Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE 2. Zr 2 O 7 TBC

Transcription

1 IOP Conference Series: Materials Science and Engineering Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE 2 Zr 2 O 7 TBC To cite this article: B Chmiela et al 2012 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. Related content - Investigation of failure mechanism of thermal barrier coatings (TBCs) deposited by EB-PVD technique M R Shahid and Musharaf Abbas - Oxidation resistance of Al2O3- nanostructured/csz composite compared to conventional CSZ and YSZ thermal barrier coatings A Keyvani and M Bahamirian - Microstructural characteristics and technological properties of YSZ-type powders designed for thermal spraying of TBC G Moskal This content was downloaded from IP address on 19/01/2018 at 16:16

2 Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE 2 Zr 2 O 7 TBC B Chmiela 1, M Sozańska and G Moskal Silesian University of Technology, Department of Materials Science and Metallurgy, ul. Krasinskiego 8b, PL-40019, Katowice, Poland bartosz.chmiela@polsl.pl Abstract. Modern aero engine turbine blades made of nickel-based superalloys are covered by thermal barrier coatings (TBC) for thermal and oxidation protection. A new generation of TBCs consist of a bond coat (thin layer of MCrAlY, where M may be Ni, Co, Fe) followed by a ceramic top coat of RE 2 Zr 2 O 7 (RE - rare earth element). In this paper we present the possibility of the electron backscatter diffraction (EBSD) method for characterisation of the microstructure and crystallographic orientation of a new TBC consisting of a Gd 2 Zr 2 O 7 top coat and a NiFeCrAlY bond coat after long thermal exposure (1100 C, 500 h). During thermal exposure, a thermally grown oxide (TGO) layer forms at the bond coat/top coat interface. The TGO is mainly composed of Al 2 O 3. But, there is a possible reaction between Gd 2 Zr 2 O 7 and Al 2 O 3, leading to Gd-Al-O phases. Phase composition plays an important role in controlling the stress evolution, TGO deformation and crack propagation. Application of SEM-EDS-EBSD techniques allows direct characterisation of the chemical composition, phase composition and crystallographic orientation in the ceramic top coat and TGO layers. This paper presents the possibilities of using the EBSD method for phase identification (Gd 2 Zr 2 O 7, spinel Ni(Al,Cr) 2 O 4, GdAlO 3 and other phases) and orientation analysis of grains in the TGO layer. 1. Introduction Aero engine turbine blades are made of nickel-base superalloys that have high creep resistance and fatigue strength. For superalloys, excellent mechanical properties are determined by the proper microstructure, which is achieved with the correct chemical composition and phase composition. But, the chemical composition of modern superalloys makes these alloys prone to topologically close-packed phase formation and hot corrosion under service conditions. Therefore, various protective coatings are applied. Ceramic thermal barrier coatings (TBCs) deposited using the air plasma spraying (APS) technique are commonly used to protect many parts of aero engines (e.g., turbine blades, vanes, combustion chambers) from high temperatures and hot corrosion [1, 2]. Application of the APS technique provides strong adhesion of the TBC to the substrate and a porous microstructure with lower thermal conductivity and improved strain accommodation. Classic TBCs consist of a bond coat (thin layer of MCrAlY alloy, where M may be Ni, Co, Fe) followed by a ceramic top coat (yttria stabilized zirconia - YSZ: ZrO 2 with 7 wt% Y 2 O 3 ). During the deposition 1 To whom any correspondence should be addressed. Published under licence by IOP Publishing Ltd 1

3 process and service conditions, a thermally grown oxide (TGO) layer forms at the top coat/bond coat interface. The TGO is a result of oxidation of the upper part of the bond coat and other chemical reactions. The TGO layer mainly consists of α-al 2 O 3 (alumina) and spinels Ni(Al,Cr) 2 O 4 [1-4]. Due to the presence of yttrium in the bond coat, the TGO may contain some phases composed of this element [2]. Several investigations into ceramic materials with better properties than YSZ led to the application of lanthanide zirconates RE 2 Zr 2 O 7 (RE = La, Nd, Sm, Gd) with the pyrochlore structure [5-8]. RE 2 Zr 2 O 7 compounds are characterized by thermal conductivities that are lower than that of YSZ [8] and very good hot corrosion resistance and thermal shock resistance. Previous studies on experimental TBCs containing lanthanide zirconates revealed the possibility of detrimental chemical reactions at the top coat/tgo interface that simulate actual thermal conditions during service [9-11]. Those reactions lead to the formation of new phases and are associated with volume changes [9], which contribute to the premature failure of the TBC. In particular, the reaction between gadolinium zirconate and alumina leads to the formation of the GdAlO 3 -phase that has the perovskite structure [9-11]. Therefore, in order to understand the degradation mechanisms of TBCs, investigating the phase composition and distribution should become standard. This paper proposes the possibility of applying scanning electron microscope, energy-dispersive X-ray spectroscopy and electron backscatter diffraction (SEM-EDS-EBSD) techniques to evaluate the microstructure, chemical composition, phase composition and crystallographic orientation [12] in experimental TBC, which is fabricated by air plasma spraying a Gd 2 Zr 2 O 7 top coat on the NiFeCrAlY bond coat. 2. Materials and experimental procedure An experimental TBC with a ceramic Gd 2 Zr 2 O 7 top coat was used in this study. A metallic bond coat layer composed of NiFeCrAlY alloy (140 µm thick) was deposited onto the substrate of a CMSX-4 superalloy by the vacuum plasma spraying technique. The top coat was deposited by the APS technique using a Gd 2 Zr 2 O 7 powder. The thickness of the top coat was about 180 µm. One of the specimens was examined in the as-deposited state, and the others were subjected to thermal exposure at a temperature of 1100 C and for various times: 2 h, 10 h, 48 h, 175 h and 500 h. After thermal exposure all specimens were air cooled. Examinations of the microstructure, chemical composition and phase composition were performed on the cross-sections of the specimens. To obtain the cross-sections, specimens were cut carefully using a precision cutter to avoid the risk of mechanical surface damage. Then, the specimens were placed in a quick-setting resin, ground and polished. Each of these specimens was first ground on waterproof abrasive papers at successively decreasing grit sizes. Next, specimens were polished in a diamond suspension (9 µm, 3 µm, 1 µm and 0.25 µm grit sizes) and Al 2 O 3 suspension (0.05 µm grit size). For EBSD analysis the specimen was additionally vibratory polished (diamond suspension: 0.05 µm grit size) for 5 hours to obtain as low surface roughness as possible [13]. The microstructure, chemical composition, phase composition and crystallographic orientation were characterized using an SEM (Hitachi S-3400N) equipped with an EDS (Thermo NORAN (System Six)) and an EBSD detector (INCA HKL Nordlys II (Channel 5)). Detailed chemical composition measurements of the TBC cross-sections were performed using the EDS technique. Variable energy of the primary electron beam was used, depending on the results obtained for primary electron energy equal to 15 kev, to guarantee the excitation of spectral lines of all analysed elements. If a given element was not identified in a chosen area, the energy of the primary electron beam was decreased to minimize the excitation volume in the material and maintain an overvoltage equal to about 2. For the cross-sectional TBC specimen treated with a thermal exposure for 500 h, phase identification through EBSD and the phase composition map were based on results from a literature survey and the EDS investigation. The specimen subject to a 500 h exposure was chosen because it had the thickest TGO layer and greatest probability of forming all potential phases. Phase identification was based on comparing the experimental Kikuchi pattern with the theoretical pattern. 2

4 Orientation analysis in micro areas of the TBC also was carried out. The energy of the primary electron beam was 20 kev, which is the optimal value for most materials [13]. Due to the small sizes of the analysed areas, the beam scanning mode was used. Additionally, to avoid the risk of overlooking small precipitates of various phases, a beam step size of 0.05 µm was utilized. Due to the use of a non-conductive material, all SEM-EDS-EBSD investigations were performed under low vacuum conditions in the SEM chamber. The gas in the chamber was air and the working distance was 10 mm. After many trials the best pressure value was determined to be 50 Pa. This pressure effectively prevents the charge effect of the specimen surface and allows the collection of good quality of Kikuchi patterns. SEM observations were performed in two modes: backscatter electron mode, revealing differences in chemical composition (BSE COMP), and environmental secondary electron mode (ESED), revealing the morphology of the material. 3. Results and discussion 3.1. SEM investigation Figure 1 presents SEM micrographs of the cross-sections of the TBC containing gadolinium zirconate. The TGO layer appeared during the deposition process (figure 1a), and its thickness increased with time of thermal exposure. After 500 h of thermal exposure, the thickness of the TGO was about 6 µm (figure 1f). Systematic degradation of the top coat was clearly visible many cracks appeared in the top coat and at the top coat/tgo interface. Figure 1. SEM micrographs of cross-sections of the TBC: a) as-deposited; b) after 2 h of thermal exposure: c) after 10 h of thermal exposure; d) after 48 h of thermal exposure; e) after 175 h of thermal exposure; and f) after 500 h of thermal exposure. The characteristic porosity just above the TGO layer was visible (figure 2). The most probable reason for this phenomenon is a chemical reaction between gadolinium zirconate and alumina due to the negative volume change. 3

5 EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis IOP Publishing IOP Conf. Series: Materials Science and Engineering 32 (2012) doi: / x/32/1/ Figure 2. Pores at the top coat/tgo interface. Cracks running in various directions (parallel and perpendicular to the top coat surface) join together, leading to spallation and delamination of the top coat after 500 h of thermal exposure (figure 3). Figure 3. Spallation of the top coat after thermal exposure (1100 C, 500 h). Back scattered electron images clearly reveal the differences in chemical composition of various areas in the TGO layer and EDS analysis confirms these findings EDS investigation EDS analyses were carried out on the cross-sections of the as-deposited specimen and thermally exposed (500 h) specimen. To estimate the skirt effect in low vacuum, several analyses (spot mode) in the TGO layer after 500 h of thermal exposure were carried out (figure 4). Based on the literature data and the knowledge about the TBC deposition and degradation processes, it is obvious that the TGO 4

6 layer consists mainly of (Al,Cr) 2 O 3 and Ni(Al,Cr) 2 O 4 compounds. EDS results confirm this findings there are only Al, O and Cr signals in the middle part of the TGO (figure 4c). Analyses near the top coat/tgo interface and near the TGO/bond coat interface reveal the presence of elements from the top coat (Gd, Zr), spinel (Ni) and bond coat (Ni, Fe, Y, Mo). Because the analysis in point 2 (figure 4c) reveals only aluminium, oxygen and chromium, it is safe to say that scattering of the electron beam (skirt effect) in quite small. Therefore, EDS analysis in given low vacuum conditions seems to be reliable. Figure 4. EDS spectra of three points within the TGO layer: a) analysed points in the crosssection of the TBC; b) point near the top coat/tgo interface; c) point in the middle of the TGO; d) point near the TGO/bond coat interface. Analysis results of the top coat and the bond coat of the as-deposited coatings are shown in table 1. Quantitative results were the mean of five analyses in the spot mode. Because a standardless analysis was performed, light elements (carbon and oxygen) were not taken into account in the quantitative analysis. The presence of carbon probably resulted from the preparation (polishing using the diamond suspension, which could fill small pores). The ratio of Gd to Zr concentrations corresponded to stoichiometric Gd 2 Zr 2 O 7, and it may be considered as binary system of ZrO 2 -Gd 2 O 3 containing 33 at% Gd 2 O 3 [5-7, 9]. 5

7 Table 1. Chemical composition of the as-deposited TBC metallic elements (at%). Al Cr Fe Ni Y Zr Mo Gd top coat bond coat EDS analysis after 500 h of thermal exposure revealed characteristic changes in the chemical composition of the top coat and TGO that are related to diffusion processes in a ceramic material and the resultant chemical reactions. The composition of the upper part of the top coat was almost identical to that before thermal exposure. But, in the lower part of the top coat (near the TGO), there was a characteristic depletion of gadolinium and zirconium and an enrichment of aluminium (table 2). Table 2. Chemical composition of the top coat layer metallic elements (at%). Al Zr Gd upper part lower part Those changes suggest the progression of diffusion and reactions between Gd 2 Zr 2 O 7 and Al 2 O 3. At the thermal exposure temperature (1100 C), reaction (1) is probably occurring and leading to the formation of the GdAlO 3 -phase gadolinium aluminate [9]: Gd 2 Zr 2 O 7 + Al 2 O 3 2GdAlO 3 + 2ZrO 2 (1) Proof of this reaction can be found from EDS and XRD analyses [10]. Figure 5 presents EDS spectra performed on the top coat and TGO layers. In some areas of the top coat/tgo interface, a phase with a qualitative composition connected to the GdAlO 3 -phase (figure 5c) was identified. One should note that a phase of this composition occurred locally and did not form a continuous layer. This observation implies that at the applied temperature, the intensity of reaction (1) is not too high, because even after 500 h of thermal exposure, the amount of this phase is small. Moreover, there was a negative volume change from reaction (1) (about 9 %), which led to the formation of additional porosity [9]. One might surmise that the rate of GdAlO 3 formation increases with time, while just below the top coat, the decreased thickness of the top coat due to spallation and the increased number of cracks increase the thermal diffusivity and coating temperature. Furthermore, EDS analyses of the TGO layer made identification of the other phase easier. Qualitative analysis of selected areas and results from the literature suggested the presence of the following phases: (Al,Cr) 2 O 3 (figure 5d), Ni(Al,Cr) 2 O 4 (figure 5e) and Y 3 Al 5 O 12 (figure 5f). The (Al,Cr) 2 O 3 -phase is sapphire, which is alumina with some Al 3+ cations substituted with Cr 3+ cations. The Ni(Al,Cr) 2 O 4 -phase is a spinel, and Y 3 Al 5 O 12 -phase is yttria-alumina garnet (YAG), forming due to the yttrium content in the bond coat layer [14]. 3.3 EBSD investigation Based on the literature and SEM-EDS results for the microstructure and phase composition, the several phases were chosen for EBSD analysis (table 3). In the area surrounding the top coat/tgo interface, a phase composition map was constructed using a phase component in the software (figures 6a and 6b). This map presents the phase distribution in the cross-section of the TBC. Raw maps were submitted to standard cleaning procedure, according to instruction for users (Oxford 6

8 Figure 5. EDS spectra of various phases of the TBC after thermal exposure (1100 C, 500 h): a) analyzed areas in the cross-section of the TBC; b) gadolinium zirconate; c) gadolinium aluminate; d) sapphire; e) spinel; and f) yttria-alumina garnet. Instruments HKL Technology, 2006). The cleaning procedure consisted of several stages: removing wild spikes (isolated points that have been incorrectly indexed) and then removing zero solutions (low level extrapolation). Finally, the orientation averaging filter (Kuwahara filter) was applied with a 3x3 size. Just below the top coat (aqua colour), small precipitates of the GdAlO 3 phase (red colour) were visible. The amount of this phase was small, and there was no continuous layer. These results indicate that during given conditions of thermal exposure, the reaction between Gd 2 Zr 2 O 7 and Al 2 O 3 occurred and produced a noticeable yield only in selected areas. This reaction may be a result of the increased thermal diffusivity of the top coat that is caused by spallation and delamination. Under the 7

9 EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis IOP Publishing IOP Conf. Series: Materials Science and Engineering 32 (2012) doi: / x/32/1/ Table 3. Phases investigated by EBSD. Phase name Chemical formula Crystallographic system Space group Unit cell parameters (nm) a0 b0 c0 Gadolinium zirconate Gadolinium aluminate (GAP) Alumina Spinel Yttria alumina garnet (YAG) Gd2Zr2O7 GdAlO3 Al2O3 NiAl2O4 Y3Al5O12 cubic orthorhombic trigonal cubic cubic Fm3m Pbnm R3c Fd 3m Ia3d Figure 6. Phase composition map of the TBC cross-section after thermal exposure (1100 C, 500 h): a) raw map; b) map after cleaning procedure; c) BSE image corresponding of the EBSD map; Gd2Zr2O7; GdAlO3; Ni(Al,Cr)2O4; (Al,Cr)2O3; white regions: not indexed; HV = 20 kv; low vacuum 50 Pa. GdAlO3-phase precipitations, mainly spinel phase (fuchsia colour) was identified. The last zone within the TGO layer consisted of sapphire (teal colour) and small amounts of spinel. Identification of these phases confirms the predictions made on the basis of the literature and EDS analyses. White regions on the phase map denote non-indexed areas. These areas were not indexed probably due to overlapping patterns, insufficient quality of the specimen surface (too high roughness) and stresses or strains in the crystal lattice [13]. 8

10 The phase distribution results in the cross-section aids in formulating the supposed mechanism of TGO formation in the TBC that contains gadolinium zirconate. A very thin TGO layer was present in the as-deposited coating, as a result of applying the top coat by air plasma spray. During thermal exposure in an air atmosphere in the furnace, Gd 2 Zr 2 O 7 serves as an ionic conductor, and due to the presence of micro-cracks, oxygen diffuses to the top coat/tgo interface. In addition, the oxygen diffusion causes oxidation of aluminium and chromium in the bond coat and growth of (Al,Cr) 2 O 3 in the upper part of the bond coat. Then, Gd 2 Zr 2 O 7 reacts in selected areas with (Al,Cr) 2 O 3, leading to the GdAlO 3 -phase, while the reaction between nickel, oxygen and sapphire leads to the spinel phase. Additionally, oxidation of yttrium and reactions between yttrium oxide and alumina lead to small YAG precipitates [1, 14]. In selected areas of the cross-section of the top coat that are free of cracks, orientation analysis was performed. Raw orientation map in all Euler colouring scheme is shown in figure 7. The same cleaning procedure as for phase map was applied. Figure 7. Raw orientation map of the cross-section of the top coat, after thermal exposure (1100 C, 500 h), with all Euler colouring scheme; HV = 20 kv; low vacuum 50 Pa. Figure 8 presents two orientation maps with inverse pole figure (IPF) colouring scheme, which allows the crystallographic orientations to be interpreted in terms of the specimen coordinate system (in this case in terms of the specimen Z axis perpendicular to their surface). The maps revealed that the top coat layer was characterized by a fine-grained microstructure. APS top coat do not display optimal mechanical properties in various directions. For this reason, in contrast to the electron beam physical vapour deposited (EB-PVD) top coat, the APS top coat layer is characterized by low deformability (especially in the direction parallel to the substrate surface), which supports crack formation, thus decreasing the lifetime of the TBC [15, 16]. But, the APS technique is more readily available and cheaper than the EB-PVD technique. The orientation analysis revealed high angle boundaries (HABs) in the top coat; the misorientation between neighbouring grains exceeded even 30 and 40 (figure 9). 4. Conclusions Thermal barrier coatings (TBCs) are characterized by low thermal conductivity and low thermal diffusivity. Application of new compounds lanthanide zirconates lead to coatings with a thermal conductivity lower than that for YSZ coatings. Unfortunately, TBCs with the Gd 2 Zr 2 O 7 compound display many drawbacks. Low service life of the coating containing Gd 2 Zr 2 O 7 is apparent. After long thermal exposure (1100 C, 500 h), the top coat in selected areas is almost completely destroyed. Large amounts of cracking result from the mismatch of thermal expansion coefficients between the top 9

11 EMAS 2011: 12th European Workshop on Modern Developments in Microbeam Analysis IOP Publishing IOP Conf. Series: Materials Science and Engineering 32 (2012) doi: / x/32/1/ Figure 8. Orientation maps of the cross-section of the top coat, after thermal exposure (1100 C, 500 h), with the IPF colouring scheme; HV = 20 kv; low vacuum 50 Pa. Figure 9. Misorientation profile (a) for the top coat after thermal exposure (1100 C, 500 h) and orientation map with highlighted profile black line (b). 10

12 coat and the bond coat and too low deformability of the top coat. Additionally, reaction between gadolinium zirconate and sapphire (accelerated by spallation of the top coat) causes increased porosity at the top coat/tgo interface. Therefore, TBCs with gadolinium zirconate may be used at a temperature lower than 1100 C; otherwise, it will be necessary to use an additional ceramic layer (YSZ) between the top coat and the bond coat, consequently decreasing the diffusion rate and preventing detrimental reactions [15, 16]. Applying the SEM-EDS-EBSD techniques provides a complete description of the microstructure, chemical composition, phase composition and orientation of the TBC. A non-conductive TBC requires the use of the low vacuum mode in the SEM; then, it is not necessary to deposit an additional conductive coat, which usually makes EDS and EBSD analysis more difficult. By performing EDS (when wavelength-dispersive X-ray spectrometry is not available), qualitative analysis of the chemical composition and correct quantitative analysis of heavy elements are possible. In addition to XRD analyses and literature data, EBSD investigations can be performed. The EBSD technique can be used to obtain a phase composition map, which is impossible with X-ray diffraction, of the TBC cross-section. Many difficulties related to specimen preparation are revealed in some non-indexed areas, but the fraction of correctly indexed points are sufficient to perform the analysis. The EBSD technique helps to evaluate applying the crystallographic orientation, which influences the mechanical properties of the ceramic top coat. Correct application of the EBSD technique requires the knowledge of as much information about the material as possible (especially about the chemical composition and most likely about the phase transformations in the material). Acknowledgements Financial support of Structural Funds in the Operational Programme-Innovative Economy (IE OP) financed from the European Regional Development Fund-Project no. POIG /09 is gratefully acknowledged. References [1] Tamarin Y 2002 Protective coatings for turbine blades. (Ohio: Materials Park) [2] Bose S 2007 High temperature coatings. (Manchester, Connecticut, USA: Elsevier Science & Technology Books) [3] Lee C H, Kim H K, Choi H S and Ahn H S 2000 Surf. Coat. Techn [4] Czech N, Fietzek H, Juez-Lorenzo M, Koralik V and Stamm W 1999 Surf. Coat. Techn [5] Wang Ch 2006 Experimental and computational phase studies of the ZrO 2 -based systems for thermal barrier coatings. (Stuttgart: Max-Planck-Institut für Metallforschung) [6] Leckie R M R 2006 Fundamental issues regarding the implementation of gadolinium zirconate in thermal barrier systems. (Santa Barbara: University of California) [7] Wilde P J and Catlow C R A 1998 Solid State Ion [8] Suresh G, Seenivasan G, Krishnaiah M V and Murti P S 1997 J. Nucl. Mater [9] Leckie R M, Krämer S, Rühle M and Levi C G 2005 Acta Mater [10] Wu J, Padture N P and Gell M 2004 Scr. Mater [11] Lakiza S, Fabrichnaya O, Wang Ch, Zinkevich M and Aldinger F 2006 J. Europ. Cer. Soc [12] Fujiyama K, Nakaseko H, Kato Y and Kimachi H 2010 J. Sol. Mech. Mater. Eng [13] Faryna M 2003 Analysis of orientation reliationships in ceramic based composites. (Krakow: Institute of Metallurgy and Materials Science, Polish Academy of Sciences) [14] Haynes J A, Rigney E D, Ferber M K and Porter W D 1996 Surf. Coat. Techn [15] Moskal G 2009 Arch. Mater. Sci. Eng [16] Vassen R, Kassner H, Stuke A, Mack D E, Jarligo M O and Stöver D 2010 Mater. Sci. Forum