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Non-destructive thermal property measurements of an APS TBC on an intact turbine blade
Surface & Coatings Technology 205 (2010) 446–451
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Non-destructive thermal property measurements of an APS TBC on an intact turbine blade Tyler Kakuda a, Andi Limarga b, Anirudha Vaidya c, Anand Kulkarni c, Ted D. Bennett a,⁎ a b c
Department of Mechanical Engineering, University of California, Santa Barbara CA, USA Materials Science, University of California, Santa Barbara CA, USA Siemens Energy, Inc., USA
a r t i c l e
i n f o
Article history: Received 11 May 2010 Accepted in revised form 2 July 2010 Available online 8 July 2010 Keywords: Thermal barrier coatings Nondestructive measurements Thermal conductivity Thermal diffusivity
a b s t r a c t The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y2O3-stabilized ZrO2 (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm2/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity. Published by Elsevier B.V.
1. Introduction Power generation companies are continuously improving the efficiency of gas turbines to meet economic and environmental goals. The trend towards higher efficiency has been achieved in part by raising the operating temperature of engines. Surface temperatures exceeding 1300 °C are commonly encountered in modern turbine engines [1]. At these temperatures, engine components are subject to many forms of degradation including oxidation, creep deformation and thermal cycle fatigue [1,2]. To minimize these harmful effects, ceramic thermal barrier coatings (TBCs) are routinely used to insulate metal components from excessive heat loads. Typical TBCs are currently composed of 7–8 wt.% Y2O3-stabilized ZrO2, which, in addition to having a characteristically low thermal conductivity, has an excellent fracture toughness and a large coefficient of thermal expansion when compared to other oxides thereby minimizing thermal stress. Since TBCs are engineered to provide sustained thermal protection for engine components, a reliable, non-intrusive, and quantitative thermal property measurement is in high demand by the industry to assess coating performance. The development of the phase of photothermal emission analysis (PopTea), utilized in this paper, addresses this need. Efforts to evaluate the “health” of TBCs by non-destructive testing include techniques such as pulsed thermography (PT) and lock-in
⁎ Corresponding author. E-mail address: [email protected] (T.D. Bennett). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2010.07.002
thermography. These techniques have been used successfully to monitor coatings for delaminations, and as an indicator for the quality of a coating deposition [3,4]. One of the largest limitations of PT and lock-in thermography is that they both rely on significant contrasts in the coating structure (severe degradation) to highlight an “unhealthy” region. Coupled with the fact that critical degradations (i.e. delaminations) can occur over short periods of time (when compared to the frequency of servicing engine parts), these techniques are only equipped to diagnose failure after degradation occurs. If TBCs are to be monitored with the goal of preventing coating failure, before the component is compromised, a more quantitative approach is necessary. A better description of coating health can be made through quantitative thermal property measurements. Changing intrinsic thermal properties can serve as a reliable indicator of the aging process, as revealed through measurements of thermal conductivity, thermal diffusivity and heat capacity of a coating. A thermal property measurement, as an assessment tool, has been difficult to implement in an industrial setting. The most commonly employed thermal property measurement is laser flash [5]. Laser flash is a thermal diffusivity measurement that records the time response of heat transfer through a material resulting from a short laser pulse. However, since the heat-sensing element must be located on the backside of the sample, this method is not suitable for coating measurements on actual engine components. Furthermore, even for specially prepared specimens, laser flash imposes significant limitations on the measurement of layered systems [6] and is sensitive to the translucency of typical coatings to infrared heat transport [7]. Other
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techniques have been proposed to address the challenges of making non-destructive measurements. For example, the photo-acoustic method heats the surface of a coating with a laser and probes the phase of the front surface temperature rise with the acoustic wave transmitted through air. From the phase of the acoustic signal, it is possible to measure the thermal diffusivity of the coating. Although this technique has been demonstrated on TBCs [8], the placement of samples in an acoustic cell limits its feasibility to larger scale parts. To confront the difficulty of reliable thermal property measurements on serviceable engine components, this paper presents phase of photothermal emission analysis (PopTea). PopTea is a temperature phase measurement, which utilizes harmonic laser heating and interrogates the temperature field through the phase of thermal emission from the coating. This measurement is non-destructive, easy to implement, and can diagnose TBC failure [9] with the potential of monitoring changes in the thermal properties of coatings over their lifetime. To demonstrate the viability of PopTea as a diagnostic tool for serviceable parts, measurements were performed on an intact turbine blade coated with an APS 7YSZ TBC.
10 μm. The CO2 laser was chosen due to its intrinsic ability to couple energy to zirconia based ceramics. An external opto-acoustic modulator is used to generate a harmonic laser intensity to heat the coating at a controlled frequency (Signal 1). After the laser beam passes through the modulator it goes through an optical expander and focusing element, which serves to spread the beam for adequate aperturing (3 to 6 mm) as well as to focus the beam through a small hole in an elliptical mirror. The optical path for the laser ends at the TBC, where heating of the coating occurs over some optical penetration depth. Emission from the heated coating is collected and focused onto an infrared detector with the elliptical mirror. The detector operates in the mid infrared regime (peak sensitivity at 5 μm), outside the range of the laser. In conjunction with a trans-impedance amplifier, the emission signal is recorded with a data acquisition card (Signal 2). The phase difference between laser (Signal 1) and thermal emission (Signal 2) is then computed for different heating frequencies.
2. PopTea measurement and model
To predict the phase of thermal emission, a heat transfer and radiation model for the TBC system is required. The temperature in the coating is derived from heat conduction through the coating and substrate materials, with heating from a Gaussian laser beam (G) confined to the coating. In a previous report, it was discovered that for coating thicknesses greater than 200 μm (typical of APS coatings), the length scale for transient heat diffusion in the substrate becomes comparable to the laser beam diameter (which is typically less than 6 mm) [11]. For this reason, a two-dimensional model for heat transfer in the substrate is required. The heat transfer problem for the temperature field in the coating and substrate is illustrated in Fig. 2. With a description of the temperature field, the emission from the coating and substrate surface can be determined from a linear radiation model, outlined in [12]. For all the complexity involved with modeling the thermal and radiative transport, it is noteworthy that only two thermal parameters fall out of the derivation that needs to be fitted with experimental data. Both thermal parameters contrast the thermal properties of the substrate to that of the coating. One of the parameters ffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (a = αsub = αcoat ) q is ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the square root of the thermal diffusivity ratio, and the other (γ = kρCp sub = kρCp coat ) is the ratio of effusivities.
The principles underlying the PopTea measurement are common to other methods that employ transient heating to measure thermal diffusivity (including laser-flash technique); the time lag in a temperature rise, that follows heating, is related to a length scale for heat transfer and the thermal diffusivity of the material. When periodic heating is used, the time lag associated with heat transfer is reported as a phase lag in the temperature field. Other distinctions that can be made between different diffusivity measurements are associated with the heating method, and the location and method of temperature field detection. PopTea utilizes heating and detection from the front surface of the coating. Heating is performed harmonically with a laser. Detection is achieved by measuring the phase of thermal emission from the TBC system. The PopTea measurement has a high specificity to the coating material that is controlled by the thermal penetration depth (ℓ) associated with the frequency of laser heating. A required length scale to make a thermal diffusivity measurement is the coating thickness, L. The existence of this length scale is revealed to the heat transfer process through a contrast in thermal properties between the coating and the underlying metal. Substrate properties are generally well known and static over the lifetime of a coating. To determine the coating thickness, a number of different non-destructive techniques can be used, including eddy current, photothermal and ultrasonic techniques [10].
2.2. Heat transfer model
The ability to measure the effusivity of the coating, simultaneous to thermal diffusivity, is an important attribute of the PopTea measurement. By determining αcoat and (kρCp)coat, both kcoat and (ρCp)coat can be determined. Furthermore, since the specific heat (Cp)coat of most common coating compositions can be found in the literature, the
2.1. Experimental setup The experimental measurement is shown schematically in Fig. 1. Heating is created using a 100 W CO2 laser having a wavelength of
Fig. 1. Experimental procedure for phase of photothermal emission analysis (PopTea).
Fig. 2. Heat transfer model for two-dimensional heat spreading in the substrate for a TBC.
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density of the coating can be inferred from the volumetric specific heat (Cp)coat measurement. For a typical set of measurements, the phase of emission is computed over a range of thermal penetration depths (i.e. laser frequency) and for different beam diameters demonstrated in Fig. 3. The dimensionless thermal penetration depth describes the diffusionp length scale ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi in the coating relative to the coating thickness: (ℓ = αcoat = 2πf = L), in which f is the laser frequency. A benefit of the two-dimensional heat transfer model is that different beam sizes can be used for heating. This provides another independent measurement for data reduction and yields less uncertainty in the measured thermal properties. Fitting the thermal parameters to the experimental data with the model described in [11,12] allows for the determination of thermal diffusivity, conductivity and density of the coating.
3. Validation of measurements Validation of measurements is an important step for any new procedure. Unfortunately, measuring a TBC with ‘known’ thermal properties is unrealistic due to the complexity of the coating process and resulting coating microstructure. Additionally, existing strategies for the thermal property measurements of coatings that might be compared have different and often conflicting requirements. For example, laser flash requires a free-standing coating or a very minimal backing material for the coating. However, the PopTea method both relies on the existence of a substrate material and requires measurements to be performed without the thermal penetration depth exceeding the thickness of that substrate (i.e. the substrate should not be too thin). Despite the difficulties of making comparative measurements, thermal diffusivity measurements were recently performed on a series of air plasma spray (APS) coatings for comparison with the laser-flash technique performed at Oak Ridge National Lab (ORNL). Coatings of various compositions were fabricated by Praxair and deposited on aluminum substrates in sets of “duplicate” pairs. An aluminum substrate material was chosen so that the substrate could be etched away from one of the duplicate samples to provide a free-standing coating for the laser-flash technique. A comparison between the diffusivity results of PopTea and laser flash was found to be in reasonable agreement, with the results falling within a 20% margin of difference. Fig. 4 summarizes the results. The free-standing coatings, prepared for laser flash, were cut into smaller
Fig. 3. Example of data set collected and analyzed by PopTea for two different beam diameters.
Fig. 4. Thermal diffusivity comparison between laser flash and phase of photothermal emission analysis.
samples for a number of discrete measurements. Error bars on the ORNL represent the spread in the discrete measurements. Only one measurement by laser flash was available for sample ‘B’, which shows the largest discrepancy between the measurement techniques. It was noted that a few of the free-standing coatings demonstrated a large amount of spread. For example, samples A, C and D all exhibited more than a 15% spread in measurements on the same coating material. On sample D, a number of measurements were made with the PopTea method at various locations on the coupon. Observation of a similar spread in measurement results suggests that coating properties may not be spatially uniform. Although a stronger comparison between the two measurements was hoped for, it is perhaps realistic to expect differences about 10% when measurements cannot be performed on the same coating under the same conditions. In the current comparison, some concerns exist as to whether the acid bath treatment for the laser-flash samples could have left residues in the coating, altering the thermal properties. Additionally, the aluminum substrates where thinner than “optimal” for the PopTea measurements (to help facilitate removal by etch for laser flash), and exhibited some warping that may have adversely affected our ability to mount them with good thermal contact to a larger aluminum block backing. Another drawback in the samples prepared for the comparative study between PopTea and laser flash was in the use of aluminum as a substrate material for the coatings. Aluminum was selected over traditional superalloys for ease of removal by etching. However, the high value of aluminum thermal conductivity led to the inability of the PopTea method to uniquely determine the γ thermal parameter, which describes the thermal contrasts between the coating material and the substrate. When the thermal contrast is too large (γ N 10), the substrate material behind the coating becomes a highly effect heat sink and the transient temperature field in the coating becomes independent of the specific value of γ. To prevent a “saturation” in the thermal contrast between the coating and substrate material on the measurement, it is desirable for γ N 10. Typical coating-superalloy systems have a thermal contrast of γ ~ 4. In this case, the thermal contrast is not so large that the temperature field in the coating is independent of γ. However, without measuring γ, the sole thermal property (a) that may be determined by PopTea yields only the thermal diffusivity of the coating. For the superalloy compositions used in today's turbine engines, γ N 10 and the PopTea method can be used to measure the two independent thermal properties of the coating. This yields values for thermal diffusivity, thermal conductivity and the volumetric heat
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Table 1 Thermal properties for aluminum and various superalloys. Substrate
L (mm)
[kz]sub (W/m/K)
[ρCp]sub (J/m3/K)
Aluminum CM247 IN738 (Turbine substrate) IN939 Rene80
1 3 3 3 3
250 8.50 9.66 10.20 10.15
2.42e + 6 3.47e + 6 3.38e + 6 3.52e + 6 3.64e + 6
capacity. Siemens provided four different samples with coatings applied to different substrate materials for measurement with the PopTea meathod. The thermal properties of the different substrates are listed in Table 1. The coatings were comparable in regard to composition and thickness (~ 300 μm). Fig. 5 shows the measurement results for the coatings on the four different substrate materials. The measured diffusivity (α), conductivity (k) and density (ρ) for each sample are contrasted with the average of all four samples, and show that the thermal properties of the four samples fall well within 10% of each other. Better than a 10% agreement is difficult to achieve, even for samples applied to the same substrate during the same deposition run [13]. 4. Turbine blade details and condition Turbine engine components may be coated with TBCs in a variety of ways that result in different physical microstructures with unique thermal properties [14]. The two most common deposition methods used for coatings are air plasma spray (APS) and electron beam vapor deposition (EB-PVD). Although EB-PVD coatings are considered more robust in service, APS coatings typically have a lower thermal conductivity, are easier to implement on larger components, and are more cost effective [15]. For this reason, the power industry commonly uses APS coatings. The turbine blade used in the current investigation was provided by Siemens Energy Inc., and measures 6.5 cm × 19 cm × 28 cm. The coating is a 7YSZ APS that has seen no service and the base metal is a single crystal Ni-based superalloy (IN939). The thermal properties of the superalloy are listed in Table 1. PopTea measurements were made at 17 locations on the blade and highlighted in Fig. 6. The thickness of the coating was determined before each measurement using the eddy current technique. For purposes of nomenclature there are five surfaces that will be referred
Fig. 5. Sensitivity of the PopTea measurement to equivalent coatings with different substrates.
Fig. 6. Turbine blade under investigation with areas of thermal property measurement highlighted and nomenclature defined.
to on the blade: suction (convex) side, pressure (concave) side, front edge, trailing edge, and base. 5. Thermal property results The different locations where the measurements were made on the turbine blade are identified in Fig. 6. The PopTea method was used to determine thermal diffusivity, thermal conductivity, and volumetric heat capacity. The known specific heat of 7YSZ (~450 J/kg/K at 24 °C) enables the density of the coating to be reported from the measurement. Measurement results for the 17 different locations on the blade are shown in Fig. 7. Based on independently fitted results for the measurements made with two different beam sizes (~3 mm and 6 mm), as well as a coupled fitting of the two beam sizes, an uncertainty was established for each thermal property measurement. Fig. 6 illustrates that the measurement locations 1 through 7 lie on the suction side/trailing edge of the blade; spots 8, 9 and 12 lie on the base; spots 10 and 11 are on the front edge; and spots 13–19 are found on the
Fig. 7. Thermal property results for 17 different locations over blade.
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pressure side/trailing edge of the blade. Spots 8 and 12 were identified for measurements, but are not reported because of difficulties accessing tight corners at the base of the blade (this limitation could be overcome with a redesign of measurement tool). A comparison between this data and values reported in literature [16] shows strong agreement. It is worth noting that the coatings measured by laser flash in [16] have similar composition (7YSZ) to the blade coating measured in this study; however, the deposition parameters (i.e. rate) are varied and the coating thicknesses are not reported.
6. Discussion The deposition of coatings on complex geometries can cause variability in coating characteristics, such as thickness and morphology, over the surface of the part. Of current interest is whether changes in morphology can be revealed by thermal property measurements. Since the local morphology is dependent on the local deposition conditions, one way to infer changes in morphology is by looking for spatial variation of thermal properties. This type of study is in high demand since a quantitative survey of the coating thermal properties over the surface of a turbine blade (either intact or sectioned) does not currently exist.
6.1. Spatial variation of thermal properties Within the margin of 10% error, the measurement results shown in Fig. 7 suggest that there is little spatial variation of thermal properties. However, closer inspection of separate regions on the blade does reveal some spatial trends. The most obvious trends are related to density and occur on the trailing edge of the concave and convex sides of the blade. The coating thickness over this blade, also has a small amount of variation, between 250–350 μm. On the suction side and leading edge of this blade the thickness increases from tip to base, whereas on the pressure side the thickness decreases from tip to base. Since the variation in thermal properties implies changing morphology in the coating, it is important to know whether the changes in thermal properties are related to variation in coating thickness. To address this question, the thermal properties of the coating are plotted against thickness in Fig. 8. The results reveal that coating thickness has little influence over thermal properties. This result is supported by earlier work on 7YPSZ APS coatings, where thermal properties had little or no correlation with the coating thickness [11].
Fig. 8. Coating thickness plotted as a function of different thermal properties.
Past micrograph studies of EB-PVD coatings on turbine blades have shown differences in the microstructure based on location [17]. In that study, dramatic differences in structure were largely confined to the turbine base, where geometric constraints cause an abnormal vapor incidence angle during coating deposition. However, the deposition of coatings with plasma spray is generally regarded to be less sensitive to the angle of deposition in terms of structure. However, it is known that the thermal properties of APS coatings can be influenced by deposition variables, such as spray distance, gas flow rate, and powder feed rate [18]. Since it is reasonable to anticipate some variability in the deposition conditions across the area of a turbine blade, a correlation with variation in measured thermal properties should be expected. 6.2. Effect of morphology on conductivity It is difficult to establish fully a detailed relationship between morphology and the thermal properties of a coating, without sectioning the blade and looking at micrographs. Even with such a study, only the course features could be investigated in the extent needed for the large area of the blade. Additionally, there is no certainty that morphological features, that a micrograph would draw attention to, would be the features of greatest importance. In the literature, one of the most commonly reported characteristics of coating morphology is the porosity. PopTea can measure the density of the blade coating, since both the specific heat and the theoretical bulk density of the coating material are well known. If the observed changes in thermal properties over the surface of the blade are related to changes in morphology, then there should exist a correlation between properties like thermal conductivity and porosity. Such a trend was determined to exist. Fig. 9 shows the thermal conductivity measurements plotted against porosity, as determined from the density measurements. The dashed line represents the best linear fit of data for all locations on the blade. Only the average values for each measurement location were plotted. The data indicates a trend in which the thermal conductivity of the coating decreases with increasing porosity. This makes physical sense, since increasing porosity means increasing the content of air gaps and interfaces in the microstructure. Plotted with the results of this work in Fig. 9 are the results of an independent study into the relationship between APS coating porosity and thermal conductivity [18]. In that work, the thermal properties of the coatings were evaluated with laser flash and the porosity was determined from the weight to volume ratio, and justified with the
Fig. 9. Morphology, represented through porosity, suggests a relationship with thermal conductivity.
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support of micrographs. Both studies have nearly the same median value of coating porosity, and show the same trend with respect to decreasing thermal conductivity with increasing porosity. The current results suggest that a larger spread in porosity is supported by the APS coating on the blade than the coating investigated in reference [18]. The earlier studies suggest that thermal conductivity has a much stronger dependency on porosity than the current study. For example, a linear extrapolation of their results suggests that the coating thermal conductivity goes to zero at a porosity of 47%. However, it is known that such factors as pore orientation, in addition to pore fraction, influence the thermal conductivity of a coating [19]. Therefore, it is reasonable to argue that the relationship between thermal conductivity and porosity alone could be nonlinear. 7. Conclusions The thermal properties of an APS coating on an intact turbine blade were measured using phase of photothermal emission analysis. This measurement represents the first determination of coating thermal properties on a serviceable turbine part. Measurements of intact turbine blades have not been feasible in the past due to inadequacies and limitations placed on more traditional methods, such as laser flash. The measurements reported in this paper demonstrate the potential for PopTea to be used in an industrial setting. The results from this blade show that thermal diffusivity, thermal conductivity, and heat capacity numbers correlate well with literature and comparative measurements. The most critical thermal property, conductivity, had a range of values from 0.9–1.4 W/m/K over the surface of the blade. Furthermore, some of the variation in thermal conductivity was linked to differences in porosity of the coating microstructure.
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Acknowledgements This work was supported by the University Turbine Research Program (UTSR), SCIES Project 07-01-SR125.
References [1] M.Y. Ali, S.Q. Nusier, G.M. Newaz, J. Mater. Sci. 39 (2004) 3383–3390. [2] B. Baufeld, U. Schulz, Surf. Coat. Technol. 201 (2006) 2667–2675. [3] S. Marinetti, D. Robba, F. Cernushi, P.G. Bison, E. Grinzato, Infrared Phys. Technol. 49 (2007) 281–285. [4] D. Wu, G. Zensinger, W. Karpen, G. Busse, Mater. Sci. Forum 210 (1996) 289–294. [5] Y. Takahashi, Int. J. Thermophys. 5 (1983) 41–52. [6] J. Absi, D.S. Smith, B. Nat-Ali, S. Grandjean, J. Berjonnaux, J. Eur. Ceram. Soc. 25 (2005) 367–373. [7] H. Wang, R.B. Dinwiddie, J. Therm. Spray Technol. 9 (2000) 210–214. [8] X. Wang, H. Hu, X. Xu, J. Heat Transfer 123 (2001) 138–144. [9] F. Yu, T.D. Bennett, J. Appl. Phys. 98 (2005). [10] L. Fabbri, M. Oksanen, J. Therm. Spray Technol. 8 (1999) 263–272. [11] T.D. Bennett, T. Kakuda, A. Kulkarni, J. Appl. Phys. 105 (2009). [12] T.D. Bennett, F. Yu, J. Appl. Phys. 98 (2005) 1–7. [13] W. Chi, S. Sampath, H. Wang, J. Am. Ceram. Soc. 91 (2008) 2636–2645. [14] D.D. Hass, A.J. Slifka, H.N.G. Wadley, Acta Mater. 49 (2001) 973–983. [15] A. Feuerstein, A. Bolcavage, Surf. Eng. Proc. 3rd Int. Surf. Eng. Conf. (2004) 291–298. [16] N. Markocsan, P. Nylen, J. Wigren, X.H. Li, J. Therm. Spray Technol. 16 (2007) 498–505. [17] Y.H. Sohn, E.Y. Lee, B.A. Nagaraj, R.R. Biederman, R.D. Sisson Jr., Surf. Coat. Technol. 146 (2001) 132–139. [18] G. Bertrand, P. Bertrand, P. Roy, C. Rio, R. Mevrel, Surf. Coat. Technol. 202 (2008) 1994–2001. [19] T.J. Lu, C.G. Levi, H.N.G. Wadley, A.G. Evans, J. Am. Ceram. Soc. 84 (2001) 2937–2946.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Non-destructive thermal property measurements of an APS TBC on an intact turbine blade Tyler Kakuda a, Andi Limarga b, Anirudha Vaidya c, Anand Kulkarni c, Ted D. Bennett a,⁎ a b c
Department of Mechanical Engineering, University of California, Santa Barbara CA, USA Materials Science, University of California, Santa Barbara CA, USA Siemens Energy, Inc., USA
a r t i c l e
i n f o
Article history: Received 11 May 2010 Accepted in revised form 2 July 2010 Available online 8 July 2010 Keywords: Thermal barrier coatings Nondestructive measurements Thermal conductivity Thermal diffusivity
a b s t r a c t The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y2O3-stabilized ZrO2 (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm2/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity. Published by Elsevier B.V.
1. Introduction Power generation companies are continuously improving the efficiency of gas turbines to meet economic and environmental goals. The trend towards higher efficiency has been achieved in part by raising the operating temperature of engines. Surface temperatures exceeding 1300 °C are commonly encountered in modern turbine engines [1]. At these temperatures, engine components are subject to many forms of degradation including oxidation, creep deformation and thermal cycle fatigue [1,2]. To minimize these harmful effects, ceramic thermal barrier coatings (TBCs) are routinely used to insulate metal components from excessive heat loads. Typical TBCs are currently composed of 7–8 wt.% Y2O3-stabilized ZrO2, which, in addition to having a characteristically low thermal conductivity, has an excellent fracture toughness and a large coefficient of thermal expansion when compared to other oxides thereby minimizing thermal stress. Since TBCs are engineered to provide sustained thermal protection for engine components, a reliable, non-intrusive, and quantitative thermal property measurement is in high demand by the industry to assess coating performance. The development of the phase of photothermal emission analysis (PopTea), utilized in this paper, addresses this need. Efforts to evaluate the “health” of TBCs by non-destructive testing include techniques such as pulsed thermography (PT) and lock-in
⁎ Corresponding author. E-mail address: [email protected] (T.D. Bennett). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2010.07.002
thermography. These techniques have been used successfully to monitor coatings for delaminations, and as an indicator for the quality of a coating deposition [3,4]. One of the largest limitations of PT and lock-in thermography is that they both rely on significant contrasts in the coating structure (severe degradation) to highlight an “unhealthy” region. Coupled with the fact that critical degradations (i.e. delaminations) can occur over short periods of time (when compared to the frequency of servicing engine parts), these techniques are only equipped to diagnose failure after degradation occurs. If TBCs are to be monitored with the goal of preventing coating failure, before the component is compromised, a more quantitative approach is necessary. A better description of coating health can be made through quantitative thermal property measurements. Changing intrinsic thermal properties can serve as a reliable indicator of the aging process, as revealed through measurements of thermal conductivity, thermal diffusivity and heat capacity of a coating. A thermal property measurement, as an assessment tool, has been difficult to implement in an industrial setting. The most commonly employed thermal property measurement is laser flash [5]. Laser flash is a thermal diffusivity measurement that records the time response of heat transfer through a material resulting from a short laser pulse. However, since the heat-sensing element must be located on the backside of the sample, this method is not suitable for coating measurements on actual engine components. Furthermore, even for specially prepared specimens, laser flash imposes significant limitations on the measurement of layered systems [6] and is sensitive to the translucency of typical coatings to infrared heat transport [7]. Other
T. Kakuda et al. / Surface & Coatings Technology 205 (2010) 446–451
447
techniques have been proposed to address the challenges of making non-destructive measurements. For example, the photo-acoustic method heats the surface of a coating with a laser and probes the phase of the front surface temperature rise with the acoustic wave transmitted through air. From the phase of the acoustic signal, it is possible to measure the thermal diffusivity of the coating. Although this technique has been demonstrated on TBCs [8], the placement of samples in an acoustic cell limits its feasibility to larger scale parts. To confront the difficulty of reliable thermal property measurements on serviceable engine components, this paper presents phase of photothermal emission analysis (PopTea). PopTea is a temperature phase measurement, which utilizes harmonic laser heating and interrogates the temperature field through the phase of thermal emission from the coating. This measurement is non-destructive, easy to implement, and can diagnose TBC failure [9] with the potential of monitoring changes in the thermal properties of coatings over their lifetime. To demonstrate the viability of PopTea as a diagnostic tool for serviceable parts, measurements were performed on an intact turbine blade coated with an APS 7YSZ TBC.
10 μm. The CO2 laser was chosen due to its intrinsic ability to couple energy to zirconia based ceramics. An external opto-acoustic modulator is used to generate a harmonic laser intensity to heat the coating at a controlled frequency (Signal 1). After the laser beam passes through the modulator it goes through an optical expander and focusing element, which serves to spread the beam for adequate aperturing (3 to 6 mm) as well as to focus the beam through a small hole in an elliptical mirror. The optical path for the laser ends at the TBC, where heating of the coating occurs over some optical penetration depth. Emission from the heated coating is collected and focused onto an infrared detector with the elliptical mirror. The detector operates in the mid infrared regime (peak sensitivity at 5 μm), outside the range of the laser. In conjunction with a trans-impedance amplifier, the emission signal is recorded with a data acquisition card (Signal 2). The phase difference between laser (Signal 1) and thermal emission (Signal 2) is then computed for different heating frequencies.
2. PopTea measurement and model
To predict the phase of thermal emission, a heat transfer and radiation model for the TBC system is required. The temperature in the coating is derived from heat conduction through the coating and substrate materials, with heating from a Gaussian laser beam (G) confined to the coating. In a previous report, it was discovered that for coating thicknesses greater than 200 μm (typical of APS coatings), the length scale for transient heat diffusion in the substrate becomes comparable to the laser beam diameter (which is typically less than 6 mm) [11]. For this reason, a two-dimensional model for heat transfer in the substrate is required. The heat transfer problem for the temperature field in the coating and substrate is illustrated in Fig. 2. With a description of the temperature field, the emission from the coating and substrate surface can be determined from a linear radiation model, outlined in [12]. For all the complexity involved with modeling the thermal and radiative transport, it is noteworthy that only two thermal parameters fall out of the derivation that needs to be fitted with experimental data. Both thermal parameters contrast the thermal properties of the substrate to that of the coating. One of the parameters ffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (a = αsub = αcoat ) q is ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the square root of the thermal diffusivity ratio, and the other (γ = kρCp sub = kρCp coat ) is the ratio of effusivities.
The principles underlying the PopTea measurement are common to other methods that employ transient heating to measure thermal diffusivity (including laser-flash technique); the time lag in a temperature rise, that follows heating, is related to a length scale for heat transfer and the thermal diffusivity of the material. When periodic heating is used, the time lag associated with heat transfer is reported as a phase lag in the temperature field. Other distinctions that can be made between different diffusivity measurements are associated with the heating method, and the location and method of temperature field detection. PopTea utilizes heating and detection from the front surface of the coating. Heating is performed harmonically with a laser. Detection is achieved by measuring the phase of thermal emission from the TBC system. The PopTea measurement has a high specificity to the coating material that is controlled by the thermal penetration depth (ℓ) associated with the frequency of laser heating. A required length scale to make a thermal diffusivity measurement is the coating thickness, L. The existence of this length scale is revealed to the heat transfer process through a contrast in thermal properties between the coating and the underlying metal. Substrate properties are generally well known and static over the lifetime of a coating. To determine the coating thickness, a number of different non-destructive techniques can be used, including eddy current, photothermal and ultrasonic techniques [10].
2.2. Heat transfer model
The ability to measure the effusivity of the coating, simultaneous to thermal diffusivity, is an important attribute of the PopTea measurement. By determining αcoat and (kρCp)coat, both kcoat and (ρCp)coat can be determined. Furthermore, since the specific heat (Cp)coat of most common coating compositions can be found in the literature, the
2.1. Experimental setup The experimental measurement is shown schematically in Fig. 1. Heating is created using a 100 W CO2 laser having a wavelength of
Fig. 1. Experimental procedure for phase of photothermal emission analysis (PopTea).
Fig. 2. Heat transfer model for two-dimensional heat spreading in the substrate for a TBC.
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density of the coating can be inferred from the volumetric specific heat (Cp)coat measurement. For a typical set of measurements, the phase of emission is computed over a range of thermal penetration depths (i.e. laser frequency) and for different beam diameters demonstrated in Fig. 3. The dimensionless thermal penetration depth describes the diffusionp length scale ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi in the coating relative to the coating thickness: (ℓ = αcoat = 2πf = L), in which f is the laser frequency. A benefit of the two-dimensional heat transfer model is that different beam sizes can be used for heating. This provides another independent measurement for data reduction and yields less uncertainty in the measured thermal properties. Fitting the thermal parameters to the experimental data with the model described in [11,12] allows for the determination of thermal diffusivity, conductivity and density of the coating.
3. Validation of measurements Validation of measurements is an important step for any new procedure. Unfortunately, measuring a TBC with ‘known’ thermal properties is unrealistic due to the complexity of the coating process and resulting coating microstructure. Additionally, existing strategies for the thermal property measurements of coatings that might be compared have different and often conflicting requirements. For example, laser flash requires a free-standing coating or a very minimal backing material for the coating. However, the PopTea method both relies on the existence of a substrate material and requires measurements to be performed without the thermal penetration depth exceeding the thickness of that substrate (i.e. the substrate should not be too thin). Despite the difficulties of making comparative measurements, thermal diffusivity measurements were recently performed on a series of air plasma spray (APS) coatings for comparison with the laser-flash technique performed at Oak Ridge National Lab (ORNL). Coatings of various compositions were fabricated by Praxair and deposited on aluminum substrates in sets of “duplicate” pairs. An aluminum substrate material was chosen so that the substrate could be etched away from one of the duplicate samples to provide a free-standing coating for the laser-flash technique. A comparison between the diffusivity results of PopTea and laser flash was found to be in reasonable agreement, with the results falling within a 20% margin of difference. Fig. 4 summarizes the results. The free-standing coatings, prepared for laser flash, were cut into smaller
Fig. 3. Example of data set collected and analyzed by PopTea for two different beam diameters.
Fig. 4. Thermal diffusivity comparison between laser flash and phase of photothermal emission analysis.
samples for a number of discrete measurements. Error bars on the ORNL represent the spread in the discrete measurements. Only one measurement by laser flash was available for sample ‘B’, which shows the largest discrepancy between the measurement techniques. It was noted that a few of the free-standing coatings demonstrated a large amount of spread. For example, samples A, C and D all exhibited more than a 15% spread in measurements on the same coating material. On sample D, a number of measurements were made with the PopTea method at various locations on the coupon. Observation of a similar spread in measurement results suggests that coating properties may not be spatially uniform. Although a stronger comparison between the two measurements was hoped for, it is perhaps realistic to expect differences about 10% when measurements cannot be performed on the same coating under the same conditions. In the current comparison, some concerns exist as to whether the acid bath treatment for the laser-flash samples could have left residues in the coating, altering the thermal properties. Additionally, the aluminum substrates where thinner than “optimal” for the PopTea measurements (to help facilitate removal by etch for laser flash), and exhibited some warping that may have adversely affected our ability to mount them with good thermal contact to a larger aluminum block backing. Another drawback in the samples prepared for the comparative study between PopTea and laser flash was in the use of aluminum as a substrate material for the coatings. Aluminum was selected over traditional superalloys for ease of removal by etching. However, the high value of aluminum thermal conductivity led to the inability of the PopTea method to uniquely determine the γ thermal parameter, which describes the thermal contrasts between the coating material and the substrate. When the thermal contrast is too large (γ N 10), the substrate material behind the coating becomes a highly effect heat sink and the transient temperature field in the coating becomes independent of the specific value of γ. To prevent a “saturation” in the thermal contrast between the coating and substrate material on the measurement, it is desirable for γ N 10. Typical coating-superalloy systems have a thermal contrast of γ ~ 4. In this case, the thermal contrast is not so large that the temperature field in the coating is independent of γ. However, without measuring γ, the sole thermal property (a) that may be determined by PopTea yields only the thermal diffusivity of the coating. For the superalloy compositions used in today's turbine engines, γ N 10 and the PopTea method can be used to measure the two independent thermal properties of the coating. This yields values for thermal diffusivity, thermal conductivity and the volumetric heat
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Table 1 Thermal properties for aluminum and various superalloys. Substrate
L (mm)
[kz]sub (W/m/K)
[ρCp]sub (J/m3/K)
Aluminum CM247 IN738 (Turbine substrate) IN939 Rene80
1 3 3 3 3
250 8.50 9.66 10.20 10.15
2.42e + 6 3.47e + 6 3.38e + 6 3.52e + 6 3.64e + 6
capacity. Siemens provided four different samples with coatings applied to different substrate materials for measurement with the PopTea meathod. The thermal properties of the different substrates are listed in Table 1. The coatings were comparable in regard to composition and thickness (~ 300 μm). Fig. 5 shows the measurement results for the coatings on the four different substrate materials. The measured diffusivity (α), conductivity (k) and density (ρ) for each sample are contrasted with the average of all four samples, and show that the thermal properties of the four samples fall well within 10% of each other. Better than a 10% agreement is difficult to achieve, even for samples applied to the same substrate during the same deposition run [13]. 4. Turbine blade details and condition Turbine engine components may be coated with TBCs in a variety of ways that result in different physical microstructures with unique thermal properties [14]. The two most common deposition methods used for coatings are air plasma spray (APS) and electron beam vapor deposition (EB-PVD). Although EB-PVD coatings are considered more robust in service, APS coatings typically have a lower thermal conductivity, are easier to implement on larger components, and are more cost effective [15]. For this reason, the power industry commonly uses APS coatings. The turbine blade used in the current investigation was provided by Siemens Energy Inc., and measures 6.5 cm × 19 cm × 28 cm. The coating is a 7YSZ APS that has seen no service and the base metal is a single crystal Ni-based superalloy (IN939). The thermal properties of the superalloy are listed in Table 1. PopTea measurements were made at 17 locations on the blade and highlighted in Fig. 6. The thickness of the coating was determined before each measurement using the eddy current technique. For purposes of nomenclature there are five surfaces that will be referred
Fig. 5. Sensitivity of the PopTea measurement to equivalent coatings with different substrates.
Fig. 6. Turbine blade under investigation with areas of thermal property measurement highlighted and nomenclature defined.
to on the blade: suction (convex) side, pressure (concave) side, front edge, trailing edge, and base. 5. Thermal property results The different locations where the measurements were made on the turbine blade are identified in Fig. 6. The PopTea method was used to determine thermal diffusivity, thermal conductivity, and volumetric heat capacity. The known specific heat of 7YSZ (~450 J/kg/K at 24 °C) enables the density of the coating to be reported from the measurement. Measurement results for the 17 different locations on the blade are shown in Fig. 7. Based on independently fitted results for the measurements made with two different beam sizes (~3 mm and 6 mm), as well as a coupled fitting of the two beam sizes, an uncertainty was established for each thermal property measurement. Fig. 6 illustrates that the measurement locations 1 through 7 lie on the suction side/trailing edge of the blade; spots 8, 9 and 12 lie on the base; spots 10 and 11 are on the front edge; and spots 13–19 are found on the
Fig. 7. Thermal property results for 17 different locations over blade.
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pressure side/trailing edge of the blade. Spots 8 and 12 were identified for measurements, but are not reported because of difficulties accessing tight corners at the base of the blade (this limitation could be overcome with a redesign of measurement tool). A comparison between this data and values reported in literature [16] shows strong agreement. It is worth noting that the coatings measured by laser flash in [16] have similar composition (7YSZ) to the blade coating measured in this study; however, the deposition parameters (i.e. rate) are varied and the coating thicknesses are not reported.
6. Discussion The deposition of coatings on complex geometries can cause variability in coating characteristics, such as thickness and morphology, over the surface of the part. Of current interest is whether changes in morphology can be revealed by thermal property measurements. Since the local morphology is dependent on the local deposition conditions, one way to infer changes in morphology is by looking for spatial variation of thermal properties. This type of study is in high demand since a quantitative survey of the coating thermal properties over the surface of a turbine blade (either intact or sectioned) does not currently exist.
6.1. Spatial variation of thermal properties Within the margin of 10% error, the measurement results shown in Fig. 7 suggest that there is little spatial variation of thermal properties. However, closer inspection of separate regions on the blade does reveal some spatial trends. The most obvious trends are related to density and occur on the trailing edge of the concave and convex sides of the blade. The coating thickness over this blade, also has a small amount of variation, between 250–350 μm. On the suction side and leading edge of this blade the thickness increases from tip to base, whereas on the pressure side the thickness decreases from tip to base. Since the variation in thermal properties implies changing morphology in the coating, it is important to know whether the changes in thermal properties are related to variation in coating thickness. To address this question, the thermal properties of the coating are plotted against thickness in Fig. 8. The results reveal that coating thickness has little influence over thermal properties. This result is supported by earlier work on 7YPSZ APS coatings, where thermal properties had little or no correlation with the coating thickness [11].
Fig. 8. Coating thickness plotted as a function of different thermal properties.
Past micrograph studies of EB-PVD coatings on turbine blades have shown differences in the microstructure based on location [17]. In that study, dramatic differences in structure were largely confined to the turbine base, where geometric constraints cause an abnormal vapor incidence angle during coating deposition. However, the deposition of coatings with plasma spray is generally regarded to be less sensitive to the angle of deposition in terms of structure. However, it is known that the thermal properties of APS coatings can be influenced by deposition variables, such as spray distance, gas flow rate, and powder feed rate [18]. Since it is reasonable to anticipate some variability in the deposition conditions across the area of a turbine blade, a correlation with variation in measured thermal properties should be expected. 6.2. Effect of morphology on conductivity It is difficult to establish fully a detailed relationship between morphology and the thermal properties of a coating, without sectioning the blade and looking at micrographs. Even with such a study, only the course features could be investigated in the extent needed for the large area of the blade. Additionally, there is no certainty that morphological features, that a micrograph would draw attention to, would be the features of greatest importance. In the literature, one of the most commonly reported characteristics of coating morphology is the porosity. PopTea can measure the density of the blade coating, since both the specific heat and the theoretical bulk density of the coating material are well known. If the observed changes in thermal properties over the surface of the blade are related to changes in morphology, then there should exist a correlation between properties like thermal conductivity and porosity. Such a trend was determined to exist. Fig. 9 shows the thermal conductivity measurements plotted against porosity, as determined from the density measurements. The dashed line represents the best linear fit of data for all locations on the blade. Only the average values for each measurement location were plotted. The data indicates a trend in which the thermal conductivity of the coating decreases with increasing porosity. This makes physical sense, since increasing porosity means increasing the content of air gaps and interfaces in the microstructure. Plotted with the results of this work in Fig. 9 are the results of an independent study into the relationship between APS coating porosity and thermal conductivity [18]. In that work, the thermal properties of the coatings were evaluated with laser flash and the porosity was determined from the weight to volume ratio, and justified with the
Fig. 9. Morphology, represented through porosity, suggests a relationship with thermal conductivity.
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support of micrographs. Both studies have nearly the same median value of coating porosity, and show the same trend with respect to decreasing thermal conductivity with increasing porosity. The current results suggest that a larger spread in porosity is supported by the APS coating on the blade than the coating investigated in reference [18]. The earlier studies suggest that thermal conductivity has a much stronger dependency on porosity than the current study. For example, a linear extrapolation of their results suggests that the coating thermal conductivity goes to zero at a porosity of 47%. However, it is known that such factors as pore orientation, in addition to pore fraction, influence the thermal conductivity of a coating [19]. Therefore, it is reasonable to argue that the relationship between thermal conductivity and porosity alone could be nonlinear. 7. Conclusions The thermal properties of an APS coating on an intact turbine blade were measured using phase of photothermal emission analysis. This measurement represents the first determination of coating thermal properties on a serviceable turbine part. Measurements of intact turbine blades have not been feasible in the past due to inadequacies and limitations placed on more traditional methods, such as laser flash. The measurements reported in this paper demonstrate the potential for PopTea to be used in an industrial setting. The results from this blade show that thermal diffusivity, thermal conductivity, and heat capacity numbers correlate well with literature and comparative measurements. The most critical thermal property, conductivity, had a range of values from 0.9–1.4 W/m/K over the surface of the blade. Furthermore, some of the variation in thermal conductivity was linked to differences in porosity of the coating microstructure.
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Acknowledgements This work was supported by the University Turbine Research Program (UTSR), SCIES Project 07-01-SR125.
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