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Characterization of oxide scales formed on HVOF NiCrAlY coatings with various oxygen contents introduced during thermal spraying
Scripta Materialia 51 (2004) 25–29 www.actamat-journals.com
Characterization of oxide scales formed on HVOF NiCrAlY coatings with various oxygen contents introduced during thermal spraying Feng Tang *, Leonardo Ajdelsztajn, Julie M. Schoenung Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616, USA Received 17 December 2003; received in revised form 15 March 2004; accepted 22 March 2004 Available online 17 April 2004
Abstract NiCrAlY coatings with oxygen contents ranging from 3 to 21 at.% introduced during HVOF spraying were prepared. Oxidation tests at 1000 °C for up to 1000 h showed that the composition and microstructure of the oxide scale formed on the coatings were significantly affected by the oxygen content in the coatings. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coatings; Oxidation; Nickel alloys; HVOF
1. Introduction MCrAlY (M ¼ Ni or Co+Ni) alloys are one of the most important coating materials for protecting Ni superalloys from hot corrosion and high temperature oxidation. These alloys have been widely used as the bond coat materials in thermal barrier coatings, which are applied to gas turbine blades. The high velocity oxyfuel (HVOF) process has been successfully used to spray the MCrAlY coatings [1,2]. The high impact velocities in the HVOF process allow for the spraying of particles in a plastic, rather than melted, state and can lead to dense and pore-free coatings [3]. The feature of plastic deformation is particularly useful when it is desirable to retain the microstructure observed in the particles in the coating. HVOF spray systems operate at atmospheric pressure, and thus, the investment and operation costs are much lower than the thermal spray systems operated in vacuum [3]. These advantages make HVOF a valued technique for thermal spray applications. However, because the sprayed materials are exposed directly to an oxidizing environment during the HVOF spraying, oxidation can occur in materials that are sensitive to oxidation. Li and Li [4] reported that the oxygen content in HVOF NiCrAlY coatings increased *
Corresponding author. Tel./fax: +1-530-752-9819. E-mail address: [email protected] (F. Tang).
exponentially with decreasing particle size. Lugscheider et al. [3] found that, among the HVOF parameters, spray distance, fuel/oxygen ratio and powder feed rate played the most important roles in controlling the oxygen content in the coatings. The oxidation during the HVOF spraying results in the formation of metal oxides on the splat boundaries. The presence of these oxides in the MCrAlY coating will degrade the resistance of the coating to corrosion [5]. The presence of these oxides has also been observed to affect mechanical properties [6]. Although the oxidation resistance is one of the most important properties for MCrAlY alloys, the effect of the oxidation during the HVOF process on the subsequent oxidation resistance of the MCrAlY coatings has not previously been systematically evaluated. In the present study, four NiCrAlY coatings with different oxygen contents (different degrees of oxidation at the splat boundaries) were prepared by HVOF spraying. Isothermal oxidation tests were conducted and the oxide scales grown on these coatings were characterized and compared.
2. Experimental procedure Two types of NiCrAlY powder, commercially available gas atomized powder and the same powder after
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.03.026
26
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
Fig. 1. BSE images of the cross-sections of (a) Coating 1 sprayed with the JP-5000 system using the gas atomized powder, (b) Coating 2 sprayed with the JP-5000 system using the cryomilled powder, (c) Coating 3 sprayed with the Jet-Kote system using the gas atomized powder, and (d) Coating 4 sprayed with the Jet-Kote system using the cryomilled powder.
cryomilling (ball milling in liquid nitrogen [7]), were used as the original materials for the HVOF thermal spraying. The NiCrAlY powders were HVOF sprayed onto Inconel 718 substrates using both a JP-5000 spray system and a Jet-Kote spray system. In this way, four different coatings were prepared. Hereafter, the coatings made from the gas atomized powder are called Coating 1 (using JP-5000) and Coating 3 (using Jet-Kote); the coatings made from the cryomilled powder are called Coating 2 (using JP-5000) and Coating 4 (using JetKote). The cross-sections of the four coatings are shown in Fig. 1. The thickness of the coatings ranged from 120 to 195 lm. The chemical composition of each coating shown in Table 1, was determined with energy dispersive X-ray spectrometry (EDS) from the coating’s crosssection. The EDS analysis was calibrated with the gas
Table 1 Chemical composition for coatings and gas atomized powder (at.%) Coating 1a Coating 2a Coating 3a Coating 4a Powderb a b
Al
Cr
Ni
O
Al:Cr:Ni:O
19 17 15 15 19.2
21 19 16 16 21.8
56 53 48 49 58.9
3 11 21 20 0.06
1:1.1:2.9:0.2 1:1.1:3.1:0.6 1:1.1:3.2:1.4 1:1.1:3.3:1.3 1:1.1:3.1:0.003
Determined by EDS analysis. Specifications provided by the supplier.
atomized particles, the chemical composition of which has been specified by the supplier. The oxygen content in the coatings changed with the type of spray system, as well with the type of the powders. It should be noted that the ratio of Al:Cr:Ni in the coating remained nearly constant in all of the coatings. The yttrium content
Fig. 2. The oxidized surfaces of (a) Coating 1 and (b) Coating 2 after 1000 h exposure, and (c) Coating 3 and (d) Coating 4 after 330 h exposure at 1000 °C. (a) and (b) are BSE images, and (c) and (d) are SE images.
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
27
could not be determined correctly with EDS because of its small value. According to the supplier, the yttrium content in the gas atomized powder was 0.6 at.%. Prior to the oxidation test, the specimens of the sprayed coatings were polished with 600-grit (20 lm) sandpaper, in order to remove the oxidized surface layer present on the coating after the thermal spraying. The polished specimens were isothermally oxidized in air at 1000 °C for up to 1000 h. The oxide scale was examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and EDS.
3. Results Fig. 2 shows the SEM images of the oxidized surfaces. Coatings 1 and 2 exhibited relatively smooth, oxidized surfaces, while Coatings 3 and 4 exhibited rather rough surfaces. By observing the cross-section, it was found that the thickness of the oxide scale was approximately 3 lm on Coating 1 (1000 °C/1000 h), 5 lm on Coating 2 (1000 °C/1000 h), and 8 lm on Coatings 3 and 4 (1000 °C/330 h). The oxidation tests on Coatings 3 and 4 were stopped after only 330 h because of the rapid rate of oxidation observed. XRD patterns from the oxidized surfaces are shown in Fig. 3. The presence of peaks corresponding to the metallic phases (c/c0 ) in the coating alloy is due to the relatively small thickness of the oxide scale. Although there are no available XRD data for Ni(Al,Cr)2 O4 , it is inferred that the peaks which appear between the peaks for NiAl2 O4 and NiCr2 O4 are, indeed, the peaks for Ni(Al,Cr)2 O4 (Fig. 3b). The XRD patterns show that the oxide scale on Coating 1 consisted mainly of a-Al2 O3 (lower patterns in Fig. 3a and b). The slight deviation of the a-Al2 O3 peaks from their normal positions reflects the internal strain in the oxide scale [8]. For Coating 2, in addition to a-Al2 O3 , Cr2 O3 and Ni(Al,Cr)2 O4 were also detected in the oxide scale (upper patterns in Figs. 3a and b). Coatings 3 and 4 had similar XRD patterns (Fig. 3c), indicating that the oxide scales on Coatings 3 and 4 consisted mainly of Cr2 O3 , with some NiCr2 O4 , while aAl2 O3 could hardly be detected. The results of EDS line-scan imaging along the thickness direction of the oxide scale indicate that the oxide scale on Coating 1 contained mainly oxygen and aluminum (Fig. 4a). A duplex oxide scale was observed on Coating 2 (Fig. 4b), and the subscale contained mainly oxygen and aluminum, while a certain amount of nickel and chromium were also detected in the upper scale. Corresponding with the XRD pattern in Fig. 3b, it is clear that subscale consisted of a-Al2 O3 and the upper scale consisted of Ni(Al,Cr)2 O4 and Cr2 O3 . The oxide scales on Coatings 3 and 4 were porous, and consisted
Fig. 3. XRD patterns from the oxidized surfaces of the four coatings. The oxidation conditions are indicated on the top of each figure.
mainly of oxygen and chromium, with a small amount of nickel (Fig. 4a and b).
4. Discussion To protect the coating alloys from further oxidation, a slow-growing dense oxide scale is desirable. For
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F. Tang et al. / Scripta Materialia 51 (2004) 25–29
Fig. 4. BSE images showing the cross-sections of the typical oxide scales on (a) Coating 1 and (b) Coating 2 after 1000 h exposure, and (c) Coating 3 and (d) Coating 4 after 330 h exposure at 1000 °C. The lower graphs are the EDS line-scan results along the straight lines shown in the BSE images.
MCrAlY alloys, it is preferred that this oxide scale be a continuous, dense a-alumina layer [9], because of the low diffusivity of oxygen and metallic elements through it [10]. Our present experimental results indicate that the composition and the microstructure of the oxide scale can be significantly affected by the oxygen content of the MCrAlY coating. The oxygen content is affected by the conditions of the HVOF spraying. Oxygen reacted with the in-flight and the impacted particles. As a result, metal oxides formed at the splat boundaries and the coating surface during the HVOF spraying. In an HVOF Inconel 718 coating, the oxide layer along the splat boundaries has been characterized to consist of (Ni, Fe)Cr2 O4 and CrNbO4 [11]. The composition of equivalent oxides in the NiCrAlY coatings prepared in the current study, which can be ob-
served as black contrast in the BSE images (see Fig. 1), cannot be determined directly. However, EDS analysis indicated that they are rich in aluminum. Therefore, it is clear that the oxygen reacted mainly with aluminum in the particles during the HVOF thermal spray process. As a result, within the particles, aluminum depletion occurred. Thus, although the total aluminum content in the coating did not change, the thermodynamic activity of aluminum (i.e., the effective concentration) decreased with increasing oxygen content in the coating. The oxide phases formed on the coatings can be justified on the basis of the thermodynamic activities of the metallic elements and the oxygen [9]. In Coating 1, as the oxygen content was low, the aluminum activity was high enough to form a pure a-Al2 O3 scale by reacting with oxygen on the coating surface. The selec-
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
tive oxidation of aluminum is possible because the free energy of formation for Al2 O3 (1244.7 kJ mol1 at 1000 °C [12]) is the highest among the possible oxides in this system. The critical value of the effective aluminum concentration needed to form a pure Al2 O3 scale on an alloy can be estimated as a function of factors such as the diffusion coefficient of Al, the oxygen permeability in the alloy, as proposed by Wagner [13]. Coating 2 had higher oxygen content than Coating 1, indicating it had a higher content of Al-rich oxides internally. As a result, the Al available for oxide scale formation was decreased. This means the Al activity was relatively lower, low enough to fall into a region where the oxide formation is sensitive to the oxygen activity. Spinel grows when the oxygen activity is higher, while Al2 O3 grows when the oxygen activity is lower than a certain level [9]. This relationship can explain the duplex oxide scale formed on Coating 2. The upper scale consisted of spinel, Ni(Al,Cr)2 O4 , because the oxygen activity was the highest at the top surface. The oxygen activity decreased with the increasing depth into the oxide scale, ultimately going below the critical value and promoting the formation of the Al2 O3 subscale. Because the chromium activity was relatively high, besides Ni(Al,Cr)2 O4 , Cr2 O3 also formed in the upper scale. A similar duplex scale with Ni(Al,Cr)2 O4 /Cr2 O3 upper scale and Al2 O3 subscale was also observed on a Ni– 8Cr–3.5Al (wt.%) alloy [14]. In Coatings 3 and 4, which have the highest oxygen contents, the aluminum activity was extremely low and the activities of chromium and nickel were high. Therefore, Cr2 O3 and NiCr2 O4 (which was formed by a further reaction of NiO with Cr2 O3 ) formed on these coatings. Cr2 O3 can vaporize by further oxidation to form CrO3 gas [15]. This may have caused the porosity in the oxide scales formed on Coatings 3 and 4.
5. Conclusions Four NiCrAlY coatings with oxygen content of 3, 11, 21, and 20 at.%, respectively, were prepared with HVOF spraying. Isothermal oxidation tests were conducted at 1000 °C for up to 1000 h and the oxide scales grown on these coatings were characterized. A dense oxide scale consisting mainly of a-Al2 O3 formed on the coating with
29
the lowest oxygen content. A duplex oxide scale with an a-Al2 O3 sub-layer and a Ni(Al,Cr)2 O4 /Cr2 O3 upper layer formed on the coating with medium oxygen content. On the two coatings with the highest oxygen content, porous Cr2 O3 /NiCr2 O4 oxide scale formed. These results indicate that the oxide scale formation on the coatings can be significantly affected by the degree of oxidation that occurs in the coatings during the HVOF spraying process. Low oxygen content in the coating is beneficial to the formation of a protective a-alumina scale.
Acknowledgements The authors are grateful to Dr. George Kim (Perpetual Technologies) and Dr. Virgil Provenzano (National Institute of Standards and Technology) for their insight into MCrAlY coatings. Financial support was provided by the Office of Naval Research (Grant N00014-02-1-0213) and the University of California Energy Institute.
References [1] Toma D, Brandl W, K€ oster U. Surf Coat Technol 1999;120–121:8. [2] Ajdelsztajn L, Picas JA, Kim GE, Bastian FL, Schoenung JM, Provenzano V. Mater Sci Eng A 2002;338:33. [3] Lugscheider E, Herbst C, Zhao L. Surf Coat Technol 1998;108– 109:16. [4] Li CJ, Li WY. Surf Coat Technol 2003;162:31. [5] Kawakita J, Kuroda S, Fukushima T, Kodama T. Sci Technol Adv Mater 2003;4:281. [6] Volenık K, Novak V, Dubsky J, Chraska P, Neufuss K. Mater Sci Eng A 1997;234–236:493. [7] He J, Schoenung JM. Mater Sci Eng A 2002;336:274. [8] Hou PY, Stringer J. Acta Metall Mater 1991;39:841. [9] Evans AG, Mumm DR, Hutchinson JW, Meier GH, Pettit FS. Prog Mater Sci 2001;46:505. [10] Messaoudi K, Huntz AM, Lesage B. Mater Sci Eng A 1998;247:248. [11] Perez P. Corros Sci 2002;44:1793. [12] Brandes EA, Brook GB. Smithells metals reference book. Reed Educational and Professional Publishing; 1992. Chapter 8. [13] Wagner C. Z Elektrochem 1959;63:772. [14] Chen GF, Lou HY. Oxid Met 2000;54:155. [15] Choi H, Yoon B, Kim H, Lee C. Surf Coat Technol 2002;150: 297.
Characterization of oxide scales formed on HVOF NiCrAlY coatings with various oxygen contents introduced during thermal spraying Feng Tang *, Leonardo Ajdelsztajn, Julie M. Schoenung Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616, USA Received 17 December 2003; received in revised form 15 March 2004; accepted 22 March 2004 Available online 17 April 2004
Abstract NiCrAlY coatings with oxygen contents ranging from 3 to 21 at.% introduced during HVOF spraying were prepared. Oxidation tests at 1000 °C for up to 1000 h showed that the composition and microstructure of the oxide scale formed on the coatings were significantly affected by the oxygen content in the coatings. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coatings; Oxidation; Nickel alloys; HVOF
1. Introduction MCrAlY (M ¼ Ni or Co+Ni) alloys are one of the most important coating materials for protecting Ni superalloys from hot corrosion and high temperature oxidation. These alloys have been widely used as the bond coat materials in thermal barrier coatings, which are applied to gas turbine blades. The high velocity oxyfuel (HVOF) process has been successfully used to spray the MCrAlY coatings [1,2]. The high impact velocities in the HVOF process allow for the spraying of particles in a plastic, rather than melted, state and can lead to dense and pore-free coatings [3]. The feature of plastic deformation is particularly useful when it is desirable to retain the microstructure observed in the particles in the coating. HVOF spray systems operate at atmospheric pressure, and thus, the investment and operation costs are much lower than the thermal spray systems operated in vacuum [3]. These advantages make HVOF a valued technique for thermal spray applications. However, because the sprayed materials are exposed directly to an oxidizing environment during the HVOF spraying, oxidation can occur in materials that are sensitive to oxidation. Li and Li [4] reported that the oxygen content in HVOF NiCrAlY coatings increased *
Corresponding author. Tel./fax: +1-530-752-9819. E-mail address: [email protected] (F. Tang).
exponentially with decreasing particle size. Lugscheider et al. [3] found that, among the HVOF parameters, spray distance, fuel/oxygen ratio and powder feed rate played the most important roles in controlling the oxygen content in the coatings. The oxidation during the HVOF spraying results in the formation of metal oxides on the splat boundaries. The presence of these oxides in the MCrAlY coating will degrade the resistance of the coating to corrosion [5]. The presence of these oxides has also been observed to affect mechanical properties [6]. Although the oxidation resistance is one of the most important properties for MCrAlY alloys, the effect of the oxidation during the HVOF process on the subsequent oxidation resistance of the MCrAlY coatings has not previously been systematically evaluated. In the present study, four NiCrAlY coatings with different oxygen contents (different degrees of oxidation at the splat boundaries) were prepared by HVOF spraying. Isothermal oxidation tests were conducted and the oxide scales grown on these coatings were characterized and compared.
2. Experimental procedure Two types of NiCrAlY powder, commercially available gas atomized powder and the same powder after
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.03.026
26
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
Fig. 1. BSE images of the cross-sections of (a) Coating 1 sprayed with the JP-5000 system using the gas atomized powder, (b) Coating 2 sprayed with the JP-5000 system using the cryomilled powder, (c) Coating 3 sprayed with the Jet-Kote system using the gas atomized powder, and (d) Coating 4 sprayed with the Jet-Kote system using the cryomilled powder.
cryomilling (ball milling in liquid nitrogen [7]), were used as the original materials for the HVOF thermal spraying. The NiCrAlY powders were HVOF sprayed onto Inconel 718 substrates using both a JP-5000 spray system and a Jet-Kote spray system. In this way, four different coatings were prepared. Hereafter, the coatings made from the gas atomized powder are called Coating 1 (using JP-5000) and Coating 3 (using Jet-Kote); the coatings made from the cryomilled powder are called Coating 2 (using JP-5000) and Coating 4 (using JetKote). The cross-sections of the four coatings are shown in Fig. 1. The thickness of the coatings ranged from 120 to 195 lm. The chemical composition of each coating shown in Table 1, was determined with energy dispersive X-ray spectrometry (EDS) from the coating’s crosssection. The EDS analysis was calibrated with the gas
Table 1 Chemical composition for coatings and gas atomized powder (at.%) Coating 1a Coating 2a Coating 3a Coating 4a Powderb a b
Al
Cr
Ni
O
Al:Cr:Ni:O
19 17 15 15 19.2
21 19 16 16 21.8
56 53 48 49 58.9
3 11 21 20 0.06
1:1.1:2.9:0.2 1:1.1:3.1:0.6 1:1.1:3.2:1.4 1:1.1:3.3:1.3 1:1.1:3.1:0.003
Determined by EDS analysis. Specifications provided by the supplier.
atomized particles, the chemical composition of which has been specified by the supplier. The oxygen content in the coatings changed with the type of spray system, as well with the type of the powders. It should be noted that the ratio of Al:Cr:Ni in the coating remained nearly constant in all of the coatings. The yttrium content
Fig. 2. The oxidized surfaces of (a) Coating 1 and (b) Coating 2 after 1000 h exposure, and (c) Coating 3 and (d) Coating 4 after 330 h exposure at 1000 °C. (a) and (b) are BSE images, and (c) and (d) are SE images.
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
27
could not be determined correctly with EDS because of its small value. According to the supplier, the yttrium content in the gas atomized powder was 0.6 at.%. Prior to the oxidation test, the specimens of the sprayed coatings were polished with 600-grit (20 lm) sandpaper, in order to remove the oxidized surface layer present on the coating after the thermal spraying. The polished specimens were isothermally oxidized in air at 1000 °C for up to 1000 h. The oxide scale was examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and EDS.
3. Results Fig. 2 shows the SEM images of the oxidized surfaces. Coatings 1 and 2 exhibited relatively smooth, oxidized surfaces, while Coatings 3 and 4 exhibited rather rough surfaces. By observing the cross-section, it was found that the thickness of the oxide scale was approximately 3 lm on Coating 1 (1000 °C/1000 h), 5 lm on Coating 2 (1000 °C/1000 h), and 8 lm on Coatings 3 and 4 (1000 °C/330 h). The oxidation tests on Coatings 3 and 4 were stopped after only 330 h because of the rapid rate of oxidation observed. XRD patterns from the oxidized surfaces are shown in Fig. 3. The presence of peaks corresponding to the metallic phases (c/c0 ) in the coating alloy is due to the relatively small thickness of the oxide scale. Although there are no available XRD data for Ni(Al,Cr)2 O4 , it is inferred that the peaks which appear between the peaks for NiAl2 O4 and NiCr2 O4 are, indeed, the peaks for Ni(Al,Cr)2 O4 (Fig. 3b). The XRD patterns show that the oxide scale on Coating 1 consisted mainly of a-Al2 O3 (lower patterns in Fig. 3a and b). The slight deviation of the a-Al2 O3 peaks from their normal positions reflects the internal strain in the oxide scale [8]. For Coating 2, in addition to a-Al2 O3 , Cr2 O3 and Ni(Al,Cr)2 O4 were also detected in the oxide scale (upper patterns in Figs. 3a and b). Coatings 3 and 4 had similar XRD patterns (Fig. 3c), indicating that the oxide scales on Coatings 3 and 4 consisted mainly of Cr2 O3 , with some NiCr2 O4 , while aAl2 O3 could hardly be detected. The results of EDS line-scan imaging along the thickness direction of the oxide scale indicate that the oxide scale on Coating 1 contained mainly oxygen and aluminum (Fig. 4a). A duplex oxide scale was observed on Coating 2 (Fig. 4b), and the subscale contained mainly oxygen and aluminum, while a certain amount of nickel and chromium were also detected in the upper scale. Corresponding with the XRD pattern in Fig. 3b, it is clear that subscale consisted of a-Al2 O3 and the upper scale consisted of Ni(Al,Cr)2 O4 and Cr2 O3 . The oxide scales on Coatings 3 and 4 were porous, and consisted
Fig. 3. XRD patterns from the oxidized surfaces of the four coatings. The oxidation conditions are indicated on the top of each figure.
mainly of oxygen and chromium, with a small amount of nickel (Fig. 4a and b).
4. Discussion To protect the coating alloys from further oxidation, a slow-growing dense oxide scale is desirable. For
28
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
Fig. 4. BSE images showing the cross-sections of the typical oxide scales on (a) Coating 1 and (b) Coating 2 after 1000 h exposure, and (c) Coating 3 and (d) Coating 4 after 330 h exposure at 1000 °C. The lower graphs are the EDS line-scan results along the straight lines shown in the BSE images.
MCrAlY alloys, it is preferred that this oxide scale be a continuous, dense a-alumina layer [9], because of the low diffusivity of oxygen and metallic elements through it [10]. Our present experimental results indicate that the composition and the microstructure of the oxide scale can be significantly affected by the oxygen content of the MCrAlY coating. The oxygen content is affected by the conditions of the HVOF spraying. Oxygen reacted with the in-flight and the impacted particles. As a result, metal oxides formed at the splat boundaries and the coating surface during the HVOF spraying. In an HVOF Inconel 718 coating, the oxide layer along the splat boundaries has been characterized to consist of (Ni, Fe)Cr2 O4 and CrNbO4 [11]. The composition of equivalent oxides in the NiCrAlY coatings prepared in the current study, which can be ob-
served as black contrast in the BSE images (see Fig. 1), cannot be determined directly. However, EDS analysis indicated that they are rich in aluminum. Therefore, it is clear that the oxygen reacted mainly with aluminum in the particles during the HVOF thermal spray process. As a result, within the particles, aluminum depletion occurred. Thus, although the total aluminum content in the coating did not change, the thermodynamic activity of aluminum (i.e., the effective concentration) decreased with increasing oxygen content in the coating. The oxide phases formed on the coatings can be justified on the basis of the thermodynamic activities of the metallic elements and the oxygen [9]. In Coating 1, as the oxygen content was low, the aluminum activity was high enough to form a pure a-Al2 O3 scale by reacting with oxygen on the coating surface. The selec-
F. Tang et al. / Scripta Materialia 51 (2004) 25–29
tive oxidation of aluminum is possible because the free energy of formation for Al2 O3 (1244.7 kJ mol1 at 1000 °C [12]) is the highest among the possible oxides in this system. The critical value of the effective aluminum concentration needed to form a pure Al2 O3 scale on an alloy can be estimated as a function of factors such as the diffusion coefficient of Al, the oxygen permeability in the alloy, as proposed by Wagner [13]. Coating 2 had higher oxygen content than Coating 1, indicating it had a higher content of Al-rich oxides internally. As a result, the Al available for oxide scale formation was decreased. This means the Al activity was relatively lower, low enough to fall into a region where the oxide formation is sensitive to the oxygen activity. Spinel grows when the oxygen activity is higher, while Al2 O3 grows when the oxygen activity is lower than a certain level [9]. This relationship can explain the duplex oxide scale formed on Coating 2. The upper scale consisted of spinel, Ni(Al,Cr)2 O4 , because the oxygen activity was the highest at the top surface. The oxygen activity decreased with the increasing depth into the oxide scale, ultimately going below the critical value and promoting the formation of the Al2 O3 subscale. Because the chromium activity was relatively high, besides Ni(Al,Cr)2 O4 , Cr2 O3 also formed in the upper scale. A similar duplex scale with Ni(Al,Cr)2 O4 /Cr2 O3 upper scale and Al2 O3 subscale was also observed on a Ni– 8Cr–3.5Al (wt.%) alloy [14]. In Coatings 3 and 4, which have the highest oxygen contents, the aluminum activity was extremely low and the activities of chromium and nickel were high. Therefore, Cr2 O3 and NiCr2 O4 (which was formed by a further reaction of NiO with Cr2 O3 ) formed on these coatings. Cr2 O3 can vaporize by further oxidation to form CrO3 gas [15]. This may have caused the porosity in the oxide scales formed on Coatings 3 and 4.
5. Conclusions Four NiCrAlY coatings with oxygen content of 3, 11, 21, and 20 at.%, respectively, were prepared with HVOF spraying. Isothermal oxidation tests were conducted at 1000 °C for up to 1000 h and the oxide scales grown on these coatings were characterized. A dense oxide scale consisting mainly of a-Al2 O3 formed on the coating with
29
the lowest oxygen content. A duplex oxide scale with an a-Al2 O3 sub-layer and a Ni(Al,Cr)2 O4 /Cr2 O3 upper layer formed on the coating with medium oxygen content. On the two coatings with the highest oxygen content, porous Cr2 O3 /NiCr2 O4 oxide scale formed. These results indicate that the oxide scale formation on the coatings can be significantly affected by the degree of oxidation that occurs in the coatings during the HVOF spraying process. Low oxygen content in the coating is beneficial to the formation of a protective a-alumina scale.
Acknowledgements The authors are grateful to Dr. George Kim (Perpetual Technologies) and Dr. Virgil Provenzano (National Institute of Standards and Technology) for their insight into MCrAlY coatings. Financial support was provided by the Office of Naval Research (Grant N00014-02-1-0213) and the University of California Energy Institute.
References [1] Toma D, Brandl W, K€ oster U. Surf Coat Technol 1999;120–121:8. [2] Ajdelsztajn L, Picas JA, Kim GE, Bastian FL, Schoenung JM, Provenzano V. Mater Sci Eng A 2002;338:33. [3] Lugscheider E, Herbst C, Zhao L. Surf Coat Technol 1998;108– 109:16. [4] Li CJ, Li WY. Surf Coat Technol 2003;162:31. [5] Kawakita J, Kuroda S, Fukushima T, Kodama T. Sci Technol Adv Mater 2003;4:281. [6] Volenık K, Novak V, Dubsky J, Chraska P, Neufuss K. Mater Sci Eng A 1997;234–236:493. [7] He J, Schoenung JM. Mater Sci Eng A 2002;336:274. [8] Hou PY, Stringer J. Acta Metall Mater 1991;39:841. [9] Evans AG, Mumm DR, Hutchinson JW, Meier GH, Pettit FS. Prog Mater Sci 2001;46:505. [10] Messaoudi K, Huntz AM, Lesage B. Mater Sci Eng A 1998;247:248. [11] Perez P. Corros Sci 2002;44:1793. [12] Brandes EA, Brook GB. Smithells metals reference book. Reed Educational and Professional Publishing; 1992. Chapter 8. [13] Wagner C. Z Elektrochem 1959;63:772. [14] Chen GF, Lou HY. Oxid Met 2000;54:155. [15] Choi H, Yoon B, Kim H, Lee C. Surf Coat Technol 2002;150: 297.