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MOCVD of thick YSZ coatings using acetylacetonates
Surface & Coatings Technology 192 (2005) 117 – 123 www.elsevier.com/locate/surfcoat
MOCVD of thick YSZ coatings using acetylacetonates S.V. Samoilenkov *, M.A. Stefan, G. Wahl Institut fu¨r Oberfla¨chentechnik und plasmatechnische Werkstoffentwicklung, TU Braunschweig, Bienroder Weg 53, 38108, Braunschweig, Germany Received 21 February 2004; accepted in revised form 10 March 2004 Available online 12 May 2004
Abstract Thick (up to 20 Am) coatings of yttria-stabilized zirconia with columnar structure have been grown by thermal MOCVD at 630 – 820 jC using acetylacetonates of Zr and Y as precursors. The samples have been characterized by Scanning Electron Microscopy (SEM), Wave Dispersion X-ray Analysis (WDX) and X-ray Diffraction (XRD) to find out the interplay of deposition conditions with coating microstructure, yttria content and phase composition. In particular, it has been found that homogenous nucleation reactions in gas phase at high temperature lead to the decrease of total growth rate and enrichment of resulting coating with yttrium. It has also been found that heterovalent substitution in ZrO2 leads to remarkable increase of a column width and a coating integrity. A simple model for the quantitative description of the process is presented. D 2004 Elsevier B.V. All rights reserved. Keywords: Zirconia; YSZ; Chemical vapor deposition
1. Introduction Zirconia together with its solid solutions is one of the most studied and technically used ceramic materials today [1,2]. This is due to its low cost, high thermal, mechanical and chemical stability, and high oxygen ion conductivity at elevated temperatures. Yttria-stabilized zirconia [ZrO2(Y2O3) = YSZ] is a material of choice for thermal barrier coatings (TBCs) used for heat protection of metal parts in gas turbines and engines [3,4]. TBC use permits to increase the turbine/engine efficiency or its lifetime. Another technologically important application of thick YSZ layers is an electrolyte layer in solid oxide fuel cells (SOFC) [5,6]. Microstructure, thickness, phase and chemical composition of the coating are of a prime importance for applications. In the case of TBCs, the porous layer with columnar microstructure consisting of tetragonal YSZ is obligatory for the best performance [3], while gas tight layers of cubic YSZ are used in SOFCs [6]. Therefore, it is important to know the interplay of deposition parameters, coatings microstructure and composition to produce the material with desirable properties. * Corresponding author. Tel.: +49-531-391-9425; fax: +49-531-3919400. E-mail address: [email protected] (S.V. Samoilenkov). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.019
Chemical Vapor Deposition (CVD) is used in technology for a wide scope of purposes. Most common precursors for the deposition of ZrO2 are zirconium halogenides, alkoholates and h-diketonates. The use of ZrCl4 for the growth of thick YSZ TBCs has been recently demonstrated by Chevillard and Drawin [7], Preauchat et al. [8] and Preauchat and Drawin [9]. Although zirconium chloride is volatile and cheap, its conversion to ZrO2 during CVD leads to formation of hazardous Cl2 and HCl. Highly volatile zirconium alkoholates are very unstable in air and this makes them difficult to handle. Additionally, alkoholates are not suited for hightemperature deposition processes due to their low thermal stability [10]. There exist a number of volatile zirconium hdiketonates. The most widely used h-diketonates are chelates with pentadione-2,4 (acetylacetone, Hacac) and 2,2,6,6-tetramethylheptandione-3,5 (Htmhd). These compounds are stable in air, possess high enough vapor pressure and are commercially available. Zr(acac)4 is thermally less stable than Zr(tmhd)4, but its low price ( c 150 EURO/kg, the price of Zr(tmhd)4 is by a factor of c 10 higher) makes acetylacetonate very attractive for CVD of thick layers. Yttrium acetylacetonate, Y(acac)3, was reported to decompose partially during sublimation, but still providing high vapor pressure [11,12]. In this study, we have attempted to elucidate some features of MOCVD of YSZ using acetylacetonates as
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volatile precursors. The interplay of growth conditions, microstructure, phase composition and film thickness is reported here.
mmol cm2 h1 units. Taking into account the density of YSZ, qYSZ c 6.0 g/cm3 and the molar mass MYSZ ¼ y MYO1:5 þ ð1 yÞ MZrO2
2. Experimental The deposition experiments have been performed in flash evaporation single source MOCVD setup (Fig. 1). The distance between the gas inlet and the substate was H = 15 cm, the inlet tube diameter was d = 2 cm and the diameter of the substrate holder was ds = 4 cm. The powder mixture of water-free Zr(acac)4 (Fluka Chemie GmbH, Germany) and Y(acac)3 (ABCR GmbH, Germany) was placed in a vibration feeding mechanism described elsewhere [13]. The coating composition was controlled by molar fraction of zirconium and yttrium precursors, nZr and nY, in the initial powder mixture. The evaporator and transport lines were heated up to 250 jC. The substrate holder was heated inductively by RF coil to reach the substrate temperature in the range of 630– 820 jC. Other deposition conditions were as follows: total precursor flow iprec = 5.27 mmol/h, argon flow iargon = 0.413 mol/h (154 sccm), oxygen flow ioxygen = 0.164 mol/h (61 sccm), total pressure ptot = 500 Pa, deposition time t = 1 h. The precursor molar fraction in the gas phase was xprec = xZr + xY = 0.009. The molar fractions in the gas are correlated with the molar fractions in the powder (n. . .) as follows: xZr = nZrxprec and xY = nYxprec. The deposition was carried out on 0.5-mm-thick plates of polycrystalline Al2O3 with a surface area of approx. 2 cm2. All samples were annealed after deposition for 10 min in air at 900 jC in order to burn out carbon-containing rests. The mass deposition rate of YSZ has been determined by weighing the substrate before and after the deposition. The Mettler AT261 balances with an accuracy of F 0.02 mg were used. Typical mass change produced by the deposition was in the range of 6 –16 mg. Because the coatings might have variable porosity, the growth rate was determined in
Fig. 1. The scheme of the experimental MOCVD setup used.
where M.. is the molar mass and y is the molar fraction of YO1.5 in YSZ, the dense layer with a thickness of sde = 1 Am corresponds roughly to 0.6 mg of the coating mass per cm2. The samples have been characterized by Scanning Electron Microscopy (SEM, CamScan 4M electron microscope), Wave Dispersion X-ray Analysis (WDX, Microspec WDX3PC analysing system) and X-ray Diffraction (XRD, Siemens D5000 diffractometer, CrKa radiation). The coating porosity e ¼ 1 SSde has been determined from the coating thickness s observed by SEM and the thickness sde calculated from the mass change. The average YSZ column width has been determined for each sample from four representative SEM cross-section images. The values of density for ZrO2 and YSZ were taken from XRD data using JCPDS data base on unit cell volume of the corresponding oxides.
3. Results and discussion 3.1. Deposition rate vs. growth temperature and yttria content Depending on conditions, there are several decomposition routes of metal acetylacetonates [18 – 21]. The decomposition process of Zr(acac)4 in inert gas starts already at 200 jC [20] and Zr(acac)4 is not the single volatile species responsible for the ZrO2 formation in MOCVD. These considerations should be even more important for less stable Y(acac)3 precursor. However, taking all precursor destruction paths into account is a difficult task. On the other hand, the residence time of precursor portion in evaporator and transport lines in the flash evaporation scheme used is rather short (being of about 2 s in our system) minimizing thus the influence of aforementioned decomposition processes. In the following, we propose a simplified description of the CVD process, which neglects the decomposition of precursor molecules during evaporation and transport to the reactor. Although such assumption is simplified, the model provides a good agreement with the experimental results obtained. The dependence of the molar deposition rate of ZrO2 ( jZr) on growth temperature is presented in Fig. 2 in an Arrhenius plot. On the right side the transfer coefficient aZr ¼ xjZrZr , where xZr is the molar fraction of Zr(acac)4 in the gas, is plotted. The curve follows well-known dependency for thermally activated CVD processes at high temperatures [14]. At 630– 720 jC, the growth rate is almost temperature-independent. This corresponds to the growth limited by mass transport (or so-called diffusioncontrolled regime). The deposition rate in the mass trans-
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max = 0.06 mmol cm2 h1 for those found experimentally: jZr jZr xZr = 0.009 (Fig. 2), aZr ¼ xZr ¼ 6:6 mmol cm2 h1. Above 700 jC, the deposition rate decreases with the temperature increase. This behaviour can be explained by side reactions, which might take place in proximity of the heated sample (drbd) and lead to formation of ZrO2 powder, which is not integrated in the film:
ZrðacacÞ4 þ O2 ! ZrO2 ðpowderÞ þ by-products ðCO2 ; CO; H2 O; etcÞ: This process competes with the film growth reaction. Both reactions might be the first-order ones with reaction constants kp = kpoexp( Ep/RT) for the powder formation and kd = kdoexp( Ed/RT) for the deposition; Ep and Ed are the activation energies.The molecular balance at the deposition surface is given by: Fig. 2. Deposition rate of ZrO2 coatings vs. reciprocal deposition temperature.
port controlled regime for the stagnation flow reactor is given by [15]: j ¼ aZr ðxZr xoZr Þ
ð1Þ
o is the precursor concentration near the surface. where xZr The maximum molar deposition rate for the stagnation flow reactor jmax can be approximately calculated using following relation (it is assumed that the precursor is o completely consumed at the substrate surface, i.e., xZr =0 near the surface):
jmax Zr ¼ aZr xZr
ð2Þ
with the transfer coefficient aZr ¼ 0:3
ffiffiffiffiffiffiffiffiffi DZr ctot pffiffiffiffiffiffi p Re 3 ScZr ds
ð3Þ
where ctot is the total molar concentration of precursor in the gas. The transfer coefficient aZr is generated by the diffusion of the molecules through the concentration boundary layer with the thickness d above the substrate surface. The diffusion coefficient DZr = 60 cm2 s1 was determined according to the methods given in Ref. [16]. Reynolds and Schmidt numbers were defined as Re = wdm1, ScZr = mDZr1, where the gas velocity w = 7.8 m s1. The kinematic viscosity m was calculated from dynamic viscosity g = 53 APa s (value for argon [17]) by the relation m = gq1, where q = 2.74 g m3 is the gas density. All these values were calculated at ptot = 500 Pa and 1000 K, giving as a result Re = 8 103 and Sc = 3.2 103. Then, according to Eqs. (2) max and (3), aZr = 14.4 mmol cm2 h1 and jZr = 0.13 mmol 2 1 cm h for xZr = 0.009. These values are quite close to
aZr ðxZr xoZr Þ ¼ ðkp ctot dr þ kd ctot ÞxoZr :
ð4Þ
o The concentration xZr near the surface is not equal to zero as in Eq. (2), but:
xoZr ¼
aZr xZr : aZr þ kp ctot dr þ kd ctot
ð5Þ
The molar deposition rate is then: jZr ¼
kd ctot amax xZr : amax þ kp ctot dr þ kd ctot
ð6Þ
At low temperatures, the condition (kpctotdr + kdctot)b amax is fulfilled [22], the deposition is controlled by surface reactions and is proportional to kd. The deposition rate decreases with the temperature decrease. This case has been described for the YSZ deposition in Ref. [23]. Exponential decrease of the deposition rate at high temperature (Fig. 2) can be explained by Eq. (6) provided kpctotdrH(amax + kdctot). Then, the deposition rate is: jZr ¼
ðEd Ep Þ kd amax xZr ~exp RT k p dr
ð7Þ
Fig. 2 verifies the exponential behaviour with (EdEp) < 0. At mean deposition temperature kdctotH(amax + kpctotdr) and the deposition rate is controlled by diffusion in the gas phase and Eq. (2) can be used. This corresponds to a horizontal part of the curve in Fig. 2. The YSZ deposition rate decreases linearly with growing Y(acac)3 content nY in the precursor mixture as it is demonstrated in Fig. 3 for the deposition temperature of 720 jC. At nY c 3 mol%, the deposition rate is by c 10% lower than that of ZrO2. In addition, the yttria mole fraction yY in the coating was determined. The partial deposition rates jZr and jY and the transfer coefficients aZr = jZr/xZr
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Fig. 4. YO1.5 content in YSZ coatings deposited at 720 jC vs. composition of initial precursor mixture. Dashed line represents the case of congruent deposition.
Fig. 5 shows the temperature dependence of the YSZ deposition rate and the variation of the yttria molar fraction in the coating for nY = 0.067. From these measurements the transfer coefficients has been calculated and plotted in Fig. 6. The deposition rate decreases exponen-
Fig. 3. (a) Deposition rate of YSZ vs. yttrium acetylacetonate molar fraction in initial precursor mixture. (b) Transfer coefficients for ZrO2 and YO1.5 vs. nY at constant deposition temperature of 720 jC.
and aY = jY/xY have been calculated from the total deposition rate, mole fraction yY and the gas mole fractions. The transfer coefficients determined are independent of nY as shown in Fig. 3b for the deposition temperature of 720 jC. The result of these differences in the transfer coefficients is that the deposition rate jYSZ decreases linearly with nY according to the relation: jYSZ ¼ aZr xprec ðaZr aY Þxprec nY as it is shown in Fig. 3a. The molar fraction of yttria in the coating increases according to the relation: aY aZr nY yY ¼ aY n 1 aZra Y Zr or, for small nY: aY yY ¼ n : aZr Y This relation is verified by Fig. 4.
Fig. 5. (a) Deposition rate and (b) YO1.5 content in YSZ coatings vs. reciprocal deposition temperature. The films were grown from precursor mixture with nY = 0.067.
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Fig. 6. Transfer coefficients for zirconia and yttria vs. deposition temperature at constant nY = 0.067.
tially with the temperature increase, similar to the behaviour found for ZrO2 growth. The transfer coefficients aZr and aY converge to almost the same value at high deposition temperature. The reason for the difference between aZr and aY could be the various diffusion coefficients, kinetic effects or incomplete evaporation of Y(acac)3. Indeed, Y(acac)3 was shown to decompose partially during evaporation [11,12]. 3.2. Coatings microstructure vs. growth temperature and yttria content All the coatings obtained in this study had a columnar structure. A typical SEM image of the sample cross-section is given in Fig. 7. It was found that the column width and uniformity over the thickness increase remarkably with only a slight doping of ZrO2 with yttria. The dependence of column width on yttria content is shown in Fig. 8a. It
Fig. 7. SEM image of cross-section of ZrO2 coating deposited at 720 jC.
Fig. 8. (a) average column width and (b) coatings porosity e vs. yttria content in the coating.
has been also observed that the coating porosity decreases from 46% to 35% as yttria content increases from 0 to 15 mol% (Fig. 8b). Let us briefly discuss this phenomenon. Column width and coating integrity ( = 1e) should increase with the increase of diffusion mobility and/or characteristic residence time of adatoms on the surface (caused, e.g., by reduced deposition rate). The increase of YSZ column width and uniformity with the increase of deposition temperature and/or decrease of the deposition rate has been previously reported for EBPVD YSZ thermal barrier coatings [24]. It has been also observed that the increase of the deposition temperature of CVD-derived YSZ layers led
Fig. 9. h – 2h XRD scans of YSZ coatings with various yttria content.
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Fig. 10. Lattice parameters of tetragonal and cubic YSZ in coatings. Dashed line represents the data for powder after Scott [29].
studied in temperature range of 630– 820 jC. It has been found that the films deposited in diffusion controlled regime (below 720 jC) were enriched with zirconia as compared to initial precursor mixture. The appearance of homogeneous nucleation reactions led to enrichment of the layers deposited above 720 jC with yttria. The strong effect of yttria concentration of the coatings microstructure (column uniformity and width, porosity) has been observed. The increase of the coating integrity and uniformity with the increase of yttria concentration was supposed to be due to diffusion enhancement occurring in YSZ due to heterovalent substitution. According to XRD studies, the monocline modification is present in YSZ coatings up to 6.4 mol% of YO1.5 content. YSZ phase in the films with higher yttria concentration was found to be tetragonal (up to 6.4 –12 mol% of YO1.5) or cubic (for higher concentration).
References to dramatic decrease of their porosity [7] and increase of column width [25]. We suppose that in present case the effect is caused by the increase of diffusion mobility due to heterovalent substitution in ZrO2. The similar behaviour has been recently demonstrated for epitaxial films of CeO2(R2O3) on sapphire substrates (R—rare earth element) [26,27]. 3.3. XRD analysis Zirconia and YSZ can exist in three crystallographic forms: monoclinic, tetragonal and cubic. The increase of temperature and yttria content leads to the increase of the cell symmetry [28]. ZrO2 coatings deposited in temperature interval 630 – 775 jC consisted of mixture of monocline and tetragonal phases. Calculated lattice parameters of the monocline ZrO2 coin˚, cided very well with those for bulk material: a = 5.31 A ˚ , c = 5.15 A ˚ , b = 99.2j. b = 5.21 A XRD scans of YSZ coatings deposited at 720 jC are presented in Fig. 9. ZrO2 coating consists of the mixture of tetragonal and monocline phases. Monocline modification is present in YSZ coatings up to 6.4 mol% of YO1.5, while tetragonal form becomes predominant as yttria content increases. The tetragonal distortion gradually decreases; YSZ in coatings with more than 12 mol% of YO1.5 is cubic (Fig. 10). This is consistent with observations of crystal structure of MOCVD-derived YSZ films deposited at 760 jC [30]. As well, the lattice parameters found correspond well to those known for bulk material.
4. Conclusions Peculiarities of MOCVD of polycrystalline ZrO2 and YSZ thick coatings on alumina substrates have been
[1] I. Birkby, R. Stevens, Key Eng. Mater. 122 – 124 (1996) 327. [2] E. Kisi (Ed.), Zirconia Engineering Ceramics, Trans Tech Publications, Switzerland, 1998, p. 1. [3] G.W. Goward, Surf. Coat. Technol. 108 – 109 (1998) 73. [4] P. Grant, in: B. Cantor, H. Assender, P. Grant (Eds.), Aerospace Materials, IOP Publishing, Bristol, 2001, p. 294. [5] H.-H. Mo¨bius, J. Solid State Electrochem. 1 (1997) 2. [6] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [7] S. Chevillard, S. Drawin, M.H. Vidal-Serif, Proc. of 85th Meeting of the Agard Structures and Material Panel (SMP)—Thermal Barriers, Aalborg, Danemark, 13 – 16 October, NASA CASI, Hanover, MD, USA, 1997, p. 1. [8] B. Preauchat, S. Drawin, S. Landais, 42e Colloque de Metallurgie de l’INSTM bMateriaux pour machines thermiques: Turbines pour l’aeronautique et pour la production d’energieH, CEA-SaclavINSTN, 22 – 24 juin (1999). [9] B. Preauchat, S. Drawin, Surf. Coat. Technol. 146 – 147 (2001) 94. [10] L.G. Hubert-Pfalzgraf, New J. Chem. 11 (1987) 663. [11] G.V. Trembovetskii, S.S. Berdonosov, I.A. Murav’eva, L.I. Martynenko, Russ. J. Inorg. Chem. 29 (1984) 1234. [12] A.S. Alikhanyan, I.P. Malkerova, Ya.Kh. Grinberg, V.B. Lazarev, V.A. Bogdanov, V.I. Gorgoraki, V.A. Schreider, Proc. Acad. Sci. USSR. Phys. Chem. Sect. 292 (1987) 43. [13] S.V. Samoylenkov, O.Yu. Gorbenko, I.E. Graboy, A.R. Kaul, Yu.D. Tretyakov, J. Mater. Chem. 6 (1996) 627. [14] G. Wahl, in: G. Elbing, G. Reiners, W. Riedel (Eds.), Jahrbuch Oberfla¨chentechnik, vol. 49, Metall-Verlag, Berlin, 1993, p. 253. [15] D.T. Chin, C.H. Tsang, J. Electrochem. Soc. 125 (1978) 1461. [16] R.C. Reid, J.M. Prausnitz, The Properties of Gases and Liquids, McGraw-Hill, New York, 1977. [17] L’Air Liquide, Encyclopedie des Gaz, Elsevier, Paris, 1976. [18] C.G. Macdonald, J.S. Shannon, Aust. J. Chem. 19 (1966) 1545. [19] A.S. Alikhanyan, I.P. Malkerova, Ya.Kh. Grinberg, V.B. Lazarev, V.A. Bogdanov, Proc. Acad. Sci. USSR. Phys. Chem. Sect. 292 (1987) 43. [20] N.B. Morozova, I.K. Igumenov, V.N. Mit’kin, K.V. Kradenov, O.G. Potapova, V.B. Lazarev, Ya.Kh. Grinberg, Russ. J. Inorg. Chem. 34 (1989) 672. [21] A.E. Turgambaeva, A.F. Bykov, I.K. Igumenov, Electrochem. Soc. Proc. 97 – 25 (1997) 139. [22] G. Wahl, in: M.L. Hitchman, K.F. Jensen (Eds.), Chemical Vapor Deposition, Academic Press, San Diego, CA, USA, 1993, p. 591.
S.V. Samoilenkov et al. / Surface & Coatings Technology 192 (2005) 117–123 [23] G. Wahl, Ch. Metz, S. Samoilenkov, J. Phys., IV France 11-Pr8 (2001) 835. [24] U. Schulz, K. Fritscher, C. Leyens, M. Peters, W.A. Kaysser, JOM 49 (1997) (Available at: http://www.tms.org/pubs/journals/JOM/ jom.html). [25] M. Pulver, W. Nemetz, G. Wahl, Surf. Coat. Technol. 125 (2000) 400. [26] I.E. Graboy, A.R. Kaul, N.V. Markov, V.V. Maleev, I.E. Korsakov, O.Yu. Gorbenko, A.A. Molodyk, Electrochem. Soc. Proc. 97 – 25 (1997) 990. [27] A.R. Kaul, A.A. Molodyk, S.V. Samoylenkov, N.V. Markov, O.Yu.
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Gorbenko, I.E. Graboy, I.E. Korsakov, G. Wahl, O. Stadel, in: G. van Tendeloo, S.N. Putilin, E.V. Antipov (Eds.), NATO Science Series: 3. High Technology, Kluwer Academic Publishing, Dordrecht, 1999, p. 271. [28] R.A. Miller, J.L. Smialek, R.G. Garlik, in: A.H. Heuer, L.W. Hobbs (Eds.), Advances in Ceramics, vol. 3, American Ceramic Society, Columbus, OH, 1981, p. 241. [29] H.G. Scott, J. Mater. Sci. 10 (1975) 1527. [30] G. Garcia, A. Figueras, R.I. Merino, V.M. Orera, J. Llibre, Thin Solid Films 370 (2000) 173.
MOCVD of thick YSZ coatings using acetylacetonates S.V. Samoilenkov *, M.A. Stefan, G. Wahl Institut fu¨r Oberfla¨chentechnik und plasmatechnische Werkstoffentwicklung, TU Braunschweig, Bienroder Weg 53, 38108, Braunschweig, Germany Received 21 February 2004; accepted in revised form 10 March 2004 Available online 12 May 2004
Abstract Thick (up to 20 Am) coatings of yttria-stabilized zirconia with columnar structure have been grown by thermal MOCVD at 630 – 820 jC using acetylacetonates of Zr and Y as precursors. The samples have been characterized by Scanning Electron Microscopy (SEM), Wave Dispersion X-ray Analysis (WDX) and X-ray Diffraction (XRD) to find out the interplay of deposition conditions with coating microstructure, yttria content and phase composition. In particular, it has been found that homogenous nucleation reactions in gas phase at high temperature lead to the decrease of total growth rate and enrichment of resulting coating with yttrium. It has also been found that heterovalent substitution in ZrO2 leads to remarkable increase of a column width and a coating integrity. A simple model for the quantitative description of the process is presented. D 2004 Elsevier B.V. All rights reserved. Keywords: Zirconia; YSZ; Chemical vapor deposition
1. Introduction Zirconia together with its solid solutions is one of the most studied and technically used ceramic materials today [1,2]. This is due to its low cost, high thermal, mechanical and chemical stability, and high oxygen ion conductivity at elevated temperatures. Yttria-stabilized zirconia [ZrO2(Y2O3) = YSZ] is a material of choice for thermal barrier coatings (TBCs) used for heat protection of metal parts in gas turbines and engines [3,4]. TBC use permits to increase the turbine/engine efficiency or its lifetime. Another technologically important application of thick YSZ layers is an electrolyte layer in solid oxide fuel cells (SOFC) [5,6]. Microstructure, thickness, phase and chemical composition of the coating are of a prime importance for applications. In the case of TBCs, the porous layer with columnar microstructure consisting of tetragonal YSZ is obligatory for the best performance [3], while gas tight layers of cubic YSZ are used in SOFCs [6]. Therefore, it is important to know the interplay of deposition parameters, coatings microstructure and composition to produce the material with desirable properties. * Corresponding author. Tel.: +49-531-391-9425; fax: +49-531-3919400. E-mail address: [email protected] (S.V. Samoilenkov). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.019
Chemical Vapor Deposition (CVD) is used in technology for a wide scope of purposes. Most common precursors for the deposition of ZrO2 are zirconium halogenides, alkoholates and h-diketonates. The use of ZrCl4 for the growth of thick YSZ TBCs has been recently demonstrated by Chevillard and Drawin [7], Preauchat et al. [8] and Preauchat and Drawin [9]. Although zirconium chloride is volatile and cheap, its conversion to ZrO2 during CVD leads to formation of hazardous Cl2 and HCl. Highly volatile zirconium alkoholates are very unstable in air and this makes them difficult to handle. Additionally, alkoholates are not suited for hightemperature deposition processes due to their low thermal stability [10]. There exist a number of volatile zirconium hdiketonates. The most widely used h-diketonates are chelates with pentadione-2,4 (acetylacetone, Hacac) and 2,2,6,6-tetramethylheptandione-3,5 (Htmhd). These compounds are stable in air, possess high enough vapor pressure and are commercially available. Zr(acac)4 is thermally less stable than Zr(tmhd)4, but its low price ( c 150 EURO/kg, the price of Zr(tmhd)4 is by a factor of c 10 higher) makes acetylacetonate very attractive for CVD of thick layers. Yttrium acetylacetonate, Y(acac)3, was reported to decompose partially during sublimation, but still providing high vapor pressure [11,12]. In this study, we have attempted to elucidate some features of MOCVD of YSZ using acetylacetonates as
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volatile precursors. The interplay of growth conditions, microstructure, phase composition and film thickness is reported here.
mmol cm2 h1 units. Taking into account the density of YSZ, qYSZ c 6.0 g/cm3 and the molar mass MYSZ ¼ y MYO1:5 þ ð1 yÞ MZrO2
2. Experimental The deposition experiments have been performed in flash evaporation single source MOCVD setup (Fig. 1). The distance between the gas inlet and the substate was H = 15 cm, the inlet tube diameter was d = 2 cm and the diameter of the substrate holder was ds = 4 cm. The powder mixture of water-free Zr(acac)4 (Fluka Chemie GmbH, Germany) and Y(acac)3 (ABCR GmbH, Germany) was placed in a vibration feeding mechanism described elsewhere [13]. The coating composition was controlled by molar fraction of zirconium and yttrium precursors, nZr and nY, in the initial powder mixture. The evaporator and transport lines were heated up to 250 jC. The substrate holder was heated inductively by RF coil to reach the substrate temperature in the range of 630– 820 jC. Other deposition conditions were as follows: total precursor flow iprec = 5.27 mmol/h, argon flow iargon = 0.413 mol/h (154 sccm), oxygen flow ioxygen = 0.164 mol/h (61 sccm), total pressure ptot = 500 Pa, deposition time t = 1 h. The precursor molar fraction in the gas phase was xprec = xZr + xY = 0.009. The molar fractions in the gas are correlated with the molar fractions in the powder (n. . .) as follows: xZr = nZrxprec and xY = nYxprec. The deposition was carried out on 0.5-mm-thick plates of polycrystalline Al2O3 with a surface area of approx. 2 cm2. All samples were annealed after deposition for 10 min in air at 900 jC in order to burn out carbon-containing rests. The mass deposition rate of YSZ has been determined by weighing the substrate before and after the deposition. The Mettler AT261 balances with an accuracy of F 0.02 mg were used. Typical mass change produced by the deposition was in the range of 6 –16 mg. Because the coatings might have variable porosity, the growth rate was determined in
Fig. 1. The scheme of the experimental MOCVD setup used.
where M.. is the molar mass and y is the molar fraction of YO1.5 in YSZ, the dense layer with a thickness of sde = 1 Am corresponds roughly to 0.6 mg of the coating mass per cm2. The samples have been characterized by Scanning Electron Microscopy (SEM, CamScan 4M electron microscope), Wave Dispersion X-ray Analysis (WDX, Microspec WDX3PC analysing system) and X-ray Diffraction (XRD, Siemens D5000 diffractometer, CrKa radiation). The coating porosity e ¼ 1 SSde has been determined from the coating thickness s observed by SEM and the thickness sde calculated from the mass change. The average YSZ column width has been determined for each sample from four representative SEM cross-section images. The values of density for ZrO2 and YSZ were taken from XRD data using JCPDS data base on unit cell volume of the corresponding oxides.
3. Results and discussion 3.1. Deposition rate vs. growth temperature and yttria content Depending on conditions, there are several decomposition routes of metal acetylacetonates [18 – 21]. The decomposition process of Zr(acac)4 in inert gas starts already at 200 jC [20] and Zr(acac)4 is not the single volatile species responsible for the ZrO2 formation in MOCVD. These considerations should be even more important for less stable Y(acac)3 precursor. However, taking all precursor destruction paths into account is a difficult task. On the other hand, the residence time of precursor portion in evaporator and transport lines in the flash evaporation scheme used is rather short (being of about 2 s in our system) minimizing thus the influence of aforementioned decomposition processes. In the following, we propose a simplified description of the CVD process, which neglects the decomposition of precursor molecules during evaporation and transport to the reactor. Although such assumption is simplified, the model provides a good agreement with the experimental results obtained. The dependence of the molar deposition rate of ZrO2 ( jZr) on growth temperature is presented in Fig. 2 in an Arrhenius plot. On the right side the transfer coefficient aZr ¼ xjZrZr , where xZr is the molar fraction of Zr(acac)4 in the gas, is plotted. The curve follows well-known dependency for thermally activated CVD processes at high temperatures [14]. At 630– 720 jC, the growth rate is almost temperature-independent. This corresponds to the growth limited by mass transport (or so-called diffusioncontrolled regime). The deposition rate in the mass trans-
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max = 0.06 mmol cm2 h1 for those found experimentally: jZr jZr xZr = 0.009 (Fig. 2), aZr ¼ xZr ¼ 6:6 mmol cm2 h1. Above 700 jC, the deposition rate decreases with the temperature increase. This behaviour can be explained by side reactions, which might take place in proximity of the heated sample (drbd) and lead to formation of ZrO2 powder, which is not integrated in the film:
ZrðacacÞ4 þ O2 ! ZrO2 ðpowderÞ þ by-products ðCO2 ; CO; H2 O; etcÞ: This process competes with the film growth reaction. Both reactions might be the first-order ones with reaction constants kp = kpoexp( Ep/RT) for the powder formation and kd = kdoexp( Ed/RT) for the deposition; Ep and Ed are the activation energies.The molecular balance at the deposition surface is given by: Fig. 2. Deposition rate of ZrO2 coatings vs. reciprocal deposition temperature.
port controlled regime for the stagnation flow reactor is given by [15]: j ¼ aZr ðxZr xoZr Þ
ð1Þ
o is the precursor concentration near the surface. where xZr The maximum molar deposition rate for the stagnation flow reactor jmax can be approximately calculated using following relation (it is assumed that the precursor is o completely consumed at the substrate surface, i.e., xZr =0 near the surface):
jmax Zr ¼ aZr xZr
ð2Þ
with the transfer coefficient aZr ¼ 0:3
ffiffiffiffiffiffiffiffiffi DZr ctot pffiffiffiffiffiffi p Re 3 ScZr ds
ð3Þ
where ctot is the total molar concentration of precursor in the gas. The transfer coefficient aZr is generated by the diffusion of the molecules through the concentration boundary layer with the thickness d above the substrate surface. The diffusion coefficient DZr = 60 cm2 s1 was determined according to the methods given in Ref. [16]. Reynolds and Schmidt numbers were defined as Re = wdm1, ScZr = mDZr1, where the gas velocity w = 7.8 m s1. The kinematic viscosity m was calculated from dynamic viscosity g = 53 APa s (value for argon [17]) by the relation m = gq1, where q = 2.74 g m3 is the gas density. All these values were calculated at ptot = 500 Pa and 1000 K, giving as a result Re = 8 103 and Sc = 3.2 103. Then, according to Eqs. (2) max and (3), aZr = 14.4 mmol cm2 h1 and jZr = 0.13 mmol 2 1 cm h for xZr = 0.009. These values are quite close to
aZr ðxZr xoZr Þ ¼ ðkp ctot dr þ kd ctot ÞxoZr :
ð4Þ
o The concentration xZr near the surface is not equal to zero as in Eq. (2), but:
xoZr ¼
aZr xZr : aZr þ kp ctot dr þ kd ctot
ð5Þ
The molar deposition rate is then: jZr ¼
kd ctot amax xZr : amax þ kp ctot dr þ kd ctot
ð6Þ
At low temperatures, the condition (kpctotdr + kdctot)b amax is fulfilled [22], the deposition is controlled by surface reactions and is proportional to kd. The deposition rate decreases with the temperature decrease. This case has been described for the YSZ deposition in Ref. [23]. Exponential decrease of the deposition rate at high temperature (Fig. 2) can be explained by Eq. (6) provided kpctotdrH(amax + kdctot). Then, the deposition rate is: jZr ¼
ðEd Ep Þ kd amax xZr ~exp RT k p dr
ð7Þ
Fig. 2 verifies the exponential behaviour with (EdEp) < 0. At mean deposition temperature kdctotH(amax + kpctotdr) and the deposition rate is controlled by diffusion in the gas phase and Eq. (2) can be used. This corresponds to a horizontal part of the curve in Fig. 2. The YSZ deposition rate decreases linearly with growing Y(acac)3 content nY in the precursor mixture as it is demonstrated in Fig. 3 for the deposition temperature of 720 jC. At nY c 3 mol%, the deposition rate is by c 10% lower than that of ZrO2. In addition, the yttria mole fraction yY in the coating was determined. The partial deposition rates jZr and jY and the transfer coefficients aZr = jZr/xZr
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Fig. 4. YO1.5 content in YSZ coatings deposited at 720 jC vs. composition of initial precursor mixture. Dashed line represents the case of congruent deposition.
Fig. 5 shows the temperature dependence of the YSZ deposition rate and the variation of the yttria molar fraction in the coating for nY = 0.067. From these measurements the transfer coefficients has been calculated and plotted in Fig. 6. The deposition rate decreases exponen-
Fig. 3. (a) Deposition rate of YSZ vs. yttrium acetylacetonate molar fraction in initial precursor mixture. (b) Transfer coefficients for ZrO2 and YO1.5 vs. nY at constant deposition temperature of 720 jC.
and aY = jY/xY have been calculated from the total deposition rate, mole fraction yY and the gas mole fractions. The transfer coefficients determined are independent of nY as shown in Fig. 3b for the deposition temperature of 720 jC. The result of these differences in the transfer coefficients is that the deposition rate jYSZ decreases linearly with nY according to the relation: jYSZ ¼ aZr xprec ðaZr aY Þxprec nY as it is shown in Fig. 3a. The molar fraction of yttria in the coating increases according to the relation: aY aZr nY yY ¼ aY n 1 aZra Y Zr or, for small nY: aY yY ¼ n : aZr Y This relation is verified by Fig. 4.
Fig. 5. (a) Deposition rate and (b) YO1.5 content in YSZ coatings vs. reciprocal deposition temperature. The films were grown from precursor mixture with nY = 0.067.
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Fig. 6. Transfer coefficients for zirconia and yttria vs. deposition temperature at constant nY = 0.067.
tially with the temperature increase, similar to the behaviour found for ZrO2 growth. The transfer coefficients aZr and aY converge to almost the same value at high deposition temperature. The reason for the difference between aZr and aY could be the various diffusion coefficients, kinetic effects or incomplete evaporation of Y(acac)3. Indeed, Y(acac)3 was shown to decompose partially during evaporation [11,12]. 3.2. Coatings microstructure vs. growth temperature and yttria content All the coatings obtained in this study had a columnar structure. A typical SEM image of the sample cross-section is given in Fig. 7. It was found that the column width and uniformity over the thickness increase remarkably with only a slight doping of ZrO2 with yttria. The dependence of column width on yttria content is shown in Fig. 8a. It
Fig. 7. SEM image of cross-section of ZrO2 coating deposited at 720 jC.
Fig. 8. (a) average column width and (b) coatings porosity e vs. yttria content in the coating.
has been also observed that the coating porosity decreases from 46% to 35% as yttria content increases from 0 to 15 mol% (Fig. 8b). Let us briefly discuss this phenomenon. Column width and coating integrity ( = 1e) should increase with the increase of diffusion mobility and/or characteristic residence time of adatoms on the surface (caused, e.g., by reduced deposition rate). The increase of YSZ column width and uniformity with the increase of deposition temperature and/or decrease of the deposition rate has been previously reported for EBPVD YSZ thermal barrier coatings [24]. It has been also observed that the increase of the deposition temperature of CVD-derived YSZ layers led
Fig. 9. h – 2h XRD scans of YSZ coatings with various yttria content.
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Fig. 10. Lattice parameters of tetragonal and cubic YSZ in coatings. Dashed line represents the data for powder after Scott [29].
studied in temperature range of 630– 820 jC. It has been found that the films deposited in diffusion controlled regime (below 720 jC) were enriched with zirconia as compared to initial precursor mixture. The appearance of homogeneous nucleation reactions led to enrichment of the layers deposited above 720 jC with yttria. The strong effect of yttria concentration of the coatings microstructure (column uniformity and width, porosity) has been observed. The increase of the coating integrity and uniformity with the increase of yttria concentration was supposed to be due to diffusion enhancement occurring in YSZ due to heterovalent substitution. According to XRD studies, the monocline modification is present in YSZ coatings up to 6.4 mol% of YO1.5 content. YSZ phase in the films with higher yttria concentration was found to be tetragonal (up to 6.4 –12 mol% of YO1.5) or cubic (for higher concentration).
References to dramatic decrease of their porosity [7] and increase of column width [25]. We suppose that in present case the effect is caused by the increase of diffusion mobility due to heterovalent substitution in ZrO2. The similar behaviour has been recently demonstrated for epitaxial films of CeO2(R2O3) on sapphire substrates (R—rare earth element) [26,27]. 3.3. XRD analysis Zirconia and YSZ can exist in three crystallographic forms: monoclinic, tetragonal and cubic. The increase of temperature and yttria content leads to the increase of the cell symmetry [28]. ZrO2 coatings deposited in temperature interval 630 – 775 jC consisted of mixture of monocline and tetragonal phases. Calculated lattice parameters of the monocline ZrO2 coin˚, cided very well with those for bulk material: a = 5.31 A ˚ , c = 5.15 A ˚ , b = 99.2j. b = 5.21 A XRD scans of YSZ coatings deposited at 720 jC are presented in Fig. 9. ZrO2 coating consists of the mixture of tetragonal and monocline phases. Monocline modification is present in YSZ coatings up to 6.4 mol% of YO1.5, while tetragonal form becomes predominant as yttria content increases. The tetragonal distortion gradually decreases; YSZ in coatings with more than 12 mol% of YO1.5 is cubic (Fig. 10). This is consistent with observations of crystal structure of MOCVD-derived YSZ films deposited at 760 jC [30]. As well, the lattice parameters found correspond well to those known for bulk material.
4. Conclusions Peculiarities of MOCVD of polycrystalline ZrO2 and YSZ thick coatings on alumina substrates have been
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