- Email: [email protected]
Engineered thermal barrier coatings deposited by suspension plasma spray
Accepted Manuscript Engineered Thermal Barrier Coatings deposited by Suspension Plasma Spray Satyapal Mahade, Nicholas Curry, Stefan Björklund, Nicolaie Markocsan, Per Nylén PII: DOI: Reference:
S0167-577X(17)31294-6 http://dx.doi.org/10.1016/j.matlet.2017.08.096 MLBLUE 23073
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 February 2017 10 August 2017 23 August 2017
Please cite this article as: S. Mahade, N. Curry, S. Björklund, N. Markocsan, P. Nylén, Engineered Thermal Barrier Coatings deposited by Suspension Plasma Spray, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet. 2017.08.096
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Engineered Thermal Barrier Coatings deposited by Suspension Plasma Spray Satyapal Mahade a*, Nicholas Curry b, Stefan Björklund a, Nicolaie Markocsan a, Per Nylén a a
Department of Engineering Science, University West, Sweden b
Treibacher Industrie AG, Austria
Abstract: Yttria stabilized zirconia (YSZ) is susceptible to CMAS (Calcium Magnesium Alumino Silicates) attack at high temperatures (>1200°C) which limits its durability. New ceramic materials which can overcome these high temperature challenges are highly desirable. This work investigates the feasibility of depositing two variations of three ceramic layered thermal barrier coatings. The first variation comprised of yttria as the top ceramic layer with gadolinium zirconate (GZ) as the intermediate layer and YSZ as the base layer. The second variation comprised of Yttrium Aluminum Garnet (YAG) as the top layer with gadolinium zirconate as the intermediate layer and YSZ as the base layer. Microstructural analysis of the as sprayed three layered TBCs were performed by SEM/EDS. Columnar microstructures with a relatively dense top layer were obtained in both the variations. The porosity content of the TBCs was measured by water intrusion and image analysis methods. Phase composition of each layer of the as sprayed TBCs was analyzed using XRD. YAG showed an amorphous phase whereas GZ showed a cubic defect fluorite phase and tetragonal phase was observed in YSZ. In the case of yttria, monoclinic and cubic phases were observed. Keywords: Gadolinium Zirconate; Suspension Plasma Spray; Yttria; Yttrium Aluminum Garnet;
1. Introduction Thermal barrier coatings (TBCs) are used in gas turbine engine applications. The efficiency of a gas turbine can be increased by opting for higher turbine inlet temperature [1]. Yttria stabilized zirconia (YSZ) is the most widely used TBC top coat material. However, above 1200°C, YSZ is susceptible to CMAS (Calcium Magnesium Alumino Silicates) degradation which results in reduced TBC durability [2]. The failure mechanism of YSZ based TBCs on interaction with CMAS is reported to be due to the selective leaching of yttria from the YSZ which results in transformation of tetragonal to monoclinic phase during cooling [2]. The phase transformation is associated with a volume change of approximately 3-5% which results in TBC spallation. Additionally, the molten CMAS infiltrates and stiffens the TBC upon cooling which results in loss of strain tolerance. Therefore, it is of interest to find alternate top coat materials for mitigating CMAS attack. Pyrochlores (gadolinium zirconate and lanthanum zirconate), Yttrium aluminum garnet (YAG), and Perovskites have been identified as the new ceramic materials which have the potential to replace YSZ [3]. However, pyrochlores are known to be inferior to YSZ in terms of erosion resistance [4] [5]. Additionally, gadolinium zirconate (GZ) has thermochemical compatibility issues with the thermally grown oxide [6]. Therefore, YSZ is used as a base layer. Suspension plasma spray (SPS) is a promising technique in depositing TBCs with columnar microstructures. Ramachandran et al. showed that a dense TBC layer can improve the erosion resistance compared to the relatively porous TBC [4]. Dense top layer in a TBC can also help in improving the CMAS attack resistance by obstructing the infiltration. Therefore, a dense layer on top of a columnar microstructure TBC seems to be desirable for enhancing the CMAS and erosion resistance. Previously, attempts were made to deposit GZ based triple layer TBC by SPS process which showed an improvement in the thermal cyclic life [7]. However, the dense GZ layer in the triple layer TBCs did not seem to improve the erosion resistance [5]. To the author’s knowledge, no attempt has been made to deposit three layered TBCs with yttria and YAG as the top dense layers by SPS process. This work attempts to deposit two variations of three layered TBCs comprising of a thin yttria and YAG top layers on intermediate GZ layer and YSZ base layer. The purpose of depositing yttria on top was to present a reservoir of yttria rich phase for improving the CMAS attack resistance. The purpose of depositing YAG layer was to explore its capability to withstand the CMAS attack. Additionally, a dense top layer in a TBC could help to improve the erosion resistance compared to the porous layer [4]. 2. Experimental details Four different suspensions, made by Treibacher Industrie AG, Austria, were used in this work. 8 wt% YSZ and GZ suspensions were supplied with a median particle size of 500nm and 25% solid loading in ethanol. The YAG suspension contained particles with a median size of 3
µm and 40% solid load in ethanol. Finally, the Yttrium Oxide suspension contained particles with a median size of 2.5 µm at 40% solid load in water. The TBCs were processed using an axial suspension plasma spray equipment with the Axial III Mettech Gun (Northwest Mettech Corp., Canada). Two different triple layer TBC architectures were deposited, as shown in Fig.1. The as sprayed TBC specimens were polished and observed in the SEM/EDS (Hitachi TM3000, Japan and Bruker for EDS). Free standing coatings were obtained and the porosity content of the TBC was measured according to the procedure discussed in our previous work [7]. Additionally, porosity content of the as sprayed TBCs was measured by image analysis method using the Image J software [8]. Twenty five different SEM micrographs at 600X were considered. XRD analysis was carried out using Seifert-TT 3003 equipment. 3. Results and Discussion Microstructural Analysis In the SEM/EDS analysis of the cross section of the as sprayed Yttria/GZ/YSZ TBC, a columnar microstructure of GZ and YSZ was observed, as shown in Fig.2 (a). The thickness of YSZ and GZ layers were 50±4 µm and 190±4 µm respectively. The top layer of yttria was found to be approximately 40±3 µm. The elemental maps of yttrium (Y), zirconium (Zr), and gadolinium (Gd) is presented along with the cross sectional SEM micrograph, as seen in Fig. 2. In the top view SEM micrograph, as seen in fig 2(b), a dense microstructure comprising dimples which happen to be the top of the columns and the area surrounding it, was observed. In the case of YAG/GZ/YSZ triple layer TBC, as shown in Fig 3 (a), a columnar microstructure of GZ/YSZ was observed. The top YAG layer was found to be approximately 30±5 µm. It could also be seen that the YAG layer showed horizontal cracks in the microstructure which could be due to the stresses generated from the liquid to solid transformation during spraying. The elemental maps of Aluminum (Al), Gadolinium (Gd), Yttrium (Y) and Zirconium (Zr) are also shown in Fig. (3). The top surface morphology of the YAG/GZ/YSZ triple layer TBC showed a relatively dense microstructure compared to the previously reported top view microstructures of YSZ and GZ [7]. Porosity Measurement The yttria top layered TBC showed a higher porosity content (approximately 14%) compared to the YAG (11%) based TBC by water intrusion method. The reason for difference in the porosity (approximately 3%) of YAG compared to yttria is not clearly understood and further investigation needs to be done in order to understand the inflight particle characteristics of the two materials (YAG and Yttria). It should be noted that the porosity content estimated by this method is due to the contribution of open pores in the TBC. With the image analysis method, similar ranking of the porosity content was observed where the YAG top layered TBC showed lower porosity (approximately 12%) than the yttria top layered TBC (approximately 16%). However, the absolute values of porosity obtained by
image analysis were higher than water intrusion method due to the fact that image analysis takes into account closed and open pores. XRD Analysis XRD analysis of individual TBC layers was carried out and the peaks were identified, as shown in Fig. 4. YSZ as sprayed TBC showed a tetragonal (t’) phase of zirconia, as seen in Fig. 4 (a). In the case of GZ as sprayed TBC, a cubic defect fluorite phase was observed. YAG layer showed an amorphous behavior as no peaks in the scanned 2 theta range were observed, according to Fig. 4 (b). It seems that YAG does not get sufficient time to crystallize while it transforms from molten to solid phase during the coating deposition process. In the case of yttria, monoclinic and cubic phases were detected, as shown in Fig.4 (c). A similar observation of monoclinic and cubic phase formation in the case of plasma sprayed yttria coatings was reported elsewhere [9]. Conclusion: In this work, it was shown that dense layers of YAG and yttria could be deposited on top of the columnar microstructured TBCs by SPS process. Additionally, the YAG layer deposits as an amorphous phase by SPS process whereas the other TBCs (yttria, GZ, YSZ) showed a crystalline phase. Future work would involve investigation of thermal cyclic lifetime, CMAS and erosion performance of the sprayed TBCs. Acknowledgements: The authors would like to thank KK foundation, Sweden (Dnr: 20140130).
References [1] R. A. Miller, “Thermal barrier coatings for aircraft engines: history and directions,” J. Therm. Spray Technol., vol. 6, no. 1, pp. 35–42, Mar. 1997. [2] J. M. Drexler, A. L. Ortiz, and N. P. Padture, “Composition effects of thermal barrier coating ceramics on their interaction with molten Ca–Mg–Al–silicate (CMAS) glass,” Acta Mater., vol. 60, no. 15, pp. 5437–5447, Sep. 2012. [3] R. Vaßen, M. O. Jarligo, T. Steinke, D. E. Mack, and D. Stöver, “Overview on advanced thermal barrier coatings,” Surf. Coat. Technol., vol. 205, no. 4, pp. 938–942, Nov. 2010. [4] C. S. Ramachandran, V. Balasubramanian, and P. V. Ananthapadmanabhan, “Erosion of atmospheric plasma sprayed rare earth oxide coatings under air suspended corundum particles,” Ceram. Int., vol. 39, no. 1, pp. 649–672, Jan. 2013. [5] S. Mahade, N. Curry, S. Björklund, N. Markocsan, P. Nylén, and R. Vaßen, “Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray,” J. Therm. Spray Technol., pp. 1–8, Dec. 2016. [6] R. M. Leckie, S. Krämer, M. Rühle, and C. G. Levi, “Thermochemical compatibility between alumina and ZrO2–GdO3/2 thermal barrier coatings,” Acta Mater., vol. 53, no. 11, pp. 3281–3292, Jun. 2005.
[7] S. Mahade, N. Curry, S. Björklund, N. Markocsan, and P. Nylén, “Failure analysis of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings subjected to thermal cyclic fatigue,” J. Alloys Compd., vol. 689, pp. 1011–1019, Dec. 2016. [8] “ImageJ,” Softonic. Available: http://imagej.en.softonic.com/ [9] J. Kitamura, Z. Tang, H. Mizuno, K. Sato, and A. Burgess, “Structural, Mechanical and Erosion Properties of Yttrium Oxide Coatings by Axial Suspension Plasma Spraying for Electronics Applications,” J. Therm. Spray Technol., vol. 20, no. 1–2, pp. 170–185, Nov. 2010. List of figures Figure.1: Schematic of the proposed TBC architectures Figure.2: SEM micrographs of yttria/GZ/YSZ TBC (a) cross sectional (b) top surface Figure.3: SEM micrographs of YAG/GZ/YSZ TBC (a) cross sectional. (b) top surface Figure.4: XRD analysis of (a) YSZ and GZ (b) YAG (c) Yttria
Highlights for Review •
Suspension plasma spray has shown the potential to deposit wide range of microstructures.
•
Dense sealing layer can be deposited on the columnar microstructured TBCs.
•
High performance TBCs could be achieved by a combination of columnar microstructure and dense layer.
S0167-577X(17)31294-6 http://dx.doi.org/10.1016/j.matlet.2017.08.096 MLBLUE 23073
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 February 2017 10 August 2017 23 August 2017
Please cite this article as: S. Mahade, N. Curry, S. Björklund, N. Markocsan, P. Nylén, Engineered Thermal Barrier Coatings deposited by Suspension Plasma Spray, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet. 2017.08.096
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Engineered Thermal Barrier Coatings deposited by Suspension Plasma Spray Satyapal Mahade a*, Nicholas Curry b, Stefan Björklund a, Nicolaie Markocsan a, Per Nylén a a
Department of Engineering Science, University West, Sweden b
Treibacher Industrie AG, Austria
Abstract: Yttria stabilized zirconia (YSZ) is susceptible to CMAS (Calcium Magnesium Alumino Silicates) attack at high temperatures (>1200°C) which limits its durability. New ceramic materials which can overcome these high temperature challenges are highly desirable. This work investigates the feasibility of depositing two variations of three ceramic layered thermal barrier coatings. The first variation comprised of yttria as the top ceramic layer with gadolinium zirconate (GZ) as the intermediate layer and YSZ as the base layer. The second variation comprised of Yttrium Aluminum Garnet (YAG) as the top layer with gadolinium zirconate as the intermediate layer and YSZ as the base layer. Microstructural analysis of the as sprayed three layered TBCs were performed by SEM/EDS. Columnar microstructures with a relatively dense top layer were obtained in both the variations. The porosity content of the TBCs was measured by water intrusion and image analysis methods. Phase composition of each layer of the as sprayed TBCs was analyzed using XRD. YAG showed an amorphous phase whereas GZ showed a cubic defect fluorite phase and tetragonal phase was observed in YSZ. In the case of yttria, monoclinic and cubic phases were observed. Keywords: Gadolinium Zirconate; Suspension Plasma Spray; Yttria; Yttrium Aluminum Garnet;
1. Introduction Thermal barrier coatings (TBCs) are used in gas turbine engine applications. The efficiency of a gas turbine can be increased by opting for higher turbine inlet temperature [1]. Yttria stabilized zirconia (YSZ) is the most widely used TBC top coat material. However, above 1200°C, YSZ is susceptible to CMAS (Calcium Magnesium Alumino Silicates) degradation which results in reduced TBC durability [2]. The failure mechanism of YSZ based TBCs on interaction with CMAS is reported to be due to the selective leaching of yttria from the YSZ which results in transformation of tetragonal to monoclinic phase during cooling [2]. The phase transformation is associated with a volume change of approximately 3-5% which results in TBC spallation. Additionally, the molten CMAS infiltrates and stiffens the TBC upon cooling which results in loss of strain tolerance. Therefore, it is of interest to find alternate top coat materials for mitigating CMAS attack. Pyrochlores (gadolinium zirconate and lanthanum zirconate), Yttrium aluminum garnet (YAG), and Perovskites have been identified as the new ceramic materials which have the potential to replace YSZ [3]. However, pyrochlores are known to be inferior to YSZ in terms of erosion resistance [4] [5]. Additionally, gadolinium zirconate (GZ) has thermochemical compatibility issues with the thermally grown oxide [6]. Therefore, YSZ is used as a base layer. Suspension plasma spray (SPS) is a promising technique in depositing TBCs with columnar microstructures. Ramachandran et al. showed that a dense TBC layer can improve the erosion resistance compared to the relatively porous TBC [4]. Dense top layer in a TBC can also help in improving the CMAS attack resistance by obstructing the infiltration. Therefore, a dense layer on top of a columnar microstructure TBC seems to be desirable for enhancing the CMAS and erosion resistance. Previously, attempts were made to deposit GZ based triple layer TBC by SPS process which showed an improvement in the thermal cyclic life [7]. However, the dense GZ layer in the triple layer TBCs did not seem to improve the erosion resistance [5]. To the author’s knowledge, no attempt has been made to deposit three layered TBCs with yttria and YAG as the top dense layers by SPS process. This work attempts to deposit two variations of three layered TBCs comprising of a thin yttria and YAG top layers on intermediate GZ layer and YSZ base layer. The purpose of depositing yttria on top was to present a reservoir of yttria rich phase for improving the CMAS attack resistance. The purpose of depositing YAG layer was to explore its capability to withstand the CMAS attack. Additionally, a dense top layer in a TBC could help to improve the erosion resistance compared to the porous layer [4]. 2. Experimental details Four different suspensions, made by Treibacher Industrie AG, Austria, were used in this work. 8 wt% YSZ and GZ suspensions were supplied with a median particle size of 500nm and 25% solid loading in ethanol. The YAG suspension contained particles with a median size of 3
µm and 40% solid load in ethanol. Finally, the Yttrium Oxide suspension contained particles with a median size of 2.5 µm at 40% solid load in water. The TBCs were processed using an axial suspension plasma spray equipment with the Axial III Mettech Gun (Northwest Mettech Corp., Canada). Two different triple layer TBC architectures were deposited, as shown in Fig.1. The as sprayed TBC specimens were polished and observed in the SEM/EDS (Hitachi TM3000, Japan and Bruker for EDS). Free standing coatings were obtained and the porosity content of the TBC was measured according to the procedure discussed in our previous work [7]. Additionally, porosity content of the as sprayed TBCs was measured by image analysis method using the Image J software [8]. Twenty five different SEM micrographs at 600X were considered. XRD analysis was carried out using Seifert-TT 3003 equipment. 3. Results and Discussion Microstructural Analysis In the SEM/EDS analysis of the cross section of the as sprayed Yttria/GZ/YSZ TBC, a columnar microstructure of GZ and YSZ was observed, as shown in Fig.2 (a). The thickness of YSZ and GZ layers were 50±4 µm and 190±4 µm respectively. The top layer of yttria was found to be approximately 40±3 µm. The elemental maps of yttrium (Y), zirconium (Zr), and gadolinium (Gd) is presented along with the cross sectional SEM micrograph, as seen in Fig. 2. In the top view SEM micrograph, as seen in fig 2(b), a dense microstructure comprising dimples which happen to be the top of the columns and the area surrounding it, was observed. In the case of YAG/GZ/YSZ triple layer TBC, as shown in Fig 3 (a), a columnar microstructure of GZ/YSZ was observed. The top YAG layer was found to be approximately 30±5 µm. It could also be seen that the YAG layer showed horizontal cracks in the microstructure which could be due to the stresses generated from the liquid to solid transformation during spraying. The elemental maps of Aluminum (Al), Gadolinium (Gd), Yttrium (Y) and Zirconium (Zr) are also shown in Fig. (3). The top surface morphology of the YAG/GZ/YSZ triple layer TBC showed a relatively dense microstructure compared to the previously reported top view microstructures of YSZ and GZ [7]. Porosity Measurement The yttria top layered TBC showed a higher porosity content (approximately 14%) compared to the YAG (11%) based TBC by water intrusion method. The reason for difference in the porosity (approximately 3%) of YAG compared to yttria is not clearly understood and further investigation needs to be done in order to understand the inflight particle characteristics of the two materials (YAG and Yttria). It should be noted that the porosity content estimated by this method is due to the contribution of open pores in the TBC. With the image analysis method, similar ranking of the porosity content was observed where the YAG top layered TBC showed lower porosity (approximately 12%) than the yttria top layered TBC (approximately 16%). However, the absolute values of porosity obtained by
image analysis were higher than water intrusion method due to the fact that image analysis takes into account closed and open pores. XRD Analysis XRD analysis of individual TBC layers was carried out and the peaks were identified, as shown in Fig. 4. YSZ as sprayed TBC showed a tetragonal (t’) phase of zirconia, as seen in Fig. 4 (a). In the case of GZ as sprayed TBC, a cubic defect fluorite phase was observed. YAG layer showed an amorphous behavior as no peaks in the scanned 2 theta range were observed, according to Fig. 4 (b). It seems that YAG does not get sufficient time to crystallize while it transforms from molten to solid phase during the coating deposition process. In the case of yttria, monoclinic and cubic phases were detected, as shown in Fig.4 (c). A similar observation of monoclinic and cubic phase formation in the case of plasma sprayed yttria coatings was reported elsewhere [9]. Conclusion: In this work, it was shown that dense layers of YAG and yttria could be deposited on top of the columnar microstructured TBCs by SPS process. Additionally, the YAG layer deposits as an amorphous phase by SPS process whereas the other TBCs (yttria, GZ, YSZ) showed a crystalline phase. Future work would involve investigation of thermal cyclic lifetime, CMAS and erosion performance of the sprayed TBCs. Acknowledgements: The authors would like to thank KK foundation, Sweden (Dnr: 20140130).
References [1] R. A. Miller, “Thermal barrier coatings for aircraft engines: history and directions,” J. Therm. Spray Technol., vol. 6, no. 1, pp. 35–42, Mar. 1997. [2] J. M. Drexler, A. L. Ortiz, and N. P. Padture, “Composition effects of thermal barrier coating ceramics on their interaction with molten Ca–Mg–Al–silicate (CMAS) glass,” Acta Mater., vol. 60, no. 15, pp. 5437–5447, Sep. 2012. [3] R. Vaßen, M. O. Jarligo, T. Steinke, D. E. Mack, and D. Stöver, “Overview on advanced thermal barrier coatings,” Surf. Coat. Technol., vol. 205, no. 4, pp. 938–942, Nov. 2010. [4] C. S. Ramachandran, V. Balasubramanian, and P. V. Ananthapadmanabhan, “Erosion of atmospheric plasma sprayed rare earth oxide coatings under air suspended corundum particles,” Ceram. Int., vol. 39, no. 1, pp. 649–672, Jan. 2013. [5] S. Mahade, N. Curry, S. Björklund, N. Markocsan, P. Nylén, and R. Vaßen, “Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray,” J. Therm. Spray Technol., pp. 1–8, Dec. 2016. [6] R. M. Leckie, S. Krämer, M. Rühle, and C. G. Levi, “Thermochemical compatibility between alumina and ZrO2–GdO3/2 thermal barrier coatings,” Acta Mater., vol. 53, no. 11, pp. 3281–3292, Jun. 2005.
[7] S. Mahade, N. Curry, S. Björklund, N. Markocsan, and P. Nylén, “Failure analysis of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings subjected to thermal cyclic fatigue,” J. Alloys Compd., vol. 689, pp. 1011–1019, Dec. 2016. [8] “ImageJ,” Softonic. Available: http://imagej.en.softonic.com/ [9] J. Kitamura, Z. Tang, H. Mizuno, K. Sato, and A. Burgess, “Structural, Mechanical and Erosion Properties of Yttrium Oxide Coatings by Axial Suspension Plasma Spraying for Electronics Applications,” J. Therm. Spray Technol., vol. 20, no. 1–2, pp. 170–185, Nov. 2010. List of figures Figure.1: Schematic of the proposed TBC architectures Figure.2: SEM micrographs of yttria/GZ/YSZ TBC (a) cross sectional (b) top surface Figure.3: SEM micrographs of YAG/GZ/YSZ TBC (a) cross sectional. (b) top surface Figure.4: XRD analysis of (a) YSZ and GZ (b) YAG (c) Yttria
Highlights for Review •
Suspension plasma spray has shown the potential to deposit wide range of microstructures.
•
Dense sealing layer can be deposited on the columnar microstructured TBCs.
•
High performance TBCs could be achieved by a combination of columnar microstructure and dense layer.