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Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings
Surface & Coatings Technology 206 (2011) 1566–1570
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
Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings B.A. Pint a,⁎, G.W. Garner a, T.M. Lowe a, J.A. Haynes a, Y. Zhang b a b
Materials Science and Technology Division, Oak Ridge National Laboratory, United States Tennessee Technological University, United States
a r t i c l e
i n f o
Available online 17 June 2011 Keywords: Thermal barrier coating Pt diffusion coating Pt aluminide coating Water vapor High temperature oxidation
a b s t r a c t To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150 °C in air with 10 vol.% water vapor (similar to natural gas combustion) and 90 vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttriastabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O2, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Despite the common practice of conducting high temperature oxidation laboratory experiments in dry environments (air or O2), there are relatively few applications that do not contain water vapor, especially for thermal barrier coatings (TBC). Thus, there has been considerable recent interest in the effect of water vapor on oxidation [1–6]. Alumina-forming alloys and coatings are considerably less affected by water vapor than chromia-forming alloys [7–11]. For turbine applications, where the combustion gas of the actual turbine environment contains ~10 vol.% water vapor [12], alumina-forming coatings are standard. The issue of water vapor is a more significant concern for turbines designed for coal-derived synthesis gas (i.e. syngas) or hydrogen fuels where the water vapor content may be higher than natural gas or jet fuel due to the combustion reaction or the use of steam diluents to assist with combustion. Typically, syngas turbines are de-rated (i.e. fired at a lower maximum temperature) due to concerns about component durability [13]. However, this derating is expected to reduce the efficiency of the turbine. The reason for the derating may be related to improper cleanup (or upset conditions) allowing sulfur or ash into the turbine or the higher water vapor content in the combustion gas. While sulfur or ash contamination could be further mitigated, the higher water vapor content is
⁎ Corresponding author. Tel.: + 1 865 576 2897; fax: + 1 865 241 0215. E-mail address: [email protected] (B.A. Pint). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.06.008
determined by the fuel and combustion conditions. Some strategies use very high water vapor levels as an approach for combustion and carbon sequestration [14]. The purpose of this study was to better understand the effect of water vapor on thermal barrier coating (TBC) performance by building on a previous study of average YSZ lifetime with two phase (γ + γ′) Pt diffusion [15–18] and Pt-modified (β-NiPtAl) aluminide bond coatings in furnace cycling at 1150 °C in dry O2 [19]. Another study had looked at alumina scale formation at 1100 °C in 0–90 vol.% water vapor on similar Pt-containing bond coatings and model alloys, however, no YSZ coatings were evaluated [3,4]. The water vapor study found similar results as prior work [2] suggesting increased scale spallation and more transient oxide formation with increasing water content. The current results surprisingly found that water vapor increased the rate of rumpling of aluminide coatings, leading to a reduced average YSZ lifetime. In contrast, the Pt diffusion coatings were unaffected by thermally cycling in air with 10% or 90% water vapor. 2. Experimental procedure Superalloy René N5 (64.7 at.% Ni-8.0% Cr-13.3% Al-0.9% Re-7.6% Co-1.6% W-2.2% Ta-0.9% Mo-132 ppma Zr, 70Y, 540Hf,17S) coupons 1.5 mm thick, ~ 16 mm in diameter and chamfered edges were first grit blasted with alumina and then electroplated with 7 ± 1 μm of Pt in a laboratory scale bath at Tennessee Technological University (TTU). For the Pt diffusion coatings, the plated coupons were then annealed in a vacuum of 10 − 4 Pa (10 − 6 Torr) for 2 h at 1175 °C resulting in a
B.A. Pint et al. / Surface & Coatings Technology 206 (2011) 1566–1570
~ 25 μm thick coating [18]. For Pt-modified aluminide coatings, the plated specimens were aluminized for 6 h at 1100 °C to form a ~ 40 μm thick β layer in a laboratory-scale chemical vapor deposition reactor, described elsewhere [20]. A ~125 μm thick YSZ top coat was deposited on one side of the coupons by electron-beam physical vapor deposition (EB-PVD) using a standard commercial process that included a light grit blasting prior to YSZ deposition. For every group of four specimens to be evaluated at each condition, three were coated with YSZ and the fourth was tested without YSZ (and without grit blasting) to study the surface morphology evolution. The specimens were hung by a Pt–Rh wire in an automated cyclic rig and were exposed for 1 h at 1150 °C in air with 10 ± 1% or 90 ± 2% vol.% water vapor and cooled in laboratory air for 10 min for each cycle. Oxidation in wet air was conducted by flowing air at 850 ml/min with distilled water atomized into the gas stream above its condensation temperature. The amount of injected water was measured to calibrate its concentration [3]. Specimen mass change was measured using a Mettler–Toledo model AG245 balance. Every 200 cycles, the specimens without YSZ were characterized by scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometer (EDS) to examine the surface scale adhesion and profilometry (six 1 cm laser line scans or 5, ~ 0.6 × 0.4 mm areas on a Veeco Wyko model NT9100 optical profilometer) to measure roughness. After the cyclic testing was completed, the specimens were metallographically sectioned and examined by light microscopy and electron probe microanalysis (EPMA) using a JEOL model 8200. 3. Results and discussion The average lifetime data is summarized in Fig. 1 including the dry O2 results from the previous study [19]. The onset of massive YSZ spallation defined the coating lifetime. For each group of three YSZcoated specimens, the average is shown with bars marking the standard deviation. As noted previously, the 0% water vapor lifetimes are higher than reported in other studies [21–23], but numerous differences in the coating fabrication, specimen geometry and thermal cycling variables make comparisons difficult. The present comparison with one superalloy substrate batch and the same standard bond coating fabrication processes represents a strongly-controlled test matrix. (The mass change data for the YSZ-coated specimens are not presented as all coated specimen masses decreased due to edge spallation of the YSZ and thus were not good indicators of performance.) The average lifetimes clearly indicate that the addition of water vapor had a more significant effect on the Pt-modified aluminide coatings than on the Pt diffusion coatings. No change in average lifetime was observed among the three environments for the Pt
Fig. 1. Average lifetimes for EB-PVD YSZ-coated N5 specimens exposed in 1 h cycles at 1150 °C in several environments. The bars note the standard deviation for 3 specimens of each type.
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diffusion coatings. For Pt-modified aluminide coatings, the average lifetime was reduced by more than 50% with the addition of 10% water vapor, compared to dry O2. With 90% water vapor, the average lifetime was slightly lower for the Pt-modified aluminide coatings but within the standard deviation. Fig. 2 shows the change in the root mean square roughness (Rq) with exposure time for the specimens without a YSZ coating. The roughness of the specimens exposed to dry O2 was measured using laser line profiles [19], while the other specimens were initially measured using a 3D optical process. As expected, the Pt-modified aluminide coatings “rumpled” [24,25] resulting in higher roughness values than the Pt diffusion coatings. Surprisingly, the rate of roughening was higher in the presence of water vapor, especially 10%, and this difference likely explains the reduced lifetime in the presence of water vapor for the β coatings. The values were so different that the specimens were also measured by laser profilometry for the last measurement (400 h in 10% H2O and 800 h in 90% H2O) to confirm that the two instruments were measuring similar values. There was a slight difference between the techniques but the results were similar. Fig. 3 qualitatively indicates the same result using plan-view SEM images of the aluminide-coated specimens without YSZ. Significant rumpling was observed after only 400, 1-h cycles in 10% water vapor (Fig. 3a) whereas after 400, 1-h cycles in 90% water vapor (Fig. 3b), the surface was not as rumpled. After 800, 1-h cycles in 90% water vapor (Fig. 3c), more rumpling was observed. In all cases, a relatively minor amount of scale spallation was observed, mainly at the coating grain boundaries where the largest deformations tended to occur. Fig. 4 compares cross-sections from failed YSZ-coated specimens with Pt-modified aluminide bond coatings in each environment. As was observed in dry O2 [19], the side of the specimen not coated with YSZ showed more roughness suggesting that the YSZ layer inhibited roughening. (This difference could be attributed to the light grit blasting on one side prior to YSZ deposition, however, roughening the surface has been found to increase deformation [26].) The crosssections only provide a qualitative comparison because not every area is identical. However, the specimen exposed in 10% water vapor appears to have more rumpling after only 420, 1-h cycles (Fig. 4a) than the specimens exposed for 900, 1-h cycles in 90% water vapor or O2 (Fig. 4b and c, respectively). Fig. 5 shows a similar group of specimens but at a higher magnification and compares the morphology with and without the YSZ overlayer. As was observed at lower magnification in Fig. 4, the specimen exposed in water vapor has a much rougher interface after a relatively short exposure time, especially on the side without the YSZ
Fig. 2. Root mean square (RMS) roughness (Rq) for the specimens without a YSZ coating as a function of 1 h cycles at 1150 °C. As expected, the Pt-modified aluminide coatings have a higher roughness than the Pt diffusion coatings. The dashed lines were measured by laser profilometry, the solid lines using optical profilometry.
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40µm
c
b
a
Fig. 3. SEM secondary electron plan view images of Pt-modified aluminide coatings on N5 after (a) 400 h in 10% H2O, (b) 400 h in 90% H2O and (c) 800 h in 90% H2O.
YSZ
250 µm
epoxy
420h in 10% H2O
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900h in 90% H2O
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900h in O2
c
Fig. 4. Light microscopy of polished sections of YSZ-coated N5 specimens with platinum-modified aluminide bond coatings after coating failure at 1150 °C after (a) 420 h in 10% H2O; (b) 900 h in 90% H2O; and (c) 900 h in O2. The YSZ-coated side is at the top in each case.
overlayer. In the light microscopy images, the β and γ′ phases are discernable. As expected, significantly more β phase was retained after only 340, 1-h cycles in 10% water vapor (Fig. 5a) compared to 800, 1-h cycles in 90% water vapor or O2 (Fig. 5b and c, respectively). At the longer exposure times, almost-continuous γ′ layers are evident
epoxy
at both the gas interface and coating-substrate interface, where interdiffusion resulted in the phase transformation from the high Al content β phase to the lower Al content γ′ phase. While the rumpling could be attributed to interdiffusion [25] or the β-γ′ phase transformation (which occurs during each thermal cycle) [27], neither
YSZ
scale
coating
a
340h in 10% H2O
b
800h in 90% H2O
c
800h in O2
epoxy Fig. 5. Light microscopy of polished sections of YSZ-coated N5 specimens with platinum-modified aluminide bond coatings after coating failure at 1150 °C after (a) 340 h in 10% H2O; (b) 800 h in 90% H2O; and (c) 800 h in O2. Higher magnification pictures show the coating beneath the YSZ at the top and the coating on the opposite side of the specimen without a YSZ overlayer at the bottom.
B.A. Pint et al. / Surface & Coatings Technology 206 (2011) 1566–1570
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YSZ
1377h in 10% H2O
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1234h in 90% H2O
b
250 µm
1500h in O2
c
Fig. 6. Light microscopy of polished sections of YSZ-coated N5 specimens with Pt diffusion bond coatings after coating failure at 1150 °C after (a) 1377 h in 10% H2O; (b) 1234 h in 90% H2O; and (c) 1500 h in O2. The YSZ-coated side is at the top in each case.
explanation appears to explain the increased roughening in the presence of water vapor. However, the increased roughening does explain the decrease in coating life observed. Further characterization is needed to compare the scale thickness as a function of exposure time in the three environments. In most cases, the YSZ coating failure appeared to occur at the coating–alumina interface. However, as in Fig. 5a, in some cases, the alumina layer was lost with the YSZ spallation. For the Pt diffusion coatings, the addition of water vapor had only a minor effect on roughness (Fig. 2). Fig. 6 compares some of the failed cross-sections with Pt diffusion coatings. No appreciable surface roughening was observed in any case. Some variation in porosity may be apparent in the specimens but this difference needs to be more carefully studied before concluding this is an effect of the environment. Higher magnification pictures of the surface are shown in Fig. 7 to contrast with the aluminide coatings in Fig. 5. Spallation appeared to occur primarily at the coating–alumina scale interface with the scale attached to the YSZ coating, Fig. 7. Nominally, the scale appeared to be similar in thickness in each environment; however, the variation in exposure time complicated this comparison. Additional characterization is needed to quantify any environment effect on the scale thickness as a function of exposure time. Previously, it was speculated that water vapor would adversely affect the low Al content Pt diffusion coatings (compared to the higher Al content β aluminide coatings), as
more Ni-rich oxide was observed to form in the presence of water vapor at 1100 °C [2–4]. However, these coatings appeared to be immune to the addition of water vapor at 1150 °C in this study. Since this temperature is much higher than the typical (~ 900 °C) metal temperatures in land-based gas turbines, this raises the issue that the high temperature selected to cause coating failure in a reasonable amount of time may also be a temperature where water vapor has a different effect than at lower temperatures. Further work is needed to understand the role of water vapor across a range of temperatures. One effect of water vapor in O2-containing environments is the possible formation of a volatile oxy-hydroxide or hydroxide [7,28]. Such a reaction leads to faster consumption of the scale-forming element due to para-linear (oxidation–evaporation) kinetics. Alumina is much more stable than chromia and more readily forms a hydroxide rather than the chromium oxy-hydroxide. Thus, the reaction of alumina with H2O to form Al(OH)3 should be limited at 1150 °C but higher in 90% water vapor than in 10% water vapor [7]. However, evidence of some evaporation reaction was evident at 1100 °C due to the alumina scale surface morphology [3]. The selection of the 90% water vapor environment produced an environment with only ~2% O2, rather than ~20% with 10% water vapor. To further explore the effect of H2O and O2 containing environments, two more conditions will be evaluated: air with 50% water vapor (~ 10% O2) and 90% water vapor with 10% O2 (O2 as the carrier gas rather than air). Results from these
Fig. 7. Light microscopy of polished sections of YSZ-coated N5 specimens with Pt diffusion bond coatings after coating failure at 1150 °C after (a) 1377 h in 10% H2O; (b) 1234 h in 90% H2O; and (c) 1100 h in O2. Higher magnification pictures show the coating and scale beneath the YSZ at the top and the coating on the opposite side of the specimen without a YSZ overlayer at the bottom.
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environments may produce a clearer picture of the role of water vapor on TBC lifetime. In addition, more focus will be placed on measuring stress in the scale after only a few thermal cycles to determine any difference due to the environment. Diffusion coatings are more widely used in aircraft engines, which are not likely to use syngas or hydrogen fuels. These initial experiments were performed on diffusion coatings as these coatings have been better characterized to date and there is full control of the coating process. The next series of experiments will examine the effect of water vapor on the lifetime of MCrAlY coatings deposited by high velocity oxygen fuel with air-plasma sprayed YSZ, which are more relevant to land-based turbines [29]. 4. Summary Furnace cycling at 1150 °C was used to compare the YSZ coating lifetime in three different environments for two types of diffusion coatings on the same batch of single crystal superalloy substrate. For Pt-diffusion bond coatings, the average EB-PVD YSZ lifetime for three specimens was similar in dry O2 and air with 10% or 90% water vapor. Initial results indicated little difference among the three environments. In contrast, the addition of 10% water vapor resulted in a N50% lifetime reduction for Pt-modified aluminide bond coatings compared to dry O2. However, the lifetime in 90% water vapor was not statistically different than in dry O2. The decrease in lifetime can be attributed to more rapid surface roughening of the Pt-modified aluminide coating in the presence of 10% H2O, which led to more rapid coating failure. However, a mechanism for this observed change in roughening has not been identified. More characterization is needed to further understand these results and additional testing in different H2O and O2 levels will provide more understanding about the role of water vapor on TBC performance. Acknowledgments The authors would like to thank K. M. Cooley and H. Longmire for assistance with the experimental work. B. Nagaraj at General Electric Aircraft Engines coated the specimens with EB-PVD YSZ. P. F. Tortorelli
provided helpful comments on the manuscript. This research was sponsored by the U.S. Department of Energy, Office of Coal and Power R&D, Office of Fossil Energy (R. Dennis program manager). Part of the research was funded by the National Science Foundation-GOALI program under grant no. 0504566 with Tennessee Tech. University. References [1] C. Leyens, K. Fritscher, R. Gehrling, M. Peters, W.A. Kaysser, Surf. Coat. Technol. 82 (1996) 133. [2] K. Onal, M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Mater. High Temp. 20 (2003) 327. [3] B.A. Pint, J.A. Haynes, Y. Zhang, K.L. More, I.G. Wright, Surf. Coat. Technol. 201 (2006) 3852. [4] B.A. Pint, J.A. Haynes, K.L. More, J.H. Schneibel, Y. Zhang, I.G. Wright, in: R.C. Reed, K.A. Green, P. Caron, T.P. Gabb, M.G. Fahrmann, E.S. Huron, S.R. Woodard (Eds.), Superalloys 2008, TMS, Warrendale, PA, 2008, p. 641. [5] J.L. Smialek, Mater. Sci. Forum 595–598 (2008) 191. [6] V. Déneux, Y. Cadoret, S. Hervier, D. Monceau, Oxid. Met. 73 (2010) 83. [7] E.J. Opila, Mater. Sci. Forum 461–464 (2004) 765. [8] B. Jönsson, C. Svedberg, Mater. Sci. Forum 251–254 (1997) 551. [9] W.J. Quadakkers, J. Zurek, M. Hänsel, JOM 61 (7) (2009) 44. [10] B.A. Pint, Y. Zhang, Mater. Corros. 62 (2011) 549. [11] B.A. Pint, M.P. Brady, Y. Yamamoto, M.L. Santella, P.J. Maziasz, W.J. Matthews, J. Eng. Gas Turbines Power 133 (2011) 102302. [12] P. Chiesa, G. Lozza, L. Mazzocchi, J. Eng. Gas Turbines Power 127 (2005) 73. [13] T.B. Gibbons, I.G. Wright, “A Review of Materials for Gas Turbines Firing Syngas Fuels”, Report # ORNL/TM-2009/137, Oak Ridge National Laboratory, Oak Ridge, TN, 2009. [14] L.A. Ruth, Mater. High Temp. 20 (2003) 7. [15] K. Bouhanek, O.A. Adesanya, F.H. Stott, P. Skeldon, D.G. Lees, G.C. Wood, Mater. Sci. Forum 369–372 (2001) 615. [16] Y. Zhang, J.A. Haynes, B.A. Pint, I.G. Wright, Surf. Coat. Technol. 200 (2005) 1259. [17] B. Gleeson, J. Propul. Power 22 (2006) 375. [18] J.A. Haynes, B.A. Pint, Y. Zhang, I.G. Wright, Surf. Coat. Technol. 203 (2008) 413. [19] B.A. Pint, J.A. Haynes, Y. Zhang, Surf. Coat. Technol. 205 (2010) 1236. [20] W.Y. Lee, Y. Zhang, I.G. Wright, B.A. Pint, P.K. Liaw, Metall. Trans. A 29 (1998) 833. [21] R.T. Wu, K. Kawagishi, H. Harada, R.C. Reed, Acta Mater. 56 (2008) 3622. [22] H.M. Tawancy, A.I. Mohamed, N.M. Abbas, R.E. Jones, D.S. Rickerby, J. Mater. Sci. 38 (2003) 3797. [23] U. Schulz, M. Menzebach, C. Leyens, Y.Q. Yang, Surf. Coat. Technol. 146–147 (2001) 117. [24] P. Deb, D.H. Boone, T.F. Manley, J. Vac. Sci. Technol., A 5 (1987) 3366. [25] V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 203 (2009) 3278. [26] I.T. Spitsberg, D.R. Mumm, A.G. Evans, Mater. Sci. Eng. A 394 (2005) 176. [27] B.A. Pint, S.A. Speakman, C.J. Rawn, Y. Zhang, JOM 58 (1) (2006) 47. [28] N. Jacobson, D. Myers, E. Opila, E. Copland, J. Phys. Chem. Solids 66 (2005) 471. [29] Y. Itoh, M. Saitoh, M. Tamura, J. Eng. Gas Turbines Power 122 (2000) 43.
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
Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings B.A. Pint a,⁎, G.W. Garner a, T.M. Lowe a, J.A. Haynes a, Y. Zhang b a b
Materials Science and Technology Division, Oak Ridge National Laboratory, United States Tennessee Technological University, United States
a r t i c l e
i n f o
Available online 17 June 2011 Keywords: Thermal barrier coating Pt diffusion coating Pt aluminide coating Water vapor High temperature oxidation
a b s t r a c t To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150 °C in air with 10 vol.% water vapor (similar to natural gas combustion) and 90 vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttriastabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O2, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Despite the common practice of conducting high temperature oxidation laboratory experiments in dry environments (air or O2), there are relatively few applications that do not contain water vapor, especially for thermal barrier coatings (TBC). Thus, there has been considerable recent interest in the effect of water vapor on oxidation [1–6]. Alumina-forming alloys and coatings are considerably less affected by water vapor than chromia-forming alloys [7–11]. For turbine applications, where the combustion gas of the actual turbine environment contains ~10 vol.% water vapor [12], alumina-forming coatings are standard. The issue of water vapor is a more significant concern for turbines designed for coal-derived synthesis gas (i.e. syngas) or hydrogen fuels where the water vapor content may be higher than natural gas or jet fuel due to the combustion reaction or the use of steam diluents to assist with combustion. Typically, syngas turbines are de-rated (i.e. fired at a lower maximum temperature) due to concerns about component durability [13]. However, this derating is expected to reduce the efficiency of the turbine. The reason for the derating may be related to improper cleanup (or upset conditions) allowing sulfur or ash into the turbine or the higher water vapor content in the combustion gas. While sulfur or ash contamination could be further mitigated, the higher water vapor content is
⁎ Corresponding author. Tel.: + 1 865 576 2897; fax: + 1 865 241 0215. E-mail address: [email protected] (B.A. Pint). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.06.008
determined by the fuel and combustion conditions. Some strategies use very high water vapor levels as an approach for combustion and carbon sequestration [14]. The purpose of this study was to better understand the effect of water vapor on thermal barrier coating (TBC) performance by building on a previous study of average YSZ lifetime with two phase (γ + γ′) Pt diffusion [15–18] and Pt-modified (β-NiPtAl) aluminide bond coatings in furnace cycling at 1150 °C in dry O2 [19]. Another study had looked at alumina scale formation at 1100 °C in 0–90 vol.% water vapor on similar Pt-containing bond coatings and model alloys, however, no YSZ coatings were evaluated [3,4]. The water vapor study found similar results as prior work [2] suggesting increased scale spallation and more transient oxide formation with increasing water content. The current results surprisingly found that water vapor increased the rate of rumpling of aluminide coatings, leading to a reduced average YSZ lifetime. In contrast, the Pt diffusion coatings were unaffected by thermally cycling in air with 10% or 90% water vapor. 2. Experimental procedure Superalloy René N5 (64.7 at.% Ni-8.0% Cr-13.3% Al-0.9% Re-7.6% Co-1.6% W-2.2% Ta-0.9% Mo-132 ppma Zr, 70Y, 540Hf,17S) coupons 1.5 mm thick, ~ 16 mm in diameter and chamfered edges were first grit blasted with alumina and then electroplated with 7 ± 1 μm of Pt in a laboratory scale bath at Tennessee Technological University (TTU). For the Pt diffusion coatings, the plated coupons were then annealed in a vacuum of 10 − 4 Pa (10 − 6 Torr) for 2 h at 1175 °C resulting in a
B.A. Pint et al. / Surface & Coatings Technology 206 (2011) 1566–1570
~ 25 μm thick coating [18]. For Pt-modified aluminide coatings, the plated specimens were aluminized for 6 h at 1100 °C to form a ~ 40 μm thick β layer in a laboratory-scale chemical vapor deposition reactor, described elsewhere [20]. A ~125 μm thick YSZ top coat was deposited on one side of the coupons by electron-beam physical vapor deposition (EB-PVD) using a standard commercial process that included a light grit blasting prior to YSZ deposition. For every group of four specimens to be evaluated at each condition, three were coated with YSZ and the fourth was tested without YSZ (and without grit blasting) to study the surface morphology evolution. The specimens were hung by a Pt–Rh wire in an automated cyclic rig and were exposed for 1 h at 1150 °C in air with 10 ± 1% or 90 ± 2% vol.% water vapor and cooled in laboratory air for 10 min for each cycle. Oxidation in wet air was conducted by flowing air at 850 ml/min with distilled water atomized into the gas stream above its condensation temperature. The amount of injected water was measured to calibrate its concentration [3]. Specimen mass change was measured using a Mettler–Toledo model AG245 balance. Every 200 cycles, the specimens without YSZ were characterized by scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometer (EDS) to examine the surface scale adhesion and profilometry (six 1 cm laser line scans or 5, ~ 0.6 × 0.4 mm areas on a Veeco Wyko model NT9100 optical profilometer) to measure roughness. After the cyclic testing was completed, the specimens were metallographically sectioned and examined by light microscopy and electron probe microanalysis (EPMA) using a JEOL model 8200. 3. Results and discussion The average lifetime data is summarized in Fig. 1 including the dry O2 results from the previous study [19]. The onset of massive YSZ spallation defined the coating lifetime. For each group of three YSZcoated specimens, the average is shown with bars marking the standard deviation. As noted previously, the 0% water vapor lifetimes are higher than reported in other studies [21–23], but numerous differences in the coating fabrication, specimen geometry and thermal cycling variables make comparisons difficult. The present comparison with one superalloy substrate batch and the same standard bond coating fabrication processes represents a strongly-controlled test matrix. (The mass change data for the YSZ-coated specimens are not presented as all coated specimen masses decreased due to edge spallation of the YSZ and thus were not good indicators of performance.) The average lifetimes clearly indicate that the addition of water vapor had a more significant effect on the Pt-modified aluminide coatings than on the Pt diffusion coatings. No change in average lifetime was observed among the three environments for the Pt
Fig. 1. Average lifetimes for EB-PVD YSZ-coated N5 specimens exposed in 1 h cycles at 1150 °C in several environments. The bars note the standard deviation for 3 specimens of each type.
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diffusion coatings. For Pt-modified aluminide coatings, the average lifetime was reduced by more than 50% with the addition of 10% water vapor, compared to dry O2. With 90% water vapor, the average lifetime was slightly lower for the Pt-modified aluminide coatings but within the standard deviation. Fig. 2 shows the change in the root mean square roughness (Rq) with exposure time for the specimens without a YSZ coating. The roughness of the specimens exposed to dry O2 was measured using laser line profiles [19], while the other specimens were initially measured using a 3D optical process. As expected, the Pt-modified aluminide coatings “rumpled” [24,25] resulting in higher roughness values than the Pt diffusion coatings. Surprisingly, the rate of roughening was higher in the presence of water vapor, especially 10%, and this difference likely explains the reduced lifetime in the presence of water vapor for the β coatings. The values were so different that the specimens were also measured by laser profilometry for the last measurement (400 h in 10% H2O and 800 h in 90% H2O) to confirm that the two instruments were measuring similar values. There was a slight difference between the techniques but the results were similar. Fig. 3 qualitatively indicates the same result using plan-view SEM images of the aluminide-coated specimens without YSZ. Significant rumpling was observed after only 400, 1-h cycles in 10% water vapor (Fig. 3a) whereas after 400, 1-h cycles in 90% water vapor (Fig. 3b), the surface was not as rumpled. After 800, 1-h cycles in 90% water vapor (Fig. 3c), more rumpling was observed. In all cases, a relatively minor amount of scale spallation was observed, mainly at the coating grain boundaries where the largest deformations tended to occur. Fig. 4 compares cross-sections from failed YSZ-coated specimens with Pt-modified aluminide bond coatings in each environment. As was observed in dry O2 [19], the side of the specimen not coated with YSZ showed more roughness suggesting that the YSZ layer inhibited roughening. (This difference could be attributed to the light grit blasting on one side prior to YSZ deposition, however, roughening the surface has been found to increase deformation [26].) The crosssections only provide a qualitative comparison because not every area is identical. However, the specimen exposed in 10% water vapor appears to have more rumpling after only 420, 1-h cycles (Fig. 4a) than the specimens exposed for 900, 1-h cycles in 90% water vapor or O2 (Fig. 4b and c, respectively). Fig. 5 shows a similar group of specimens but at a higher magnification and compares the morphology with and without the YSZ overlayer. As was observed at lower magnification in Fig. 4, the specimen exposed in water vapor has a much rougher interface after a relatively short exposure time, especially on the side without the YSZ
Fig. 2. Root mean square (RMS) roughness (Rq) for the specimens without a YSZ coating as a function of 1 h cycles at 1150 °C. As expected, the Pt-modified aluminide coatings have a higher roughness than the Pt diffusion coatings. The dashed lines were measured by laser profilometry, the solid lines using optical profilometry.
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40µm
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Fig. 3. SEM secondary electron plan view images of Pt-modified aluminide coatings on N5 after (a) 400 h in 10% H2O, (b) 400 h in 90% H2O and (c) 800 h in 90% H2O.
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Fig. 4. Light microscopy of polished sections of YSZ-coated N5 specimens with platinum-modified aluminide bond coatings after coating failure at 1150 °C after (a) 420 h in 10% H2O; (b) 900 h in 90% H2O; and (c) 900 h in O2. The YSZ-coated side is at the top in each case.
overlayer. In the light microscopy images, the β and γ′ phases are discernable. As expected, significantly more β phase was retained after only 340, 1-h cycles in 10% water vapor (Fig. 5a) compared to 800, 1-h cycles in 90% water vapor or O2 (Fig. 5b and c, respectively). At the longer exposure times, almost-continuous γ′ layers are evident
epoxy
at both the gas interface and coating-substrate interface, where interdiffusion resulted in the phase transformation from the high Al content β phase to the lower Al content γ′ phase. While the rumpling could be attributed to interdiffusion [25] or the β-γ′ phase transformation (which occurs during each thermal cycle) [27], neither
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epoxy Fig. 5. Light microscopy of polished sections of YSZ-coated N5 specimens with platinum-modified aluminide bond coatings after coating failure at 1150 °C after (a) 340 h in 10% H2O; (b) 800 h in 90% H2O; and (c) 800 h in O2. Higher magnification pictures show the coating beneath the YSZ at the top and the coating on the opposite side of the specimen without a YSZ overlayer at the bottom.
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Fig. 6. Light microscopy of polished sections of YSZ-coated N5 specimens with Pt diffusion bond coatings after coating failure at 1150 °C after (a) 1377 h in 10% H2O; (b) 1234 h in 90% H2O; and (c) 1500 h in O2. The YSZ-coated side is at the top in each case.
explanation appears to explain the increased roughening in the presence of water vapor. However, the increased roughening does explain the decrease in coating life observed. Further characterization is needed to compare the scale thickness as a function of exposure time in the three environments. In most cases, the YSZ coating failure appeared to occur at the coating–alumina interface. However, as in Fig. 5a, in some cases, the alumina layer was lost with the YSZ spallation. For the Pt diffusion coatings, the addition of water vapor had only a minor effect on roughness (Fig. 2). Fig. 6 compares some of the failed cross-sections with Pt diffusion coatings. No appreciable surface roughening was observed in any case. Some variation in porosity may be apparent in the specimens but this difference needs to be more carefully studied before concluding this is an effect of the environment. Higher magnification pictures of the surface are shown in Fig. 7 to contrast with the aluminide coatings in Fig. 5. Spallation appeared to occur primarily at the coating–alumina scale interface with the scale attached to the YSZ coating, Fig. 7. Nominally, the scale appeared to be similar in thickness in each environment; however, the variation in exposure time complicated this comparison. Additional characterization is needed to quantify any environment effect on the scale thickness as a function of exposure time. Previously, it was speculated that water vapor would adversely affect the low Al content Pt diffusion coatings (compared to the higher Al content β aluminide coatings), as
more Ni-rich oxide was observed to form in the presence of water vapor at 1100 °C [2–4]. However, these coatings appeared to be immune to the addition of water vapor at 1150 °C in this study. Since this temperature is much higher than the typical (~ 900 °C) metal temperatures in land-based gas turbines, this raises the issue that the high temperature selected to cause coating failure in a reasonable amount of time may also be a temperature where water vapor has a different effect than at lower temperatures. Further work is needed to understand the role of water vapor across a range of temperatures. One effect of water vapor in O2-containing environments is the possible formation of a volatile oxy-hydroxide or hydroxide [7,28]. Such a reaction leads to faster consumption of the scale-forming element due to para-linear (oxidation–evaporation) kinetics. Alumina is much more stable than chromia and more readily forms a hydroxide rather than the chromium oxy-hydroxide. Thus, the reaction of alumina with H2O to form Al(OH)3 should be limited at 1150 °C but higher in 90% water vapor than in 10% water vapor [7]. However, evidence of some evaporation reaction was evident at 1100 °C due to the alumina scale surface morphology [3]. The selection of the 90% water vapor environment produced an environment with only ~2% O2, rather than ~20% with 10% water vapor. To further explore the effect of H2O and O2 containing environments, two more conditions will be evaluated: air with 50% water vapor (~ 10% O2) and 90% water vapor with 10% O2 (O2 as the carrier gas rather than air). Results from these
Fig. 7. Light microscopy of polished sections of YSZ-coated N5 specimens with Pt diffusion bond coatings after coating failure at 1150 °C after (a) 1377 h in 10% H2O; (b) 1234 h in 90% H2O; and (c) 1100 h in O2. Higher magnification pictures show the coating and scale beneath the YSZ at the top and the coating on the opposite side of the specimen without a YSZ overlayer at the bottom.
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environments may produce a clearer picture of the role of water vapor on TBC lifetime. In addition, more focus will be placed on measuring stress in the scale after only a few thermal cycles to determine any difference due to the environment. Diffusion coatings are more widely used in aircraft engines, which are not likely to use syngas or hydrogen fuels. These initial experiments were performed on diffusion coatings as these coatings have been better characterized to date and there is full control of the coating process. The next series of experiments will examine the effect of water vapor on the lifetime of MCrAlY coatings deposited by high velocity oxygen fuel with air-plasma sprayed YSZ, which are more relevant to land-based turbines [29]. 4. Summary Furnace cycling at 1150 °C was used to compare the YSZ coating lifetime in three different environments for two types of diffusion coatings on the same batch of single crystal superalloy substrate. For Pt-diffusion bond coatings, the average EB-PVD YSZ lifetime for three specimens was similar in dry O2 and air with 10% or 90% water vapor. Initial results indicated little difference among the three environments. In contrast, the addition of 10% water vapor resulted in a N50% lifetime reduction for Pt-modified aluminide bond coatings compared to dry O2. However, the lifetime in 90% water vapor was not statistically different than in dry O2. The decrease in lifetime can be attributed to more rapid surface roughening of the Pt-modified aluminide coating in the presence of 10% H2O, which led to more rapid coating failure. However, a mechanism for this observed change in roughening has not been identified. More characterization is needed to further understand these results and additional testing in different H2O and O2 levels will provide more understanding about the role of water vapor on TBC performance. Acknowledgments The authors would like to thank K. M. Cooley and H. Longmire for assistance with the experimental work. B. Nagaraj at General Electric Aircraft Engines coated the specimens with EB-PVD YSZ. P. F. Tortorelli
provided helpful comments on the manuscript. This research was sponsored by the U.S. Department of Energy, Office of Coal and Power R&D, Office of Fossil Energy (R. Dennis program manager). Part of the research was funded by the National Science Foundation-GOALI program under grant no. 0504566 with Tennessee Tech. University. References [1] C. Leyens, K. Fritscher, R. Gehrling, M. Peters, W.A. Kaysser, Surf. Coat. Technol. 82 (1996) 133. [2] K. Onal, M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Mater. High Temp. 20 (2003) 327. [3] B.A. Pint, J.A. Haynes, Y. Zhang, K.L. More, I.G. Wright, Surf. Coat. Technol. 201 (2006) 3852. [4] B.A. Pint, J.A. Haynes, K.L. More, J.H. Schneibel, Y. Zhang, I.G. Wright, in: R.C. Reed, K.A. Green, P. Caron, T.P. Gabb, M.G. Fahrmann, E.S. Huron, S.R. Woodard (Eds.), Superalloys 2008, TMS, Warrendale, PA, 2008, p. 641. [5] J.L. Smialek, Mater. Sci. Forum 595–598 (2008) 191. [6] V. Déneux, Y. Cadoret, S. Hervier, D. Monceau, Oxid. Met. 73 (2010) 83. [7] E.J. Opila, Mater. Sci. Forum 461–464 (2004) 765. [8] B. Jönsson, C. Svedberg, Mater. Sci. Forum 251–254 (1997) 551. [9] W.J. Quadakkers, J. Zurek, M. Hänsel, JOM 61 (7) (2009) 44. [10] B.A. Pint, Y. Zhang, Mater. Corros. 62 (2011) 549. [11] B.A. Pint, M.P. Brady, Y. Yamamoto, M.L. Santella, P.J. Maziasz, W.J. Matthews, J. Eng. Gas Turbines Power 133 (2011) 102302. [12] P. Chiesa, G. Lozza, L. Mazzocchi, J. Eng. Gas Turbines Power 127 (2005) 73. [13] T.B. Gibbons, I.G. Wright, “A Review of Materials for Gas Turbines Firing Syngas Fuels”, Report # ORNL/TM-2009/137, Oak Ridge National Laboratory, Oak Ridge, TN, 2009. [14] L.A. Ruth, Mater. High Temp. 20 (2003) 7. [15] K. Bouhanek, O.A. Adesanya, F.H. Stott, P. Skeldon, D.G. Lees, G.C. Wood, Mater. Sci. Forum 369–372 (2001) 615. [16] Y. Zhang, J.A. Haynes, B.A. Pint, I.G. Wright, Surf. Coat. Technol. 200 (2005) 1259. [17] B. Gleeson, J. Propul. Power 22 (2006) 375. [18] J.A. Haynes, B.A. Pint, Y. Zhang, I.G. Wright, Surf. Coat. Technol. 203 (2008) 413. [19] B.A. Pint, J.A. Haynes, Y. Zhang, Surf. Coat. Technol. 205 (2010) 1236. [20] W.Y. Lee, Y. Zhang, I.G. Wright, B.A. Pint, P.K. Liaw, Metall. Trans. A 29 (1998) 833. [21] R.T. Wu, K. Kawagishi, H. Harada, R.C. Reed, Acta Mater. 56 (2008) 3622. [22] H.M. Tawancy, A.I. Mohamed, N.M. Abbas, R.E. Jones, D.S. Rickerby, J. Mater. Sci. 38 (2003) 3797. [23] U. Schulz, M. Menzebach, C. Leyens, Y.Q. Yang, Surf. Coat. Technol. 146–147 (2001) 117. [24] P. Deb, D.H. Boone, T.F. Manley, J. Vac. Sci. Technol., A 5 (1987) 3366. [25] V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 203 (2009) 3278. [26] I.T. Spitsberg, D.R. Mumm, A.G. Evans, Mater. Sci. Eng. A 394 (2005) 176. [27] B.A. Pint, S.A. Speakman, C.J. Rawn, Y. Zhang, JOM 58 (1) (2006) 47. [28] N. Jacobson, D. Myers, E. Opila, E. Copland, J. Phys. Chem. Solids 66 (2005) 471. [29] Y. Itoh, M. Saitoh, M. Tamura, J. Eng. Gas Turbines Power 122 (2000) 43.