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Effect of the top coat on the phase transformation of thermally grown oxide in thermal barrier coatings
Scripta Materialia 55 (2006) 1051–1054 www.actamat-journals.com
Effect of the top coat on the phase transformation of thermally grown oxide in thermal barrier coatings X. Zhao, T. Hashimoto and P. Xiao* Materials Science Centre, School of Materials, University of Manchester, Manchester M1 7HS, UK Received 23 June 2006; revised 31 July 2006; accepted 1 August 2006 Available online 7 September 2006
The phase transformation of the thermally grown oxide (TGO) formed on a Pt enriched c + c 0 bond coat in electron beam physical vapour deposited thermal barrier coatings (TBCs) was studied by photo-stimulated luminescence spectroscopy. The presence of the TBC retards the h to a transformation of the TGO and leads to a higher oxidation rate. The reasons for these phenomena are discussed. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings (TBCs); Thermally grown oxides (TGO); Phase transformation; Alumina
Thermal barrier coatings (TBCs), produced by the electron beam-physical vapour deposition (EBPVD) technique, have been widely used in the hot sections of aircraft engine turbines to increase turbine efficiency and to extend the life of metal components [1–3]. The ceramic coating consists of yttria-stabilised zirconia (YSZ) with columnar structure to provide strain tolerance. To further protect the substrate from oxidation and hot corrosion, a bond coat, either a MCrAlY (M = Ni, Co) or a (Ni, Pt)Al, is applied between the YSZ coating and the metal substrate. During exposure to high temperatures, a thermally grown oxide (TGO) forms between the coating and the bond coat. In the absence of mechanical damage, the failure of TBCs typically occurs at the interface between the bond coat and the TGO by buckling or edge delamination [1,4]. Two main mechanisms govern these failures, i.e. Al depletion [5] and rumpling of the TGO [1,2]. Nevertheless, the failure induced by the first category rarely occurs due to the adoption of high Al activity bond coat. For a single phase b-(Ni, Pt)Al bond coat, most of the failure modes are associated with the local stresses induced by rumpling. However, for a Pt-enriched c + c 0 bond coat, the failure induced by rumpling is largely suppressed due to its low volume shrinkage during phase transformation, higher creep and yield strength * Corresponding author. Tel.: +44 161 200 5941; fax: +44 161 200 3586; e-mail: [email protected]
[6]. The mechanisms, which can cause the interfacial separation, are still being intensively investigated. Although the thermal barrier systems exhibit several competing mechanisms of failure, most are associated with the oxidation of the bond coat [7]. Similar to other nickel aluminide, platinum-modified aluminide tends to form a metastable phase such as c- or h-Al2O3 in the earlier oxidation stage [8]. Studies have shown that aluminum oxide first forms at low temperatures as an amorphous oxide and then subsequently transforms to the stable, high temperature polymorph, a-Al2O3 [9]. On several bond coat alloys, including NiAl and (Ni, Pt)Al, the amorphous alumina transforms first to c-Al2O3, then to h-Al2O3 and finally to a-Al2O3 [10– 12]. The existence of metastable phases in the TGO has several unfavorable effects such as high growth rate [13] and poor adherence. In addition, if there exists metastable phase in the TGO, the following transformation is geometrically constrained. As each of the phase transformations in alumina is accompanied by a decrease in volume, the stresses generated within the alumina TGO will be tensile and, under favorable conditions, may be sufficient to cause interface separation. As suggested by Clarke, there exists a critical thickness of the untransformed thermally grown oxides above which interfacial separation can occur [14]. As with any compressively stressed film, the eventual failure by buckling requires a region along the interface to be already separated [2,15]. The phase transformation of the TGO could cause the crack formation at
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.08.017
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the interface, which leads to failure eventually. However, the effect of the TBC on the TGO phase transformation has not been systemically investigated. In this contribution, the phase transformation of the TGO in EBPVD TBCs with a c + c 0 bond coat was studied and compared with the bond coat without a TBC, using photo-stimulated luminescence spectroscopy (PSLS) [16,17], coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As will be described, the presence of the TBC not only retards the phase transformation rate of the TGO from theta to alpha, but also changes the TGO morphology. The possible reason for this phenomenon is discussed. The TBCs samples consisted of 7–8 wt.% yttriastabilised zirconia (YSZ) layer with a thickness of about 130–150 lm, and a Pt-enriched c + c 0 bond coat on CMSX-4 (nominal composition of Ni–9.6Co–6.4Cr– 6.4W–6.6Ta–5.6Al–2.9Re–1.03Ti–0.2Hf in wt.%) substrate. The bond coat was lightly grit blasted prior to the deposition of YSZ by EBPVD process and some of the bond coat without deposition was used as comparison. All the samples were cut into 5 · 5 mm with the substrate thickness of 1 mm and subjected to oxidation at 1150 C in air. The crystallographic forms of the TGO were characterized by PSLS at room temperature using the Renishaw Ramanscope 2000 (RenishawTM, UK). During the measurements, the laser (He–Ne, 632.8 nm) was focused on the top surface of a TBC or TGO and the laser spot size was set to be about 2–5 lm. The reference sample was a stress free sapphire single crystal. The TGO thickness was measured by SEM (Philips XL30). The impurities and phases in the TGO were identified by analytical transmission microscopy (TEM/STEM, TecnaiTM G2 F30 U-TWIN). The TEM specimen of the TGO was prepared by cutting across the TGO from the cross-section of TBCs, using a focused ion beam (FIB, FEI Nova 600 DualBeam system); the detailed preparation process was described elsewhere [18]. To investigate the effect of the TBC on the TGO growth rate, both the bond coat with and without a TBC were oxidised at 1150 C. Figure 1 shows the oxi-
Figure 1. (A) TGO thickness as function of oxidation time at 1150 C for bond coats with and without TBC; (B) SEM cross-section of the TGO with EBPVD TBC showing a thicker TGO and (C) SEM crosssection of TGO from oxidation of bond coat without TBC at 1150 C for 20 h.
dation kinetics and cross-sectional images of two typical samples. The TGO growth on a TBC coated bond coat was much faster than that of the uncoated samples of the same composition, which is in agreement with previous studies on oxidation of a b-(Ni, Pt)Al bond coat [7,19]. However, this phenomenon is still poorly understood. The growth kinetics of the TGO could be affected by the composition and roughness of the bond coat [8], whereas both bond coats were fabricated using the same procedures. Thus, the only reason for such different growth rates might be the presence of the TBC. It has been shown that the existence of metastable alumina dramatically increases the TGO oxidation rate [13]. In order to determine whether there was metastable alumina in the TGO, luminescence spectroscopy was used due to its sensitivity to the phase transformation of alumina [16,17]. In this experiment, the PSLS was performed on the TGO through the TBC. For comparison, the luminescence spectra of the TGO on the uncoated bond coat are also presented. As shown in Figure 2(A), the TGO consisted of a-Al2O3 in as-deposited TBCs. However, some h-Al2O3 formed after a short time at 1150 C. According to the relative peak intensity corresponding to h- and a-Al2O3, the fraction of h-Al2O3 in the TGO decreased with increasing oxidation time, while that of a-Al2O3 increased. In contrast, no h-Al2O3 was observed in the TGO from oxidation of the bond coat without a TBC at this temperature (Fig. 2(B)). Therefore, the higher growth rate of the TGO on TBC coated sample could be related to the generation of metastable phase during oxidation. The results presented in Figure 2 were surprising, according to earlier reports on b-(Ni, Pt)Al or NiAl bond coat, the metastable h-Al2O3 typically forms at a low temperature (800–1100 C) [8,13,20], but will transform to stable a-Al2O3 after a short period at 1150 C. However, in this experiment, a significant of h-Al2O3 was produced on a TBC coated c + c 0 bond coat even after oxidation at 1150 C for 20 h, while for the uncoated bond coat, no h-Al2O3 was observed even after oxidation at 1150 C for 5 h. Indeed, the TGO on the uncoated bond coat was determined to be a-Al2O3 even after oxidation at 1000 C for 1 h. Since the fabrication temperature for the bond coat is higher than 1100 C, the primary phase in the TGO is a-Al2O3. Thus, the
Figure 2. Photo-stimulated luminescence spectra from the TGO formed on a c + c 0 bond coat oxidised at 1150 C in air: (A) TBC coated samples and (B) uncoated samples; both were measured from the top surface of the TBC.
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TBC has the ability to slow the phase transformation of the TGO from h- to a-Al2O3. Previous studies has shown that the TGO growth rate also depends on its lateral grain size [21], because the grain boundaries are the fast diffusion paths for oxygen and aluminum ions [22,23]. It is worthwhile investigating the difference in the grain morphology of the TGO grown on these two kinds of samples. As shown in Figure 3, the TGO formed on the TBC coated bond coat exhibits two distinct microstructural domains: columnar grains next to the bond coat and equiaxial grains next to the TBC. Whereas the TGO formed on the bare bond coat almost consists of equiaxial grains. Moreover, the lateral grain size seems larger than that of columnar grains formed on the TBC coated bond coat. Since the TGO consisting of coarser grains has a smaller grain boundary area available for oxygen diffusion [21], it will lead to a slower oxidation rate, as observed with the TGO grown on a bare bond coat. Therefore, the existence of the TBC also affects the TGO grain morphology and results in a columnar structure. Figure 4 shows the PSLS spectra from measurements on a free standing TBC, which was prepared by dissolving the substrate in hydrochloric acid. Two features are particular noteworthy: a significant amount of h-Al2O3 was found in the TGO when measuring from the top surface of the TBC, while no such signal was observed when measuring from the bottom; the measured stresses, which were based on the R2 peak shift of a-Al2O3, from the top are lower than that measured from the bottom (Fig. 4(A)). The first feature implies that the h-Al2O3 might exist at the interface between the TBC and TGO, since the laser was scattered by the YSZ columns, and the signal mainly comes from the top surface of the TGO. In addition, the lower stress measured from the top surface might be attributed to the h-Al2O3 in the TGO. Because the a-Al2O3 usually nucleated within the h-Al2O3 matrix [17], a tensile stress was generated on the a-Al2O3 due to the volume shrinkage from
Figure 3. The TGO microstructure formed on (A) TBC coated bond coat showing columnar grains next to the alloy, while (B) TGO grown on uncoated bond coat exhibiting almost equiaxial grains. Both were oxidised at 1150 C 10 h.
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Figure 4. (A) The PSLS spectra and corresponding stresses measured from the top surface and bottom of a free standing TBC, respectively. (B) Bright field TEM image of the TGO and selected area diffraction patterns showing the presence of h-Al2O3.
h- to a-Al2O3 and balanced part of the compressive stresses applied by the TBC. Therefore, the stress measured from the top is lower than that from bottom. TEM was used to verify the existence of h-Al2O3 in the TGO. As shown by the selected area diffraction pattern in Figure 4(B), the h-Al2O3 was found near the YSZ (equiaxial zone), while the alumina far away from YSZ (columnar zone) was determined to be alpha phase, which is consistent with the PSLS measurement. Ragan and Clarke studied different dopants such as Cr, Fe, Y and Er on the kinetics of c- to a-Al2O3 transformation [24,25]. They found Y, Cr and Er ions could decrease the c- to a-transformation rate and do so in proportion to their concentration. Pint and co-workers investigated the effect of various cation dopants on the h to a transformation of Al2O3 grown on b-NiAl they found the larger ions such as Y, Zr, La and Hf appeared to slow such a transformation [26–28]. Therefore, the presence of Zr ions in TGO was assumed to decrease the phase transformation rate from h to a in this experiment. The energy dispersive spectroscopy analysis using analytical microscopy (STEM) confirmed the presence of Zr ions. As shown in Figure 5(A), Zr does exist in the TGO (the Cu signal comes from the copper grid and the Ga signal is from the Ga ion beam during the milling process by FIB). In addition, the content of Zr was higher near the YSZ than that near the bond coat, and the highest Zr content was found in the grain boundary of the alumina in the TGO near the YSZ/ TGO interface. Therefore, due to the higher Zr concentration near the YSZ, the transformation from h to a was more retarded than that near the bond coat. This explains why most of the h phase exists at the TBC/ TGO interface. Earlier studies on c 0 -Ni3Al have shown that the TGO growth mechanism and kinetics are similar to other alumina forming alloys such as the MCrAl (where M
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The authors would like to thank Prof. R.J. Young for the use of the spectroscopy instrument and Dr. R.M. Langford for the use of the FIB. They would also acknowledge Drs. R. Jones and D.S. Rickerby from Rolls-Royce Plc. for provision of the samples.
Figure 5. (A) Energy dispersive spectra measured by STEM on the different positions of the TGO. (B) Schematic of the diffusion model of bond coat oxidation, the grain boundaries act as ‘short-circuit’ paths.
is Ni, Fe or Co) alloys [11,29]. In the early stage, the TGO growth is mainly inter-diffusion of cation and oxygen, at the same time, the rare earth (RE) elements, such as Zr, Y, begin to segregate to the alumina grain boundary where they block cation transport and possibly promote the oxygen transport [30–32] (schematic in Fig. 5(B)). Thus the RE elements change the TGO growth from the one in which inter-diffusion predominates to one in which oxygen predominates. Because of the primary inward-growth and a preferred growth direction, the microstructure of TGO grown on TBC coated bond coat is further modified by the formation of columnar grains adjacent to the alloy [22]. The standard grit-blasting procedure, used in practice to prepare the bond coat for TBC deposition, has the effect of promoting the formation of an a-Al2O3 TGO [8]. It does, however, also result in a rough surface and nano-sized voids in the TGO. During the TBC deposition, the YSZ forms randomly orientated nanosized nuclei in the rough surface or voids. Meanwhile, either during deposition or an early stage of oxidation, the Al cations diffused outward and encapsulated these YSZ nuclei. Subsequently, a mixed oxidation zone (equiaxial grains) formed near the YSZ/TGO interface [33]. Due to the presence of Zr, the phase transformation from h- to a-Al2O3 was inhibited, thus some of h-Al2O3 was found near the YSZ/TGO interface. On the other hand, the Zr ions segregated to the alumina grain boundary and blocked the outward diffusion of cation ion [30–32], then the TGO growth changed from the metal and oxygen inter-diffusion to the oxygen diffusion dominated process due to the higher diffusivity of O anions in oxides [34]. Since there is no Zr (or a minor amount) far away from the YSZ/TGO interface, the phase transformation from h to a is so quick that only a-Al2O3 was detected. In this paper, the phase transformation of the TGO on a Pt-enriched c + c 0 bond coat was determined by photo-stimulated luminescence spectroscopy. It was found the TBC could inhibit the phase transformation of the TGO from h- to a-Al2O3, affect the TGO grain structure, and result in a higher oxidation rate than the uncoated samples. The concentration of Zr was higher near the YSZ/TGO interface, but lower far away from that interface. Consequently, the h- to a-Al2O3 transformation near the YSZ was more inhibited and led to the presence of h-Al2O3 near the YSZ.
[1] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. [2] A.G. Evans, M.Y. He, J.W. Hutchinson, Prog. Mater. Sci. 46 (2001) 249. [3] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [4] P.K. Wright, A.G. Evans, Curr. Opin. Solid State Mater. Sci. 4 (1999) 255. [5] E.A.G. Shillington, D.R. Clarke, Acta Mater. 47 (1999) 1297. [6] J.R. Nicholls, MRS Bull. (2003) 659. [7] V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 146–147 (2001) 124. [8] V.K. Tolpygo, D.R. Clarke, Mater. High. Temp. 17 (2000) 59. [9] M.W. Brumm, H.J. Grabke, Corros. Sci. 33 (1992) 1677. [10] C.G. Levi, E. Sommer, S.G. Terry, A. Catanoiu, M. Ruhle, J. Am. Ceram. Soc. 86 (2003) 676. [11] J. Doychak, M. Ruhle, Oxid. Met. 31 (1989) 431. [12] G.C. Rybicki, J.L. Smialek, Oxid. Met. 31 (1989) 275. [13] K.M.N. Prasanna, A.S. Khanna, R. Chandra, W.J. Quadakkers, Oxid. Met. 46 (1996) 465. [14] D.R. Clarke, V. Sergo, M.Y. He, in: J.M. Hampikian, N.B. Dahotre (Eds.), In Elevated temperature coatings: Science and Technology III, The Minerals, Metals & Materials Society, 1999, p. 67. [15] A.G. Evans, J.W. Hutchinson, Int. J. Solids. Struct. 20 (1984) 455. [16] Q. Wen, D.M. Lipkin, D.R. Clarke, J. Am. Ceram. Soc. 81 (1998) 3345. [17] D.M. Lipkin, H. Schaffer, F. Adar, D.R. Clarke, Appl. Phys. Lett. 70 (1997) 2550. [18] R.M. Langford, A.K. Petford-Long, J. Vac. Sci. Technol. A 19 (2001) 2186. [19] V.K. Tolpygo, D.R. Clarke, Surf. Coat. Technol. 200 (2005) 1276. [20] X. Peng, D.R. Clarke, F. Wang, Oxid. Met. 60 (2003) 225. [21] A. Andoh, S. Taniguchi, T. Shibata, Mater. Sci. Forum. 251–254 (1997) 785. [22] B.A. Pint, Oxid. Met. 45 (1996) 1. [23] H. Hindam, D.P. Whittle, Oxid. Met. 18 (1982) 245. [24] D.D. Ragan, T. Mates, D.R. Clarke, J. Am. Ceram. Soc. 86 (2003) 541. [25] D.R. Clarke, Phys. Stat. Sol. A 166 (1998) 183. [26] B.A. Pint, J.R. Martin, L.W. Hobbs, Solid State Ion. 78 (1995) 99. [27] B.A. Pint, L.W. Hobbs, J. Electrochem. Soc. 141 (1994) 2443. [28] B.A. Pint, M. Treska, L.W. Hobbs, Oxid. Met. 47 (1997) 1. [29] J.D. Kuenzly, D.L. Douglass, Oxid. Met. 12 (1974) 503. [30] B.A. Pint, Oxid. Met. 45 (1996) 1. [31] F.H. Stott, G.C. Wood, Mater. Sci. Eng. 87 (1987) 267. [32] D.G. Lee, Oxid. Met. 27 (1987) 75. [33] K.S. Murphy, K.L. More, M.J. Lance, Surf. Coat. Technol. 146–147 (2001) 152. [34] B.A. Pint, J.R. Martin, L.W. Hobbs, Oxid. Met. 39 (1993) 167.
Effect of the top coat on the phase transformation of thermally grown oxide in thermal barrier coatings X. Zhao, T. Hashimoto and P. Xiao* Materials Science Centre, School of Materials, University of Manchester, Manchester M1 7HS, UK Received 23 June 2006; revised 31 July 2006; accepted 1 August 2006 Available online 7 September 2006
The phase transformation of the thermally grown oxide (TGO) formed on a Pt enriched c + c 0 bond coat in electron beam physical vapour deposited thermal barrier coatings (TBCs) was studied by photo-stimulated luminescence spectroscopy. The presence of the TBC retards the h to a transformation of the TGO and leads to a higher oxidation rate. The reasons for these phenomena are discussed. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings (TBCs); Thermally grown oxides (TGO); Phase transformation; Alumina
Thermal barrier coatings (TBCs), produced by the electron beam-physical vapour deposition (EBPVD) technique, have been widely used in the hot sections of aircraft engine turbines to increase turbine efficiency and to extend the life of metal components [1–3]. The ceramic coating consists of yttria-stabilised zirconia (YSZ) with columnar structure to provide strain tolerance. To further protect the substrate from oxidation and hot corrosion, a bond coat, either a MCrAlY (M = Ni, Co) or a (Ni, Pt)Al, is applied between the YSZ coating and the metal substrate. During exposure to high temperatures, a thermally grown oxide (TGO) forms between the coating and the bond coat. In the absence of mechanical damage, the failure of TBCs typically occurs at the interface between the bond coat and the TGO by buckling or edge delamination [1,4]. Two main mechanisms govern these failures, i.e. Al depletion [5] and rumpling of the TGO [1,2]. Nevertheless, the failure induced by the first category rarely occurs due to the adoption of high Al activity bond coat. For a single phase b-(Ni, Pt)Al bond coat, most of the failure modes are associated with the local stresses induced by rumpling. However, for a Pt-enriched c + c 0 bond coat, the failure induced by rumpling is largely suppressed due to its low volume shrinkage during phase transformation, higher creep and yield strength * Corresponding author. Tel.: +44 161 200 5941; fax: +44 161 200 3586; e-mail: [email protected]
[6]. The mechanisms, which can cause the interfacial separation, are still being intensively investigated. Although the thermal barrier systems exhibit several competing mechanisms of failure, most are associated with the oxidation of the bond coat [7]. Similar to other nickel aluminide, platinum-modified aluminide tends to form a metastable phase such as c- or h-Al2O3 in the earlier oxidation stage [8]. Studies have shown that aluminum oxide first forms at low temperatures as an amorphous oxide and then subsequently transforms to the stable, high temperature polymorph, a-Al2O3 [9]. On several bond coat alloys, including NiAl and (Ni, Pt)Al, the amorphous alumina transforms first to c-Al2O3, then to h-Al2O3 and finally to a-Al2O3 [10– 12]. The existence of metastable phases in the TGO has several unfavorable effects such as high growth rate [13] and poor adherence. In addition, if there exists metastable phase in the TGO, the following transformation is geometrically constrained. As each of the phase transformations in alumina is accompanied by a decrease in volume, the stresses generated within the alumina TGO will be tensile and, under favorable conditions, may be sufficient to cause interface separation. As suggested by Clarke, there exists a critical thickness of the untransformed thermally grown oxides above which interfacial separation can occur [14]. As with any compressively stressed film, the eventual failure by buckling requires a region along the interface to be already separated [2,15]. The phase transformation of the TGO could cause the crack formation at
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.08.017
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the interface, which leads to failure eventually. However, the effect of the TBC on the TGO phase transformation has not been systemically investigated. In this contribution, the phase transformation of the TGO in EBPVD TBCs with a c + c 0 bond coat was studied and compared with the bond coat without a TBC, using photo-stimulated luminescence spectroscopy (PSLS) [16,17], coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As will be described, the presence of the TBC not only retards the phase transformation rate of the TGO from theta to alpha, but also changes the TGO morphology. The possible reason for this phenomenon is discussed. The TBCs samples consisted of 7–8 wt.% yttriastabilised zirconia (YSZ) layer with a thickness of about 130–150 lm, and a Pt-enriched c + c 0 bond coat on CMSX-4 (nominal composition of Ni–9.6Co–6.4Cr– 6.4W–6.6Ta–5.6Al–2.9Re–1.03Ti–0.2Hf in wt.%) substrate. The bond coat was lightly grit blasted prior to the deposition of YSZ by EBPVD process and some of the bond coat without deposition was used as comparison. All the samples were cut into 5 · 5 mm with the substrate thickness of 1 mm and subjected to oxidation at 1150 C in air. The crystallographic forms of the TGO were characterized by PSLS at room temperature using the Renishaw Ramanscope 2000 (RenishawTM, UK). During the measurements, the laser (He–Ne, 632.8 nm) was focused on the top surface of a TBC or TGO and the laser spot size was set to be about 2–5 lm. The reference sample was a stress free sapphire single crystal. The TGO thickness was measured by SEM (Philips XL30). The impurities and phases in the TGO were identified by analytical transmission microscopy (TEM/STEM, TecnaiTM G2 F30 U-TWIN). The TEM specimen of the TGO was prepared by cutting across the TGO from the cross-section of TBCs, using a focused ion beam (FIB, FEI Nova 600 DualBeam system); the detailed preparation process was described elsewhere [18]. To investigate the effect of the TBC on the TGO growth rate, both the bond coat with and without a TBC were oxidised at 1150 C. Figure 1 shows the oxi-
Figure 1. (A) TGO thickness as function of oxidation time at 1150 C for bond coats with and without TBC; (B) SEM cross-section of the TGO with EBPVD TBC showing a thicker TGO and (C) SEM crosssection of TGO from oxidation of bond coat without TBC at 1150 C for 20 h.
dation kinetics and cross-sectional images of two typical samples. The TGO growth on a TBC coated bond coat was much faster than that of the uncoated samples of the same composition, which is in agreement with previous studies on oxidation of a b-(Ni, Pt)Al bond coat [7,19]. However, this phenomenon is still poorly understood. The growth kinetics of the TGO could be affected by the composition and roughness of the bond coat [8], whereas both bond coats were fabricated using the same procedures. Thus, the only reason for such different growth rates might be the presence of the TBC. It has been shown that the existence of metastable alumina dramatically increases the TGO oxidation rate [13]. In order to determine whether there was metastable alumina in the TGO, luminescence spectroscopy was used due to its sensitivity to the phase transformation of alumina [16,17]. In this experiment, the PSLS was performed on the TGO through the TBC. For comparison, the luminescence spectra of the TGO on the uncoated bond coat are also presented. As shown in Figure 2(A), the TGO consisted of a-Al2O3 in as-deposited TBCs. However, some h-Al2O3 formed after a short time at 1150 C. According to the relative peak intensity corresponding to h- and a-Al2O3, the fraction of h-Al2O3 in the TGO decreased with increasing oxidation time, while that of a-Al2O3 increased. In contrast, no h-Al2O3 was observed in the TGO from oxidation of the bond coat without a TBC at this temperature (Fig. 2(B)). Therefore, the higher growth rate of the TGO on TBC coated sample could be related to the generation of metastable phase during oxidation. The results presented in Figure 2 were surprising, according to earlier reports on b-(Ni, Pt)Al or NiAl bond coat, the metastable h-Al2O3 typically forms at a low temperature (800–1100 C) [8,13,20], but will transform to stable a-Al2O3 after a short period at 1150 C. However, in this experiment, a significant of h-Al2O3 was produced on a TBC coated c + c 0 bond coat even after oxidation at 1150 C for 20 h, while for the uncoated bond coat, no h-Al2O3 was observed even after oxidation at 1150 C for 5 h. Indeed, the TGO on the uncoated bond coat was determined to be a-Al2O3 even after oxidation at 1000 C for 1 h. Since the fabrication temperature for the bond coat is higher than 1100 C, the primary phase in the TGO is a-Al2O3. Thus, the
Figure 2. Photo-stimulated luminescence spectra from the TGO formed on a c + c 0 bond coat oxidised at 1150 C in air: (A) TBC coated samples and (B) uncoated samples; both were measured from the top surface of the TBC.
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TBC has the ability to slow the phase transformation of the TGO from h- to a-Al2O3. Previous studies has shown that the TGO growth rate also depends on its lateral grain size [21], because the grain boundaries are the fast diffusion paths for oxygen and aluminum ions [22,23]. It is worthwhile investigating the difference in the grain morphology of the TGO grown on these two kinds of samples. As shown in Figure 3, the TGO formed on the TBC coated bond coat exhibits two distinct microstructural domains: columnar grains next to the bond coat and equiaxial grains next to the TBC. Whereas the TGO formed on the bare bond coat almost consists of equiaxial grains. Moreover, the lateral grain size seems larger than that of columnar grains formed on the TBC coated bond coat. Since the TGO consisting of coarser grains has a smaller grain boundary area available for oxygen diffusion [21], it will lead to a slower oxidation rate, as observed with the TGO grown on a bare bond coat. Therefore, the existence of the TBC also affects the TGO grain morphology and results in a columnar structure. Figure 4 shows the PSLS spectra from measurements on a free standing TBC, which was prepared by dissolving the substrate in hydrochloric acid. Two features are particular noteworthy: a significant amount of h-Al2O3 was found in the TGO when measuring from the top surface of the TBC, while no such signal was observed when measuring from the bottom; the measured stresses, which were based on the R2 peak shift of a-Al2O3, from the top are lower than that measured from the bottom (Fig. 4(A)). The first feature implies that the h-Al2O3 might exist at the interface between the TBC and TGO, since the laser was scattered by the YSZ columns, and the signal mainly comes from the top surface of the TGO. In addition, the lower stress measured from the top surface might be attributed to the h-Al2O3 in the TGO. Because the a-Al2O3 usually nucleated within the h-Al2O3 matrix [17], a tensile stress was generated on the a-Al2O3 due to the volume shrinkage from
Figure 3. The TGO microstructure formed on (A) TBC coated bond coat showing columnar grains next to the alloy, while (B) TGO grown on uncoated bond coat exhibiting almost equiaxial grains. Both were oxidised at 1150 C 10 h.
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Figure 4. (A) The PSLS spectra and corresponding stresses measured from the top surface and bottom of a free standing TBC, respectively. (B) Bright field TEM image of the TGO and selected area diffraction patterns showing the presence of h-Al2O3.
h- to a-Al2O3 and balanced part of the compressive stresses applied by the TBC. Therefore, the stress measured from the top is lower than that from bottom. TEM was used to verify the existence of h-Al2O3 in the TGO. As shown by the selected area diffraction pattern in Figure 4(B), the h-Al2O3 was found near the YSZ (equiaxial zone), while the alumina far away from YSZ (columnar zone) was determined to be alpha phase, which is consistent with the PSLS measurement. Ragan and Clarke studied different dopants such as Cr, Fe, Y and Er on the kinetics of c- to a-Al2O3 transformation [24,25]. They found Y, Cr and Er ions could decrease the c- to a-transformation rate and do so in proportion to their concentration. Pint and co-workers investigated the effect of various cation dopants on the h to a transformation of Al2O3 grown on b-NiAl they found the larger ions such as Y, Zr, La and Hf appeared to slow such a transformation [26–28]. Therefore, the presence of Zr ions in TGO was assumed to decrease the phase transformation rate from h to a in this experiment. The energy dispersive spectroscopy analysis using analytical microscopy (STEM) confirmed the presence of Zr ions. As shown in Figure 5(A), Zr does exist in the TGO (the Cu signal comes from the copper grid and the Ga signal is from the Ga ion beam during the milling process by FIB). In addition, the content of Zr was higher near the YSZ than that near the bond coat, and the highest Zr content was found in the grain boundary of the alumina in the TGO near the YSZ/ TGO interface. Therefore, due to the higher Zr concentration near the YSZ, the transformation from h to a was more retarded than that near the bond coat. This explains why most of the h phase exists at the TBC/ TGO interface. Earlier studies on c 0 -Ni3Al have shown that the TGO growth mechanism and kinetics are similar to other alumina forming alloys such as the MCrAl (where M
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The authors would like to thank Prof. R.J. Young for the use of the spectroscopy instrument and Dr. R.M. Langford for the use of the FIB. They would also acknowledge Drs. R. Jones and D.S. Rickerby from Rolls-Royce Plc. for provision of the samples.
Figure 5. (A) Energy dispersive spectra measured by STEM on the different positions of the TGO. (B) Schematic of the diffusion model of bond coat oxidation, the grain boundaries act as ‘short-circuit’ paths.
is Ni, Fe or Co) alloys [11,29]. In the early stage, the TGO growth is mainly inter-diffusion of cation and oxygen, at the same time, the rare earth (RE) elements, such as Zr, Y, begin to segregate to the alumina grain boundary where they block cation transport and possibly promote the oxygen transport [30–32] (schematic in Fig. 5(B)). Thus the RE elements change the TGO growth from the one in which inter-diffusion predominates to one in which oxygen predominates. Because of the primary inward-growth and a preferred growth direction, the microstructure of TGO grown on TBC coated bond coat is further modified by the formation of columnar grains adjacent to the alloy [22]. The standard grit-blasting procedure, used in practice to prepare the bond coat for TBC deposition, has the effect of promoting the formation of an a-Al2O3 TGO [8]. It does, however, also result in a rough surface and nano-sized voids in the TGO. During the TBC deposition, the YSZ forms randomly orientated nanosized nuclei in the rough surface or voids. Meanwhile, either during deposition or an early stage of oxidation, the Al cations diffused outward and encapsulated these YSZ nuclei. Subsequently, a mixed oxidation zone (equiaxial grains) formed near the YSZ/TGO interface [33]. Due to the presence of Zr, the phase transformation from h- to a-Al2O3 was inhibited, thus some of h-Al2O3 was found near the YSZ/TGO interface. On the other hand, the Zr ions segregated to the alumina grain boundary and blocked the outward diffusion of cation ion [30–32], then the TGO growth changed from the metal and oxygen inter-diffusion to the oxygen diffusion dominated process due to the higher diffusivity of O anions in oxides [34]. Since there is no Zr (or a minor amount) far away from the YSZ/TGO interface, the phase transformation from h to a is so quick that only a-Al2O3 was detected. In this paper, the phase transformation of the TGO on a Pt-enriched c + c 0 bond coat was determined by photo-stimulated luminescence spectroscopy. It was found the TBC could inhibit the phase transformation of the TGO from h- to a-Al2O3, affect the TGO grain structure, and result in a higher oxidation rate than the uncoated samples. The concentration of Zr was higher near the YSZ/TGO interface, but lower far away from that interface. Consequently, the h- to a-Al2O3 transformation near the YSZ was more inhibited and led to the presence of h-Al2O3 near the YSZ.
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