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Microstructure investigation on gradient porous thermal barrier coating prepared by EB-PVD
Scripta mater. 44 (2001) 683– 687 www.elsevier.com/locate/scriptamat
MICROSTRUCTURE INVESTIGATION ON GRADIENT POROUS THERMAL BARRIER COATING PREPARED BY EB-PVD Hongbo Guo, Xiaofang Bi, Shengkai Gong and Huibin Xu Department of Materials Science and Engineering, Beijing, University of Aeronautics and Astronautics, Beijing, 100083, China (Received June 6, 2000) (Accepted in revised form October 3, 2000) Keywords: EB-PVD; Gradient thermal barrier coating; Micro-hardness; Micro-porosity
Introduction Thermal barrier coatings (TBCs) are increasingly applied to hot-section components of gas turbines to increase gas turbine inlet temperature (TIT) (1–3). Traditional two-layered thermal barrier coating for turbine blades consists of an oxidation protective bond coat and thermally insulating Yttria Stabilized Zirconia (YSZ) topcoat. The coating is thermally unstable under long time exposure at elevated temperature due to a thermally grown oxide scale (TGO) caused by the formation of alumina at the bond coat/YSZ topcoat interface during selective oxidation of the bond coat (4 –7). Stresses arising from different thermal coefficients between the TGO and bond coat lead to coating failure along the interface between the TGO and the bond coat. Comparing with the traditional two-layered coating, a continuously graded coating has the advantage of forming continuous microstructure along the cross-section of coating and consequently avoiding stress concentrations in the coating. In this paper, gradient thermal barrier coating were prepared by the co-deposition of a tablet of mixtures of Al2O3-YSZ and YSZ onto NiCoCrAlY bond coat by means of EB-PVD and its microstructures was investigated.
Experimental Procedure The substrate material was Ni-based superalloy with the nominal composition listed in Table 1. A Ni-22Co-21Cr-8Al-1Y coating with a thickness of about 40m was first deposited using EB-PVD onto the substrate. Subsequently, a tablet pressed from mixed powders of Al2O3-YSZ and an ingot of ZrO2-8wt.%Y2O3 were evaporated by EB-PVD. As the vapor pressures of the mixtures of the tablet are different at the evaporation temperature, selective evaporation of the molten tablet of Al2O3-YSZ took place: first Al2O3 and then YSZ. As-deposited coatings were heat-treated for 4hr at 1323K. Scanning electron microscope (SEM) with energy disperse spectrum (EDS) was used to analyze the composition distribution across the thickness of the coating and TEM observed the microstructures. The phases in the gradient coating were analyzed by X-ray Diffraction (XRD) with CuK␣ radiation and micro-hardnesses across the thickness of the coating were obtained by micro-hardness tester of ⌸MT-3 with the load of 50g. 1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00646-1
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TABLE 1 Nominal Composition of Substrate of Ni-based Superalloy Ni
Cr
Fe
Ti
Mo
Mn
Si
C
W
Balance
22
33
0.8
2.3
0.6
0.7
0.06
1.5
Results Fig. 1 shows the observation of SEM micrograph across the thickness of the gradient thermal barrier coating along with EDS analysis. In the gradient coating, the left side is a NiCoCrAlY bond coat and then followed by a transition layer with the thickness of about 10m. In the YSZ topcoat, a typical columnar microstructure was observed. According to the EDS analysis, the composition distribution such as Ni, Al, O and Zr changed continuously across the thickness of the coating. From the XRD analysis of the transition layer, the transition layer consists of t-ZrO2 phase and ␣-Al2O3 phase, as shown in Fig. 2. In addition, ␥⬘ ⫹ ␥ phases from Ni-based solid solution in the NiCoCrAlY bond coat can be also detected. Fig. 3 is a magnification of cross-sectional micrograph of the transition layer. In the transition layer, a thin Al2O3 layer with the thickness of less than 2m and a two-phase ZrO2⫹Al2O3 region about 5m are observed and the outer layer is YSZ topcoat. The microstructure of the gradient thermal barrier coating transfers gradually from metallic bond coat to YSZ topcoat, due to the formation of the transition layer. Fig. 4 shows a bright-field image of the two-phase ZrO2⫹Al2O3 region, along with a selected area diffraction pattern from an ␣-Al2O3 particle in this region of the as-deposited coating. It is clear that
Figure 1. Cross-section micrograph and composition distribution of Ni, Al, O and Zr across the thickness of the as-deposited gradient coating.
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Figure 2. XRD patterns of the transition layer in the as-deposited gradient coating.
␣-Al2O3 phase are dispersed in the matrix of ZrO2 in the form of particles. Fig. 5 shows micro-hardness distribution in a traditional two-layered EB-PVD thermal barrier coating and the Al2O3-YSZ gradient coatings deposited at 1123K and 1223K, respectively. The micro-hardness of the two-layered coating is almost the same as that of ceramic layer, while those of the gradient coatings are gradually increasing towards the surface of the YSZ topcoats. The improvement in micro-hardness of the gradient coating attributes to the density increase and the microporosity decrease in the transition region towards the YSZ topcoat (3). Furthermore, the micro-hardness of the gradient coating deposited at 1223K is found to be smaller than those at 1123K. It is considered that the decline in micro-hardness is caused by the formation of porous structure in the gradient coating. Discussion During the evaporation of the molten tablet of mixtures of Al2O3-ZrO2, due to the higher vapor pressure of Al2O3 than that of ZrO2, Al2O3 was first evaporated from the tablet, and then the evaporation of ZrO2 gradually increased with the increase of the evaporating temperature. Deposition of this heterogeneous vapor flow on substrate resulted in the formation of a gradient component concentrations and gradient microstructure across the section of the condensate. The mechanism of micro-pore formation in the two-phase ZrO2⫹Al2O3 region of the coating can be regarded as the result of the “shadow” area formed
Figure 3. SEM micrograph of cross-section of the transition layer in the as-deposited gradient coating.
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Figure 4. (a) Bright-field image of the two-phase Al2O3⫹ZrO2 region of the as-deposited gradient thermal barrier coating. (b) Electron diffraction patterns obtained from Al2O3 particles in this region.
during ZrO2 phase condensation on the Al2O3 surface, as demonstrated in Fig. 6. The micro-porous structure is initially formed in the two-phase ZrO2⫹Al2O3 region, and then, is transformed into the YSZ topcoat in the form of gradient micro-porous structure. Therefore, the distribution of micro-hardness of the gradient coating reveals an increase towards the outer ceramic layer. Furthermore, the rise of substrate temperature (Ts) leads to larger particle size of Al2O3 and greater dimensions of “shadows.” After condensation, coagulation of the “shadow” areas results in formation of micro-pores with higher microporosity in the ceramic layer, accordingly, the lower density of the layer. Therefore, the micro-hardnesses of the gradient coating deposited at 1223K became smaller than those deposited at 1123K. Conclusions A new type of gradient thermal barrier coating was prepared by co-deposition of mixtures of Al2O3-ZrO2 onto NiCoCrAlY bond coat by means of EB-PVD. ●
1. Microstructure of the coating gradually transferred from metallic bond coat to YSZ topcoat and composition distribution across the thickness of the coating changed continuously. In the two-
Figure 5. Micro-hardness distribution in traditional two-Layered thermal barrier coating and the as-deposited Al2O3-YSZ gradient thermal barrier coating deposited at 1123K and 1223K, respectively.
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Figure 6. Mechanism of “shadow” areas formation in the two-phase ZrO2⫹Al2O3 region of the gradient coating during ZrO2 deposition on the surface of Al2O3 particle.
●
phase Al2O3⫹ZrO2 transition region, ␣-Al2O3 phase was dispersed in the matrix of ZrO2 in the form of particle. 2. Due to the formation of gradient micro-porous structure in the gradient coating, micro-hardness distribution in the ceramic layer of the coating revealed an increase towards the outer YSZ topcoat, and with the rise of substrate temperature, the micro-hardness became smaller. Acknowledgments
This research is sponsored by National Natural Science Foundation of China (NSFC) and Aviation Science Foundation of China (ASFC). References 1. 2. 3. 4. 5. 6. 7.
F. Jamarani and M. Korotkin, Surf. Coat. Technol. 54/55, 58 (1992). H. Xu, S. Gong, and L. Deng, Thin Solid Films. 334, 98 (1998). H. Xu, H. Guo, and S. Gong, Surf. Coat. Technol. 130, 133 (2000). B. A. Movchan, I. S. Malashenko, and K. Yu. Yakovchuk, Surf. Coat. Technol. 67, 55 (1994). R. A. Miller and C. E. Lowell, Thin Solid Films. 195, 26 (1982). S. M. Meier, D. M. Nissley, K. D. Sheffler, and T. A. Cruse, Trans. ASME. 114, 258 (1992). Kh. G. Schmitt-Thomas, Surf. Coat. Tech. 68/69, 113 (1994).
MICROSTRUCTURE INVESTIGATION ON GRADIENT POROUS THERMAL BARRIER COATING PREPARED BY EB-PVD Hongbo Guo, Xiaofang Bi, Shengkai Gong and Huibin Xu Department of Materials Science and Engineering, Beijing, University of Aeronautics and Astronautics, Beijing, 100083, China (Received June 6, 2000) (Accepted in revised form October 3, 2000) Keywords: EB-PVD; Gradient thermal barrier coating; Micro-hardness; Micro-porosity
Introduction Thermal barrier coatings (TBCs) are increasingly applied to hot-section components of gas turbines to increase gas turbine inlet temperature (TIT) (1–3). Traditional two-layered thermal barrier coating for turbine blades consists of an oxidation protective bond coat and thermally insulating Yttria Stabilized Zirconia (YSZ) topcoat. The coating is thermally unstable under long time exposure at elevated temperature due to a thermally grown oxide scale (TGO) caused by the formation of alumina at the bond coat/YSZ topcoat interface during selective oxidation of the bond coat (4 –7). Stresses arising from different thermal coefficients between the TGO and bond coat lead to coating failure along the interface between the TGO and the bond coat. Comparing with the traditional two-layered coating, a continuously graded coating has the advantage of forming continuous microstructure along the cross-section of coating and consequently avoiding stress concentrations in the coating. In this paper, gradient thermal barrier coating were prepared by the co-deposition of a tablet of mixtures of Al2O3-YSZ and YSZ onto NiCoCrAlY bond coat by means of EB-PVD and its microstructures was investigated.
Experimental Procedure The substrate material was Ni-based superalloy with the nominal composition listed in Table 1. A Ni-22Co-21Cr-8Al-1Y coating with a thickness of about 40m was first deposited using EB-PVD onto the substrate. Subsequently, a tablet pressed from mixed powders of Al2O3-YSZ and an ingot of ZrO2-8wt.%Y2O3 were evaporated by EB-PVD. As the vapor pressures of the mixtures of the tablet are different at the evaporation temperature, selective evaporation of the molten tablet of Al2O3-YSZ took place: first Al2O3 and then YSZ. As-deposited coatings were heat-treated for 4hr at 1323K. Scanning electron microscope (SEM) with energy disperse spectrum (EDS) was used to analyze the composition distribution across the thickness of the coating and TEM observed the microstructures. The phases in the gradient coating were analyzed by X-ray Diffraction (XRD) with CuK␣ radiation and micro-hardnesses across the thickness of the coating were obtained by micro-hardness tester of ⌸MT-3 with the load of 50g. 1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00646-1
684
GRADED POROUS TBC BY EB-PVD
Vol. 44, No. 4
TABLE 1 Nominal Composition of Substrate of Ni-based Superalloy Ni
Cr
Fe
Ti
Mo
Mn
Si
C
W
Balance
22
33
0.8
2.3
0.6
0.7
0.06
1.5
Results Fig. 1 shows the observation of SEM micrograph across the thickness of the gradient thermal barrier coating along with EDS analysis. In the gradient coating, the left side is a NiCoCrAlY bond coat and then followed by a transition layer with the thickness of about 10m. In the YSZ topcoat, a typical columnar microstructure was observed. According to the EDS analysis, the composition distribution such as Ni, Al, O and Zr changed continuously across the thickness of the coating. From the XRD analysis of the transition layer, the transition layer consists of t-ZrO2 phase and ␣-Al2O3 phase, as shown in Fig. 2. In addition, ␥⬘ ⫹ ␥ phases from Ni-based solid solution in the NiCoCrAlY bond coat can be also detected. Fig. 3 is a magnification of cross-sectional micrograph of the transition layer. In the transition layer, a thin Al2O3 layer with the thickness of less than 2m and a two-phase ZrO2⫹Al2O3 region about 5m are observed and the outer layer is YSZ topcoat. The microstructure of the gradient thermal barrier coating transfers gradually from metallic bond coat to YSZ topcoat, due to the formation of the transition layer. Fig. 4 shows a bright-field image of the two-phase ZrO2⫹Al2O3 region, along with a selected area diffraction pattern from an ␣-Al2O3 particle in this region of the as-deposited coating. It is clear that
Figure 1. Cross-section micrograph and composition distribution of Ni, Al, O and Zr across the thickness of the as-deposited gradient coating.
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Figure 2. XRD patterns of the transition layer in the as-deposited gradient coating.
␣-Al2O3 phase are dispersed in the matrix of ZrO2 in the form of particles. Fig. 5 shows micro-hardness distribution in a traditional two-layered EB-PVD thermal barrier coating and the Al2O3-YSZ gradient coatings deposited at 1123K and 1223K, respectively. The micro-hardness of the two-layered coating is almost the same as that of ceramic layer, while those of the gradient coatings are gradually increasing towards the surface of the YSZ topcoats. The improvement in micro-hardness of the gradient coating attributes to the density increase and the microporosity decrease in the transition region towards the YSZ topcoat (3). Furthermore, the micro-hardness of the gradient coating deposited at 1223K is found to be smaller than those at 1123K. It is considered that the decline in micro-hardness is caused by the formation of porous structure in the gradient coating. Discussion During the evaporation of the molten tablet of mixtures of Al2O3-ZrO2, due to the higher vapor pressure of Al2O3 than that of ZrO2, Al2O3 was first evaporated from the tablet, and then the evaporation of ZrO2 gradually increased with the increase of the evaporating temperature. Deposition of this heterogeneous vapor flow on substrate resulted in the formation of a gradient component concentrations and gradient microstructure across the section of the condensate. The mechanism of micro-pore formation in the two-phase ZrO2⫹Al2O3 region of the coating can be regarded as the result of the “shadow” area formed
Figure 3. SEM micrograph of cross-section of the transition layer in the as-deposited gradient coating.
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Figure 4. (a) Bright-field image of the two-phase Al2O3⫹ZrO2 region of the as-deposited gradient thermal barrier coating. (b) Electron diffraction patterns obtained from Al2O3 particles in this region.
during ZrO2 phase condensation on the Al2O3 surface, as demonstrated in Fig. 6. The micro-porous structure is initially formed in the two-phase ZrO2⫹Al2O3 region, and then, is transformed into the YSZ topcoat in the form of gradient micro-porous structure. Therefore, the distribution of micro-hardness of the gradient coating reveals an increase towards the outer ceramic layer. Furthermore, the rise of substrate temperature (Ts) leads to larger particle size of Al2O3 and greater dimensions of “shadows.” After condensation, coagulation of the “shadow” areas results in formation of micro-pores with higher microporosity in the ceramic layer, accordingly, the lower density of the layer. Therefore, the micro-hardnesses of the gradient coating deposited at 1223K became smaller than those deposited at 1123K. Conclusions A new type of gradient thermal barrier coating was prepared by co-deposition of mixtures of Al2O3-ZrO2 onto NiCoCrAlY bond coat by means of EB-PVD. ●
1. Microstructure of the coating gradually transferred from metallic bond coat to YSZ topcoat and composition distribution across the thickness of the coating changed continuously. In the two-
Figure 5. Micro-hardness distribution in traditional two-Layered thermal barrier coating and the as-deposited Al2O3-YSZ gradient thermal barrier coating deposited at 1123K and 1223K, respectively.
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Figure 6. Mechanism of “shadow” areas formation in the two-phase ZrO2⫹Al2O3 region of the gradient coating during ZrO2 deposition on the surface of Al2O3 particle.
●
phase Al2O3⫹ZrO2 transition region, ␣-Al2O3 phase was dispersed in the matrix of ZrO2 in the form of particle. 2. Due to the formation of gradient micro-porous structure in the gradient coating, micro-hardness distribution in the ceramic layer of the coating revealed an increase towards the outer YSZ topcoat, and with the rise of substrate temperature, the micro-hardness became smaller. Acknowledgments
This research is sponsored by National Natural Science Foundation of China (NSFC) and Aviation Science Foundation of China (ASFC). References 1. 2. 3. 4. 5. 6. 7.
F. Jamarani and M. Korotkin, Surf. Coat. Technol. 54/55, 58 (1992). H. Xu, S. Gong, and L. Deng, Thin Solid Films. 334, 98 (1998). H. Xu, H. Guo, and S. Gong, Surf. Coat. Technol. 130, 133 (2000). B. A. Movchan, I. S. Malashenko, and K. Yu. Yakovchuk, Surf. Coat. Technol. 67, 55 (1994). R. A. Miller and C. E. Lowell, Thin Solid Films. 195, 26 (1982). S. M. Meier, D. M. Nissley, K. D. Sheffler, and T. A. Cruse, Trans. ASME. 114, 258 (1992). Kh. G. Schmitt-Thomas, Surf. Coat. Tech. 68/69, 113 (1994).