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Oxide films by combustion pyrolysis of solution precursors
Materials Science and Engineering A359 (2003) 18 /23 www.elsevier.com/locate/msea
Oxide films by combustion pyrolysis of solution precursors R. Kavitha *, S.R. Hegde, Vikram Jayaram Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Received 6 September 2002; received in revised form 16 January 2003
Abstract A combustion pyrolysis technique has been developed to deposit oxide films onto metal and ceramic substrates. A precursor solution is atomised and injected into an oxy-acetylene flame where thermal decomposition takes place to directly yield a film on a substrate. This technique is used to deposit different kinds of films including Al2O3, ZnO, metastable solid solutions of ZnO /MgO and ZrO2 /Y2O3. The conditions that were necessary to obtain uniform and adherent films were established. Films were characterised by X-ray diffractometry, scanning electron microscopy and micro-hardness. # 2003 Elsevier B.V. All rights reserved. Keywords: Combustion pyrolysis; Al2O3; ZnO; Metastable solid solutions; ZnO /MgO; ZrO2 /Y2O3
1. Introduction Over the years numerous techniques of thin film deposition have been developed that are capable of producing oxide films of thicknesses in the order of microns (1 /100). These methods include plasma/melt spraying, spray pyrolysis [1], chemical vapour deposition (CVD) and its derivatives [2 /4], evaporation and sputtering [5]. Recent developments include techniques that combine solution precursors with thermal sources such as combustion flames [6 /9] or plasmas [10]. In this paper, we report the development of a variant of the flame-assisted technique, combustion pyrolysis, for the deposition of thin films. The coatings were prepared by thermally decomposing an aqueous solution of metal salts, such as acetates or nitrates. This precursor solution route offers the convenience of easily available and inexpensive compounds, as opposed to the more complex ones that are generally used for techniques such as MOCVD [2]. There is also no need for fine, free flowing powders that are required as feedstock in thermal or plasma spraying [11]. Other advantages include the ease with which multi-component oxides may be tailored by simply changing the cation concentration in the solution, the ability to deposit thin films in * Corresponding author. E-mail address: [email protected] (R. Kavitha). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00213-2
open atmosphere without the need for any furnace or reaction/vacuum chamber and, finally, the ability to deposit non-equilibrium phases owing to the relatively low temperatures and rapid decomposition involved. As a result the operating and capital costs are reduced compared with many of the other techniques that are presently used. The use of combustion flames to decompose metal salts is not new. Nielsen et al. [12] described the use of flames, using natural gas and air, for the production of metal oxide powders as far back as 1960. Metal salts, dissolved either in water or in organic solvents have been used for the production of single oxide or mixed oxide powders, generally of spherical shape with diameters ranging from 0.01 to 5 mm. Recently, several variants of this method have emerged for the production of both films as well as powders. Hunt and coworkers carried out combustion chemical vapour deposition (CCVD) [13] on a number of different and complex oxide systems. They utilised extremely fine glass capillaries for atomisation with the result that feed rates and deposition rates were somewhat low (microns per hour) [6 /9]. Ichinose et al. deposited perovskite thin films [14,15] by spraying ultrasonically atomised solutions into a combustion flame. Tikkanen et al. describe the production of nanoparticles by a liquid flame spray process [16] wherein precursor dissolved in a solvent is injected and atomised in an oxygen /hydrogen flame to
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yield fine particles. A high-velocity oxy fuel (HVOF) spray torch has been used to melt particles and accelerate the molten droplets to form a thin splat on a substrate [11]. Low-pressure flame deposition, reported by Glumac involves pyrolysis of precursors in a flat, low pressure flame that yields clusters/nanoparticles in the gas phase that sinter to form a coating on the substrate [17]. Recently, Choy and Vyas [18] successfully deposited ZrO2 /Y2O3 thermal barrier coating on a Nibased superalloy by flame-assisted vapour deposition technique. The technique that we have developed and which is the subject of this paper seeks to address some of the drawbacks inherent in the other methods reported above, notably in providing a simple, open air system with reasonably high deposition rates with the capability to generate coatings of both equilibrium phases as well as those of metastable solid solutions.
2. Experimental procedure The different types of experimental arrangements that were attempted for deposition are described, followed by the details of the systems that were produced in an optimised set-up. 2.1. Deposition arrangement Two types of experimental arrangement are described. In both types atomisation of the precursor solution is air-assisted. In the first arrangement (Fig. 1) a peristaltic pump is used to feed the solution at a very low flow rate (3 /5 ml min 1) into a conventional welding torch fitted with a nozzle of 1 mm diameter. An oxy-acetylene gas mixture atomises solution that flows into the nozzle. This type of atomiser belongs to the internal mixing configuration in which gas and precursor solution mix within the nozzle before discharging through the outlet orifice. The discharged droplets are ignited with an acetylene pilot flame, which is kept in place during combustion to prevent flame blow off. The substrate is
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placed on a copper substrate holder held at a suitable distance from the nozzle. The primary drawback of the arrangement is that at the low rates of throughput necessary the flow of the precursor solution into the welding torch nozzle is discontinuous. Furthermore, because of the internal mixing of the precursor solution and the oxy-acetylene gas within the nozzle this type of atomiser suffers from the problem of a back-pressure that prevents free flow of the solution [19]. In the present experimental arrangement (Fig. 2), the peristaltic pump is replaced by an atomiser and separation chamber. Atomisation of the precursor solution is performed by an external-mixing air assisted two-fluid atomiser as shown in the inset in Fig. 2. Precursor solution flows through the centre pipe and compressed air through the outer pipe. There is no mixing of the precursor solution and atomising gas inside the nozzle. Compressed air flowing at a rate of /17 l min 1 impinges and exerts a drag force on the precursor solution outside the liquid discharge orifice, thereby atomising the precursor solution into fine droplets. In the separation chamber, which is a cylindrical vessel of 15 cm diameter and 30 cm height, atomised droplets delivered into the chamber collide against each other and also on the walls of the container. During this process the droplet size is further reduced and the atomising gas carries only the finer droplets to the welding torch nozzle. Heavier droplets that settle at the bottom are collected and reused. This set-up eliminated the problems faced previously. Flame ignition remains the same as described in set up 1. Input flow rates of fuel, oxygen and compressed air are monitored through gas flow meters. The flame temperature and structure such as turbulence and flame length determines the extent of reaction of the atomised droplet in the flame, while the characteristics of the flame are decided by the oxygen to fuel gas ratio. The surface temperature of the substrate was monitored by a K-type thermocouple inserted through a hole drilled at the centre of the substrate. Temperature control during deposition was maintained to within 20 8C. The deposition process was controlled by managing several variables, including substrate temperature, precursor concentration and deposition time. 2.2. Materials and characterisation techniques
Fig. 1. Schematic diagram of the flame pyrolysis deposition process (Set-up 1); Internal mixing configuration.
Amorphous silica and Ni-based superalloy (Nimonic90) were used as substrates. A variety of oxide films were deposited, of which we confine our report here to Al2O3, ZnO, ZnO /20 mol% MgO and ZrO2 /Y2O3. The surface roughness of the superalloy substrate was maintained at 600-grit finish. The substrates were cleaned with alcohol and wiped clean before deposition. Aqueous solutions of metal acetates were used for ZnO and ZnO /20 mol% MgO films and the concentration of the
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Fig. 2. Schematic diagram of the flame pyrolyisis deposition process (set-up 2); inset picture shows an external mixing atomiser.
solutions were 100 and 50 g l 1, respectively. Solutions of nitrates of aluminium (50 g l 1) and zirconium/ yttrium (35 g l 1) in deionised water were used as the precursor for Al2O3 and ZrO2 /Y2O3 coatings. The ratio of zirconium nitrate and yttrium nitrate was adjusted to obtain ZrO2 /8 wt.% Y2O3. The films were deposited for 30 min. Crystallinity and phases in the deposited films were established by X-ray diffraction (JEOL 8030, Cu / Ka). The surface morphology and cross-sectional view of the deposited films were observed using scanning electron microscopy (JEOL JSM 840A). Cross-sections were prepared by sandwiching the film to a blank substrate, followed by slicing and polishing. Vickers indentations (HMV-2000 Shimadzu) were used to measure the film hardness at various loads.
3. Results and discussion 3.1. Alumina films The as-deposited alumina film at 650 8C was found to be amorphous. Post deposition heat treatment of the film at 1200 8C for 5 h resulted in transformation of the amorphous film to crystalline alpha alumina while the amorphous silica substrate had also crystallised to tridymite as shown by the XRD patterns in Fig. 3. This result is consistent with that seen for alumina films produced by various other techniques such as sputtering [20], plasma enhanced CVD [2], ion-beam induced CVD [3] and metal /organic CVD [4], in which direct deposition of alpha alumina is rarely observed and post deposition annealing is generally needed to arrive at this structure. The average hardness of the amorphous alumina films is /4 GPa. The hardness of alpha alumina was found to be as high as 10 GPa in some regions, while an average value of 8.5 GPa was obtained
from 15 observations made on different regions of the film. Indentation imprints on amorphous alumina film produced by Vickers test is shown in Fig. 4, it is seen that the film remains intact and that the effect of indentation was mainly in ‘pushing’ the film down. The results indicate that alpha alumina is approximately twice as hard as the amorphous alumina film. Chou [20] had observed similar behaviour on alumina films deposited by r.f. Magnetron sputtering. For comparison, the hardness of fully dense polycrystalline alumina ranges from 18 to 20 GPa indicating that the films in the present case are porous. 3.2. ZnO and ZnO /20 mol%MgO films ZnO films were deposited at temperatures of 150, 350 and 650 8C. The strong influence of substrate temperature (Ts) on structure is depicted in Fig. 5. The film deposited at 150 8C did not exhibit any peaks of the wurtzite phase, while those deposited at 350 8C reveal single phase, polycrystalline wurtzite. Films deposited at 650 8C exhibit enhanced intensity of the (0002) peak indicating that the film prefers to grow with the c-axis normal to the substrate. In an analysis of the thermal decomposition of zinc acetate [21] it has been reported that the decomposition of zinc acetate is complete only at 300 8C. Others [1,22] observe similar behaviour on ZnO films deposited at different temperatures by spray pyrolysis. With increase in the deposition temperature the grain size increases [23], resulting in sharp diffraction peaks. From the above observations it is clear that Ts plays an important role in determining the structure of ZnO films. ZnO /20 mol% MgO films were deposited at 500 and 600 8C. At the lower temperature, only single phase wurtzite is present, as seen by the XRD pattern in Fig. 6a. At 600 8C one can observe phase separation with the appearance of rocksalt (Fig. 6b). Although ZnO and
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Fig. 3. XRD patterns of as-deposited and heat treated alumina films.
MgO display different crystal structures, wurtzite and rocksalt, respectively, the similarity in Pauling’s ionic ˚ ) and Mg2 (0.72 A ˚ ) allows radii between Zn2 (0.75 A some solubility. It is seen from the phase diagram [24] that the equilibrium solid solubility of MgO in wurtzite is limited to 5% at 900 8C. However, it has been shown that solubility can be metastably extended to as much as 30 /35% by spray pyrolysis [25] or by laser ablation [26]. Hence it may be inferred that the rapid process of evaporation and decomposition of the fine droplets, together with the low deposition temperature can lead to the formation of films of extended solubility, as earlier demonstrated for powders [25]. The surface morphology of the single phase wurtzite film in Fig. 7 shows small crystallites spread uniformly all over the substrate and this morphology is similar to the surface morphology exhibited by thin films deposited by spray pyrolysis [23].
From the cross-sectional view of the film (Fig. 8) the thickness was measured to be 17 mm. The porosity that is observed near the interface with the substrate might be due to the low substrate temperature at the beginning of deposition.
Fig. 4. Indentation imprint on as-deposited alumina (amorphous) film produced by Vickers test under 1000 g load for 10 s.
Fig. 5. X-ray diffractograms of ZnO films deposited at three different temperatures.
3.3. ZrO2 /Y2O3 films The surface morphology and cross section of the coating deposited for 30 min at a substrate temperature of 550 8C are shown in Figs. 9 and 10. The cross
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Fig. 6. X-ray diffraction patterns of single phase (wurtzite) and phase segregated ZnO /20 mol% MgO films.
Fig. 7. SEM micrograph of ZnO /20 mol% MgO (wurtzite) film deposited at 500 8C.
Fig. 8. Cross-sectional SEM image of ZnO /20 mol% MgO film (wurtzite) film of 17 mm thickness.
sectional microstructure is entirely different from that of electron-beam PVD (EBPVD) coatings and air plasma spray (APS) coatings. It is well known that plasma spraying produces laminar coatings while EBPVD coatings are columnar [27]. The film produced by the present method is granular with porosity and microcracks. From the XRD pattern, Fig. 11, it is clear that the film contains a negligible amount of monoclinic phase. It is difficult to distinguish the cubic and tetragonal phases since the strong peaks overlap significantly and also because of the change in the lattice parameter of zirconia with addition of yttria [28]. It may be concluded that the predominant phase is cubic or tetragonal or a mixture of both the phases and that, the monoclinic phase, if present should be a minor constituent. The average hardness of the coating in cross section is 130 Hv. This hardness value is several times lower than the hardness (1160 KHN) of bulk zirconia [29]. Also,
Fig. 9. Surface morphology of ZrO2 /Y2O3 film.
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Acknowledgements Financial support for this work was provided by a grant from the Aeronautical Research and Development Board, Government of India.
References
Fig. 10. Cross-sectional view of ZrO2 /Y2O3 film.
Fig. 11. XRD patterns of blank substrate and ZrO2 /Y2O3 film with substrate.
hardness of plasma spray deposited 3 mol% YSZ was found to be 550 /700 Hv [30]. The lower hardness values of the coatings are attributed to the presence of porosity and microcracks, which may be seen in the cross sectional micrograph.
4. Conclusions A new versatile and inexpensive technique has been developed for the deposition of oxide films by combustion of aqueous precursors. The optimised configuration of deposition incorporates the injection of atomised solutions, via a separation chamber, into an oxyacetylene mixture prior to combustion. Metastable extension of the wurtzite solid solution by up to 20 mol% MgO has been demonstrated in ZnO /MgO films by suitable control of deposition temperature. Coatings of ZrO2 /Y2O3 of an average thickness of about 60 mm have been fabricated at a low substrate temperature of / 550 8C.
[1] J.L. Van Heerden, R. Swanepoel, Thin Solid Films 299 (1997) 72. [2] H.L. Wang, C.H. Lin, M.H. Hon, Thin Solid Films 310 (1997) 260. [3] A. Caballero, D. Leinen, A. Fernandez, A.R. Gonzalez-Elipe, Surf. Coat. Technol. 80 (1996) 23. [4] E. Ciliberto, I. Fragala, R. Rizza, G. Spoto, Appl. Phys. Lett. 67 (1995). [5] K.B. Sundaram, A. Khan, Thin Solid Films 295 (1997) 87. [6] A.T. Hunt, Appl. Phys. Lett. 63 (1993) 266. [7] J.M. Hampikian, W.B. Carter, Mater. Sci. Eng. A 267 (1999) 7. [8] D.W. Stollberg, W.B. Carter, J.M. Hampikian, Surf. Coat. Technol. 94 /95 (1997) 137. [9] T.A. Polley, W.B. Carter, D.B. Poker, Thin Solid Films 357 (1999) 132. [10] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [11] S. Sampath, MRS Bull. 7 (2000) 17. [12] M.L. Nielsen, W.E. Kuhn (Ed.), Ultra Fine Particles, Wiley, New York, 1963, p. 181. [13] S. Shanmugam, A. Hunt, D. Motley, Ceram. Bull. 85 (2002) 36. [14] H. Ichinose, Y. Shiwa, M. Nagano, Jpn. J. Appl. Phys. 33 (1994) 5903. [15] H. Ichinose, Y. Shiwa, M. Nagano, Jpn. J. Appl. Phys. 33 (1994) 5907. [16] J. Tikkanen, K.A. Gross, C.C. Berndt, V. Pitakanen, J. Keskinen, S. Raghu, M. Rahala, J. Karthikeyan, Surf. Coat. Technol. 90 (1997) 210. [17] G. Skandan, N. Glumac, Y.-J. Chen, Fredric, Cosandey, E. Heims, B.H. Kear, J. Am. Ceram. Soc.8 (1998) 2753. [18] K.L. Choy, Key Eng. Mater. 161 /163 (1999) 653. [19] A.H. Leferbve, Atomization and Sprays, Hemisphere Publishing Company, USA, 1989, p. 7. [20] T.C. Chou, T.G. Nieh, S.D. McAdams, G.M. Pharr, Scr. Metal. 25 (1991) 2203. [21] A. Paul, H.N. Acharya, J. Mater. Sci. 27 (1992) 1716. [22] S.A. Studeniken, N. Golego, M. Cocivera, J. Appl. Phys. 83 (1998) 2104. [23] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [24] S. Raghavan, J.P. Hajra, G.N.K. Iyengar, K.P. Abraham, Thermochim. Acta 89 (1991) 151. [25] V. Jayaram, B. Sirisha Rani, Mater. Sci. Eng. A 304 /306 (2001) 800. [26] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [27] J.R. Nicholls, M.J. Deakin, D.S. Rickerby, Wear 233 /235 (1999) 352. [28] R.P. Ingel, D. Lewis, III, J. Am. Ceram. Soc. 69 (1986) 325. [29] R.C. Weast (Ed.), Hand Book of Chemistry and Physics, 55th ed., CRC Press, USA, 1974 /1975, F-22. [30] J.-H. Shin, D.-S. Lim, H.-S. Ahn, Surf. Coat. Technol. 133 /134 (2000) 403.
Oxide films by combustion pyrolysis of solution precursors R. Kavitha *, S.R. Hegde, Vikram Jayaram Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Received 6 September 2002; received in revised form 16 January 2003
Abstract A combustion pyrolysis technique has been developed to deposit oxide films onto metal and ceramic substrates. A precursor solution is atomised and injected into an oxy-acetylene flame where thermal decomposition takes place to directly yield a film on a substrate. This technique is used to deposit different kinds of films including Al2O3, ZnO, metastable solid solutions of ZnO /MgO and ZrO2 /Y2O3. The conditions that were necessary to obtain uniform and adherent films were established. Films were characterised by X-ray diffractometry, scanning electron microscopy and micro-hardness. # 2003 Elsevier B.V. All rights reserved. Keywords: Combustion pyrolysis; Al2O3; ZnO; Metastable solid solutions; ZnO /MgO; ZrO2 /Y2O3
1. Introduction Over the years numerous techniques of thin film deposition have been developed that are capable of producing oxide films of thicknesses in the order of microns (1 /100). These methods include plasma/melt spraying, spray pyrolysis [1], chemical vapour deposition (CVD) and its derivatives [2 /4], evaporation and sputtering [5]. Recent developments include techniques that combine solution precursors with thermal sources such as combustion flames [6 /9] or plasmas [10]. In this paper, we report the development of a variant of the flame-assisted technique, combustion pyrolysis, for the deposition of thin films. The coatings were prepared by thermally decomposing an aqueous solution of metal salts, such as acetates or nitrates. This precursor solution route offers the convenience of easily available and inexpensive compounds, as opposed to the more complex ones that are generally used for techniques such as MOCVD [2]. There is also no need for fine, free flowing powders that are required as feedstock in thermal or plasma spraying [11]. Other advantages include the ease with which multi-component oxides may be tailored by simply changing the cation concentration in the solution, the ability to deposit thin films in * Corresponding author. E-mail address: [email protected] (R. Kavitha). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00213-2
open atmosphere without the need for any furnace or reaction/vacuum chamber and, finally, the ability to deposit non-equilibrium phases owing to the relatively low temperatures and rapid decomposition involved. As a result the operating and capital costs are reduced compared with many of the other techniques that are presently used. The use of combustion flames to decompose metal salts is not new. Nielsen et al. [12] described the use of flames, using natural gas and air, for the production of metal oxide powders as far back as 1960. Metal salts, dissolved either in water or in organic solvents have been used for the production of single oxide or mixed oxide powders, generally of spherical shape with diameters ranging from 0.01 to 5 mm. Recently, several variants of this method have emerged for the production of both films as well as powders. Hunt and coworkers carried out combustion chemical vapour deposition (CCVD) [13] on a number of different and complex oxide systems. They utilised extremely fine glass capillaries for atomisation with the result that feed rates and deposition rates were somewhat low (microns per hour) [6 /9]. Ichinose et al. deposited perovskite thin films [14,15] by spraying ultrasonically atomised solutions into a combustion flame. Tikkanen et al. describe the production of nanoparticles by a liquid flame spray process [16] wherein precursor dissolved in a solvent is injected and atomised in an oxygen /hydrogen flame to
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yield fine particles. A high-velocity oxy fuel (HVOF) spray torch has been used to melt particles and accelerate the molten droplets to form a thin splat on a substrate [11]. Low-pressure flame deposition, reported by Glumac involves pyrolysis of precursors in a flat, low pressure flame that yields clusters/nanoparticles in the gas phase that sinter to form a coating on the substrate [17]. Recently, Choy and Vyas [18] successfully deposited ZrO2 /Y2O3 thermal barrier coating on a Nibased superalloy by flame-assisted vapour deposition technique. The technique that we have developed and which is the subject of this paper seeks to address some of the drawbacks inherent in the other methods reported above, notably in providing a simple, open air system with reasonably high deposition rates with the capability to generate coatings of both equilibrium phases as well as those of metastable solid solutions.
2. Experimental procedure The different types of experimental arrangements that were attempted for deposition are described, followed by the details of the systems that were produced in an optimised set-up. 2.1. Deposition arrangement Two types of experimental arrangement are described. In both types atomisation of the precursor solution is air-assisted. In the first arrangement (Fig. 1) a peristaltic pump is used to feed the solution at a very low flow rate (3 /5 ml min 1) into a conventional welding torch fitted with a nozzle of 1 mm diameter. An oxy-acetylene gas mixture atomises solution that flows into the nozzle. This type of atomiser belongs to the internal mixing configuration in which gas and precursor solution mix within the nozzle before discharging through the outlet orifice. The discharged droplets are ignited with an acetylene pilot flame, which is kept in place during combustion to prevent flame blow off. The substrate is
19
placed on a copper substrate holder held at a suitable distance from the nozzle. The primary drawback of the arrangement is that at the low rates of throughput necessary the flow of the precursor solution into the welding torch nozzle is discontinuous. Furthermore, because of the internal mixing of the precursor solution and the oxy-acetylene gas within the nozzle this type of atomiser suffers from the problem of a back-pressure that prevents free flow of the solution [19]. In the present experimental arrangement (Fig. 2), the peristaltic pump is replaced by an atomiser and separation chamber. Atomisation of the precursor solution is performed by an external-mixing air assisted two-fluid atomiser as shown in the inset in Fig. 2. Precursor solution flows through the centre pipe and compressed air through the outer pipe. There is no mixing of the precursor solution and atomising gas inside the nozzle. Compressed air flowing at a rate of /17 l min 1 impinges and exerts a drag force on the precursor solution outside the liquid discharge orifice, thereby atomising the precursor solution into fine droplets. In the separation chamber, which is a cylindrical vessel of 15 cm diameter and 30 cm height, atomised droplets delivered into the chamber collide against each other and also on the walls of the container. During this process the droplet size is further reduced and the atomising gas carries only the finer droplets to the welding torch nozzle. Heavier droplets that settle at the bottom are collected and reused. This set-up eliminated the problems faced previously. Flame ignition remains the same as described in set up 1. Input flow rates of fuel, oxygen and compressed air are monitored through gas flow meters. The flame temperature and structure such as turbulence and flame length determines the extent of reaction of the atomised droplet in the flame, while the characteristics of the flame are decided by the oxygen to fuel gas ratio. The surface temperature of the substrate was monitored by a K-type thermocouple inserted through a hole drilled at the centre of the substrate. Temperature control during deposition was maintained to within 20 8C. The deposition process was controlled by managing several variables, including substrate temperature, precursor concentration and deposition time. 2.2. Materials and characterisation techniques
Fig. 1. Schematic diagram of the flame pyrolysis deposition process (Set-up 1); Internal mixing configuration.
Amorphous silica and Ni-based superalloy (Nimonic90) were used as substrates. A variety of oxide films were deposited, of which we confine our report here to Al2O3, ZnO, ZnO /20 mol% MgO and ZrO2 /Y2O3. The surface roughness of the superalloy substrate was maintained at 600-grit finish. The substrates were cleaned with alcohol and wiped clean before deposition. Aqueous solutions of metal acetates were used for ZnO and ZnO /20 mol% MgO films and the concentration of the
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Fig. 2. Schematic diagram of the flame pyrolyisis deposition process (set-up 2); inset picture shows an external mixing atomiser.
solutions were 100 and 50 g l 1, respectively. Solutions of nitrates of aluminium (50 g l 1) and zirconium/ yttrium (35 g l 1) in deionised water were used as the precursor for Al2O3 and ZrO2 /Y2O3 coatings. The ratio of zirconium nitrate and yttrium nitrate was adjusted to obtain ZrO2 /8 wt.% Y2O3. The films were deposited for 30 min. Crystallinity and phases in the deposited films were established by X-ray diffraction (JEOL 8030, Cu / Ka). The surface morphology and cross-sectional view of the deposited films were observed using scanning electron microscopy (JEOL JSM 840A). Cross-sections were prepared by sandwiching the film to a blank substrate, followed by slicing and polishing. Vickers indentations (HMV-2000 Shimadzu) were used to measure the film hardness at various loads.
3. Results and discussion 3.1. Alumina films The as-deposited alumina film at 650 8C was found to be amorphous. Post deposition heat treatment of the film at 1200 8C for 5 h resulted in transformation of the amorphous film to crystalline alpha alumina while the amorphous silica substrate had also crystallised to tridymite as shown by the XRD patterns in Fig. 3. This result is consistent with that seen for alumina films produced by various other techniques such as sputtering [20], plasma enhanced CVD [2], ion-beam induced CVD [3] and metal /organic CVD [4], in which direct deposition of alpha alumina is rarely observed and post deposition annealing is generally needed to arrive at this structure. The average hardness of the amorphous alumina films is /4 GPa. The hardness of alpha alumina was found to be as high as 10 GPa in some regions, while an average value of 8.5 GPa was obtained
from 15 observations made on different regions of the film. Indentation imprints on amorphous alumina film produced by Vickers test is shown in Fig. 4, it is seen that the film remains intact and that the effect of indentation was mainly in ‘pushing’ the film down. The results indicate that alpha alumina is approximately twice as hard as the amorphous alumina film. Chou [20] had observed similar behaviour on alumina films deposited by r.f. Magnetron sputtering. For comparison, the hardness of fully dense polycrystalline alumina ranges from 18 to 20 GPa indicating that the films in the present case are porous. 3.2. ZnO and ZnO /20 mol%MgO films ZnO films were deposited at temperatures of 150, 350 and 650 8C. The strong influence of substrate temperature (Ts) on structure is depicted in Fig. 5. The film deposited at 150 8C did not exhibit any peaks of the wurtzite phase, while those deposited at 350 8C reveal single phase, polycrystalline wurtzite. Films deposited at 650 8C exhibit enhanced intensity of the (0002) peak indicating that the film prefers to grow with the c-axis normal to the substrate. In an analysis of the thermal decomposition of zinc acetate [21] it has been reported that the decomposition of zinc acetate is complete only at 300 8C. Others [1,22] observe similar behaviour on ZnO films deposited at different temperatures by spray pyrolysis. With increase in the deposition temperature the grain size increases [23], resulting in sharp diffraction peaks. From the above observations it is clear that Ts plays an important role in determining the structure of ZnO films. ZnO /20 mol% MgO films were deposited at 500 and 600 8C. At the lower temperature, only single phase wurtzite is present, as seen by the XRD pattern in Fig. 6a. At 600 8C one can observe phase separation with the appearance of rocksalt (Fig. 6b). Although ZnO and
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Fig. 3. XRD patterns of as-deposited and heat treated alumina films.
MgO display different crystal structures, wurtzite and rocksalt, respectively, the similarity in Pauling’s ionic ˚ ) and Mg2 (0.72 A ˚ ) allows radii between Zn2 (0.75 A some solubility. It is seen from the phase diagram [24] that the equilibrium solid solubility of MgO in wurtzite is limited to 5% at 900 8C. However, it has been shown that solubility can be metastably extended to as much as 30 /35% by spray pyrolysis [25] or by laser ablation [26]. Hence it may be inferred that the rapid process of evaporation and decomposition of the fine droplets, together with the low deposition temperature can lead to the formation of films of extended solubility, as earlier demonstrated for powders [25]. The surface morphology of the single phase wurtzite film in Fig. 7 shows small crystallites spread uniformly all over the substrate and this morphology is similar to the surface morphology exhibited by thin films deposited by spray pyrolysis [23].
From the cross-sectional view of the film (Fig. 8) the thickness was measured to be 17 mm. The porosity that is observed near the interface with the substrate might be due to the low substrate temperature at the beginning of deposition.
Fig. 4. Indentation imprint on as-deposited alumina (amorphous) film produced by Vickers test under 1000 g load for 10 s.
Fig. 5. X-ray diffractograms of ZnO films deposited at three different temperatures.
3.3. ZrO2 /Y2O3 films The surface morphology and cross section of the coating deposited for 30 min at a substrate temperature of 550 8C are shown in Figs. 9 and 10. The cross
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Fig. 6. X-ray diffraction patterns of single phase (wurtzite) and phase segregated ZnO /20 mol% MgO films.
Fig. 7. SEM micrograph of ZnO /20 mol% MgO (wurtzite) film deposited at 500 8C.
Fig. 8. Cross-sectional SEM image of ZnO /20 mol% MgO film (wurtzite) film of 17 mm thickness.
sectional microstructure is entirely different from that of electron-beam PVD (EBPVD) coatings and air plasma spray (APS) coatings. It is well known that plasma spraying produces laminar coatings while EBPVD coatings are columnar [27]. The film produced by the present method is granular with porosity and microcracks. From the XRD pattern, Fig. 11, it is clear that the film contains a negligible amount of monoclinic phase. It is difficult to distinguish the cubic and tetragonal phases since the strong peaks overlap significantly and also because of the change in the lattice parameter of zirconia with addition of yttria [28]. It may be concluded that the predominant phase is cubic or tetragonal or a mixture of both the phases and that, the monoclinic phase, if present should be a minor constituent. The average hardness of the coating in cross section is 130 Hv. This hardness value is several times lower than the hardness (1160 KHN) of bulk zirconia [29]. Also,
Fig. 9. Surface morphology of ZrO2 /Y2O3 film.
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Acknowledgements Financial support for this work was provided by a grant from the Aeronautical Research and Development Board, Government of India.
References
Fig. 10. Cross-sectional view of ZrO2 /Y2O3 film.
Fig. 11. XRD patterns of blank substrate and ZrO2 /Y2O3 film with substrate.
hardness of plasma spray deposited 3 mol% YSZ was found to be 550 /700 Hv [30]. The lower hardness values of the coatings are attributed to the presence of porosity and microcracks, which may be seen in the cross sectional micrograph.
4. Conclusions A new versatile and inexpensive technique has been developed for the deposition of oxide films by combustion of aqueous precursors. The optimised configuration of deposition incorporates the injection of atomised solutions, via a separation chamber, into an oxyacetylene mixture prior to combustion. Metastable extension of the wurtzite solid solution by up to 20 mol% MgO has been demonstrated in ZnO /MgO films by suitable control of deposition temperature. Coatings of ZrO2 /Y2O3 of an average thickness of about 60 mm have been fabricated at a low substrate temperature of / 550 8C.
[1] J.L. Van Heerden, R. Swanepoel, Thin Solid Films 299 (1997) 72. [2] H.L. Wang, C.H. Lin, M.H. Hon, Thin Solid Films 310 (1997) 260. [3] A. Caballero, D. Leinen, A. Fernandez, A.R. Gonzalez-Elipe, Surf. Coat. Technol. 80 (1996) 23. [4] E. Ciliberto, I. Fragala, R. Rizza, G. Spoto, Appl. Phys. Lett. 67 (1995). [5] K.B. Sundaram, A. Khan, Thin Solid Films 295 (1997) 87. [6] A.T. Hunt, Appl. Phys. Lett. 63 (1993) 266. [7] J.M. Hampikian, W.B. Carter, Mater. Sci. Eng. A 267 (1999) 7. [8] D.W. Stollberg, W.B. Carter, J.M. Hampikian, Surf. Coat. Technol. 94 /95 (1997) 137. [9] T.A. Polley, W.B. Carter, D.B. Poker, Thin Solid Films 357 (1999) 132. [10] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [11] S. Sampath, MRS Bull. 7 (2000) 17. [12] M.L. Nielsen, W.E. Kuhn (Ed.), Ultra Fine Particles, Wiley, New York, 1963, p. 181. [13] S. Shanmugam, A. Hunt, D. Motley, Ceram. Bull. 85 (2002) 36. [14] H. Ichinose, Y. Shiwa, M. Nagano, Jpn. J. Appl. Phys. 33 (1994) 5903. [15] H. Ichinose, Y. Shiwa, M. Nagano, Jpn. J. Appl. Phys. 33 (1994) 5907. [16] J. Tikkanen, K.A. Gross, C.C. Berndt, V. Pitakanen, J. Keskinen, S. Raghu, M. Rahala, J. Karthikeyan, Surf. Coat. Technol. 90 (1997) 210. [17] G. Skandan, N. Glumac, Y.-J. Chen, Fredric, Cosandey, E. Heims, B.H. Kear, J. Am. Ceram. Soc.8 (1998) 2753. [18] K.L. Choy, Key Eng. Mater. 161 /163 (1999) 653. [19] A.H. Leferbve, Atomization and Sprays, Hemisphere Publishing Company, USA, 1989, p. 7. [20] T.C. Chou, T.G. Nieh, S.D. McAdams, G.M. Pharr, Scr. Metal. 25 (1991) 2203. [21] A. Paul, H.N. Acharya, J. Mater. Sci. 27 (1992) 1716. [22] S.A. Studeniken, N. Golego, M. Cocivera, J. Appl. Phys. 83 (1998) 2104. [23] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [24] S. Raghavan, J.P. Hajra, G.N.K. Iyengar, K.P. Abraham, Thermochim. Acta 89 (1991) 151. [25] V. Jayaram, B. Sirisha Rani, Mater. Sci. Eng. A 304 /306 (2001) 800. [26] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [27] J.R. Nicholls, M.J. Deakin, D.S. Rickerby, Wear 233 /235 (1999) 352. [28] R.P. Ingel, D. Lewis, III, J. Am. Ceram. Soc. 69 (1986) 325. [29] R.C. Weast (Ed.), Hand Book of Chemistry and Physics, 55th ed., CRC Press, USA, 1974 /1975, F-22. [30] J.-H. Shin, D.-S. Lim, H.-S. Ahn, Surf. Coat. Technol. 133 /134 (2000) 403.