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Effects of substrate rotation speed during deposition on the thermal cycle life of thermal barrier coatings fabricated by electron beam physical vapor deposition
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Surface & Coatings Technology 202 (2008) 3507 – 3512 www.elsevier.com/locate/surfcoat
Effects of substrate rotation speed during deposition on the thermal cycle life of thermal barrier coatings fabricated by electron beam physical vapor deposition Mineaki Matsumoto a,⁎, Kunihiko Wada b , Norio Yamaguchi a , Takeharu Kato a , Hideaki Matsubara a a
Materials Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan b Power and Industrial Systems R&D Center, Toshiba Corporation, 2-4, Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan Received 18 July 2007; accepted in revised form 18 December 2007 Available online 2 January 2008
Abstract The effects of substrate rotation speed during deposition of an Y2O3 stabilized ZrO2 (YSZ) layer fabricated by electron beam physical vapor deposition (EB-PVD) on the microstructure, elastic modulus and lifetime were investigated. The microstructure and elastic modulus of EB-PVD YSZ coatings were highly influenced by the rotation speed of the substrates. The elastic modulus of the coatings was found to decrease as the rotation speed was increased, which led to a longer thermal cycle life. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coatings; Physical vapor deposition; Elastic modulus; Sintering
1. Introduction Electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) are widely used to protect the hot section parts of aircraft engine turbines by reducing the temperature of the metal substrates. The current state-of-theart TBCs typically consist of an Y2O3 stabilized ZrO2 (YSZ) top coat and a metallic bond coat. It is well established that the segmented columnar structure of EB-PVD TBCs induces high strain compliance, which results in superior thermal shock resistance [1,2]. The development of columnar structures in EBPVD TBCs is highly influenced by processing parameters such as the substrate rotation speed, deposition rate and substrate temperature [3–8]. Shultz et al. clearly demonstrated that TBCs with columns of intermediate width had longer lives than those of TBCs with columns of coarse or fine width. However, the contribution of each processing parameter (rotation speed, substrate temperature and deposition rate) to the lifetime of the
⁎ Corresponding author. Tel.: +81 52 871 3500; fax: +81 52 871 3599. E-mail address: [email protected] (M. Matsumoto). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.12.035
TBCs remains unclear because these process parameters were changed simultaneously in their study [3]. In this paper, the effects of rotation speed of substrates during deposition on the microstructure, elastic modulus and thermal cycle life of EBPVD TBCs are investigated to clarify the effect of rotation on these properties. 2. Experimental Disk-shaped substrates (Inconel 738LC) with dimensions of φ25 mm × 3 mm were coated with a Ni–Al diffusion layer to a thickness of about 50 μm using an industrial CVD process. A top coat of ZrO2–4 mol%Y2O3 (YSZ) was deposited on the substrates to a thickness of about 200 μm using an EB-PVD coater (Von Ardenne, Tuba 150) consisting of a 150 kW electron gun and separated chambers for loading, preheating and coating. The substrate temperature during deposition was about 950 °C, and the rotation speed of the substrates during deposition was changed in the range of 0∼30 revolutions per minute (rpm). The deposition rates of the coatings performed with and without substrate rotation were about 360 μm/h and 900 μm/h, respectively.
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A scanning electron microscope (SEM, Hitachi S-4500) and a transmission electron microscope (TEM, Topcon EM-002B) were employed to investigate the microstructure of the coatings. The focused ion beam technique was applied for the preparation of site-specific TBC specimens with an emphasis on the columnar grains just above the interface between the substrate and the top coat. The microstructure observation were performed in the plane perpendicular to the axis of rotation during deposition. The effective elastic moduli of the as-deposited and annealed coatings were determined by the indentation technique using a Vickers indenter (Fischerscope, H100 V). The indentation tests were performed on polished cross sections of the samples at a maximum load of 1 N with a holding period duration of 60 s. The elastic modulus was determined from the profile of the load-displacement curve during unloading. It should be noted that the obtained elastic modulus (reduced elastic modulus, Er) is given by, Er ¼
E 1 v2
ð1Þ
where E and ν are the intrinsic elastic modulus and Poissions ratio, respectively. Furnace cycle tests of the samples were performed using a vertical furnace. The heating time was 40 min at 1150 °C, and then the samples were quenched by compressed air for 5 min under 100 °C. The lifetimes of the coatings were determined by the cycle numbers when the coating failure occurred.
3. Results and discussion 3.1. Microstructure Fig. 1 shows the effect of substrate rotation on the morphology of the coatings. The coating deposited at 0 rpm shows a dense structure. In contrast, the columnar structures are
clearly seen in the samples deposited on the rotating substrates. It is pointed out that the EB-PVD YSZ coating deposited at 0 rpm has dense and randomly-oriented columns, whereas the columns produced in the coatings deposited on rotating substrates are oriented in b001N direction and have intercolumnar gaps that are introduced due to the “shadowing effect”. Namely, the tip edge of a columnar grain blocks the vapor flux and produces a shadow during rotation, which results in the formation of inter-columnar gaps [4–7]. For the sample deposited at 1 rpm, the columns have wavy curved structures (hereafter referred to as “C”-shaped structure). These wavy sections, which are periodic throughout the thickness of the coating, have a periodic length corresponding to one revolution of the substrate during deposition [2,4,6,7]. In contrast, the columnar grains of the sample deposited at 20 rpm do not show C-shaped structure. Fig. 2 presents the microstructure at the top coat/bond coat interface of a sample deposited at 10 rpm. It is clear that the periodic length of the C-shaped structure becomes much smaller than that of the sample deposited at 1 rpm. Because the deposition rate of the coatings on rotating substrate was observed to be independent of the rotation speed, the periodic length was inversely proportional to the rotation speed. Therefore, the C-shaped structure becomes smaller as the rotation speed is raised, and the column grains will grow up vertically when the periodic length becomes small enough under high rotation speed [3]. The relationship between the density and rotation speed of the coatings is shown in Fig. 3. The density of the coatings is determined by mass and dimensions measurements. The stationary-deposited coating shows a relative density of about 95%. In contrast, the densities of samples deposited on rotating substrates are 75∼80%, which is about 20% lower than that of the stationary-deposited coating. It is clear that the rotation of the substrates reduces the density of the coatings by causing the formation of inter-columnar gaps.
Fig. 1. Microstructure of EB-PVD YSZ coatings deposited at different rotational speeds. The observation was performed in the plane perpendicular to the axis of rotation during deposition.
M. Matsumoto et al. / Surface & Coatings Technology 202 (2008) 3507–3512
Fig. 2. Microstructure at the top coat/bond coat interface of a sample deposited at 10 rpm.
3.2. Elastic modulus and thermal cycle life The elastic modulus of EB-PVD YSZ will be anisotropic due to the presence of C-shaped structure [2,3] and the difference in the microstructure within the coatings (such as column width) [3]. To avoid the effect of the anisotropy derived from the Cshaped structure, the measurements by indentation technique were performed in the plane perpendicular or parallel to the axis of rotation during deposition. The points of measurements in the coatings were taken to be 20 μm (top) , 100 μm (middle) and 180 μm (bottom) above the interface between the top coat and bond coat in order to evaluate the effect of the difference in microstructure within the coatings on the modulus. The indents were successfully made within 5 μm of the aimed position. The imprint diameter of the indenter was about 20 μm which is much larger than the width of the column grains as shown in Fig. 4, Therefore, the measured values represents the microscopic in-plane elastic moduli of the YSZ layers, which include the effects of several kinds of defects, such as inter-columnar gaps, pores and feather-like structures. Fig. 5 presents the elastic modulus of as-deposited coatings measured in the plane perpendicular to the rotation axis. Five
Fig. 3. Density of EB-PVD YSZ coatings as a function of substrate rotation speed during deposition.
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Fig. 4. SEM images of Vickers indenter imprints: a) at a surface side (top) and b) at bond coat side (bottom). Note that the imprint diameter is much larger than the column width.
measurements were performed for each position. The obtained moduli are 30∼130 GPa, which are similar values reported for EB-PVD TBCs by other researchers and much lower than those of YSZ bulk materials (200∼250 GPa) [2,8]. The stationary-deposited sample shows high elastic modulus, and the modulus decreases as the rotation speed is increased. It is clear that the gaps introduced by the rotation effectively decrease elastic modulus of the coatings, leading to a coating with a highly compliant microstructure. The measurement performed in the plane parallel to the rotation axis showed the same tendency, although the value of the modulus was slightly lower. There are several possibilities which can account for the decrease in elastic modulus observed with increasing rotation speed. Wada et al. pointed out that the gap area between the columns of coatings grown with rotation becomes larger as the rotation speed is raised [10]. In this scenario, the density of the coatings must decrease as the rotation speed is increased. There
Fig. 5. Elastic modulus of as-deposited coatings as a function of substrate rotation speed during deposition. The measurements were performed in the plane perpendicular to the axis of rotation during deposition. The points of measurements in the coatings were taken to be 20 μm (top) , 100 μm (middle) and 180 μm (bottom) above the interface between the top coat and bond coat.
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Fig. 6. Cross sectional image of columnar grains formed in an EB-PVD YSZ coating deposited at a) 1 rpm and b) 20 rpm. The observed areas were about 20 μm above the interface between the top coat and bond coat.
are some papers that have reported that the density of EB-PVD YSZ coatings decrease with increasing rotation speed [7]. In this study, however, the measured density of the coatings grown with rotation does not show such tendency. This may be due to the difficulty in measuring the density of the coatings, which results in the large amount of scatter in the measurements as shown in Fig. 3, and makes it difficult to prove the scenario proposed by Wada et al. Another possible reason is that the uniform distribution of inter-columnar gaps obtained at high rotation speeds lowers the effective elastic modulus of the coatings. Fig. 6 compares microstructures of coatings deposited at 1 rpm and 20 rpm at higher magnification. As shown in Figs. 1 and 6, the coating deposited at 1 rpm is not uniformly porous but the porosity appears to be preferentially accumulated in the regions of the highest VIA conditions (the VIA is near 0° or 180°). Namely, the columns become a dense structure when the vapor incident
Fig. 7. Substrate temperature during deposition of EB-PVD YSZ coating: a) 1 rpm and b) 20 rpm.
angle (VIA) is 90°, whereas a porous structure is formed when the VIA is nearer to 0° or 180°. The adjacent columns are considered to be in strong contact with each other in denser regions formed at the VIA is about 90° (the middle part of the Cshaped structure), which will lead to an increase in the modulus of the coatings. At high rotation speed, the morphology of inter-columnar porosity changes from distinct inverse “C” patterns to rows of elongated vertical voids, and the adjacent columns are scarcely in contact with each other as shown in Fig. 6. Therefore, the high rotation speed will produce highly compliant microstructure with a low in-plane modulus by introducing the vertical inter-columnar gaps. In addition, a distribution in the substrate temperature of about 3∼4°C corresponding to the rotation of substrates during deposition was observed at low rotation speeds as shown in Fig. 7. The surface temperature of coatings during deposition at low rotation speed will show much larger distribution. This temperature distribution occurred because the substrate temperature is strongly affected by the radiation emitted from the melt
Fig. 8. Effects of annealing on the elastic modulus of coatings produced by APS and EB-PVD. The measured positions in the coatings were about 20 μm above the top coat/bond coat interface.
M. Matsumoto et al. / Surface & Coatings Technology 202 (2008) 3507–3512
Fig. 9. Thermal cycle life of EB-PVD YSZ coatings as a function of substrate rotation speed during deposition.
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was correlated strongly with the elastic modulus of EB-PVD YSZ coatings [2]. In this case, the longer life of the coatings grown under high rotational speed conditions is attributed to the lower residual stress achieved by the control of the columnar structure. Recently, Suzuki et al. evaluated residual stresses of the EB-PVD YSZ coatings by X-ray diffraction method, and found that the residual stress decreased as the rotation speed during deposition is raised [13]. Their report strongly supports our results. The disappearance of C-shaped structure at high rotation speed also leads to a decrease in harmful defects because the accumulated inter-columnar pores formed at low rotation speed, as shown in Figs. 1 and 6, can act as crack initiations and paths at the top coat/bond coat interface. In terms of the defect size, it is also beneficial to deposit coatings under high rotation speed conditions. 4. Conclusions
pool of ZrO2 and the condensation heat of vapor. This increase in substrate temperature during deposition was occurred when the VIA was around 90° because the effective area of the substrate facing the melt pool is at its largest. The condition at VIA = 90° also corresponds to the formation of the densest region of the Cshaped structure, which is expected to enhance the formation and sintering of this particular dense region. On the contrary, a uniform substrate temperature of about 950 °C was observed at high rotation speed because the total time that the substrate faced the melt pool of ZrO2 per rotation became shorter. The uniform substrate temperature reduces surface diffusion and sintering of the columns, and will also lower the modulus of the coatings. It is difficult to separate these factors from each other but these reasons would all contribute to the observed reduction in the modulus of the coatings. The elastic moduli of the coatings after annealing at 1100 °C and 1150 °C for 10 h are compared in Fig. 8, where the modulus of the YSZ coating produced by air plasma spraying (APS) are also shown for comparison. After the annealing, the APS coating shows a significant increase in modulus of about 200% due to sintering of splat boundaries as previously reported [11]. For the EB-PVD coatings, the stationary-deposited coating also shows a significant increase in modulus due to sintering. In contrast, the moduli of samples deposited on rotating substrates increased only 10∼20% by the heat treatments, showing that EB-PVD coatings has high resistance to sintering. The reason for this sintering resistance is due to the fact that the coatings deposited on rotating substrates have larger content of vertical inter-columnar porosity up to 25%. Furthermore, Wada et al. revealed that the columns shows complicated sintering behavior, referred to as “mud cracking” [9]. Namely, the local gap closure induces gap openings in other regions of the EBPVD coating during sintering under conditions constrained by substrates [9,12], which makes the macroscopic elastic modulus almost constant [9]. Fig. 9 shows the effect of substrate rotation speed on the thermal cycle life of coatings, where three specimens were tested for each sample. It is apparent that the life increases as the rotation speed is increased. It is reported that the residual stress
The microstructure, elastic modulus and thermal cycle life of ZrO2–4 mol%Y2O3 coatings produced by EB-PVD were investigated as a function of substrate rotation speed during deposition. The results are summarized as follows; (1) In the case of the stationary deposition, the obtained coatings show a dense structure with a relative density of about 95%. In contrast, a columnar structure with inter-columnar gaps is introduced in coatings deposited on rotating substrates. The columnar grains show a “C”-shaped structure, which straightens and becomes more uniform as the rotating speed is raised. (2) The thermal cycle life of the coatings is prolonged as the rotation speed is raised. Higher substrate rotation speeds leads to the formation of elongated vertical gaps, which lead to a decrease in the elastic modulus and the defect size of the coatings. It is also shown that the coatings grown on rotating substrates show slower increase in modulus at high temperature in comparison with the stationary-deposited and plasmasprayed coatings. The low elastic modulus, high resistance to sintering and smaller defect size of coatings deposited at high rotation speeds are considered to be the main reasons for the obtained longer thermal cycle life. Acknowledgement This work was supported by NEDO under the “Nanotechnology Materials Program/the Nanostructure Coating Project” promoted by METI, Japan. References [1] M. Peter, C. Leyens, U. Schulz, W.A. Kaysser, Adv. Eng. Mater. 3 (2001) 193. [2] C.A. Johnson, J.A. Ruun, R. Bruce, D. Wortman, J. Surf. Coat. Tech. 108–109 (1998) 80. [3] U. Shulz, K. Fritscher, C. Leyens, M. Peters, J. Eng. Gas Turbines Power 124 (2002) 229. [4] J. Cho, S.G. Terry, R. LeSar, C.G. Levi, Mater. Sci. Eng. A391 (2005) 390. [5] U. Schulz, S.G. Terry, C.G. Levi, Mater. Sci. Eng. A360 (2003) 319. [6] K. Wada, N. Yamaguchi, H. Matsubara, Surf. Coat.Tech. 191 (2005) 367. [7] N. Yamaguchi, K. Wada, K. Kimura, H. Matsubara, J. Ceram. Soc. Jpn. 111 (2003) 882.
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[8] U. Schulz, J. Munzer, U. Kaden, Ceramic Eng. Sci. Proc. (2002) 353. [9] K. Wada, Y. Ishiwata, T. Iwahashi, M. Fukuda, H. Matsubara, Proc. of the Asian Congress on Gas Turbines (2005). [10] K. Wada, Y. Ishiwata, T. Iwahashi, M. Fukuda, H. Matsubara, Materia Japan 45 (2006) 222.
[11] S. Guo, Y. Kagawa, Scripta Mater. 50 (2004) 1401. [12] V. Lughi, V.K. Tolpygo, D.R. Clarke, Mater. Sci. Eng. A368 (2004) 212. [13] K. Suzuki, K. Wada, H. Matsubara, T. Shobu, M. Kawamura, K. Tanaka, J. Mater. Soc. Jpn. 56 (2007) 588.
Surface & Coatings Technology 202 (2008) 3507 – 3512 www.elsevier.com/locate/surfcoat
Effects of substrate rotation speed during deposition on the thermal cycle life of thermal barrier coatings fabricated by electron beam physical vapor deposition Mineaki Matsumoto a,⁎, Kunihiko Wada b , Norio Yamaguchi a , Takeharu Kato a , Hideaki Matsubara a a
Materials Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan b Power and Industrial Systems R&D Center, Toshiba Corporation, 2-4, Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan Received 18 July 2007; accepted in revised form 18 December 2007 Available online 2 January 2008
Abstract The effects of substrate rotation speed during deposition of an Y2O3 stabilized ZrO2 (YSZ) layer fabricated by electron beam physical vapor deposition (EB-PVD) on the microstructure, elastic modulus and lifetime were investigated. The microstructure and elastic modulus of EB-PVD YSZ coatings were highly influenced by the rotation speed of the substrates. The elastic modulus of the coatings was found to decrease as the rotation speed was increased, which led to a longer thermal cycle life. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coatings; Physical vapor deposition; Elastic modulus; Sintering
1. Introduction Electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) are widely used to protect the hot section parts of aircraft engine turbines by reducing the temperature of the metal substrates. The current state-of-theart TBCs typically consist of an Y2O3 stabilized ZrO2 (YSZ) top coat and a metallic bond coat. It is well established that the segmented columnar structure of EB-PVD TBCs induces high strain compliance, which results in superior thermal shock resistance [1,2]. The development of columnar structures in EBPVD TBCs is highly influenced by processing parameters such as the substrate rotation speed, deposition rate and substrate temperature [3–8]. Shultz et al. clearly demonstrated that TBCs with columns of intermediate width had longer lives than those of TBCs with columns of coarse or fine width. However, the contribution of each processing parameter (rotation speed, substrate temperature and deposition rate) to the lifetime of the
⁎ Corresponding author. Tel.: +81 52 871 3500; fax: +81 52 871 3599. E-mail address: [email protected] (M. Matsumoto). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.12.035
TBCs remains unclear because these process parameters were changed simultaneously in their study [3]. In this paper, the effects of rotation speed of substrates during deposition on the microstructure, elastic modulus and thermal cycle life of EBPVD TBCs are investigated to clarify the effect of rotation on these properties. 2. Experimental Disk-shaped substrates (Inconel 738LC) with dimensions of φ25 mm × 3 mm were coated with a Ni–Al diffusion layer to a thickness of about 50 μm using an industrial CVD process. A top coat of ZrO2–4 mol%Y2O3 (YSZ) was deposited on the substrates to a thickness of about 200 μm using an EB-PVD coater (Von Ardenne, Tuba 150) consisting of a 150 kW electron gun and separated chambers for loading, preheating and coating. The substrate temperature during deposition was about 950 °C, and the rotation speed of the substrates during deposition was changed in the range of 0∼30 revolutions per minute (rpm). The deposition rates of the coatings performed with and without substrate rotation were about 360 μm/h and 900 μm/h, respectively.
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M. Matsumoto et al. / Surface & Coatings Technology 202 (2008) 3507–3512
A scanning electron microscope (SEM, Hitachi S-4500) and a transmission electron microscope (TEM, Topcon EM-002B) were employed to investigate the microstructure of the coatings. The focused ion beam technique was applied for the preparation of site-specific TBC specimens with an emphasis on the columnar grains just above the interface between the substrate and the top coat. The microstructure observation were performed in the plane perpendicular to the axis of rotation during deposition. The effective elastic moduli of the as-deposited and annealed coatings were determined by the indentation technique using a Vickers indenter (Fischerscope, H100 V). The indentation tests were performed on polished cross sections of the samples at a maximum load of 1 N with a holding period duration of 60 s. The elastic modulus was determined from the profile of the load-displacement curve during unloading. It should be noted that the obtained elastic modulus (reduced elastic modulus, Er) is given by, Er ¼
E 1 v2
ð1Þ
where E and ν are the intrinsic elastic modulus and Poissions ratio, respectively. Furnace cycle tests of the samples were performed using a vertical furnace. The heating time was 40 min at 1150 °C, and then the samples were quenched by compressed air for 5 min under 100 °C. The lifetimes of the coatings were determined by the cycle numbers when the coating failure occurred.
3. Results and discussion 3.1. Microstructure Fig. 1 shows the effect of substrate rotation on the morphology of the coatings. The coating deposited at 0 rpm shows a dense structure. In contrast, the columnar structures are
clearly seen in the samples deposited on the rotating substrates. It is pointed out that the EB-PVD YSZ coating deposited at 0 rpm has dense and randomly-oriented columns, whereas the columns produced in the coatings deposited on rotating substrates are oriented in b001N direction and have intercolumnar gaps that are introduced due to the “shadowing effect”. Namely, the tip edge of a columnar grain blocks the vapor flux and produces a shadow during rotation, which results in the formation of inter-columnar gaps [4–7]. For the sample deposited at 1 rpm, the columns have wavy curved structures (hereafter referred to as “C”-shaped structure). These wavy sections, which are periodic throughout the thickness of the coating, have a periodic length corresponding to one revolution of the substrate during deposition [2,4,6,7]. In contrast, the columnar grains of the sample deposited at 20 rpm do not show C-shaped structure. Fig. 2 presents the microstructure at the top coat/bond coat interface of a sample deposited at 10 rpm. It is clear that the periodic length of the C-shaped structure becomes much smaller than that of the sample deposited at 1 rpm. Because the deposition rate of the coatings on rotating substrate was observed to be independent of the rotation speed, the periodic length was inversely proportional to the rotation speed. Therefore, the C-shaped structure becomes smaller as the rotation speed is raised, and the column grains will grow up vertically when the periodic length becomes small enough under high rotation speed [3]. The relationship between the density and rotation speed of the coatings is shown in Fig. 3. The density of the coatings is determined by mass and dimensions measurements. The stationary-deposited coating shows a relative density of about 95%. In contrast, the densities of samples deposited on rotating substrates are 75∼80%, which is about 20% lower than that of the stationary-deposited coating. It is clear that the rotation of the substrates reduces the density of the coatings by causing the formation of inter-columnar gaps.
Fig. 1. Microstructure of EB-PVD YSZ coatings deposited at different rotational speeds. The observation was performed in the plane perpendicular to the axis of rotation during deposition.
M. Matsumoto et al. / Surface & Coatings Technology 202 (2008) 3507–3512
Fig. 2. Microstructure at the top coat/bond coat interface of a sample deposited at 10 rpm.
3.2. Elastic modulus and thermal cycle life The elastic modulus of EB-PVD YSZ will be anisotropic due to the presence of C-shaped structure [2,3] and the difference in the microstructure within the coatings (such as column width) [3]. To avoid the effect of the anisotropy derived from the Cshaped structure, the measurements by indentation technique were performed in the plane perpendicular or parallel to the axis of rotation during deposition. The points of measurements in the coatings were taken to be 20 μm (top) , 100 μm (middle) and 180 μm (bottom) above the interface between the top coat and bond coat in order to evaluate the effect of the difference in microstructure within the coatings on the modulus. The indents were successfully made within 5 μm of the aimed position. The imprint diameter of the indenter was about 20 μm which is much larger than the width of the column grains as shown in Fig. 4, Therefore, the measured values represents the microscopic in-plane elastic moduli of the YSZ layers, which include the effects of several kinds of defects, such as inter-columnar gaps, pores and feather-like structures. Fig. 5 presents the elastic modulus of as-deposited coatings measured in the plane perpendicular to the rotation axis. Five
Fig. 3. Density of EB-PVD YSZ coatings as a function of substrate rotation speed during deposition.
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Fig. 4. SEM images of Vickers indenter imprints: a) at a surface side (top) and b) at bond coat side (bottom). Note that the imprint diameter is much larger than the column width.
measurements were performed for each position. The obtained moduli are 30∼130 GPa, which are similar values reported for EB-PVD TBCs by other researchers and much lower than those of YSZ bulk materials (200∼250 GPa) [2,8]. The stationary-deposited sample shows high elastic modulus, and the modulus decreases as the rotation speed is increased. It is clear that the gaps introduced by the rotation effectively decrease elastic modulus of the coatings, leading to a coating with a highly compliant microstructure. The measurement performed in the plane parallel to the rotation axis showed the same tendency, although the value of the modulus was slightly lower. There are several possibilities which can account for the decrease in elastic modulus observed with increasing rotation speed. Wada et al. pointed out that the gap area between the columns of coatings grown with rotation becomes larger as the rotation speed is raised [10]. In this scenario, the density of the coatings must decrease as the rotation speed is increased. There
Fig. 5. Elastic modulus of as-deposited coatings as a function of substrate rotation speed during deposition. The measurements were performed in the plane perpendicular to the axis of rotation during deposition. The points of measurements in the coatings were taken to be 20 μm (top) , 100 μm (middle) and 180 μm (bottom) above the interface between the top coat and bond coat.
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Fig. 6. Cross sectional image of columnar grains formed in an EB-PVD YSZ coating deposited at a) 1 rpm and b) 20 rpm. The observed areas were about 20 μm above the interface between the top coat and bond coat.
are some papers that have reported that the density of EB-PVD YSZ coatings decrease with increasing rotation speed [7]. In this study, however, the measured density of the coatings grown with rotation does not show such tendency. This may be due to the difficulty in measuring the density of the coatings, which results in the large amount of scatter in the measurements as shown in Fig. 3, and makes it difficult to prove the scenario proposed by Wada et al. Another possible reason is that the uniform distribution of inter-columnar gaps obtained at high rotation speeds lowers the effective elastic modulus of the coatings. Fig. 6 compares microstructures of coatings deposited at 1 rpm and 20 rpm at higher magnification. As shown in Figs. 1 and 6, the coating deposited at 1 rpm is not uniformly porous but the porosity appears to be preferentially accumulated in the regions of the highest VIA conditions (the VIA is near 0° or 180°). Namely, the columns become a dense structure when the vapor incident
Fig. 7. Substrate temperature during deposition of EB-PVD YSZ coating: a) 1 rpm and b) 20 rpm.
angle (VIA) is 90°, whereas a porous structure is formed when the VIA is nearer to 0° or 180°. The adjacent columns are considered to be in strong contact with each other in denser regions formed at the VIA is about 90° (the middle part of the Cshaped structure), which will lead to an increase in the modulus of the coatings. At high rotation speed, the morphology of inter-columnar porosity changes from distinct inverse “C” patterns to rows of elongated vertical voids, and the adjacent columns are scarcely in contact with each other as shown in Fig. 6. Therefore, the high rotation speed will produce highly compliant microstructure with a low in-plane modulus by introducing the vertical inter-columnar gaps. In addition, a distribution in the substrate temperature of about 3∼4°C corresponding to the rotation of substrates during deposition was observed at low rotation speeds as shown in Fig. 7. The surface temperature of coatings during deposition at low rotation speed will show much larger distribution. This temperature distribution occurred because the substrate temperature is strongly affected by the radiation emitted from the melt
Fig. 8. Effects of annealing on the elastic modulus of coatings produced by APS and EB-PVD. The measured positions in the coatings were about 20 μm above the top coat/bond coat interface.
M. Matsumoto et al. / Surface & Coatings Technology 202 (2008) 3507–3512
Fig. 9. Thermal cycle life of EB-PVD YSZ coatings as a function of substrate rotation speed during deposition.
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was correlated strongly with the elastic modulus of EB-PVD YSZ coatings [2]. In this case, the longer life of the coatings grown under high rotational speed conditions is attributed to the lower residual stress achieved by the control of the columnar structure. Recently, Suzuki et al. evaluated residual stresses of the EB-PVD YSZ coatings by X-ray diffraction method, and found that the residual stress decreased as the rotation speed during deposition is raised [13]. Their report strongly supports our results. The disappearance of C-shaped structure at high rotation speed also leads to a decrease in harmful defects because the accumulated inter-columnar pores formed at low rotation speed, as shown in Figs. 1 and 6, can act as crack initiations and paths at the top coat/bond coat interface. In terms of the defect size, it is also beneficial to deposit coatings under high rotation speed conditions. 4. Conclusions
pool of ZrO2 and the condensation heat of vapor. This increase in substrate temperature during deposition was occurred when the VIA was around 90° because the effective area of the substrate facing the melt pool is at its largest. The condition at VIA = 90° also corresponds to the formation of the densest region of the Cshaped structure, which is expected to enhance the formation and sintering of this particular dense region. On the contrary, a uniform substrate temperature of about 950 °C was observed at high rotation speed because the total time that the substrate faced the melt pool of ZrO2 per rotation became shorter. The uniform substrate temperature reduces surface diffusion and sintering of the columns, and will also lower the modulus of the coatings. It is difficult to separate these factors from each other but these reasons would all contribute to the observed reduction in the modulus of the coatings. The elastic moduli of the coatings after annealing at 1100 °C and 1150 °C for 10 h are compared in Fig. 8, where the modulus of the YSZ coating produced by air plasma spraying (APS) are also shown for comparison. After the annealing, the APS coating shows a significant increase in modulus of about 200% due to sintering of splat boundaries as previously reported [11]. For the EB-PVD coatings, the stationary-deposited coating also shows a significant increase in modulus due to sintering. In contrast, the moduli of samples deposited on rotating substrates increased only 10∼20% by the heat treatments, showing that EB-PVD coatings has high resistance to sintering. The reason for this sintering resistance is due to the fact that the coatings deposited on rotating substrates have larger content of vertical inter-columnar porosity up to 25%. Furthermore, Wada et al. revealed that the columns shows complicated sintering behavior, referred to as “mud cracking” [9]. Namely, the local gap closure induces gap openings in other regions of the EBPVD coating during sintering under conditions constrained by substrates [9,12], which makes the macroscopic elastic modulus almost constant [9]. Fig. 9 shows the effect of substrate rotation speed on the thermal cycle life of coatings, where three specimens were tested for each sample. It is apparent that the life increases as the rotation speed is increased. It is reported that the residual stress
The microstructure, elastic modulus and thermal cycle life of ZrO2–4 mol%Y2O3 coatings produced by EB-PVD were investigated as a function of substrate rotation speed during deposition. The results are summarized as follows; (1) In the case of the stationary deposition, the obtained coatings show a dense structure with a relative density of about 95%. In contrast, a columnar structure with inter-columnar gaps is introduced in coatings deposited on rotating substrates. The columnar grains show a “C”-shaped structure, which straightens and becomes more uniform as the rotating speed is raised. (2) The thermal cycle life of the coatings is prolonged as the rotation speed is raised. Higher substrate rotation speeds leads to the formation of elongated vertical gaps, which lead to a decrease in the elastic modulus and the defect size of the coatings. It is also shown that the coatings grown on rotating substrates show slower increase in modulus at high temperature in comparison with the stationary-deposited and plasmasprayed coatings. The low elastic modulus, high resistance to sintering and smaller defect size of coatings deposited at high rotation speeds are considered to be the main reasons for the obtained longer thermal cycle life. Acknowledgement This work was supported by NEDO under the “Nanotechnology Materials Program/the Nanostructure Coating Project” promoted by METI, Japan. References [1] M. Peter, C. Leyens, U. Schulz, W.A. Kaysser, Adv. Eng. Mater. 3 (2001) 193. [2] C.A. Johnson, J.A. Ruun, R. Bruce, D. Wortman, J. Surf. Coat. Tech. 108–109 (1998) 80. [3] U. Shulz, K. Fritscher, C. Leyens, M. Peters, J. Eng. Gas Turbines Power 124 (2002) 229. [4] J. Cho, S.G. Terry, R. LeSar, C.G. Levi, Mater. Sci. Eng. A391 (2005) 390. [5] U. Schulz, S.G. Terry, C.G. Levi, Mater. Sci. Eng. A360 (2003) 319. [6] K. Wada, N. Yamaguchi, H. Matsubara, Surf. Coat.Tech. 191 (2005) 367. [7] N. Yamaguchi, K. Wada, K. Kimura, H. Matsubara, J. Ceram. Soc. Jpn. 111 (2003) 882.
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[8] U. Schulz, J. Munzer, U. Kaden, Ceramic Eng. Sci. Proc. (2002) 353. [9] K. Wada, Y. Ishiwata, T. Iwahashi, M. Fukuda, H. Matsubara, Proc. of the Asian Congress on Gas Turbines (2005). [10] K. Wada, Y. Ishiwata, T. Iwahashi, M. Fukuda, H. Matsubara, Materia Japan 45 (2006) 222.
[11] S. Guo, Y. Kagawa, Scripta Mater. 50 (2004) 1401. [12] V. Lughi, V.K. Tolpygo, D.R. Clarke, Mater. Sci. Eng. A368 (2004) 212. [13] K. Suzuki, K. Wada, H. Matsubara, T. Shobu, M. Kawamura, K. Tanaka, J. Mater. Soc. Jpn. 56 (2007) 588.