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The effect of water vapor on the oxidation behavior of Ni–Pt–Al coatings and alloys
Surface & Coatings Technology 201 (2006) 3852 – 3856 www.elsevier.com/locate/surfcoat
The effect of water vapor on the oxidation behavior of Ni–Pt–Al coatings and alloys B.A. Pint a,⁎, J.A. Haynes a , Y. Zhang b , K.L. More a , I.G. Wright a a
Metals and Ceramics Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, P. O. Box 2008, M.S. 6156, Oak Ridge, TN 37831-6156, United States b Department of Mech. Eng., Tennessee Technological University, P. O. Box 5014, Cookeville, TN 38505-0001, United States Available online 12 September 2006
Abstract Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the combustion gas than natural gas fired turbines. The effect of increased water vapor on alumina-forming coatings and model alloys was investigated at 1100 °C in furnace cyclic testing. Increasing the water vapor content from 10% to 50 vol.% increased the amount of scale spallation on undoped aluminaforming alloys. Compared to dry O2, increased spallation was observed for β and γ/γ' phase coatings on the substrates of alloys 142 and N5. In all cases, the addition of water vapor appeared to reduce the formation of alumina whiskers and ridges at the scale–gas interface, but did not significantly change the alumina growth rate. The addition of water vapor may have a detrimental effect on the selective oxidation of Al in γ/γ' alloys and coatings. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum oxide; Water vapor; High temperature oxidation; Platinum; Chemical vapor deposition; TEM
1. Introduction With the recent increase in natural gas prices, there is intense interest in the use of syngas [1] or hydrogen derived from coal gasification [2] to fire combined-cycle power plants. Both scenarios will increase the water vapor content of the combustion gas at the turbine inlet from 10–15% for natural gas to ∼60% for hydrogen. While current oxidation-resistant coatings operate at the lower H2O levels, there is concern that the higher H2O levels will reduce performance, particularly of thermal barrier coatings (TBC). Furthermore, new bond coating compositions are being investigated, such as the high Pt, low Al, γ/γ' coatings, [3–5] and their performance in humid atmospheres has not been studied. Oxidation resistant bond coatings for first-stage blades and vanes in state-of-the-art turbines form alumina scales. Thermodynamically, alumina is less affected by water vapor than are chromia or silica scales. [6] Previous experimental work has examined the role of water vapor on bare superalloys, [7–11] bond coatings (aluminide [7–9] and MCrAlY [12]) and other alumina-formers. [12–17] Most of those studies examined ⁎ 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 © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.244
water vapor contents of ≤10% and the main effects were increased scale spallation for spallation-prone substrates, faster oxidation and more transient or base–alloy oxide formation. Results using 50% H2O in air [8] suggested that these effects of water vapor increased with increasing H2O content. The effects on transient oxidation are of particular concern for γ/γ' coatings, which rely on high Pt levels to allow these relatively low Al content coatings to form alumina scales with little Nirich oxide formation. [4,18] This paper provides some initial observations on the oxidation behavior, in air + 10% and 50% water vapor, of β and γ/γ' coatings on single crystal (SX) and directionally solidified (DS) superalloy substrates and model alloys, which include NiAl, γ/γ' and NiCrAlYHf compositions. 2. Experimental procedure All of the model alloys were inductively melted and cast at Oak Ridge National Laboratory using a chilled copper mold and annealed for 4 h at 1300 °C in a sealed quartz ampoule or at ∼10− 4 Pa vacuum. As-cast alloy compositions (atomic percent is used throughout) are given in Table 1. Coupons ≈15 mm in diameter and 1–1.5 mm thick were cut and polished to a 0.3 μm finish. Simple β, Pt-modified β and simple γ/γ' coatings on
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Table 1 Chemical compositions of the cast alloys in atomic percent determined by induction coupled plasma analysis and combustion analysis
Ni–48Al + Hf Ni–39Al–5Pt Ni–50Al–5Pt Ni–50Al Ni–22Al–10Pt + 0.4Hf NiCrAlYHf Alloy N5 Alloy 142
Ni
Al
Hf
51.6 55.8 45.1 49.9 68.1 61.4 64.5 59.5
48.3 38.7 49.6 50.1 21.7 18.9 13.7 13.9
0.056 b0.01 b0.01 b0.01 0.393 0.046 0.047 0.518
Pt 5.6 5.2 9.7
Cr
C (ppm)
S (ppm)
Other
b0.01 b0.01 0.05 b0.01 0.01 19.5 8.0 7.8
340 270 420 360 120 420 2490 6000
b4 b4 9 b4 b4 5 6 11
0.007Zr 0.006Y, 0.02Si 7.1Co, 2.2Ta, 1.6W, 0.9Re, 0.9Mo, 0.007Y 11.9Co, 2.1Ta, 1.5W, 0.9Re, 0.9Mo
similar sized coupons of René N5 (SX) and 142 (DS) were fabricated by chemical vapor deposition (β coatings) or vacuum annealing (γ/γ' coatings), as described previously. [5,19] The β-phase coatings contained ∼40% Al/∼ 5% Pt and the simple γ/γ' coatings ∼18% Al/∼ 18% Pt. Cyclic oxidation experiments were conducted at 1100 °C in an automated test rig, where the specimens were hung by Pt–Rh wire in flowing dry O2 or wet air environment, and subjected to thermal cycles of 1 h at temperature and 10 min cooling. [20] 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. Water contents of 10 ± 1 and 50 ± 2 vol.% were used for these experiments. After oxidation, specimens were characterized by field emission gun scanning electron microscopy (SEM) and electron microprobe analysis (EPMA). They were coated with Cu prior to sectioning for metallography. Selected specimens were thinned by focused ion-beam to be examined by transmission electron microscopy (TEM).
agreement with prior work, [7–9,11,16] little effect was observed for adherent alumina formers such as Ni–48Al + Hf and NiCrAlYHf. Unlike Ni–48Al + Hf, the small mass losses for the NiCrAlYHf specimens after 1000 cycles coincide with observed scale spallation, Fig. 2a, because of their higher thermal expansion coefficient compared to NiAl. [21] The undoped (i.e. no Y or Hf addition) castings, Ni–50Al and Ni–39Al–5Pt, showed increased spallation in humid air, while Ni–50Al–5Pt formed an adherent oxide with a slightly higher mass gain with 10% H2O. As there was little effect with 10% H2O, characterization focused mainly on the air + 50% H2O environment. Fig. 3 shows the summaries for some of the model alloys. The Hf- and Ydoped alumina formers showed only a slightly higher mass gain
3. Results and discussion Fig. 1 shows the results for model alloys tested at 1100 °C in air + 10% H2O compared to specimens tested in dry O2. In
Fig. 1. Specimen mass changes for various cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 10% H2O (dashed lines).
Fig. 2. SEM secondary electron plan view images of the alumina scale formed after exposures at 1100 °C (a) NiCrAlYHf: 1000 h in 10% H2O (b) Ni–50Al– 5Pt: 2000 h in O2; and (c) Ni–50Al–5Pt: 1000 h in 10% H2O.
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Fig. 4. SEM secondary electron plan view images of the alumina scale formed on Ni–48Al + Hf oxidized for 1000, 1 h cycles at 1100 °C in (a) dry O2 and (b) air + 50% H2O.
the scale was also observed in dry O2, and was attributed to that fraction of the scale which initially formed as θ–Al2O3 and transformed to α–Al2O3. [22,24] The larger porosity near the gas interface of the scale formed on NiCrAlYHf (Fig. 5b) also was similar to that observed in dry O2, and is associated with the
Fig. 3. Specimen mass changes for various cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 10% or 50% H2O (dashed lines).
in air + 50% H2O, Fig. 3a. The higher water vapor content increased the amount of spallation for undoped Ni–50Al, Fig. 3b. A lower mass gain was observed for Ni–50Al–5Pt which may indicate increased spallation particularly after 1000 cycles. For all of the undoped alloys, whiskers were typically observed at the scale–gas interface when oxidized in dry O2, e.g. Fig. 2b, similar to previous observations. [22,23] However, no whiskers were observed in the 10% or 50% H2O environments, e.g. Fig. 2c. Similarly, fine ridges were typical at oxide grain boundaries on Hf-doped Ni–48Al, [22–24] Fig. 4a, but were absent with the addition of water vapor, e.g. Fig. 4b. These observations likely reflect a small amount of Al(OH)3 evaporation [25] at the scale–gas interface. In order to further study the effect of water vapor on the alumina scale, TEM cross-sections were made on the scale formed on Ni–48Al + Hf and NiCrAlYHf after 100, 1 h cycles in 50% H2O, Fig. 5. The microstructures were not significantly different than those observed in previous work in dry O2. [26,27] The scale on Ni–48Al + Hf exhibited a slightly finer grain width (0.5 μm) in humid air (Fig. 5a), and no ridges were observed at the gas interface, consistent with the plan view observation, Fig. 4b. The fine round porosity in the outer part of
Fig. 5. TEM bright field images of the scales formed after 100, 1 h cycles at 1100 °C in air + 50% H2O on (a) Ni–48Al + Hf and (b) NiCrAlYHf.
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thin Ni-rich transient oxide that formed on this material. The underlying columnar alumina microstructure appeared unaffected by the addition of water vapor. Fig. 6 shows the mass change results for the coated substrates in dry O2 and air + 50% H2O compared to some of the model alloys. In general, all of the coatings on N5 behaved similarly to Ni–48Al + Hf, with little effect of water vapor on mass change. This is not surprising since the alumina grain boundaries formed on Pt-modified aluminide coatings have been shown to contain segregated Hf ions. [28] A higher mass gain was observed for the simple γ/γ' coating on alloy 142 in dry O2, and the highest mass gain was observed for the same coating with the addition of water vapor, Fig. 6. A similar increase was observed for cast Ni–22Al–10Pt–0.4Hf. As these simple γ/γ' coatings and castings have relatively low Al contents, the addition of water vapor may have affected the selective oxidation of Al, as has been reported previously. [8,12,16] However, a similar effect was not observed for the γ/γ' coating on an N5 substrate, Fig. 6. The lowest mass gain was observed for a CVD Pt-modified β-aluminide coating on alloy 142 exposed in dry O2. This behavior may be associated with the higher Hf content in the 142 substrate compared to N5 (Table 1) resulting in a more optimized Hf content in the scale, and a greater reduction in the alumina scale growth rate. [23,28,29] After 200, 1 h cycles, the scale on this substrate was very adherent, Fig. 7a, similar to the other coatings in dry O2. However, a similar specimen exposed to air + 50% H2O showed spallation, Fig. 7b, although not enough to show a mass loss, Fig. 6. Similar spallation behavior was noted on the other coatings exposed to water vapor. Longer testing may reveal a more distinct mass change difference between the two environments. Because of the suppression of whisker and ridge formation on the cast materials in the presence of water vapor, Figs. 2 and 4,
Fig. 6. Specimen mass changes for various β-aluminide (simple or Pt-modified) or γ/γ' (+Pt label) coatings and cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 50% H2O (dashed lines). One CVD PtAl specimen substrate was a version of N5 without Y (N5−).
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Fig. 7. SEM secondary electron plan view images of the scale formed on coatings after exposure in 1 h cycles at 1100 °C; (a) CVD Pt-modified aluminide on 142 after 200 h in dry O2; (b) CVD Pt-modidfied aluminide on 142 after 200 h in air + 50% H2O; and (c) simple γ/γ' coating on N5 after 400 h in air + 50% H2O.
the morphology of the scale formed on the coatings was examined. In each case, the alumina scales formed in air + 50% H2O showed no ridge formation, e.g. Fig. 7c. Ridge formation was observed on the coatings exposed in dry O2 and typically is observed on alumina scales formed on aluminide coatings. [19,28,30,31] Thus, there appears to be a consistent minor modification of the alumina formation in the presence of water vapor. Previous work suggests that this difference was not due to an effect of oxidation in air rather than O2. [20] While the increase in scale spallation from the coated superalloys was minor, extrapolating to longer times suggests that decreased TBC lifetime (due to alumina scale spallation beneath the zirconia top coat) may be expected in environments with higher water vapor contents, such as syngas- or hydrogenfired turbines. The results for cast materials suggest that reactive element (RE) doping, which is well-known to improve scale adhesion, [29] significantly reduces the detrimental effect of water vapor observed in undoped alumina-formers, Fig. 3. Optimization of RE doping in coatings may be critical to improving coating performance in the presence of high water vapor contents. While Hf- and Zr-doping has been attempted with some success for CVD aluminide coatings, [32,33] commercial coatings are still being developed. Coatings based on γ/γ' may have an advantage due to their higher RE solubility. [4] However, the effect of water vapor on the
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transient oxidation behavior of these low Al content coatings needs to be further characterized. Formation of a Ni-rich layer oxide layer at the gas interface, particularly as the coatings are depleted in Pt due to interdiffusion, [5] would form a weak link for TBC adhesion due to porosity formation at the interface with alumina. [34,35] 4. Summary Alumina forming alloys and coatings were cyclicly oxidized at 1100 °C in air + 10% H2O and 50% H2O and the results were compared to the behavior in dry O2. For Y- and Hf-doped alloys, the effect of water vapor was minor with no increase in the scale growth rate. For undoped alumina-formers with less spallation resistance, the amount of spallation increased with increasing water vapor content. Both with and without dopants, the addition of water vapor reduced the amount of alumina whiskers and ridges at the scale–gas interface. For β- and γ/γ'phase coatings on alloy N5 or 142 substrates, the addition of water vapor appeared to cause some increased spallation compared to exposures in dry O2, and may have a detrimental effect on the selective oxidation of Al in γ/γ' coatings. Longer exposures are needed to clarify the effects of higher water vapor contents of the coated superalloys. Acknowledgement The author would like to thank J. Moser, K. Cooley, J. Vought, H. Longmire, L. Walker, T. Brummett and K. S. Reeves for assistance with the experimental work. M. P. Brady, S. Dryepondt and 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 and D, Office of Fossil Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. References [1] “FutureGen — A Sequestration and Hydrogen Research Initiative,” U. S. Department of Energy, Washington, DC 2003. www.fossil.energy.gov/ programs/powersystems/futuregen/futuregen_factsheet.pdf. [2] P. Chiesa, G. Lozza, L. Mazzocchi, J. Eng. Gas Turbine Power 127 (2005) 73. [3] K. Bouhanek, O.A. Adesanya, F.H. Stott, P. Skeldon, D.G. Lees, G.C. Wood, Mater. Sci. Forum 369–372 (2001) 615.
[4] B. Gleeson, W. Wang, S. Hayashi, D. Sordelet, Mater. Sci. Forum 461–464 (2004) 213. [5] Y. Zhang, J.A. Haynes, B.A. Pint, I.G. Wright, Surf. Coat. Technol. 200 (2005) 1259. [6] E.J. Opila, Mater. Sci. Forum 461–464 (2004) 765. [7] R. Janakiraman, G.H. Meier, F.S. Pettit, Met. Mater. Trans. 30A (1999) 2905. [8] M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Met. Mater. Trans. 34A (2003) 2609. [9] K. Onal, M.C. Maris-Sida, G.H. Meier, F.S. Pettit, Mater. High Temp. 20 (2003) 327. [10] M.A. Smith, W.E. Frazier, B.A. Pregger, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 203 (1995) 388. [11] J.L. Smialek, JOM 58 (1) (2006) 29. [12] C. Leyens, K. Fritscher, R. Gehrling, M. Peters, W.A. Kaysser, Surf. Coat. Technol. 82 (1996) 133. [13] I. Kvernes, M. Olivera, P. Kofstad, Corros. Sci. 17 (1977) 237. [14] H. Buscail, S. Heinze, P. Dufour, J.P. Larpin, Oxid. Met. 47 (1997) 445. [15] H. Al-Badairy, G.J. Tatlock, Mater. High Temp. 17 (2000) 133. [16] B.A. Pint, K.L. More, P.F. Tortorelli, ASME Paper #2002-GT-30543, Presented at the International Gas Turbine and Aeroengine Congress and Exhibition, Amsterdam, Netherlands, June 3–6, 2002. [17] J.R. Regina, J.N. DuPont, A.R. Marder, Oxid. Met. 61 (2004) 69. [18] B.A. Pint, Surf. Coat. Technol. 188–189 (2004) 71. [19] Y. Zhang, W.Y. Lee, J.A. Haynes, I.G. Wright, B.A. Pint, K.M. Cooley, P.K. Liaw, Metall. Trans., A, Phys. Metall. Mater. Sci. 30 (1999) 2679. [20] B.A. Pint, P.F. Tortorelli, I.G. Wright, Oxid. Met. 58 (2002) 73. [21] J.A. Haynes, B.A. Pint, W.D. Porter, I.G. Wright, Mater. High Temp. 21 (2004) 87. [22] B.A. Pint, M. Treska, L.W. Hobbs, Oxid. Met. 47 (1997) 1. [23] B.A. Pint, Oxid. Met. 49 (1998) 531. [24] J. Doychak, in: J.H. Westbrook, R.L. Fleischer (Eds.), Intermetallic Compounds, Vol. 1: Principles, John Wiley & Sons, New York, NY, 1994, p. 977. [25] E.J. Opila, D.L. Myers, J. Am. Ceram. Soc. 87 (2004) 1701. [26] B.A. Pint, K.L. More, I.G. Wright, P.F. Tortorelli, Mater. High Temp. 17 (2000) 165. [27] B.A. Pint, K.L. More, I.G. Wright, Mater. High Temp. 20 (2003) 375. [28] J.A. Haynes, B.A. Pint, K.L. More, Y. Zhang, I.G. Wright, Oxid. Met. 58 (2002) 513. [29] B.A. Pint, Oxid. Met. 45 (1996) 1. [30] V.K. Tolpygo, D.R. Clarke, Mater. High Temp. 17 (2000) 59. [31] V.K. Tolpygo, D.R. Clarke, Acta Mater. 52 (2004) 5129. [32] B.M. Warnes, Surf. Coat. Technol. 146–147 (2001) 7. [33] S. Naveos, G. Oberlaender, Y. Cadoret, P. Josso, M.P. Bacos, Mater. Sci. Forum 461–464 (2004) 375. [34] J. Doychak, M. Rühle, Oxid. Met. 31 (1989) 431. [35] B.A. Pint, A.J. Garratt-Reed, L.W. Hobbs, Oxid. Met. 56 (2001) 119.
The effect of water vapor on the oxidation behavior of Ni–Pt–Al coatings and alloys B.A. Pint a,⁎, J.A. Haynes a , Y. Zhang b , K.L. More a , I.G. Wright a a
Metals and Ceramics Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, P. O. Box 2008, M.S. 6156, Oak Ridge, TN 37831-6156, United States b Department of Mech. Eng., Tennessee Technological University, P. O. Box 5014, Cookeville, TN 38505-0001, United States Available online 12 September 2006
Abstract Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the combustion gas than natural gas fired turbines. The effect of increased water vapor on alumina-forming coatings and model alloys was investigated at 1100 °C in furnace cyclic testing. Increasing the water vapor content from 10% to 50 vol.% increased the amount of scale spallation on undoped aluminaforming alloys. Compared to dry O2, increased spallation was observed for β and γ/γ' phase coatings on the substrates of alloys 142 and N5. In all cases, the addition of water vapor appeared to reduce the formation of alumina whiskers and ridges at the scale–gas interface, but did not significantly change the alumina growth rate. The addition of water vapor may have a detrimental effect on the selective oxidation of Al in γ/γ' alloys and coatings. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum oxide; Water vapor; High temperature oxidation; Platinum; Chemical vapor deposition; TEM
1. Introduction With the recent increase in natural gas prices, there is intense interest in the use of syngas [1] or hydrogen derived from coal gasification [2] to fire combined-cycle power plants. Both scenarios will increase the water vapor content of the combustion gas at the turbine inlet from 10–15% for natural gas to ∼60% for hydrogen. While current oxidation-resistant coatings operate at the lower H2O levels, there is concern that the higher H2O levels will reduce performance, particularly of thermal barrier coatings (TBC). Furthermore, new bond coating compositions are being investigated, such as the high Pt, low Al, γ/γ' coatings, [3–5] and their performance in humid atmospheres has not been studied. Oxidation resistant bond coatings for first-stage blades and vanes in state-of-the-art turbines form alumina scales. Thermodynamically, alumina is less affected by water vapor than are chromia or silica scales. [6] Previous experimental work has examined the role of water vapor on bare superalloys, [7–11] bond coatings (aluminide [7–9] and MCrAlY [12]) and other alumina-formers. [12–17] Most of those studies examined ⁎ 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 © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.244
water vapor contents of ≤10% and the main effects were increased scale spallation for spallation-prone substrates, faster oxidation and more transient or base–alloy oxide formation. Results using 50% H2O in air [8] suggested that these effects of water vapor increased with increasing H2O content. The effects on transient oxidation are of particular concern for γ/γ' coatings, which rely on high Pt levels to allow these relatively low Al content coatings to form alumina scales with little Nirich oxide formation. [4,18] This paper provides some initial observations on the oxidation behavior, in air + 10% and 50% water vapor, of β and γ/γ' coatings on single crystal (SX) and directionally solidified (DS) superalloy substrates and model alloys, which include NiAl, γ/γ' and NiCrAlYHf compositions. 2. Experimental procedure All of the model alloys were inductively melted and cast at Oak Ridge National Laboratory using a chilled copper mold and annealed for 4 h at 1300 °C in a sealed quartz ampoule or at ∼10− 4 Pa vacuum. As-cast alloy compositions (atomic percent is used throughout) are given in Table 1. Coupons ≈15 mm in diameter and 1–1.5 mm thick were cut and polished to a 0.3 μm finish. Simple β, Pt-modified β and simple γ/γ' coatings on
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Table 1 Chemical compositions of the cast alloys in atomic percent determined by induction coupled plasma analysis and combustion analysis
Ni–48Al + Hf Ni–39Al–5Pt Ni–50Al–5Pt Ni–50Al Ni–22Al–10Pt + 0.4Hf NiCrAlYHf Alloy N5 Alloy 142
Ni
Al
Hf
51.6 55.8 45.1 49.9 68.1 61.4 64.5 59.5
48.3 38.7 49.6 50.1 21.7 18.9 13.7 13.9
0.056 b0.01 b0.01 b0.01 0.393 0.046 0.047 0.518
Pt 5.6 5.2 9.7
Cr
C (ppm)
S (ppm)
Other
b0.01 b0.01 0.05 b0.01 0.01 19.5 8.0 7.8
340 270 420 360 120 420 2490 6000
b4 b4 9 b4 b4 5 6 11
0.007Zr 0.006Y, 0.02Si 7.1Co, 2.2Ta, 1.6W, 0.9Re, 0.9Mo, 0.007Y 11.9Co, 2.1Ta, 1.5W, 0.9Re, 0.9Mo
similar sized coupons of René N5 (SX) and 142 (DS) were fabricated by chemical vapor deposition (β coatings) or vacuum annealing (γ/γ' coatings), as described previously. [5,19] The β-phase coatings contained ∼40% Al/∼ 5% Pt and the simple γ/γ' coatings ∼18% Al/∼ 18% Pt. Cyclic oxidation experiments were conducted at 1100 °C in an automated test rig, where the specimens were hung by Pt–Rh wire in flowing dry O2 or wet air environment, and subjected to thermal cycles of 1 h at temperature and 10 min cooling. [20] 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. Water contents of 10 ± 1 and 50 ± 2 vol.% were used for these experiments. After oxidation, specimens were characterized by field emission gun scanning electron microscopy (SEM) and electron microprobe analysis (EPMA). They were coated with Cu prior to sectioning for metallography. Selected specimens were thinned by focused ion-beam to be examined by transmission electron microscopy (TEM).
agreement with prior work, [7–9,11,16] little effect was observed for adherent alumina formers such as Ni–48Al + Hf and NiCrAlYHf. Unlike Ni–48Al + Hf, the small mass losses for the NiCrAlYHf specimens after 1000 cycles coincide with observed scale spallation, Fig. 2a, because of their higher thermal expansion coefficient compared to NiAl. [21] The undoped (i.e. no Y or Hf addition) castings, Ni–50Al and Ni–39Al–5Pt, showed increased spallation in humid air, while Ni–50Al–5Pt formed an adherent oxide with a slightly higher mass gain with 10% H2O. As there was little effect with 10% H2O, characterization focused mainly on the air + 50% H2O environment. Fig. 3 shows the summaries for some of the model alloys. The Hf- and Ydoped alumina formers showed only a slightly higher mass gain
3. Results and discussion Fig. 1 shows the results for model alloys tested at 1100 °C in air + 10% H2O compared to specimens tested in dry O2. In
Fig. 1. Specimen mass changes for various cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 10% H2O (dashed lines).
Fig. 2. SEM secondary electron plan view images of the alumina scale formed after exposures at 1100 °C (a) NiCrAlYHf: 1000 h in 10% H2O (b) Ni–50Al– 5Pt: 2000 h in O2; and (c) Ni–50Al–5Pt: 1000 h in 10% H2O.
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Fig. 4. SEM secondary electron plan view images of the alumina scale formed on Ni–48Al + Hf oxidized for 1000, 1 h cycles at 1100 °C in (a) dry O2 and (b) air + 50% H2O.
the scale was also observed in dry O2, and was attributed to that fraction of the scale which initially formed as θ–Al2O3 and transformed to α–Al2O3. [22,24] The larger porosity near the gas interface of the scale formed on NiCrAlYHf (Fig. 5b) also was similar to that observed in dry O2, and is associated with the
Fig. 3. Specimen mass changes for various cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 10% or 50% H2O (dashed lines).
in air + 50% H2O, Fig. 3a. The higher water vapor content increased the amount of spallation for undoped Ni–50Al, Fig. 3b. A lower mass gain was observed for Ni–50Al–5Pt which may indicate increased spallation particularly after 1000 cycles. For all of the undoped alloys, whiskers were typically observed at the scale–gas interface when oxidized in dry O2, e.g. Fig. 2b, similar to previous observations. [22,23] However, no whiskers were observed in the 10% or 50% H2O environments, e.g. Fig. 2c. Similarly, fine ridges were typical at oxide grain boundaries on Hf-doped Ni–48Al, [22–24] Fig. 4a, but were absent with the addition of water vapor, e.g. Fig. 4b. These observations likely reflect a small amount of Al(OH)3 evaporation [25] at the scale–gas interface. In order to further study the effect of water vapor on the alumina scale, TEM cross-sections were made on the scale formed on Ni–48Al + Hf and NiCrAlYHf after 100, 1 h cycles in 50% H2O, Fig. 5. The microstructures were not significantly different than those observed in previous work in dry O2. [26,27] The scale on Ni–48Al + Hf exhibited a slightly finer grain width (0.5 μm) in humid air (Fig. 5a), and no ridges were observed at the gas interface, consistent with the plan view observation, Fig. 4b. The fine round porosity in the outer part of
Fig. 5. TEM bright field images of the scales formed after 100, 1 h cycles at 1100 °C in air + 50% H2O on (a) Ni–48Al + Hf and (b) NiCrAlYHf.
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thin Ni-rich transient oxide that formed on this material. The underlying columnar alumina microstructure appeared unaffected by the addition of water vapor. Fig. 6 shows the mass change results for the coated substrates in dry O2 and air + 50% H2O compared to some of the model alloys. In general, all of the coatings on N5 behaved similarly to Ni–48Al + Hf, with little effect of water vapor on mass change. This is not surprising since the alumina grain boundaries formed on Pt-modified aluminide coatings have been shown to contain segregated Hf ions. [28] A higher mass gain was observed for the simple γ/γ' coating on alloy 142 in dry O2, and the highest mass gain was observed for the same coating with the addition of water vapor, Fig. 6. A similar increase was observed for cast Ni–22Al–10Pt–0.4Hf. As these simple γ/γ' coatings and castings have relatively low Al contents, the addition of water vapor may have affected the selective oxidation of Al, as has been reported previously. [8,12,16] However, a similar effect was not observed for the γ/γ' coating on an N5 substrate, Fig. 6. The lowest mass gain was observed for a CVD Pt-modified β-aluminide coating on alloy 142 exposed in dry O2. This behavior may be associated with the higher Hf content in the 142 substrate compared to N5 (Table 1) resulting in a more optimized Hf content in the scale, and a greater reduction in the alumina scale growth rate. [23,28,29] After 200, 1 h cycles, the scale on this substrate was very adherent, Fig. 7a, similar to the other coatings in dry O2. However, a similar specimen exposed to air + 50% H2O showed spallation, Fig. 7b, although not enough to show a mass loss, Fig. 6. Similar spallation behavior was noted on the other coatings exposed to water vapor. Longer testing may reveal a more distinct mass change difference between the two environments. Because of the suppression of whisker and ridge formation on the cast materials in the presence of water vapor, Figs. 2 and 4,
Fig. 6. Specimen mass changes for various β-aluminide (simple or Pt-modified) or γ/γ' (+Pt label) coatings and cast alloys during 1 h cycles at 1100 °C in dry O2 (solid lines) or air + 50% H2O (dashed lines). One CVD PtAl specimen substrate was a version of N5 without Y (N5−).
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Fig. 7. SEM secondary electron plan view images of the scale formed on coatings after exposure in 1 h cycles at 1100 °C; (a) CVD Pt-modified aluminide on 142 after 200 h in dry O2; (b) CVD Pt-modidfied aluminide on 142 after 200 h in air + 50% H2O; and (c) simple γ/γ' coating on N5 after 400 h in air + 50% H2O.
the morphology of the scale formed on the coatings was examined. In each case, the alumina scales formed in air + 50% H2O showed no ridge formation, e.g. Fig. 7c. Ridge formation was observed on the coatings exposed in dry O2 and typically is observed on alumina scales formed on aluminide coatings. [19,28,30,31] Thus, there appears to be a consistent minor modification of the alumina formation in the presence of water vapor. Previous work suggests that this difference was not due to an effect of oxidation in air rather than O2. [20] While the increase in scale spallation from the coated superalloys was minor, extrapolating to longer times suggests that decreased TBC lifetime (due to alumina scale spallation beneath the zirconia top coat) may be expected in environments with higher water vapor contents, such as syngas- or hydrogenfired turbines. The results for cast materials suggest that reactive element (RE) doping, which is well-known to improve scale adhesion, [29] significantly reduces the detrimental effect of water vapor observed in undoped alumina-formers, Fig. 3. Optimization of RE doping in coatings may be critical to improving coating performance in the presence of high water vapor contents. While Hf- and Zr-doping has been attempted with some success for CVD aluminide coatings, [32,33] commercial coatings are still being developed. Coatings based on γ/γ' may have an advantage due to their higher RE solubility. [4] However, the effect of water vapor on the
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transient oxidation behavior of these low Al content coatings needs to be further characterized. Formation of a Ni-rich layer oxide layer at the gas interface, particularly as the coatings are depleted in Pt due to interdiffusion, [5] would form a weak link for TBC adhesion due to porosity formation at the interface with alumina. [34,35] 4. Summary Alumina forming alloys and coatings were cyclicly oxidized at 1100 °C in air + 10% H2O and 50% H2O and the results were compared to the behavior in dry O2. For Y- and Hf-doped alloys, the effect of water vapor was minor with no increase in the scale growth rate. For undoped alumina-formers with less spallation resistance, the amount of spallation increased with increasing water vapor content. Both with and without dopants, the addition of water vapor reduced the amount of alumina whiskers and ridges at the scale–gas interface. For β- and γ/γ'phase coatings on alloy N5 or 142 substrates, the addition of water vapor appeared to cause some increased spallation compared to exposures in dry O2, and may have a detrimental effect on the selective oxidation of Al in γ/γ' coatings. Longer exposures are needed to clarify the effects of higher water vapor contents of the coated superalloys. Acknowledgement The author would like to thank J. Moser, K. Cooley, J. Vought, H. Longmire, L. Walker, T. Brummett and K. S. Reeves for assistance with the experimental work. M. P. Brady, S. Dryepondt and 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 and D, Office of Fossil Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. References [1] “FutureGen — A Sequestration and Hydrogen Research Initiative,” U. S. Department of Energy, Washington, DC 2003. www.fossil.energy.gov/ programs/powersystems/futuregen/futuregen_factsheet.pdf. [2] P. Chiesa, G. Lozza, L. Mazzocchi, J. Eng. Gas Turbine Power 127 (2005) 73. [3] K. Bouhanek, O.A. Adesanya, F.H. Stott, P. Skeldon, D.G. Lees, G.C. Wood, Mater. Sci. Forum 369–372 (2001) 615.
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