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Effective spectral emissivity measurements of superalloys and YSZ thermal barrier coating at high temperatures using a 1.6μm single wavelength pyrometer
Optics and Lasers in Engineering 30 (1998) 77—91
Effective spectral emissivity measurements of superalloys and YSZ thermal barrier coating at high temperatures using a 1.6 lm single wavelength pyrometer Sami Alaruri, Lisa Bianchini, Andrew Brewington Allison Engine Company, Rolls Royce Aerospace Group, Box 420, M/S W03A, Indianapolis, IN 46206-0420, USA Received 13 August 1997; accepted 25 November 1997
Abstract A method which employs an integrating sphere and a single-wavelength (1.6 lm) pyrometer for measuring the spectral effective emissivities of superalloys in the temperature range ("650—1050°C) is described. The spectral effective emissivities for five superalloys, namely, MARM-247, MARM-509, CMSX-4, Inconel-718, N-155 and two Rene´-N6 samples coated with YSZ thermal barrier coating were measured. Correcting the pyrometer measurements for the variations in the object emissivity would reduce the uncertainty in the temperature measurements to ($1%. ( 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Single-wavelength pyrometers are used extensively as a diagnostic and health monitoring non-contact temperature measurement tool in the development and research of advanced high-temperature military and commercial gas turbine engines [1]. In addition, these intensity-based IR sensors are widely used for gathering non-contact temperature measurements in petrochemical, material processing and laser-machining industries [1]. In contrast with the widely used temperature sensors, namely, thermocouples, pyrometers have several advantages over thermocouples. First, the absence of wires makes pyrometers attractive for non-contact surface temperature measurements of rotating parts (i.e. blades). Second, the temperature measurements gathered using pyrometers are immune to the electromagnetic interference generated from the surrounding environment. Third, temperature measurements can be collected non-intrusively and without perturbing any of the system parameters (as an example gas flow patterns). 0143-8166/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved PII S 0 1 43 -8 16 6 ( 97 ) 0 01 0 8- 5
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In spite of these advantages single-wavelength radiation thermometry instruments (i.e. single-wavelength pyrometers) suffer from three major problems: (1) inferring the object temperature from the spectral radiance measurements without knowing the emissivity of the object at different temperatures; (2) correcting temperature measurements for the extraneous reflection and emission components (i.e. background radiance) which can be produced by flames, walls, or particles in the field of view; and (3) correcting for the attenuation of the optical signal due to the variable transmissivity of the optical path. To circumvent some of these problems multi-wavelength [1, 2] and reflectance [1] pyrometers were developed. Despite the several advances in the development of these instruments in recent years, measurements gathered using these instruments lack accuracy, especially when used in flame environments for viewing low emissivity (i.e. high reflectivity) components such as ceramics [1]. In view of these difficulties, the development of algorithms for correcting the measurements gathered using pyrometers for the emissivity variations and the contribution due to the reflected radiation components was deemed necessary. In the work herein, a practical method for determining experimentally the effective spectral emissivity of engine components at high-temperature using a singlewavelength pyrometer is described. Emissivity measurements pertaining to Ni, Co, Ni—Fe, and Ni—Co—Fe-based superalloys and yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) are discussed. In addition, an error analysis for the collected measurements is presented.
2. Experimental The spectral effective emissivity for selected superalloys namely, MARM 247 [3], MARM 509 [3], CMSX-4 [4], Inconel 718 [3], N155 [3], and YSZ TBC deposited on Rene´-N6 superalloy (coated with a thin layer of platinum aluminide (PtAl) bond-coat) was examined in air over the temperature range &650—1050°C. The TBC layer was applied to the PtAl bond-coat using an electron beam physical vapor deposition (EB-PVD) technique. As illustrated in Fig. 1, a disc (&2 cm in diameter) made from the alloy under investigation was instrumented with two type K thermocouples (nickel—chromium versus nickel—aluminum alloy). The sensing ends of the two thermocouples were spot-welded into the center of the disc. In the case of the YSZ TBC disc the sensing ends of the two thermocouples were adhered to the center of the sample using ceramic cement (Ceramabond 569, Aremco Products, Inc.). For commercial type K thermocouples a calibration tolerance of $0.4% is specified over the temperature range 0—1250°C by ASTM standards. Furthermore, a block of metal was spot-welded to the back of the disc to ensure the generation of a quasi-isothermal surface area. The surface area of each superalloy disc was roughened before instrumenting the disc with thermocouples using a fine sand blaster. To prevent the infrared radiation generated from the surrounding environment from interfering with the collected signals, the disc was mounted at one of the open-ends of a 30 cm long alumina tube (&4.6 mm wall thickness). The other open end of the alumina tube was
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Fig. 1. Schematic diagram of the experimental set-up used for measuring the effective spectral emissivity of the superalloys and the YSZ samples using a single wavelength (1.6 lm) pyrometer.
coupled to the entrance port of an integrating sphere (Newport-P/N 819-IS-2). The integrating sphere inner surface is coated with a Spectralon which has a 0.991 reflectance (o) at 1.6 lm (8° hemispherical) [5]. Herein, it is worth noting that the integrating sphere was introduced to the measurement setup to ensure that the non-Lambertian radiant flux emitted from the surface area of the disc [6] is transformed into a circular lambertian source at the exit port. During all experiments the monolithic thermoplastic integrating sphere was air-cooled by blowing air at the surface area facing the furnace to maintain the thermal stability of the sphere. By placing the disc and the isothermal block inside a three-zone furnace (Lindberg, model 55347), the photons emitted from the surface area of the coupon at different temperature settings were measured. The temperature of the three-zone furnace was controlled via a programmable temperature controller (Eurothermmodel 818). Further, the analog output of the pyrometer amplifier-filter module and the readings of the two thermocouples were monitored using a data acquisition system (HP 75000 B) equipped with 16-channel thermocouple and high-voltage relay multiplexer cards (HP E1345-66201). During each data collection cycle the data-acquisition system was programmed to calculate the average of ten readings for each channel. In addition, the standard error was calculated for each measurement. A commercial pyrometer system (Land Turbine Sensors, Inc.-Type 699.057) which consists of three major components: collection optics, detector and data-acquisition electronics, was used to measure the throughput of the integrating sphere. All measurements were gathered with the pyrometer emissivity setting set at 1.0. Before any measurements were collected, the pyrometer system was calibrated with respect to a blackbody cavity (Mikron-model 330). The blackbody cavity was heated with molybdenum disilicide elements and the temperature of the cavity was monitored using a platinum B-type thermocouple. The accuracy of the blackbody was rated at $0.25% of the $1 digit for temperature readings above 600°C, whereas, the
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Fig. 2. Calibration curve for the pyrometer used in this work. The solid line represents the straight line calculated using the calibration measurements.
emissivity of the cavity was rated at 0.990$0.005. Fig. 2 depicts the calibration curve of the instrument used in this work. As shown in Fig. 1, the radiant flux emitted from the heated disc was collected and coupled into a bundle of 200 lm core-diameter waveguides by means of a short focal length sapphire lens ( f"12.5 cm). Radiation emerging from the distal end of the waveguides was coupled into a InGaAs detector. By placing a band-pass filter in front of the detector, the output of which was fed into an amplifier-filter module for signal processing, the spectral response of the detector was restricted to a 75 nm spectral band centered at 1.6 lm. 3. Results and discussion Using least-squares linear regression analysis, a functional relationship expressing the temperature of the blackbody cavity as a function of the pyrometer spectral radiance temperature was calculated. The calculated functional form which is illustrated in Fig. 2 is given by ¹ "(1.012$0.005)¹j!(17.400$2.462) (1) BB where ¹ is the temperature indicated by the blackbody cavity and ¹j is the BB spectral radiance temperature indicated by the pyrometer. The calculated coefficient
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of determination (r2), the standard deviation of the slope and the standard deviation of the intercept were 0.999, $0.005, and $2.462, respectively. Next, the calibrated pyrometer was used to measure the spectral effective emissivities for each alloy over the temperature range &650—1050°C. The relationship expressing the effective spectral emissivity in terms of the pyrometer spectral radiance temperature and the temperature of the alloy as indicated by the thermocouples [Eq. (5)] is deduced from the arguments which will follow. The spectral radiance (sterance), ¸(j, ¹), of a blackbody radiator (i.e. emissivity(1) at temperature ¹ is described by Planck’s equation j¸(j, ¹)"(2hc2)j~5/[e(hc@jkT)!1]
(2)
where h is Planck’s constant, c is the speed of light in vacuum, j is the wavelength in meters, k is Boltzmann’s constant and ¹ is the absolute temperature of the blackbody in degrees Kelvin. In the case of a greybody (i.e. emissivity (1) the output signal of the pyrometer system, /(j,¹), may be expressed, as a first-order approximation, as [7] U(j, ¹)"We(j, ¹)¸ (j, ¹) '3%:"0$:
(3)
where W is the instrument constant and e(j, ¹) is the effective spectral emissivity of the greybody at a given temperature and spectral bandwidth. Further, the emissivity depends on surface quality and geometry of the object. The instrument constant, W, which appears in Eq. (3) is generally evaluated in terms of the path transmittance (q ), 1 the detector detectivity (D*), the detector effective area (A ), the spectral band width of 4 the system (*j) and the solid angle subtended by the detector (X ). 4 Likewise, the output signal measured for a blackbody by a pyrometer may be represented as U(j, ¹)"W¸(j, ¹)
(4)
From Eqs. (2)—(4) the effective spectral emissivity of the greybody can be written as e(j, ¹)"e*(1@T~1@Tj)C2@j)+
(5)
where the value of C is 14388 lm K, the value of j is 1.6 lm, ¹j is the temperature of 2 the greybody as indicated by the thermocouples, and ¹j is the spectral radiance temperature indicated by the pyrometer. Consequently, the temperature of the greybody can written as ¹"(j/C Ln e(j, ¹)#1/¹j)~1 2
(6)
Here, it is worth noting that the derivation of Eq. (5) implies the replacement of Planck’s law [Eq. (2)] by Wien’s approximation which is valid for short wavelengths and low temperatures (i.e. C Aj¹). Wien’s approximation introduces an error of the 2 order of 0.1% and does not affect the correctness of the results significantly. By employing Eq. (5) and utilizing the temperature measurements, the effective spectral emissivities for the alloys noted previously were determined experimentally.
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The results are presented in Figs. 3—7. The solid line in each graph depicts the calculated empirical fit using the relationships listed in Table 1. The calculated coefficients of determination for these empirical fits ranged between 0.90 and 0.85. Also, Table 1 tabulates the elemental percentage weight composition, the calculated average effective spectral emissivity and the standard error of the emissivity measurement for each alloy. Since the standard deviation of the emissivity measurements did not exceed $5%, it can be assumed as a first-order approximation that the emissivity for each alloy was constant over the temperature range &650—1050°C. By comparing the measured average effective spectral emissivities for CMSX-4 (e"0.806$0.003), MARM-247 (e"0.817$0.002) and Inconel-718 (e"0.853$ 0.009) as a function of the percentage weight concentration of Ni, it can be seen that the value of the spectral effective emissivity increased with the decrease in the alloy’s Ni concentration. As shown in Table 1, the percentage concentration of Ni for CMSX-4, MARM-247 and Inconel-718 is 61.7, 59.0 and 52.5%, respectively. To test whether the three mean emissivities (i.e. e ,e ,e ) are CMSX-4 MARM-247 I/#0/%--718 significantly different or not, two hypotheses were tested by subjecting the emissivity means to a t-distribution test [i.e. H (null hypothesis): e "e for critical region 0 1 2 t (t — accept hypothesis H and reject H ; H : e Oe for critical region #!-#6-!5%$ #3*5*#!0 1 1 1 2 t 't — reject hypothesis H and accept H ] [8]. As such, t was #!-#6-!5%$ #3*5*#!0 1 #!-#6-!5%$ greater than t for the three possible emissivity mean combinations, that is #3*5*#!e vs e ;e vs e ; and e vs e . Thus, the null CMSX-4 MARM-247 CMSX-4 I/#0/%--718 MARM-247 I/#0/%--718 hypothesis was rejected and the alternative hypothesis was accepted for the three possible emissivity combinations. This result, at 85% confidence level, implies that the emissivity values are indeed significantly different. Further, based upon this result one may conclude that a 14.9% decrease in the Ni concentration would result in a
Fig. 3. Scatter diagram showing the measured effective spectral emissivity for MARM-247 over the temperature range &65—1050°C.
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Fig. 4. Scatter diagram showing the measured effective spectral emissivity for MARM-509 over the temperature range &65—1050°C.
Fig. 5. Scatter diagram showing the measured effective spectral emissivity for CMSX-4 over the temperature range &65—1050°C.
5.5% increase in the value of the measured spectral effective emissivity. Herein, it is worth noting that throughout this work the hypothesis tests were carried out under the assumption that the emissivity measurements were random samples drawn from a normal distribution.
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Fig. 6. Scatter diagram showing the measured effective spectral emissivity for Inconel-718 over the temperature range &65—1050°C.
Fig. 7. Scatter diagram showing the measured effective spectral emissivity for N-155 over the temperature range &65—1050°C.
Likewise, the emissivity measurements were examined as a function of the Co weight percentage concentration in three alloys, namely, MARM-509 (54.5% Co), MARM-247 (10.0% Co) and CMSX-4 (9.0% Co). Clearly, a decrease in the percentage weight concentration of Co resulted in a decrease in the measured spectral effective emissivity of the alloy. In a similar manner, the three mean emissivities were
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Table 1 Regression fits, the percentage elemental composition, average effective spectral emissivity and emissivity standard errors calculated for each superalloy Alloy % composition
Empirical fit
Average emissivity
MARM-247 (Ni-based alloy) 59.0 Ni—10.0 Co-(0.5 Fe
e"3.5]10~3¹ !4.7]10~6 ¹2j j #2.1]10~9¹3 j e"4.8]10~3¹ !7.1]10~6 ¹2j j #3.2]10~9¹3 j e"3.9]10~3¹ !5.7]10~6 ¹2j j #2.6]10~9¹3 j e"2.5]10~3¹ !2.5]10~6 ¹2j j #9.7]10~10¹3 j e"39.9/¹ #2.5]10~3 ¹j j !3.2]10~6¹2#1.4]10~9¹3 j j
0.817$0.002
MARM-509 (Co-based alloy) 10.0 Ni—54.5 Co CMSX-4 (Ni-based alloy) 61.7 Ni—9.0 Co Inconel-718 (Ni—Fe-based alloy) 52.5 Ni—18.5 Fe N-155 (Ni-Co—Fe-based alloy) 20 Ni—20 Co—30 Fe
0.926$0.006 0.806$0.003 0.853$0.005 0.697$0.002
Fig. 8. Scatter diagram showing the measured effective spectral emissivity for the two (S1 and S2) heated YSZ samples.
tested by assuming a null hypothesis (H : e "e ) and an alternative hypothesis 0 1 2 vs (H : e Oe ). The results of the statistical inference indicated that e 1 1 2 MARM-509 e and e vs e were significantly different and the null hypothesis MARM-247 MARM-509 CMSX-4 was rejected for each combination. However, in the case of e MARM-509
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Table 2 Calculated average effective spectral emissivities and their standard errors for the two YSZ samples after each heat cycle Sample no.
Run no.
Duration of heat cycle (h)
Average emissivity
S1 S1 S2 S2 S2
1 2 1 3 4
8 24 8 24 32
0.358$0.019 0.725$0.014 0.439$0.077 0.758$0.009 0.704$0.008
Fig. 9. Relative K-ratio scanning electron microscope output showing the elemental composition of a YSZ sample before exposing the sample to heat (Zr"zirconium; Hf"hafnium; Y" yttrium).
the null hypothesis was accepted at a 99.5% confidence level. Consequently, vs e CMSX-4 e and e are not significantly different in value. As shown in Table 1, it MARM-247 CMSX-4 can be seen that a decrease of approximately 81.6% in the Co percentage concentration yields a decrease of 11.8% in the value of the spectral effective emissivity of the sample [9]. Furthermore, the effective spectral emissivities for two YSZ TBC coated samples were measured as a function of temperature using the apparatus shown in Fig. 1. The obtained results are depicted in Fig. 8. Each set of data (i.e. run) was collected over 8 h time period. As shown in Fig. 8 and listed in Table 2 an average increase of
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Fig. 10. Relative K-ratio scanning electron microscope output showing the elemental composition of a heat cycled YSZ coupon (Al"aluminum; Zr"zirconium; Hf"hafnium; Y" yttrium).
approximately 45.3% in the measured effective spectral emissivity values was observed after the first heat cycle. After run d 1 (i.e. 8 h heat cycle) the effective spectral emissivity values for both samples ranged between 0.70 and 0.78. Such an increase in the measured emissivity values between the first run (i.e. run d 1 for S1 and S2) and the later runs was attributed to the migration of aluminum and aluminum oxide from the PtAl bond-coat into the TBC layer. The migration of aluminum from the bondcoat into the surface area of the TBC structure was detected using a scanning electron microscope (JXA-35, JEOL). As illustrated in Fig. 9 the elemental surface structure measured for a fresh YSZ sample does not indicate the presence of aluminum in the surface area. Whereas the presence of aluminum in the surface area of a sample heat cycled for 32 h (S2 (run d4)) is evident as shown in Fig. 10. It was reported that the migration of aluminum and aluminum oxide from the bond-coat into the TBC leads to the spallation of TBC, thereby leading to temperature rise of the airfoil and premature failure of engine components [10]. To test whether the three mean emissivities [i.e. e , e and e where e i" 1,2 2,3 2,4 i,j sample number (i"1, 2) and j"run number (1, 2, 2 ,4)] are significantly different, two hypotheses were tested by subjecting the emissivity means to a t-distribution test [i.e. H (null hypothesis): e "e for critical region t (t , accept hypoth0 i,j i,j #!-#6-!5%$ #3*5*#!esis and reject H ; H : e Oe for critical t 't region, reject hypothesis 1 1 i,j i,j #!-#6-!5%$ #3*5*#!H and accept H ]. As such, t was greater than t for the three possible 0 1 #!-#6-!5%$ #3*5*#!-
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Fig. 11. Transmittance and reflectance measurements for the YSZ layer (&0.330 mm thick) deposited on a sapphire disc using a EB-PVD method. Also, the transmittance measurements for a 1mm thick sapphire disc over the spectral region 250—2750 nm are shown.
emissivity mean combinations, that is e vs e , e vs e ; and e vs e . Thus, the 1,2 2,3 1,2 2,4 2,3 2,4 null hypothesis was rejected and the alternative hypothesis was accepted for the three possible emissivity combinations. This result, at 99.5% confidence level, implies that the emissivity values are indeed significantly different and the surface area of the TBC layer did not reach stability after several heat cycles. However, for practical purposes an average emissivity value of 0.729 $ 0.013 (average for e , e and e ) can be 1,2 2,3 2,4 used as the effective spectral emissivity for the YSZ layer at 1.6 lm. Further, based upon these findings one may conclude that the optical characteristics for TBC change drastically due to the migration of aluminum from the bond-coat into the TBC layer. After a short heat cycle the emissivity values for the TBC layer become comparable in value to that measured for superalloys over the same temperature range. Consequently, the transmittance of the YSZ layer which is very high at long wavelengths, as shown in Fig. 11, will be reduced drastically. Such a result suggests that using long
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Fig. 12. Relative errors (% relative error " M(pyrometer reading!thermocouple reading)/thermocouple reading)N100) calculated for each superalloy with the pyrometer emissivity set at unity.
wavelength pyrometry measurements for monitoring the surface temperature of TBC-coated engine parts does not offer a clear advantage over short wavelength TBC pyrometry measurements. Additionally, in an actual turbine engine environment the columnar structure of the TBC-layer would allow particulate matter from the engine flow path to be deposited between the YSZ columns. As a result the optical properties of the TBC layer will be transformed into properties similar to that obtained from metallic surfaces. Also it is worth noting that due to the high thermal expansion coefficients of these particulate matters relative to the YSZ columns, they can contribute in a significant manner to the spallation of the TBC layer. Lastly, we examined the percentage relative errors which resulted from setting the pyrometer emissivity control at one (i.e. assuming e"1). The relative errors were calculated by normalizing the difference between the pyrometer and the thermocouple readings by the temperature measurements obtained using type K thermocouples. As illustrated in Figs. 12, such an assumption, as the data for N-155 alloy suggests, can lead to $ 6.3% error (&$63°C at 1000°C) in the temperature measurements. The magnitude of such errors can be reduced to ($1% when the correct emissivity of the superalloy is selected. Further, as shown in Fig. 13, the percentage relative errors for the first two heat cycles for the two YSZ samples varied between 10 and 16% over the 650—1000°C temperature range. The values of the percentage relative error dropped to an average value of 4.98$0.08% [average for run d2(S1), run d3(S2), and run d 4(S2)] after the first heat cycle. This result implies that a reduction in the magnitude of the percentage relative error can be achieved by heat cycling the TBC coated turbine engine components before gathering temperature measurements in an engine. However, the impact of the heat cycle on the lifetime of the engine part and the integrity of the TBC layer is not clear at this stage.
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Fig. 13. Relative errors (% relative error"M(pyrometer reading!thermocouple reading)/thermocouple reading)N100) calculated for the two YSZ samples with the pyrometer emissivity set at unity.
4. Conclusions Analysis of the emissivity measurements indicated that at 1.6 lm a decrease in the Ni concentration (61—52% weight composition of the alloy) would result in an increase in the value of the measured emissivity. Whereas a decrease in the Co concentration (54.5—9.0% the weight composition of the alloy) would result in a decrease in the emissivity of the alloy. Further, the average effective spectral emissivity of YSZ thermal barrier coating increased from 0.3991$ 0.0289 to 0.7287$ 0.0128 after a short heat cycle. A significant reduction in the contribution of the emissivity error component and, consequently, the reflection component can be achieved by heat cycling YSZ TBC-coated engine parts prior to taking any temperature measurements using pyrometers. Acknowledgement The authors wish to express their thanks to Mr. Pete Linko and his colleagues at General Electric Aircraft Engines for supplying us with the TBC coated samples.
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References [1] NASA Lewis Research Center, Remote Temperature Sensing Workshop, 1994. [2] Hiernaut J, Beukers R, Heinz W, Selslag R, Hoch M, Ohse R. Submillisecond six-wavelength pyrometer for high-temperature measurements in the range 2000 to 5000 K. 10 ETPC Proc., vol. 18. 1986;617—25. [3] Davis JR, ASM Materials Engineering Dictionary, 1992. [4] Fullagar K, Broomfield R, Hulands M, Harris K, Erickson G, Sikkenga S, Aeroengine test experience with CMSX-4 alloy single crystal turbine blades. 39th ASME/IGTI Int. Gas Turbine and Aeroengine Congress and Exposition, The Hague, Netherlands, 13—16 June, 1994. [5] Newport Corporation, Newport catalog, Irvine, California, 1992; k-28—30. [6] DeWitt D, Gene Nutter D, Theory and Practice of Radiation Thermometry. New York: Wiley, 1988:91—187. [7] Wyatt C, Radiometric System Design. Macmillan, New York, 1987. [8] Guenther W, Concepts of Statistical Inference. 2nd edn, McGraw-Hill Kogakusha Ltd, Tokyo, Japan, 1973. [9] Alaruri S, Bianchini L, Brewington A, Jilg T, Belcher B, Integrating sphere method for determining the effective spectral emissivity of superalloys at high temperature using a single wavelength pyrometer. Opt. Engng. 1996;35(9):2736. [10] Daleo J, Boone D, Failure mechanisms of coating systems applied to advanced turbine engine components. The 42nd ASME Gas Turbine and Aeroengine Congress, 97-GT-486, Orlando, FL, 2—5, June 1997.
Effective spectral emissivity measurements of superalloys and YSZ thermal barrier coating at high temperatures using a 1.6 lm single wavelength pyrometer Sami Alaruri, Lisa Bianchini, Andrew Brewington Allison Engine Company, Rolls Royce Aerospace Group, Box 420, M/S W03A, Indianapolis, IN 46206-0420, USA Received 13 August 1997; accepted 25 November 1997
Abstract A method which employs an integrating sphere and a single-wavelength (1.6 lm) pyrometer for measuring the spectral effective emissivities of superalloys in the temperature range ("650—1050°C) is described. The spectral effective emissivities for five superalloys, namely, MARM-247, MARM-509, CMSX-4, Inconel-718, N-155 and two Rene´-N6 samples coated with YSZ thermal barrier coating were measured. Correcting the pyrometer measurements for the variations in the object emissivity would reduce the uncertainty in the temperature measurements to ($1%. ( 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Single-wavelength pyrometers are used extensively as a diagnostic and health monitoring non-contact temperature measurement tool in the development and research of advanced high-temperature military and commercial gas turbine engines [1]. In addition, these intensity-based IR sensors are widely used for gathering non-contact temperature measurements in petrochemical, material processing and laser-machining industries [1]. In contrast with the widely used temperature sensors, namely, thermocouples, pyrometers have several advantages over thermocouples. First, the absence of wires makes pyrometers attractive for non-contact surface temperature measurements of rotating parts (i.e. blades). Second, the temperature measurements gathered using pyrometers are immune to the electromagnetic interference generated from the surrounding environment. Third, temperature measurements can be collected non-intrusively and without perturbing any of the system parameters (as an example gas flow patterns). 0143-8166/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved PII S 0 1 43 -8 16 6 ( 97 ) 0 01 0 8- 5
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In spite of these advantages single-wavelength radiation thermometry instruments (i.e. single-wavelength pyrometers) suffer from three major problems: (1) inferring the object temperature from the spectral radiance measurements without knowing the emissivity of the object at different temperatures; (2) correcting temperature measurements for the extraneous reflection and emission components (i.e. background radiance) which can be produced by flames, walls, or particles in the field of view; and (3) correcting for the attenuation of the optical signal due to the variable transmissivity of the optical path. To circumvent some of these problems multi-wavelength [1, 2] and reflectance [1] pyrometers were developed. Despite the several advances in the development of these instruments in recent years, measurements gathered using these instruments lack accuracy, especially when used in flame environments for viewing low emissivity (i.e. high reflectivity) components such as ceramics [1]. In view of these difficulties, the development of algorithms for correcting the measurements gathered using pyrometers for the emissivity variations and the contribution due to the reflected radiation components was deemed necessary. In the work herein, a practical method for determining experimentally the effective spectral emissivity of engine components at high-temperature using a singlewavelength pyrometer is described. Emissivity measurements pertaining to Ni, Co, Ni—Fe, and Ni—Co—Fe-based superalloys and yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) are discussed. In addition, an error analysis for the collected measurements is presented.
2. Experimental The spectral effective emissivity for selected superalloys namely, MARM 247 [3], MARM 509 [3], CMSX-4 [4], Inconel 718 [3], N155 [3], and YSZ TBC deposited on Rene´-N6 superalloy (coated with a thin layer of platinum aluminide (PtAl) bond-coat) was examined in air over the temperature range &650—1050°C. The TBC layer was applied to the PtAl bond-coat using an electron beam physical vapor deposition (EB-PVD) technique. As illustrated in Fig. 1, a disc (&2 cm in diameter) made from the alloy under investigation was instrumented with two type K thermocouples (nickel—chromium versus nickel—aluminum alloy). The sensing ends of the two thermocouples were spot-welded into the center of the disc. In the case of the YSZ TBC disc the sensing ends of the two thermocouples were adhered to the center of the sample using ceramic cement (Ceramabond 569, Aremco Products, Inc.). For commercial type K thermocouples a calibration tolerance of $0.4% is specified over the temperature range 0—1250°C by ASTM standards. Furthermore, a block of metal was spot-welded to the back of the disc to ensure the generation of a quasi-isothermal surface area. The surface area of each superalloy disc was roughened before instrumenting the disc with thermocouples using a fine sand blaster. To prevent the infrared radiation generated from the surrounding environment from interfering with the collected signals, the disc was mounted at one of the open-ends of a 30 cm long alumina tube (&4.6 mm wall thickness). The other open end of the alumina tube was
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Fig. 1. Schematic diagram of the experimental set-up used for measuring the effective spectral emissivity of the superalloys and the YSZ samples using a single wavelength (1.6 lm) pyrometer.
coupled to the entrance port of an integrating sphere (Newport-P/N 819-IS-2). The integrating sphere inner surface is coated with a Spectralon which has a 0.991 reflectance (o) at 1.6 lm (8° hemispherical) [5]. Herein, it is worth noting that the integrating sphere was introduced to the measurement setup to ensure that the non-Lambertian radiant flux emitted from the surface area of the disc [6] is transformed into a circular lambertian source at the exit port. During all experiments the monolithic thermoplastic integrating sphere was air-cooled by blowing air at the surface area facing the furnace to maintain the thermal stability of the sphere. By placing the disc and the isothermal block inside a three-zone furnace (Lindberg, model 55347), the photons emitted from the surface area of the coupon at different temperature settings were measured. The temperature of the three-zone furnace was controlled via a programmable temperature controller (Eurothermmodel 818). Further, the analog output of the pyrometer amplifier-filter module and the readings of the two thermocouples were monitored using a data acquisition system (HP 75000 B) equipped with 16-channel thermocouple and high-voltage relay multiplexer cards (HP E1345-66201). During each data collection cycle the data-acquisition system was programmed to calculate the average of ten readings for each channel. In addition, the standard error was calculated for each measurement. A commercial pyrometer system (Land Turbine Sensors, Inc.-Type 699.057) which consists of three major components: collection optics, detector and data-acquisition electronics, was used to measure the throughput of the integrating sphere. All measurements were gathered with the pyrometer emissivity setting set at 1.0. Before any measurements were collected, the pyrometer system was calibrated with respect to a blackbody cavity (Mikron-model 330). The blackbody cavity was heated with molybdenum disilicide elements and the temperature of the cavity was monitored using a platinum B-type thermocouple. The accuracy of the blackbody was rated at $0.25% of the $1 digit for temperature readings above 600°C, whereas, the
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Fig. 2. Calibration curve for the pyrometer used in this work. The solid line represents the straight line calculated using the calibration measurements.
emissivity of the cavity was rated at 0.990$0.005. Fig. 2 depicts the calibration curve of the instrument used in this work. As shown in Fig. 1, the radiant flux emitted from the heated disc was collected and coupled into a bundle of 200 lm core-diameter waveguides by means of a short focal length sapphire lens ( f"12.5 cm). Radiation emerging from the distal end of the waveguides was coupled into a InGaAs detector. By placing a band-pass filter in front of the detector, the output of which was fed into an amplifier-filter module for signal processing, the spectral response of the detector was restricted to a 75 nm spectral band centered at 1.6 lm. 3. Results and discussion Using least-squares linear regression analysis, a functional relationship expressing the temperature of the blackbody cavity as a function of the pyrometer spectral radiance temperature was calculated. The calculated functional form which is illustrated in Fig. 2 is given by ¹ "(1.012$0.005)¹j!(17.400$2.462) (1) BB where ¹ is the temperature indicated by the blackbody cavity and ¹j is the BB spectral radiance temperature indicated by the pyrometer. The calculated coefficient
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of determination (r2), the standard deviation of the slope and the standard deviation of the intercept were 0.999, $0.005, and $2.462, respectively. Next, the calibrated pyrometer was used to measure the spectral effective emissivities for each alloy over the temperature range &650—1050°C. The relationship expressing the effective spectral emissivity in terms of the pyrometer spectral radiance temperature and the temperature of the alloy as indicated by the thermocouples [Eq. (5)] is deduced from the arguments which will follow. The spectral radiance (sterance), ¸(j, ¹), of a blackbody radiator (i.e. emissivity(1) at temperature ¹ is described by Planck’s equation j¸(j, ¹)"(2hc2)j~5/[e(hc@jkT)!1]
(2)
where h is Planck’s constant, c is the speed of light in vacuum, j is the wavelength in meters, k is Boltzmann’s constant and ¹ is the absolute temperature of the blackbody in degrees Kelvin. In the case of a greybody (i.e. emissivity (1) the output signal of the pyrometer system, /(j,¹), may be expressed, as a first-order approximation, as [7] U(j, ¹)"We(j, ¹)¸ (j, ¹) '3%:"0$:
(3)
where W is the instrument constant and e(j, ¹) is the effective spectral emissivity of the greybody at a given temperature and spectral bandwidth. Further, the emissivity depends on surface quality and geometry of the object. The instrument constant, W, which appears in Eq. (3) is generally evaluated in terms of the path transmittance (q ), 1 the detector detectivity (D*), the detector effective area (A ), the spectral band width of 4 the system (*j) and the solid angle subtended by the detector (X ). 4 Likewise, the output signal measured for a blackbody by a pyrometer may be represented as U(j, ¹)"W¸(j, ¹)
(4)
From Eqs. (2)—(4) the effective spectral emissivity of the greybody can be written as e(j, ¹)"e*(1@T~1@Tj)C2@j)+
(5)
where the value of C is 14388 lm K, the value of j is 1.6 lm, ¹j is the temperature of 2 the greybody as indicated by the thermocouples, and ¹j is the spectral radiance temperature indicated by the pyrometer. Consequently, the temperature of the greybody can written as ¹"(j/C Ln e(j, ¹)#1/¹j)~1 2
(6)
Here, it is worth noting that the derivation of Eq. (5) implies the replacement of Planck’s law [Eq. (2)] by Wien’s approximation which is valid for short wavelengths and low temperatures (i.e. C Aj¹). Wien’s approximation introduces an error of the 2 order of 0.1% and does not affect the correctness of the results significantly. By employing Eq. (5) and utilizing the temperature measurements, the effective spectral emissivities for the alloys noted previously were determined experimentally.
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The results are presented in Figs. 3—7. The solid line in each graph depicts the calculated empirical fit using the relationships listed in Table 1. The calculated coefficients of determination for these empirical fits ranged between 0.90 and 0.85. Also, Table 1 tabulates the elemental percentage weight composition, the calculated average effective spectral emissivity and the standard error of the emissivity measurement for each alloy. Since the standard deviation of the emissivity measurements did not exceed $5%, it can be assumed as a first-order approximation that the emissivity for each alloy was constant over the temperature range &650—1050°C. By comparing the measured average effective spectral emissivities for CMSX-4 (e"0.806$0.003), MARM-247 (e"0.817$0.002) and Inconel-718 (e"0.853$ 0.009) as a function of the percentage weight concentration of Ni, it can be seen that the value of the spectral effective emissivity increased with the decrease in the alloy’s Ni concentration. As shown in Table 1, the percentage concentration of Ni for CMSX-4, MARM-247 and Inconel-718 is 61.7, 59.0 and 52.5%, respectively. To test whether the three mean emissivities (i.e. e ,e ,e ) are CMSX-4 MARM-247 I/#0/%--718 significantly different or not, two hypotheses were tested by subjecting the emissivity means to a t-distribution test [i.e. H (null hypothesis): e "e for critical region 0 1 2 t (t — accept hypothesis H and reject H ; H : e Oe for critical region #!-#6-!5%$ #3*5*#!0 1 1 1 2 t 't — reject hypothesis H and accept H ] [8]. As such, t was #!-#6-!5%$ #3*5*#!0 1 #!-#6-!5%$ greater than t for the three possible emissivity mean combinations, that is #3*5*#!e vs e ;e vs e ; and e vs e . Thus, the null CMSX-4 MARM-247 CMSX-4 I/#0/%--718 MARM-247 I/#0/%--718 hypothesis was rejected and the alternative hypothesis was accepted for the three possible emissivity combinations. This result, at 85% confidence level, implies that the emissivity values are indeed significantly different. Further, based upon this result one may conclude that a 14.9% decrease in the Ni concentration would result in a
Fig. 3. Scatter diagram showing the measured effective spectral emissivity for MARM-247 over the temperature range &65—1050°C.
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Fig. 4. Scatter diagram showing the measured effective spectral emissivity for MARM-509 over the temperature range &65—1050°C.
Fig. 5. Scatter diagram showing the measured effective spectral emissivity for CMSX-4 over the temperature range &65—1050°C.
5.5% increase in the value of the measured spectral effective emissivity. Herein, it is worth noting that throughout this work the hypothesis tests were carried out under the assumption that the emissivity measurements were random samples drawn from a normal distribution.
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Fig. 6. Scatter diagram showing the measured effective spectral emissivity for Inconel-718 over the temperature range &65—1050°C.
Fig. 7. Scatter diagram showing the measured effective spectral emissivity for N-155 over the temperature range &65—1050°C.
Likewise, the emissivity measurements were examined as a function of the Co weight percentage concentration in three alloys, namely, MARM-509 (54.5% Co), MARM-247 (10.0% Co) and CMSX-4 (9.0% Co). Clearly, a decrease in the percentage weight concentration of Co resulted in a decrease in the measured spectral effective emissivity of the alloy. In a similar manner, the three mean emissivities were
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Table 1 Regression fits, the percentage elemental composition, average effective spectral emissivity and emissivity standard errors calculated for each superalloy Alloy % composition
Empirical fit
Average emissivity
MARM-247 (Ni-based alloy) 59.0 Ni—10.0 Co-(0.5 Fe
e"3.5]10~3¹ !4.7]10~6 ¹2j j #2.1]10~9¹3 j e"4.8]10~3¹ !7.1]10~6 ¹2j j #3.2]10~9¹3 j e"3.9]10~3¹ !5.7]10~6 ¹2j j #2.6]10~9¹3 j e"2.5]10~3¹ !2.5]10~6 ¹2j j #9.7]10~10¹3 j e"39.9/¹ #2.5]10~3 ¹j j !3.2]10~6¹2#1.4]10~9¹3 j j
0.817$0.002
MARM-509 (Co-based alloy) 10.0 Ni—54.5 Co CMSX-4 (Ni-based alloy) 61.7 Ni—9.0 Co Inconel-718 (Ni—Fe-based alloy) 52.5 Ni—18.5 Fe N-155 (Ni-Co—Fe-based alloy) 20 Ni—20 Co—30 Fe
0.926$0.006 0.806$0.003 0.853$0.005 0.697$0.002
Fig. 8. Scatter diagram showing the measured effective spectral emissivity for the two (S1 and S2) heated YSZ samples.
tested by assuming a null hypothesis (H : e "e ) and an alternative hypothesis 0 1 2 vs (H : e Oe ). The results of the statistical inference indicated that e 1 1 2 MARM-509 e and e vs e were significantly different and the null hypothesis MARM-247 MARM-509 CMSX-4 was rejected for each combination. However, in the case of e MARM-509
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Table 2 Calculated average effective spectral emissivities and their standard errors for the two YSZ samples after each heat cycle Sample no.
Run no.
Duration of heat cycle (h)
Average emissivity
S1 S1 S2 S2 S2
1 2 1 3 4
8 24 8 24 32
0.358$0.019 0.725$0.014 0.439$0.077 0.758$0.009 0.704$0.008
Fig. 9. Relative K-ratio scanning electron microscope output showing the elemental composition of a YSZ sample before exposing the sample to heat (Zr"zirconium; Hf"hafnium; Y" yttrium).
the null hypothesis was accepted at a 99.5% confidence level. Consequently, vs e CMSX-4 e and e are not significantly different in value. As shown in Table 1, it MARM-247 CMSX-4 can be seen that a decrease of approximately 81.6% in the Co percentage concentration yields a decrease of 11.8% in the value of the spectral effective emissivity of the sample [9]. Furthermore, the effective spectral emissivities for two YSZ TBC coated samples were measured as a function of temperature using the apparatus shown in Fig. 1. The obtained results are depicted in Fig. 8. Each set of data (i.e. run) was collected over 8 h time period. As shown in Fig. 8 and listed in Table 2 an average increase of
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Fig. 10. Relative K-ratio scanning electron microscope output showing the elemental composition of a heat cycled YSZ coupon (Al"aluminum; Zr"zirconium; Hf"hafnium; Y" yttrium).
approximately 45.3% in the measured effective spectral emissivity values was observed after the first heat cycle. After run d 1 (i.e. 8 h heat cycle) the effective spectral emissivity values for both samples ranged between 0.70 and 0.78. Such an increase in the measured emissivity values between the first run (i.e. run d 1 for S1 and S2) and the later runs was attributed to the migration of aluminum and aluminum oxide from the PtAl bond-coat into the TBC layer. The migration of aluminum from the bondcoat into the surface area of the TBC structure was detected using a scanning electron microscope (JXA-35, JEOL). As illustrated in Fig. 9 the elemental surface structure measured for a fresh YSZ sample does not indicate the presence of aluminum in the surface area. Whereas the presence of aluminum in the surface area of a sample heat cycled for 32 h (S2 (run d4)) is evident as shown in Fig. 10. It was reported that the migration of aluminum and aluminum oxide from the bond-coat into the TBC leads to the spallation of TBC, thereby leading to temperature rise of the airfoil and premature failure of engine components [10]. To test whether the three mean emissivities [i.e. e , e and e where e i" 1,2 2,3 2,4 i,j sample number (i"1, 2) and j"run number (1, 2, 2 ,4)] are significantly different, two hypotheses were tested by subjecting the emissivity means to a t-distribution test [i.e. H (null hypothesis): e "e for critical region t (t , accept hypoth0 i,j i,j #!-#6-!5%$ #3*5*#!esis and reject H ; H : e Oe for critical t 't region, reject hypothesis 1 1 i,j i,j #!-#6-!5%$ #3*5*#!H and accept H ]. As such, t was greater than t for the three possible 0 1 #!-#6-!5%$ #3*5*#!-
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Fig. 11. Transmittance and reflectance measurements for the YSZ layer (&0.330 mm thick) deposited on a sapphire disc using a EB-PVD method. Also, the transmittance measurements for a 1mm thick sapphire disc over the spectral region 250—2750 nm are shown.
emissivity mean combinations, that is e vs e , e vs e ; and e vs e . Thus, the 1,2 2,3 1,2 2,4 2,3 2,4 null hypothesis was rejected and the alternative hypothesis was accepted for the three possible emissivity combinations. This result, at 99.5% confidence level, implies that the emissivity values are indeed significantly different and the surface area of the TBC layer did not reach stability after several heat cycles. However, for practical purposes an average emissivity value of 0.729 $ 0.013 (average for e , e and e ) can be 1,2 2,3 2,4 used as the effective spectral emissivity for the YSZ layer at 1.6 lm. Further, based upon these findings one may conclude that the optical characteristics for TBC change drastically due to the migration of aluminum from the bond-coat into the TBC layer. After a short heat cycle the emissivity values for the TBC layer become comparable in value to that measured for superalloys over the same temperature range. Consequently, the transmittance of the YSZ layer which is very high at long wavelengths, as shown in Fig. 11, will be reduced drastically. Such a result suggests that using long
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Fig. 12. Relative errors (% relative error " M(pyrometer reading!thermocouple reading)/thermocouple reading)N100) calculated for each superalloy with the pyrometer emissivity set at unity.
wavelength pyrometry measurements for monitoring the surface temperature of TBC-coated engine parts does not offer a clear advantage over short wavelength TBC pyrometry measurements. Additionally, in an actual turbine engine environment the columnar structure of the TBC-layer would allow particulate matter from the engine flow path to be deposited between the YSZ columns. As a result the optical properties of the TBC layer will be transformed into properties similar to that obtained from metallic surfaces. Also it is worth noting that due to the high thermal expansion coefficients of these particulate matters relative to the YSZ columns, they can contribute in a significant manner to the spallation of the TBC layer. Lastly, we examined the percentage relative errors which resulted from setting the pyrometer emissivity control at one (i.e. assuming e"1). The relative errors were calculated by normalizing the difference between the pyrometer and the thermocouple readings by the temperature measurements obtained using type K thermocouples. As illustrated in Figs. 12, such an assumption, as the data for N-155 alloy suggests, can lead to $ 6.3% error (&$63°C at 1000°C) in the temperature measurements. The magnitude of such errors can be reduced to ($1% when the correct emissivity of the superalloy is selected. Further, as shown in Fig. 13, the percentage relative errors for the first two heat cycles for the two YSZ samples varied between 10 and 16% over the 650—1000°C temperature range. The values of the percentage relative error dropped to an average value of 4.98$0.08% [average for run d2(S1), run d3(S2), and run d 4(S2)] after the first heat cycle. This result implies that a reduction in the magnitude of the percentage relative error can be achieved by heat cycling the TBC coated turbine engine components before gathering temperature measurements in an engine. However, the impact of the heat cycle on the lifetime of the engine part and the integrity of the TBC layer is not clear at this stage.
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Fig. 13. Relative errors (% relative error"M(pyrometer reading!thermocouple reading)/thermocouple reading)N100) calculated for the two YSZ samples with the pyrometer emissivity set at unity.
4. Conclusions Analysis of the emissivity measurements indicated that at 1.6 lm a decrease in the Ni concentration (61—52% weight composition of the alloy) would result in an increase in the value of the measured emissivity. Whereas a decrease in the Co concentration (54.5—9.0% the weight composition of the alloy) would result in a decrease in the emissivity of the alloy. Further, the average effective spectral emissivity of YSZ thermal barrier coating increased from 0.3991$ 0.0289 to 0.7287$ 0.0128 after a short heat cycle. A significant reduction in the contribution of the emissivity error component and, consequently, the reflection component can be achieved by heat cycling YSZ TBC-coated engine parts prior to taking any temperature measurements using pyrometers. Acknowledgement The authors wish to express their thanks to Mr. Pete Linko and his colleagues at General Electric Aircraft Engines for supplying us with the TBC coated samples.
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