to entrapped porosity. The optical band-gap of YSZ is reported to be ~5 eV [7]. The optical absorption spectrum of GZO is unknown but is believed to be similar to
Fig. 1. The optical absorption and scattering cross-sections for a YSZ material (top), redrawn following Ref. [8]. Superimposed are the wavelengths of two of the lasers used to excite luminescence from our sensors. Bottom, superimposed blackbody radiation curves for the indicated temperatures. The shaded band indicates the wavelength window over which any temperature sensor must operate in the presence of thermal radiation. At shorter wavelengths, absorption and the optical band-gap (Eg) of YSZ limit both luminescence excitation and emission. At longer wavelengths, sensing will be limited by the ability to distinguish the luminescence signal above the thermal radiation.
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
YSZ and other complex oxides, with band-gap absorption in the mid-UV and absorption in the mid-IR. The optical absorption properties of both materials at elevated temperatures have not been reported. The presence of porosity within the coatings causes optical scattering, particularly in the plasma-sprayed coatings. The frequency dependence of the optical absorption and optical scattering for a typical YSZ material are shown in Fig. 1 (top) [8]. As will be demonstrated, the difference in absorption in the visible and the UV, illustrated in Fig. 1, provides an opportunity to use lasers of different frequency to probe different depths into YSZ coatings for a variety of sensor applications. Also pertinent to the issue of sensing is that the primary TBC failure modes are spallation and delamination of the coating [9]. In both modes, the coating separates from the component with the actual failure path being within the ceramic coating, typically close to the interface with the thermally grown oxide. (In large thermal gradients, the failure path can shift to different planes in the coating.) Failure thus causes a local reduction in the thickness of the TBC with the consequence that the temperature of the metal under the spalled region increases because of the lower thermal resistance afforded by the thinner coating.
3. Sensing concepts Several concepts for monitoring the bhealthQ of thermal barrier coatings can be envisaged. These range from the possibility of assessing damage and wear to in-situ measurement of temperatures. Broadly speaking the con-
95
cepts can be classified as being either bvisualizationQ sensors or bmeasurementQ sensors. Possibly the simplest bvisualizationQ sensor is as an inspection method at room temperature, for instance standard boroscope inspection during periodic maintenance, to assess whether the coating has locally spalled away. In cases where the spalled regions are very large, they can be seen visually. This is the basis for boroscope inspection today. However, when the spalled regions are smaller, the concept of a bred-lineQ sensor facilitates greater contrast. The concept is shown in Fig. 2 and consists of an inner layer of the coating doped with a chromophore adjacent to the bond-coat. Where the coating is intact, the doped region is buried and so can only be probed with a laser wavelength to which the coating is transparent. Where the coating has spalled away or has been eroded, the doped layer is exposed and can be illuminated with a laser in the UV as well as in the visible. An elaboration of the bred-lineQ sensor is what might be termed the brainbowQ sensor, shown schematically in Fig. 3, consisting of a series of layers, each with a different dopant that luminesces at a different frequency. Under optical excitation in the visible, characteristic luminescence from each of the layers can be collected. As the thermal barrier coating erodes away, successive doped layers will be removed and so the visible luminescence will characterize the remaining layers, enabling the remaining thickness of the coating to be assessed. Alternatively, using mid-UV excitation above the optical band gap, it should be possible to detect the outermost layer in the brainbowQ sensor and thereby directly monitor the progress of erosion.
Fig. 2. Schematic diagram of the use of a buried luminescence layer as a bred-lineQ sensor. The same configuration, with a temperature sensitive chromophore, can be used to monitor the temperature of the coating in contact with the underlying bond-coat metal.
96
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 3. Schematic diagram of the brainbowQ sensor, consisting of a multilayer of luminescent dopants. As individual doped layers are eroded, worn away or spalled, the spectrum changes with the characteristic luminescence from those disappearing as they are eroded away, as shown schematically.
Both sensor configurations also form the basis of temperature measurement using dopants whose luminescence characteristics are temperature-dependent. The bredlineQ sensor configuration then provides a means, in principle, of monitoring the temperature of the coating in contact with the bond-coat. More ambitious, but also highly desirable would be the measurement of the temperature gradient through the thickness of the coating by monitoring the luminescence simultaneously from layers at both the inner and outer portions of the coating. Highly desirable, also, would be the measurement of the temperature at the outer surface of the coating. There are two traditional luminescence methods of determining temperature [10]. One is to measure the ratio of the intensities of different spectral lines as this ratio can be temperature dependent. The other is by measurement of the luminescence lifetime. In both cases, calibration curves must be established first so as to determine the temperature from these spectral measurements. Both work at U.C. Santa Barbara by ourselves and by Eldridge at NASA Glenn have demonstrating using the luminescence lifetime method to determine temperature of YSZ and GZO materials and coatings.
4. Phase compatibility As a thermal barrier coating is in direct, physical contact with the thermally grown aluminum oxide, phase compatibility of the coating material with aluminum oxide places very severe restrictions on the selection of possible luminescent materials and their microstructural location within a coating system. Luminescent materials that are not thermodynamically compatible with the coating will react and decompose with inevitable degradation in lumines-
cence and also, possibly, coating life. For the two commonest thermal barrier coatings materials, YSZ and GZO, the pertinent phase compatible diagrams with aluminum oxide are shown in Fig. 4 drawn from the work of Leckie et al. [11] and Leckie and Levi [12]. The figure consists to two pseudo-ternary phase diagrams joined along their common Al2O3–ZrO2 pseudo-binary. Of the phases in the compatibility diagrams of Fig. 4, the three best known to be hosts for luminescence ions [13,14], yttria (Y2O3), yttrium aluminate (YAlO3) and yttrium aluminum garnet (YAG), are seen to be incompatible with both the thermally grown oxide, aluminum oxide and t-ZrO2 (YSZ). This indicates that none of these phases can be used as stable luminescence sensors in direct contact with the thermally grown oxide formed by oxidation of the bond-coat and YSZ. If the thermal barrier coating were to be made from the pyrochlore phase (indicated as Py in Fig. 4), then it would be possible to use the GdAlO3 phase but the pyrochlore phase itself is not compatible with alumina. The lack of stability in oxidizing atmospheres at high temperatures obviously precludes the use of other well-known phosphors, such as the oxy-sulphides. These phase compatibility restrictions indicate that the coating materials themselves must be used as the hosts for the luminescing chromophore. Fortunately, both YSZ and GZO can accommodate tri-valent rare-earth ions by partial substitution for the Y3+ and Gd3+ ions, respectively, because of their similar ionic size and valence [15]. Indeed, there a number of reports of the photoluminescence of a number of rare-earth ions in YSZ [16– 18]. It should be noted that the phase compatibility restrictions are less severe for sensing the outer portions of the coating since the luminescence sensing material does not have to be compatible with alumina. Then the
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
97
Fig. 4. Diagram representing the thermodynamic phase stability of alumina with zirconia containing yttria or gadolina. The symbols refer to the crystal structure of the compounds: P—perovskite, C—cubic, G—garnet, F—fluorite, Py—pyrochlore. (Redrawn after Leckie et al. [11] and Leckie and Levi [12]).
perovskite phases, such as YAG, could be deposited as a sensing layer on YSZ coatings.
5. Selection of luminescence chromophore dopants As mentioned above, the trivalent rare-earth ions have some limited solubility in both the YSZ and GZO phases. As they are also well known to be luminescent ions [13,14] and widely used in phosphors and displays, the trivalent rare-earth ions are the natural choice for the class of dopants for use in luminescence sensing of thermal barrier coatings. For room temperature sensor applications, such as the bredlineQ sensor, there are several choices of the rare-earth dopant. For the brainbowQ sensors, the dopants in the different layers must be chosen so that there is little overlap between the luminescence lines of the different dopants. Guidance in selecting potential rare-earth ions dopants for YSZ and GZO coatings is provided by the Dieke diagram (Fig. 5) tabulating the luminescence from different energy levels of the indicated rare-earth ions in lanthanum chloride. This diagram indicates that for room temperature sensor applications almost all the rare-earth ions can serve as potential luminescence dopants as they luminesce over the entire visible spectrum and into the infra-red. For sensing applications at temperature, the selection of appropriate dopants is more restrictive since the luminescence signal must be detectable against the black-body radiation from the surroundings and the hot gas as well as the coating itself. So, to be detectable the luminescence must be at wavelengths below the high-energy edge of the black-body curve. This is shown in Fig. 1 (bottom) for a number of representative black-body temperatures and indicates that ions that luminescence at wavelengths below ~650 nm are required to avoid being swamped by blackbody radiation.
To measure temperature, the choice of the rare-earth ions is still more restrictive, particularly if the luminescence lifetime method is used. As yet, little is known about the temperature dependence of the luminescence lifetime from different rare-earth ions in either YSZ or GZO. However, it is known, for instance, that the temperature dependence of luminescence lifetime from Cr ions is dependent on the host crystal structure and can vary over several hundred degrees [19].
6. Concentration effects Once an appropriate chromophore ion has been identified, it is essential to establish its useful range of concentration for the particular sensor application. For visualization sensing, the doping concentration that maximizes the luminescence signal is most desirable, subject to phase stability considerations. On the other hand, for in-situ temperature measurement, although maximizing the luminescence intensity is clearly desirable, it is important that the selected concentration is one for which the lifetime decay varies over the temperature range of interest. The concentration at which the luminescence intensity is a maximum is set by the phenomenon of concentration quenching. This is illustrated by the intensity of different luminescence lines from Sm-doped YSZ excited using an argon-ion laser at 514 nm (Fig. 6). The intensity data as a function of concentration, normalized to the maximum intensity in each spectrum, is plotted in Fig. 7. The curves through the data correspond to a fit using the Johnson– Williams equation [20]:
g¼
cð 1 cÞ z c þ A ð 1 cÞ
ð1Þ
98
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 5. Dieke diagram for the energy levels of some of the candidate rare-earth ions that are soluble within the crystal structure of both YSZ and GZO coatings. For reference, the energies of candidate excitation lasers are also indicated.
Notionally, the exponent z represents an effective coordination number of isolated, like ions around the excited ion and the parameter A represents a ratio of optical cross sections but, in practice, they are usually used as fitting parameters since neither is usually known a-priori. The
Fig. 6. Luminescence spectrum in the visible for YSZ doped with 2 m/o SmO1.5 and excited at 514 nm.
Fig. 7. Luminescence intensity of Sm3+ ions in YSZ as a function of concentration illustrating the phenomenon of concentration quenching. The different curves correspond to the different luminescence lines at the wavelengths labeled. The curves through the data correspond to Eq. (1). Excitation at room temperature with an argon-ion laser operating at 514 nm.
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
Fig. 8. Luminescence intensity versus concentration of Er3+ in YSZ illustrating the absence of concentration quenching for the emission lines at 645 and 678 nm but quenching for the lines at 543 and 562 nm.
values of z and A vary for the different spectral transitions in Fig. 7 but are typically of the order of 30 and 0.006. Also, as indicated by the data in Fig. 7, the Sm-dopant concentration at which the luminescence intensity peaks depends on the particular spectral transition. Interestingly, there are dopants and combinations of excitation and luminescence wavelengths over which no apparent concentration quenching occurs. This unexpected behavior occurs, for example, for the 654 and 678 nm lines from Er-doped YSZ whereas the lines at 543 and 562 nm exhibit concentration quenching (Fig. 8). At this stage in the development of high-temperature luminescence sensing, insufficient data on the concentration dependence of the luminescence lifetime decay and its temperature dependence has been accumulated to be specific about the optimum range of chromophore concentration.
99
Fig. 9. Luminescence from a buried ~10 Am layer of Er-doped YSZ in an EB-PVD YSZ coating and excited with a 514-nm laser. The presence of the R-line luminescence from the TGO in the same spectra confirms that the excitation laser penetrated to the buried sensor layer. No luminescence is excited with the 248-nm laser as it cannot penetrate into the TBC.
the Er3+ was detected through the full thickness of the coating (Fig. 9). In contrast, using the illumination at 248 nm, no luminescence from the Er-doped layer was detectable since the absorption length at 248 nm is substantially smaller than the coating thickness. This
7. Demonstration of sensor concepts In this section, we illustrate the basic concepts described in the preceding sections. Two types of samples were used. One was a 8YSZ thermal barrier coating deposited by electron beam deposition at UCSB. This consisted of a thin (~10 Am) layer of Er-doped YSZ deposited on an oxidized metal alloy and then over-coated with ~120 Am of undoped YSZ coating to make a standard thickness YSZ coating. For this demonstration, the composition of the sensor layer was Zr0.9625Y0.07Er0.005O1.9625. The other was a series of ceramic multilayer of differently doped layers created by sequential deposition of doped powder and sintering at 1500 8C. Both doped YSZ and doped GZO multilayers were produced this way to simulate the brainbowQ coatings based on both materials systems. When the buried Er-doped sensor was illuminated using the 514-nm excitation, the characteristic luminescence from
Fig. 10. Demonstration of two prototype brainbow sensorsQ containing ErO1.5 and EuO1.5 one fabricated out of YSZ and the other out of GZO. Excitation at 514 nm.
100
M.M. Gentleman, D.R. Clarke / Surface & Coatings Technology 188–189 (2004) 93–100
straightforward demonstration illustrates the combined use of a buried luminescence layer and selection of excitation wavelengths to discriminate between the surface and innerlayers of a thermal barrier coating. Incidentally, the presence in the spectrum of the R-line luminescence from the thermally growth oxide confirms that the 514-nm excitation penetrates through the TBC substantiating that the Er luminescence is from next to the TGO. Fig. 10 is an example of the luminescence spectra from two prototypical brainbowQ sensors, one based on YSZ and the other on GZO, both excited at 514 nm, consisting of ErO1.5 and EuO1.5 doped layers. As indicated by the labeling, the spectral lines from the individual layers are distinct and discernable.
8. Summary The restrictions of phase compatibility with the thermally grown aluminum oxide and the TBC coating itself place severe constraints on the possible luminescence sensor materials and suggest that the best chromophores are likely to be those that are incorporated into the crystal structure of the thermal barrier coating. For both YSZ and the rare-earth pyrochlore coatings, rare-earth dopants can serve as chromophores and a number of concepts for bvisualizationQ and bquantitativeQ luminescence sensing are demonstrated based on the optical properties of these coating materials and the luminescence properties of the rare-earth dopants. For temperature sensing, the choice of rare-earth dopants is much more restrictive as will be described elsewhere.
Acknowledgements The authors are grateful to Rafael Leckie and Professor Carlos Levi for the use of the electron-beam deposition
system at UCSB in fabricating the Er-doped YSZ bred-lineQ buried sensor illustrated in this work. Support for Ms. Gentleman during the course of this work has been provided by the NSF IGERT program at UCSB on Optical Materials and by the ONR-MURI-TBC program, also at UCSB.
References [1] R.A. Miller, NASA Report CP 3312 17 (1995). [2] K.H. Stern, Metallurgical and Ceramic Protective Coatings, Chapman and Hall, 1996. [3] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Petit, Prog. Mater. Sci. 46 (2001) 505. [4] D.R. Clarke, C. Levi, Annu. Rev. Mater. Res. 33 (2003) 383. [5] Maloney, M., 2000. U.S. Patent 6,117,560. [6] A.H. Heuer, L.W. Hobbs (Eds.), Science and Technology of Zirconia, Am. Ceram. Soc., Columbus, Ohio, 1981. [7] D.C. Harris, Materials for Infrared Windows and Domes, SPIE Optical Engineering Press, 1999. [8] S. Wahiduzzaman, T. Morel, Final Report to Oak Ridge National Laboratory, # ORNL/sub/88-22042/2 available as DE92-041384 from NTIS. [9] R. Hillery (Ed.), NRC report, National Academy Press, 1996. [10] Z.Y. Zhang, K.T.V. Grattan, Fiber Optic Fluorescence Thermometry, Chapman and Hall, 1994. [11] R.M.R. Leckie, N.R. Rebollo, J.Y. Yang, C.G. Levi, Microstructure stability issues in emerging TBC materials, in: W.A. Kaysser (Ed.), Proceedings of the International Symposium on Thermal Barrier Coatings and Titanium Aluminides, DLR, Bonn, Germany, 2002. [12] R.M.R. Leckie, C.G. Levi, to be published. [13] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, New York, 1994. [14] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Oxford University Press, 1989. [15] R.D. Shannon, Acta Crystallogr., A 32 (1976) 751. [16] V. Sergo, D.R. Clarke, unpublished research (1994). [17] J.P. Feist, A.L. Heyes, K.L. Choy, J. Mei, J.R. Nicholls, Proc. Inst. Mech. Eng. 215 (2001) 333. [18] J. Eldridge, NASA Glenn, private communication (2003). [19] Compiled and shown as Fig. 2.10 in Ref. [10]. [20] P.D. Johnson, F.E. Williams, J. Chem. Phys. 18 (1950) 1477.