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Structural characterisation of thermal barrier coatings deposited using electrostatic spray assisted vapour deposition method
Materials Science and Engineering A277 (2000) 206 – 212 www.elsevier.com/locate/msea
Structural characterisation of thermal barrier coatings deposited using electrostatic spray assisted vapour deposition method J.D. Vyas, K.-L. Choy * Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW 7 2BP, UK Received 3 March 1999; received in revised form 16 July 1999
Abstract Gas turbine engines provide one of the harshest environments for engineering components today. Thermal barrier coatings (TBCs) such as 8wt.% Y2O3 –ZrO2 (YSZ) are able to protect the blades and can lead to further increases in operating temperature by providing an effective thermal barrier on the blades. YSZ coatings were deposited using a novel and cost-effective electrostatic spray-assisted vapour deposition (ESAVD) technique. This method involves spraying atomised precursor droplets across an electric field, where they undergo decomposition and/or chemical reactions in the vapour phase near the vicinity of the heated substrate. A solution containing zirconium and yttrium alkoxide precursors was used for the deposition of YSZ films. The coatings were characterised using a combination of scanning electron microscopy, X-ray diffraction (XRD) and Raman spectroscopy. Thick and uniform YSZ films with columnar growth features were successfully deposited using the ESAVD method. The Raman investigations showed the presence of carbon in the as-deposited coatings. However, the carbon was removed when the YSZ coating was heat-treated at 1000°C for 2 h to improve the adhesion of the TBC coating to the bond coat. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Yttria stabilised zirconia; Electrostatic spray assisted vapour deposition; Thermal barrier coatings; Raman spectroscopy
1. Introduction The gas turbine engine provides one of the harshest environments (high temperature and direct stress) challenging material systems today. As engine components are subjected to rigorous mechanical loading conditions, high temperatures, corrosive (hot gases surrounding the blade are highly oxidising and may contain contaminants such as chlorides and sulphates-leading to hot corrosion) and/or erosive media (sand) [1]. The efficiency and performance of a gas turbine engine are directly related to the difference in temperature by the Carnot cycle and hence the operating temperature. Therefore, thermal barrier coatings (TBCs) are able to protect the blades and can lead to further increases in efficiency through the operating temperature by providing an effective thermal barrier on the blades [2]. TBCs typically consist of an intermediate layer (metallic bond coat) and a ceramic topcoat. The function of * Corresponding author. Tel./fax: +44-171-594-6750. E-mail address: [email protected] (K.-L. Choy)
the bond coat is essentially to provide a ‘key’ for firm adhesion of the topcoat (TBC) to the Ni-based material by having compatible coefficients of thermal expansion (within 11–13× 10 − 6 K − 1 range). The bond coat also helps to inhibit oxidation and provides good diffusion stability with the substrate. Doped zirconia is commonly used for the overcoat material because of its many benefits such as low thermal conductivity, relatively high thermal expansion coefficient and excellent chemical resistance [3,4]. The benefits of such systems have been well documented in many papers using different stabilisers or dopants [5,6]. However, all agree that the best composition and stabiliser is 8wt.% yttria stabilised zirconia (YSZ) [7,8]. The main commercial deposition methods for YSZ coatings are electron beam physical vapour deposition (EB-PVD) [9,10] and thermal spraying [9,11]. The thermal spraying includes flame spray processes, both air [9,12,13] and vacuum plasma spraying [11] and detonation gun process [11]. However, these methods are generally expensive to install and their maintenance costs are high. This is especially true for the EB-PVD
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Table 1 Advantages and disadvantages of thermal spraying and electron beam physical vapour deposition (EB-PVD) Main method
Benefits
Problems
Thermal spraying
Cheaper compared to EB-PVD Some stress–strain tolerance offered Columnar structure
Splash-like structure Require sophisticated coating chambers Line-of-sight process Very expensive (capital cost\US$ 25M) Line-of sight process Requires sophisticated chambers
EB-PVD
Much better stress– strain tolerance
method. Therefore, there is a need to develop simple and cost-effective coating methods. Table 1 summarises the advantages and disadvantages of thermal spraying and EB-PVD. Recently, a novel cost-effective coating technique called electrostatic spray assisted vapour deposition (ESAVD) has been developed by Choy and Bai [14]. Process principles involves spraying atomised precursor droplets across an electric field, where they undergo decomposition and/or chemical reactions in the vapour phase near the vicinity of the heated substrate. This produces a stable solid film with excellent adhesion onto the substrate [15]. The detailed deposition mechanism of the ESAVD process and its advantages over other aerosol deposition methods, including pyrolysis and electrostatic spray deposition have been reviewed elsewhere [14 – 16]. Therefore, this method is employed for the deposition of thick thermal barrier coatings for aerospace applications.
2. Experimental Prior to the deposition, the aluminised Ni-based substrates (Nimonic 75) were ultrasonically cleaned in 2-propanol. A 0.05 M precursor solution was prepared by dissolving zirconium (IV) butoxide and yttrium 2-ethylhexanoate in butanol to deposit a coating with a nominal composition of 8wt.%Y2O3 – 92wt.%ZrO2 (8wt.% YSZ) using the ESAVD method. Fig. 1 shows a schematic diagram of the ESAVD apparatus used for depositing yttria partially stabilised zirconia. The precursor solution was atomised and sprayed across an electric field. By carefully controlling the processing conditions the atomised spray droplets undergo decomposition and/or heterogeneous chemical reactions near the heated substrate, to produce a solid well-adherent coating. The deposition temperature at the substrate surface was varied from 500 to 600°C under an electric field between 5 and 20 kV cm − 1.
Fig. 1. A schematic diagram of the electrostatic spray-assisted vapour deposition (ESAVD) apparatus.
The as-deposited and heat-treated (2 h at 1000°C in air) YSZ coatings were analysed using a combination of chemical analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The scanning electron microscope (T220A) was used to examine the surface morphology of the polished coating, both in topography and cross-sections. Chemical analysis based on the inductively coupled plasma, combined with atomic emission spectroscopy (ICP-AES) was used to determine the stochiometry of the YSZ coatings. A Phillips PW1700 series diffractometer (CuKa radiation of wavelength 1.54178 A, ) was employed to perform the phase analysis. The reported d-spacings from the JCPDS cards, for tetragonal (c24-1164 and c17-923), cubic (c 27-997 and c301468) and monoclinic (c 13-307) were the compared with observed peaks. A Raman Renishaw 2000 Spectrograph was used for the detection of carbon and for YSZ coating analysis. The instrument collects information on the scattered radiation through a multi-element detector. The spectra were excited with a red He–Ne ion laser tuned to a Raman wavelength (l) of 633 nm. Spectra of each sample were taken over the range of 1200–1800 cm − 1 for carbon detection, and 120–740 cm − 1 for 8wt.% YSZ coating analysis.
3. Results and discussion The possible deposition mechanism in the ESAVD process has been summarised elsewhere [17,18]. In general, for the deposition of TBC coatings, the process conditions are tailored to enable the chemical reactions to form Y2O3 –ZrO2 to take place near the vicinity of the heated Ni-based substrate (i.e. heterogeneous CVD reactions). If the deposition temperature is too high, Y2O3 –ZrO2 particles will form in the gas phase and favour the deposition of powdery films with poor adhesion. If the deposition temperature is too low, splashing of the droplets on substrate surface takes place and vaporisation of solvent occurs to leave behind a dry precipitate (intermediates). Further heat treatment at
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higher temperature will cause the precursor to undergo decomposition, and followed by subsequent sintering to give porous films with poor adhesion.
3.1. SEM Fig. 2 shows the scanning electron mircograph of the as-deposited 8wt.% YSZ by ESAVD. The surface morphology in Fig. 2(a) reveals a ‘cauliflower’ type of structure. The cross-section in Fig. 2(b) shows the columnar-like growth features (column width approximately 9–18 mm at base and 16 – 20 mm at top) with microcracks ranging from 2 to 10 mm within the thick ceramic coating (approximately 50 mm). A porosity of 20% was calculated for the microstructure in Fig. 2(b) using the grid method. In comparison, the plasmasprayed structure is quite porous (10 – 15%) as well and extensively microcracked [9]. This porosity is primarily responsible for the generally lower thermal conductivity values observed for air plasma sprayed TBCs. The
superior erosion properties of EB-PVD TBCs are attributed to the columnar structure in the ceramic coating [19]. The TBCs deposited using the ESAVD method incorporate microstructural features such as porosity and microcracking and still maintain the beneficial columnar-like characteristics thereby taking advantage from both APS (thermal conductivity) and EB-PVD (erosion) techniques. Much thicker coatings (approximately 400 mm) have since been deposited using the ESAVD method and will be reported in the near future. The coatings deposited by ESAVD method were qualitatively evaluated to provide a rapid assessment of their adhesion to the Ni-based substrates. They were heated to 1200°C and cooled rapidly to room temperature. The procedure was repeated and the TBCs remained in tact for more than 12 cycles. The deposition rate was found to be about 0.5 mm min − 1, which is comparable to EB-PVD, which has a rate of approximately 1–2 mm min − 1, depending on geometry and processing conditions. As a note the deposition rate for plasma spraying is 5–10 mm per pass as the gun will move around the component [20].
3.2. ICP-AES For the compositional analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES), three to four measurements on the heat-treated (2 h at 1000°C) YSZ films were made and an average value was reported within the limits of the error. The ICPAES analysis showed a presence of (8.989 0.05) wt.% yttria in zirconia. A minor error possibly due to preparation/dilution of the solution, e.g. weighing of powders and/or preparation of the precursor solution because the zirconium tetra-butoxide is a high viscosity liquid at room temperature.
3.3. XRD
Fig. 2. A typical scanning electron micrograph (SEM) of the as-deposited yttria stabilised zirconia (YSZ) coatings; (a) surface morphology, (b) cross-section.
XRD was used to characterise the phases present. JCPDS cards for tetragonal (c24-1164 and c17-923), cubic ( c 27-997 and c30-1468) and monoclinic (c13307) were used to identify the presence of phases. Typical XRD patterns for TBCs deposited using the ESAVD method, both before and after heat-treatment, are shown in Fig. 3. In the as-deposited condition, the tetragonal and cubic phases are present. However, some of these peaks are also common to the monoclinic phase. The broad peaks indicate either nanosize crystallites or stress (which would be to some extent relieved upon heating) within the TBC. Once the samples were heat-treated, grain growth occurred which gave sharper peaks, having higher intensity counts because of the larger crystallite size. The Raman analysis did not show the monoclinic phase in the 8wt.% YSZ TBCs (Fig. 5). The TBC coating was randomly orientated because the
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due to small crystallite size was greater than that due to instrumentation but becomes more important for larger crystallite sizes at higher temperatures.
3.4. Raman analysis
Fig. 3. Typical X-ray diffraction (XRD) traces for 8wt.% yttria stabilised zirconia (YSZ) coatings; (a) as-deposited, (b) after heattreatment at 1000°C for 2 h in air.
strongest peaks [t,c(111), t,c(220), t(131), c(311) and t,c(200)] in TBC matched very closely with the tetragonal and cubic phases. Some of the cubic phases might overlap with the tetragonal phases because they have very similar d values (differences in crystal structure are small) and consequently there are peak overlaps. The Scherrer formula (Eq. (1)) was used to estimate the crystallite size (L) of the crystals (i.e. sizes of regions of lattice ordering) from the measured width of the diffraction curve in the as-deposited TBC (Fig. 3(a)). L=
0.29l b1/2 cos uB
(1)
where l= wavelength (1.542 A, for CuKa radiation), b1/2 = peak width at half maximum (in radians) and uB =angle of diffraction (in radians). For the (111) plane in the as-deposited coating, the measured width at 30° (2u) was about b1/2 =2.1° (2u) at full width half maximum (FWHM). Substituting these values in Eq. (1), the crystallite size for the (111) plane was of the order of 4 nm assuming that line broadening was not related to stress. If the same procedure is repeated for the heat-treated TBC (Fig. 3(b)) but corrected for instrument broadening only [b1/2(corrected)= b1/2(TBC) − b1,/2(instrumental)] and by considering the FWHM from a standard silicon peak having large crystallite size. The crystallite size was then almost doubled between corrected and uncorrected data (Table 2). Clearly, the correction factor was not necessary for the as-deposited condition because the peak broadening
3.4.1. Carbon detection in the as-deposited condition In all experiments, the as-deposited coatings were found to be grey, brown or black. The black samples were thought to contain carbon. These were analysed using Raman spectroscopy. Fig. 4(a) shows a typical Raman spectrum of TBCs deposited using ESAVD at temperatures between 500 and 600°C. There is a broad band centred at around 1500 cm − 1 which is attributed to amorphous carbon [21]. Moreover, this broad band may overshadow the microcrystalline graphitic bands, which occur at approximately 1580 and 1360 cm − 1 [21,22]. Therefore, carbon detected in the TBCs is likely to consist of a mixture of amorphous and crystalline graphitic phases. Furthermore, carbon was not picked up by XRD. This was because either (a) it was below the detection limit: 2–3%, or (b) it was amorphous, or both. Therefore, Raman analysis was used to detect small levels of carbon because the Raman beam can be focused on local areas to give a spatial resolution of approximately 1–3 mm. When the TBCs were heat-treated at an elevated temperature of 1000°C for 2 h, the colour of the coatings changes from black to white. The Raman spectrum for the heat-treated coating, Fig. 4(b), shows that the carbon/graphitic bands are no longer present. This indicated that during the deposition process, carbon was incorporated into the films. Thus, the microstructure of TBCs deposited by ESAVD might be affected during the heat-treatment process, as the carbon is burned off. Amorphous carbon begins to oxidise at about 500°C, and graphitic carbon at a slightly higher temperature ( 550°C), with the oxidation rate reaching a maximum at approximately 700 and 800°C, respectively [23,24]. Therefore, heat-treating the TBCs at 1000°C in air ensures that most of the carbon present is removed by oxidation and the structure becomes more crystalline and possibly slightly porous as well (Fig. 3(b)). Table 2 Crystallite size of 8wt.% yttria stabilised zirconia (YSZ) thermal barrier coating (TBC) deposited by electrostatic spray-assisted vapour deposition (ESAVD) TBC condition
As-deposited Heat-treated
Crystallite size in the (111) plane; L(111) (nm) Uncorrected
Corrected for instrument broadening only
4 24
4 40
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Fig. 4. Raman spectra for the (a) as-deposited coating, (b) coating heat-treated at 1000°C for 2 h in air.
If carbon incorporation in the films deposited by ESAVD is to be avoided then it may be necessary to place a further limit on the selection of precursors (which are normally alkoxides having extensive Cbranching) and solvents to those having fewer carbon atoms and/or carbon double bonds. In the latter case it may be difficult to break the double bond in time before a coating begins to form on the substrate surface and thus trapping the carbon. Having fewer carbon atoms in the solvents may prove to be considerably more important than in the precursors. For example, in butanol solvent with only four carbon atoms would have about 2.1× 1025 number of C atoms in 1000 ml of solution. In contrast to a precursor such as yttrium (III) 2-ethylhexanoate with 24 carbon atoms in the molecular formula would have 6.5 × 1022 number of C atoms in 1000 ml of solution. Therefore, a selection of precursors and solvents based on this added criteria and deposition in an oxygen-rich atmosphere can help to minimise the carbon contamination problem. The colour of the as-deposited EB-PVD coatings can also be found to be grey (like in the ESAVD samples) if not enough oxygen is available during deposition which is carried out under argon. This inert atmosphere is needed to prevent oxidation of coating materials prior to reaching the substrate. This lack of oxygen causes the composition to be non-stochiometic, e.g. ZrO2 might be ZrO1.5, and heat-treatment in air at about 700–800°C will change the colour from grey to white. However, such separate heat-treatment is not usually required because, after deposition by EB-PVD method, the samples are heat-treated (1100°C for 1 h in air) to improve the bonding between coating and bond coat [25]. Adherent oxide coatings (25 – 75 mm thick) of zirconia have been deposited on graphite substrates previously [26] by the application of ion beam deposition method using zirconium isopropoxide
(Zr[OCH(CH3)2]4·(CH3)2CHOH, FW= 387.67) and zirconium tetra-tertiary butoxides (Zr[OC(CH3)3]4, FW= 383.68) precursors. This latter precursor was also used in this study as a source of zirconium. The deposits obtained from isopropoxides were grey–white to black in colour and electrically conductive. When the deposits were moderately heated in air, they became white, powdery in texture, and electrically non-conductive. The oxide coatings obtained from tertiary butoxides were grey–white to white in colour and electrically non-conductive. They underwent no change when heated in air. However, in most cases, the as-deposited TBCs via ESAVD method have a grey–white to black colour. Therefore, it is more likely that during the ESAVD deposition process, the lack of oxygen available from the atmosphere (air) resulted in an incomplete combustion and/or chemical reactions of the constituents present in the solution. Thus, heat-treatment at 1000°C ensures the completion of any such reactions and to yield a coating that is white in colour. Moreover, such heat-treatment also helps to improve the adhesion of the TBCs to the bond coat.
3.4.2. Phase detection of 8wt.% YSZ by Raman spectroscopy Raman spectra for 8wt.% YSZ deposited using ESAVD, and an EB-PVD sample [27] of the same nominal composition is included in Fig. 5 for comparison. The frequency shifts of the tetragonal and cubic vibrational modes are slightly modified due to thermal broadening [27]. It is clear that there are peaks in almost identical places for the two coated samples prepared by very different techniques. This indicates that ESAVD method is capable of depositing coatings that have similar phases. The spectrum for samples deposited by the ESAVD method (and EB-PVD) clearly show that there are no indications of the monoclinic phase being present in the 8wt.% YSZ. Further-
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more, it has been shown that the major phase in the coatings deposited by ESAVD is tetragonal for which the data (vertical lines in Fig. 5) are in good agreement with previous Raman work [27 – 30]. The Raman analysis work done by Benner and Nagelberg [27] for 12wt.% YSZ is also included (Fig. 5(c) to indicate the position of the peaks for the cubic phase.
4. Conclusions Partially stabilised Y2O3 – ZrO2 with columnar-like growth features was successfully deposited using ESAVD method at a growth rate greater then 0.5 mm min − 1. The yttria composition of (8.9890.05) wt.% was analysed using ICP-AES. The XRD data showed the presence of both tetragonal (t) and cubic (c) phases though the presence of monoclinic (m) phase could not be ruled out completely as there were some peaks, which had very similar d-values with all three phases; t, c and m. The major phase was tetragonal. This was confirmed by Raman spectroscopy on the YSZ films. The crystallite size of the YSZ in the as-deposited condition was found to be 4 nm, which increased by 10-fold after heat-treatment, owing to grain growth. Further Raman investigations indicated that carbon was incorporated during the deposition process. The
Fig. 5. Raman spectra for (a) a typical 8wt.% yttria stabilised zirconia (YSZ) electrostatic spray-assisted vapour deposition (ESAVD) coating (after heat-treatment at 1000°C for 2 h in air), (b) 8wt.% YSZ electron beam physical vapour deposition (EB-PVD) (from Benner and Nagelberg [21]), (c) 12wt.% YSZ EB-PVD [21].
211
carbon incorporation during the deposition was probably due to incomplete combustion of the precursor. The Raman spectra for 8wt.% YSZ were in good agreement with previous work and unlike XRD conclusively determined the absence of the monoclinic phase. It may be possible for a future TBC to incorporate such microstructural features as porosity, microcracking and still maintain the beneficial columnar-like characteristics thereby taking advantage from both APS and EB-PVD methods.
Acknowledgements The authors wish to thank EPSRC and Rolls Royce for the CASE studentship. They are also grateful to Dr W. Bai and Dr P. Rogers for useful discussions, to Professor R. Stradling of the Physics Department at Imperial College for the use of the Raman spectrograph, and to Mr R. Sweeney for the use of XRD equipment and advice on the interpretation of XRD traces.
References [1] T. DeMasi-Marcin, D.K. Gupta, Surf. Coat. Technol. 68/69 (1994) 1. [2] R. Taylor, J.R. Brandon, P. Morrell, Surf. Coat. Technol. 50 (1992) 141. [3] E. Strangman, Thin Solid Films 127 (1985) 93. [4] R. Brandon, R. Taylor, Surf. Coat. Technol. 39/40 (1989) 143. [5] S.M. Meier, D.K. Gupta, K.D. Sheffler, J. Mater. 43 (1991) 50. [6] V. Joshi, M.P. Srivastava, Surf. Eng. 11 (3) (1995) 233. [7] S. Alpe`rine, L. Lelait, J. Eng. Gas Turbines Power, Trans. ASME 116 (1994) 258. [8] S. Stecura, Thin Solid Films 182 (1989) 121. [9] S.M. Meier, D.K. Gupta, J. Eng. Gas Turbines Power, Trans. ASME 116 (1994) 250. [10] U. Schulz, K. Fritscher, M. Peters, Surf. Coat. Tech. 82 (1996) 259. [11] R.C. Tucker, In: ASM Handbook Committee (Eds.), The Materials Information Society, ASM Handbook, Surf. Eng., 5 (1994) 497. [12] J. Wigren, L. Pejryd, In: C. Coddet (Ed.), Thermal Spray, Meeting the Challenges of the 21st Century, MCSC ASM International, USA, Proceedings of the 15th International Thermal Spray Conference, 25 – 29 May 1998, Nice, France, p. 1531. [13] W. Gru¨nling, W. Mannsmann, J. Phys. IV, C7 supplement to J. de Physique III 3 (1993) 903. [14] K.-L. Choy, W. Bai, British Patent File No. 9525505.5 (1995). [15] K.-L. Choy, Mater. World 6 (3) (1998) 144. [16] K- L. Choy, Adv. Mater. (1999), submitted. [17] K.-L. Choy, J.D. Vyas, In: K.K. Niihara, T. Sekino, E. Yasuda, T. Sasa (Eds.), The Science of Engineering Ceramics II, Proceedings of the 2nd International Symposium on the Science of Engineering Ceramics (EnCera ’98), Osaka, Japan, 6–9 September 1998, Trans. Tech. Publications, USA, p. 653. [18] K.-L. Choy, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nonotechnology, Academic Press, London, 1999. [19] J.R. Nicholls, Y. Jaslier, D.S. Rickerby, Mats. High Temp. 15 (1998) 15.
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[20] Plasma Technik User Manual. [21] W.M. Lau, S.-T. Lee, Q. Fuguang, in: T.S. Sudarshan, J.F. Braza (Eds.), Surface Modification Technologies V, The Institute of Materials, London, 1992, p. 283. [22] S.J. Bull, P.R. Chalker, C. Johnston, in: T.S. Sudarshan, J.F. Braza (Eds.), Surface Modification Technologies V, The Institute of Materials, London, 1992, p. 245. [23] G. Savage, Carbon-Carbon Composites, Chapman and Hall, London, 1993, p. 203, 220, 222. [24] Thomas, C.R. (Ed.), Essentials of Carbon–Carbon Composites, Royal Society of Chemistry, London, 1993, pp. 33, 95 – 98, 204.
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[25] J. Higgins, Private communication, Rolls Royce, Derby, January 1999. [26] Powell, C.F., Oxley, J.H., Blocher, J.M., Vapour Deposition, Wiley, New York, 1966, p. 400. [27] R.E. Benner, A.S. Nagelberg, Thin Solid Films 84 (1981) 89. [28] B.-K Kim, J.-W. Hahn, K.R. Han, J. Mater. Sci. Let. 16 (1997) 669. [29] C.A. Leach, J. Mater. Sci. 24 (1989) 1380. [30] V.G. Keramidas, W.B. White, J. Am. Ceram. Soc. 57 (1974) 22.
Structural characterisation of thermal barrier coatings deposited using electrostatic spray assisted vapour deposition method J.D. Vyas, K.-L. Choy * Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW 7 2BP, UK Received 3 March 1999; received in revised form 16 July 1999
Abstract Gas turbine engines provide one of the harshest environments for engineering components today. Thermal barrier coatings (TBCs) such as 8wt.% Y2O3 –ZrO2 (YSZ) are able to protect the blades and can lead to further increases in operating temperature by providing an effective thermal barrier on the blades. YSZ coatings were deposited using a novel and cost-effective electrostatic spray-assisted vapour deposition (ESAVD) technique. This method involves spraying atomised precursor droplets across an electric field, where they undergo decomposition and/or chemical reactions in the vapour phase near the vicinity of the heated substrate. A solution containing zirconium and yttrium alkoxide precursors was used for the deposition of YSZ films. The coatings were characterised using a combination of scanning electron microscopy, X-ray diffraction (XRD) and Raman spectroscopy. Thick and uniform YSZ films with columnar growth features were successfully deposited using the ESAVD method. The Raman investigations showed the presence of carbon in the as-deposited coatings. However, the carbon was removed when the YSZ coating was heat-treated at 1000°C for 2 h to improve the adhesion of the TBC coating to the bond coat. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Yttria stabilised zirconia; Electrostatic spray assisted vapour deposition; Thermal barrier coatings; Raman spectroscopy
1. Introduction The gas turbine engine provides one of the harshest environments (high temperature and direct stress) challenging material systems today. As engine components are subjected to rigorous mechanical loading conditions, high temperatures, corrosive (hot gases surrounding the blade are highly oxidising and may contain contaminants such as chlorides and sulphates-leading to hot corrosion) and/or erosive media (sand) [1]. The efficiency and performance of a gas turbine engine are directly related to the difference in temperature by the Carnot cycle and hence the operating temperature. Therefore, thermal barrier coatings (TBCs) are able to protect the blades and can lead to further increases in efficiency through the operating temperature by providing an effective thermal barrier on the blades [2]. TBCs typically consist of an intermediate layer (metallic bond coat) and a ceramic topcoat. The function of * Corresponding author. Tel./fax: +44-171-594-6750. E-mail address: [email protected] (K.-L. Choy)
the bond coat is essentially to provide a ‘key’ for firm adhesion of the topcoat (TBC) to the Ni-based material by having compatible coefficients of thermal expansion (within 11–13× 10 − 6 K − 1 range). The bond coat also helps to inhibit oxidation and provides good diffusion stability with the substrate. Doped zirconia is commonly used for the overcoat material because of its many benefits such as low thermal conductivity, relatively high thermal expansion coefficient and excellent chemical resistance [3,4]. The benefits of such systems have been well documented in many papers using different stabilisers or dopants [5,6]. However, all agree that the best composition and stabiliser is 8wt.% yttria stabilised zirconia (YSZ) [7,8]. The main commercial deposition methods for YSZ coatings are electron beam physical vapour deposition (EB-PVD) [9,10] and thermal spraying [9,11]. The thermal spraying includes flame spray processes, both air [9,12,13] and vacuum plasma spraying [11] and detonation gun process [11]. However, these methods are generally expensive to install and their maintenance costs are high. This is especially true for the EB-PVD
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Table 1 Advantages and disadvantages of thermal spraying and electron beam physical vapour deposition (EB-PVD) Main method
Benefits
Problems
Thermal spraying
Cheaper compared to EB-PVD Some stress–strain tolerance offered Columnar structure
Splash-like structure Require sophisticated coating chambers Line-of-sight process Very expensive (capital cost\US$ 25M) Line-of sight process Requires sophisticated chambers
EB-PVD
Much better stress– strain tolerance
method. Therefore, there is a need to develop simple and cost-effective coating methods. Table 1 summarises the advantages and disadvantages of thermal spraying and EB-PVD. Recently, a novel cost-effective coating technique called electrostatic spray assisted vapour deposition (ESAVD) has been developed by Choy and Bai [14]. Process principles involves spraying atomised precursor droplets across an electric field, where they undergo decomposition and/or chemical reactions in the vapour phase near the vicinity of the heated substrate. This produces a stable solid film with excellent adhesion onto the substrate [15]. The detailed deposition mechanism of the ESAVD process and its advantages over other aerosol deposition methods, including pyrolysis and electrostatic spray deposition have been reviewed elsewhere [14 – 16]. Therefore, this method is employed for the deposition of thick thermal barrier coatings for aerospace applications.
2. Experimental Prior to the deposition, the aluminised Ni-based substrates (Nimonic 75) were ultrasonically cleaned in 2-propanol. A 0.05 M precursor solution was prepared by dissolving zirconium (IV) butoxide and yttrium 2-ethylhexanoate in butanol to deposit a coating with a nominal composition of 8wt.%Y2O3 – 92wt.%ZrO2 (8wt.% YSZ) using the ESAVD method. Fig. 1 shows a schematic diagram of the ESAVD apparatus used for depositing yttria partially stabilised zirconia. The precursor solution was atomised and sprayed across an electric field. By carefully controlling the processing conditions the atomised spray droplets undergo decomposition and/or heterogeneous chemical reactions near the heated substrate, to produce a solid well-adherent coating. The deposition temperature at the substrate surface was varied from 500 to 600°C under an electric field between 5 and 20 kV cm − 1.
Fig. 1. A schematic diagram of the electrostatic spray-assisted vapour deposition (ESAVD) apparatus.
The as-deposited and heat-treated (2 h at 1000°C in air) YSZ coatings were analysed using a combination of chemical analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The scanning electron microscope (T220A) was used to examine the surface morphology of the polished coating, both in topography and cross-sections. Chemical analysis based on the inductively coupled plasma, combined with atomic emission spectroscopy (ICP-AES) was used to determine the stochiometry of the YSZ coatings. A Phillips PW1700 series diffractometer (CuKa radiation of wavelength 1.54178 A, ) was employed to perform the phase analysis. The reported d-spacings from the JCPDS cards, for tetragonal (c24-1164 and c17-923), cubic (c 27-997 and c301468) and monoclinic (c 13-307) were the compared with observed peaks. A Raman Renishaw 2000 Spectrograph was used for the detection of carbon and for YSZ coating analysis. The instrument collects information on the scattered radiation through a multi-element detector. The spectra were excited with a red He–Ne ion laser tuned to a Raman wavelength (l) of 633 nm. Spectra of each sample were taken over the range of 1200–1800 cm − 1 for carbon detection, and 120–740 cm − 1 for 8wt.% YSZ coating analysis.
3. Results and discussion The possible deposition mechanism in the ESAVD process has been summarised elsewhere [17,18]. In general, for the deposition of TBC coatings, the process conditions are tailored to enable the chemical reactions to form Y2O3 –ZrO2 to take place near the vicinity of the heated Ni-based substrate (i.e. heterogeneous CVD reactions). If the deposition temperature is too high, Y2O3 –ZrO2 particles will form in the gas phase and favour the deposition of powdery films with poor adhesion. If the deposition temperature is too low, splashing of the droplets on substrate surface takes place and vaporisation of solvent occurs to leave behind a dry precipitate (intermediates). Further heat treatment at
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higher temperature will cause the precursor to undergo decomposition, and followed by subsequent sintering to give porous films with poor adhesion.
3.1. SEM Fig. 2 shows the scanning electron mircograph of the as-deposited 8wt.% YSZ by ESAVD. The surface morphology in Fig. 2(a) reveals a ‘cauliflower’ type of structure. The cross-section in Fig. 2(b) shows the columnar-like growth features (column width approximately 9–18 mm at base and 16 – 20 mm at top) with microcracks ranging from 2 to 10 mm within the thick ceramic coating (approximately 50 mm). A porosity of 20% was calculated for the microstructure in Fig. 2(b) using the grid method. In comparison, the plasmasprayed structure is quite porous (10 – 15%) as well and extensively microcracked [9]. This porosity is primarily responsible for the generally lower thermal conductivity values observed for air plasma sprayed TBCs. The
superior erosion properties of EB-PVD TBCs are attributed to the columnar structure in the ceramic coating [19]. The TBCs deposited using the ESAVD method incorporate microstructural features such as porosity and microcracking and still maintain the beneficial columnar-like characteristics thereby taking advantage from both APS (thermal conductivity) and EB-PVD (erosion) techniques. Much thicker coatings (approximately 400 mm) have since been deposited using the ESAVD method and will be reported in the near future. The coatings deposited by ESAVD method were qualitatively evaluated to provide a rapid assessment of their adhesion to the Ni-based substrates. They were heated to 1200°C and cooled rapidly to room temperature. The procedure was repeated and the TBCs remained in tact for more than 12 cycles. The deposition rate was found to be about 0.5 mm min − 1, which is comparable to EB-PVD, which has a rate of approximately 1–2 mm min − 1, depending on geometry and processing conditions. As a note the deposition rate for plasma spraying is 5–10 mm per pass as the gun will move around the component [20].
3.2. ICP-AES For the compositional analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES), three to four measurements on the heat-treated (2 h at 1000°C) YSZ films were made and an average value was reported within the limits of the error. The ICPAES analysis showed a presence of (8.989 0.05) wt.% yttria in zirconia. A minor error possibly due to preparation/dilution of the solution, e.g. weighing of powders and/or preparation of the precursor solution because the zirconium tetra-butoxide is a high viscosity liquid at room temperature.
3.3. XRD
Fig. 2. A typical scanning electron micrograph (SEM) of the as-deposited yttria stabilised zirconia (YSZ) coatings; (a) surface morphology, (b) cross-section.
XRD was used to characterise the phases present. JCPDS cards for tetragonal (c24-1164 and c17-923), cubic ( c 27-997 and c30-1468) and monoclinic (c13307) were used to identify the presence of phases. Typical XRD patterns for TBCs deposited using the ESAVD method, both before and after heat-treatment, are shown in Fig. 3. In the as-deposited condition, the tetragonal and cubic phases are present. However, some of these peaks are also common to the monoclinic phase. The broad peaks indicate either nanosize crystallites or stress (which would be to some extent relieved upon heating) within the TBC. Once the samples were heat-treated, grain growth occurred which gave sharper peaks, having higher intensity counts because of the larger crystallite size. The Raman analysis did not show the monoclinic phase in the 8wt.% YSZ TBCs (Fig. 5). The TBC coating was randomly orientated because the
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due to small crystallite size was greater than that due to instrumentation but becomes more important for larger crystallite sizes at higher temperatures.
3.4. Raman analysis
Fig. 3. Typical X-ray diffraction (XRD) traces for 8wt.% yttria stabilised zirconia (YSZ) coatings; (a) as-deposited, (b) after heattreatment at 1000°C for 2 h in air.
strongest peaks [t,c(111), t,c(220), t(131), c(311) and t,c(200)] in TBC matched very closely with the tetragonal and cubic phases. Some of the cubic phases might overlap with the tetragonal phases because they have very similar d values (differences in crystal structure are small) and consequently there are peak overlaps. The Scherrer formula (Eq. (1)) was used to estimate the crystallite size (L) of the crystals (i.e. sizes of regions of lattice ordering) from the measured width of the diffraction curve in the as-deposited TBC (Fig. 3(a)). L=
0.29l b1/2 cos uB
(1)
where l= wavelength (1.542 A, for CuKa radiation), b1/2 = peak width at half maximum (in radians) and uB =angle of diffraction (in radians). For the (111) plane in the as-deposited coating, the measured width at 30° (2u) was about b1/2 =2.1° (2u) at full width half maximum (FWHM). Substituting these values in Eq. (1), the crystallite size for the (111) plane was of the order of 4 nm assuming that line broadening was not related to stress. If the same procedure is repeated for the heat-treated TBC (Fig. 3(b)) but corrected for instrument broadening only [b1/2(corrected)= b1/2(TBC) − b1,/2(instrumental)] and by considering the FWHM from a standard silicon peak having large crystallite size. The crystallite size was then almost doubled between corrected and uncorrected data (Table 2). Clearly, the correction factor was not necessary for the as-deposited condition because the peak broadening
3.4.1. Carbon detection in the as-deposited condition In all experiments, the as-deposited coatings were found to be grey, brown or black. The black samples were thought to contain carbon. These were analysed using Raman spectroscopy. Fig. 4(a) shows a typical Raman spectrum of TBCs deposited using ESAVD at temperatures between 500 and 600°C. There is a broad band centred at around 1500 cm − 1 which is attributed to amorphous carbon [21]. Moreover, this broad band may overshadow the microcrystalline graphitic bands, which occur at approximately 1580 and 1360 cm − 1 [21,22]. Therefore, carbon detected in the TBCs is likely to consist of a mixture of amorphous and crystalline graphitic phases. Furthermore, carbon was not picked up by XRD. This was because either (a) it was below the detection limit: 2–3%, or (b) it was amorphous, or both. Therefore, Raman analysis was used to detect small levels of carbon because the Raman beam can be focused on local areas to give a spatial resolution of approximately 1–3 mm. When the TBCs were heat-treated at an elevated temperature of 1000°C for 2 h, the colour of the coatings changes from black to white. The Raman spectrum for the heat-treated coating, Fig. 4(b), shows that the carbon/graphitic bands are no longer present. This indicated that during the deposition process, carbon was incorporated into the films. Thus, the microstructure of TBCs deposited by ESAVD might be affected during the heat-treatment process, as the carbon is burned off. Amorphous carbon begins to oxidise at about 500°C, and graphitic carbon at a slightly higher temperature ( 550°C), with the oxidation rate reaching a maximum at approximately 700 and 800°C, respectively [23,24]. Therefore, heat-treating the TBCs at 1000°C in air ensures that most of the carbon present is removed by oxidation and the structure becomes more crystalline and possibly slightly porous as well (Fig. 3(b)). Table 2 Crystallite size of 8wt.% yttria stabilised zirconia (YSZ) thermal barrier coating (TBC) deposited by electrostatic spray-assisted vapour deposition (ESAVD) TBC condition
As-deposited Heat-treated
Crystallite size in the (111) plane; L(111) (nm) Uncorrected
Corrected for instrument broadening only
4 24
4 40
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Fig. 4. Raman spectra for the (a) as-deposited coating, (b) coating heat-treated at 1000°C for 2 h in air.
If carbon incorporation in the films deposited by ESAVD is to be avoided then it may be necessary to place a further limit on the selection of precursors (which are normally alkoxides having extensive Cbranching) and solvents to those having fewer carbon atoms and/or carbon double bonds. In the latter case it may be difficult to break the double bond in time before a coating begins to form on the substrate surface and thus trapping the carbon. Having fewer carbon atoms in the solvents may prove to be considerably more important than in the precursors. For example, in butanol solvent with only four carbon atoms would have about 2.1× 1025 number of C atoms in 1000 ml of solution. In contrast to a precursor such as yttrium (III) 2-ethylhexanoate with 24 carbon atoms in the molecular formula would have 6.5 × 1022 number of C atoms in 1000 ml of solution. Therefore, a selection of precursors and solvents based on this added criteria and deposition in an oxygen-rich atmosphere can help to minimise the carbon contamination problem. The colour of the as-deposited EB-PVD coatings can also be found to be grey (like in the ESAVD samples) if not enough oxygen is available during deposition which is carried out under argon. This inert atmosphere is needed to prevent oxidation of coating materials prior to reaching the substrate. This lack of oxygen causes the composition to be non-stochiometic, e.g. ZrO2 might be ZrO1.5, and heat-treatment in air at about 700–800°C will change the colour from grey to white. However, such separate heat-treatment is not usually required because, after deposition by EB-PVD method, the samples are heat-treated (1100°C for 1 h in air) to improve the bonding between coating and bond coat [25]. Adherent oxide coatings (25 – 75 mm thick) of zirconia have been deposited on graphite substrates previously [26] by the application of ion beam deposition method using zirconium isopropoxide
(Zr[OCH(CH3)2]4·(CH3)2CHOH, FW= 387.67) and zirconium tetra-tertiary butoxides (Zr[OC(CH3)3]4, FW= 383.68) precursors. This latter precursor was also used in this study as a source of zirconium. The deposits obtained from isopropoxides were grey–white to black in colour and electrically conductive. When the deposits were moderately heated in air, they became white, powdery in texture, and electrically non-conductive. The oxide coatings obtained from tertiary butoxides were grey–white to white in colour and electrically non-conductive. They underwent no change when heated in air. However, in most cases, the as-deposited TBCs via ESAVD method have a grey–white to black colour. Therefore, it is more likely that during the ESAVD deposition process, the lack of oxygen available from the atmosphere (air) resulted in an incomplete combustion and/or chemical reactions of the constituents present in the solution. Thus, heat-treatment at 1000°C ensures the completion of any such reactions and to yield a coating that is white in colour. Moreover, such heat-treatment also helps to improve the adhesion of the TBCs to the bond coat.
3.4.2. Phase detection of 8wt.% YSZ by Raman spectroscopy Raman spectra for 8wt.% YSZ deposited using ESAVD, and an EB-PVD sample [27] of the same nominal composition is included in Fig. 5 for comparison. The frequency shifts of the tetragonal and cubic vibrational modes are slightly modified due to thermal broadening [27]. It is clear that there are peaks in almost identical places for the two coated samples prepared by very different techniques. This indicates that ESAVD method is capable of depositing coatings that have similar phases. The spectrum for samples deposited by the ESAVD method (and EB-PVD) clearly show that there are no indications of the monoclinic phase being present in the 8wt.% YSZ. Further-
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more, it has been shown that the major phase in the coatings deposited by ESAVD is tetragonal for which the data (vertical lines in Fig. 5) are in good agreement with previous Raman work [27 – 30]. The Raman analysis work done by Benner and Nagelberg [27] for 12wt.% YSZ is also included (Fig. 5(c) to indicate the position of the peaks for the cubic phase.
4. Conclusions Partially stabilised Y2O3 – ZrO2 with columnar-like growth features was successfully deposited using ESAVD method at a growth rate greater then 0.5 mm min − 1. The yttria composition of (8.9890.05) wt.% was analysed using ICP-AES. The XRD data showed the presence of both tetragonal (t) and cubic (c) phases though the presence of monoclinic (m) phase could not be ruled out completely as there were some peaks, which had very similar d-values with all three phases; t, c and m. The major phase was tetragonal. This was confirmed by Raman spectroscopy on the YSZ films. The crystallite size of the YSZ in the as-deposited condition was found to be 4 nm, which increased by 10-fold after heat-treatment, owing to grain growth. Further Raman investigations indicated that carbon was incorporated during the deposition process. The
Fig. 5. Raman spectra for (a) a typical 8wt.% yttria stabilised zirconia (YSZ) electrostatic spray-assisted vapour deposition (ESAVD) coating (after heat-treatment at 1000°C for 2 h in air), (b) 8wt.% YSZ electron beam physical vapour deposition (EB-PVD) (from Benner and Nagelberg [21]), (c) 12wt.% YSZ EB-PVD [21].
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carbon incorporation during the deposition was probably due to incomplete combustion of the precursor. The Raman spectra for 8wt.% YSZ were in good agreement with previous work and unlike XRD conclusively determined the absence of the monoclinic phase. It may be possible for a future TBC to incorporate such microstructural features as porosity, microcracking and still maintain the beneficial columnar-like characteristics thereby taking advantage from both APS and EB-PVD methods.
Acknowledgements The authors wish to thank EPSRC and Rolls Royce for the CASE studentship. They are also grateful to Dr W. Bai and Dr P. Rogers for useful discussions, to Professor R. Stradling of the Physics Department at Imperial College for the use of the Raman spectrograph, and to Mr R. Sweeney for the use of XRD equipment and advice on the interpretation of XRD traces.
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