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Surface coating technology for biomaterials—morphology and nano-structure control
International Congress Series 1284 (2005) 248 – 256
www.ics-elsevier.com
Surface coating technology for biomaterials—morphology and nano-structure control Takashi Goto * Institute for Materials Research, Tohoku University, 2-1-1 Katahira Aoba-ku Sendai 980-8577, Japan
Abstract. Thin film coatings have been widely employed, particularly in the semiconductor industry; however, thick oxide coatings also have promising applications including biomaterial coatings. High-speed deposition processes, commonly plasma spray or electron beam physical vapor deposition, have been utilized for thick coatings, whereas another deposition route has been pursued to obtain high-performance thick coatings. This paper introduces a new high-speed deposition process, laser chemical vapor deposition (LCVD), for thick coatings and its versatility of morphology and nano-structure control are advantageous to adherence of ceramic coatings to metal substrates. LCVD has achieved extremely high deposition rates ranging from 100 to 3000 Am/h for various oxide coatings such as ZrO2, Y2O3, Al2O3 and TiO2. Excellent adherence can be developed by columnar texture containing a large amount of nano-pores in the grain of oxide coatings. D 2005 Elsevier B.V. All rights reserved. Keywords: Laser chemical vapor deposition; Morphology; Nano-structure; Oxide coating; Columnar texture; Nano-pore
1. Introduction Since thin-film coating has been an essential key technology for semiconductor devises, a variety of coating processes have been developed. On the other hand, thick-film coating process has not matured well in spite of wide-range engineering applications including biomaterial coatings. The thickness of biomaterial coatings could be changed from submicron to several hundred micro-meters depending on the circumstance of usage for coatings, and therefore, an appropriate coating process should be chosen. Fig. 1 * Tel.: +81 22 215 2105; fax: +81 22 215 2107. E-mail address: [email protected]. 0531-5131/ D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2005.06.007
T. Goto / International Congress Series 1284 (2005) 248–256
249
Fig. 1. A systematic diagram coating processes for biomaterials.
summarizes the coating processes employed for biomaterials. The process temperature for the coating should be less than about 600 8C due to the degradation of mechanical strength of metal substrates, usually Ti-base alloys. Many kinds of processes such as sol–gel [1], thermal spray [2], electrophoretic deposition [3] and dip-coating [4] have been applied for biomaterial coatings, where lower process temperature and stronger adherence are significantly important. Among these coating processes, chemical vapor deposition (CVD) can be a promising process due to the controllability of morphology and nano-structure of deposits ranging from dense, columnar and porous coatings [5]. As CVD has been regarded as a thin-film deposition technique, thick coatings have been scarcely fabricated by conventional CVD [6]. On the other hand, due to the stringent requirement of high-performance thick oxide coatings, particularly thermal barrier coatings (TBCs) – commonly yttria-stabilized zirconia (YSZ) coatings on Ni-base super-alloys – thick YSZ coatings have been obtained at high deposition rates around 50 [7] to 100 Am/h [8]. For further enhancement of deposition rate, auxiliary energy of plasma has been supplied in CVD process, and high deposition rate of 200 Am/h for YSZ coatings has been achieved by using plasma CVD [9]. Another auxiliary energy of laser may significantly enhance the deposition rates in CVD [10]; however, no work using laser CVD (LDVD) has been reported for thick oxide coatings on wide-area substrates. We have recently developed a new LCVD process to fabricate several oxide coatings at extremely high deposition rates up to 3000 Am/h with wellcontrolled morphology and nano-structure with strong adherence to metal substrates [11]. Since the deposition rate generally decreases with decreasing deposition temperature, the process temperature (i.e., deposition temperature) can be lowered by using a high-speed deposition process. In biomaterial coatings, the deposition temperature should be less than 600 to 700 8C as the substrate metal of Ti-base alloy will be degraded at high temperatures. This paper briefly reviews the recent study of thick oxide YSZ coatings on metal substrates by using CVD and LCVD as a case study for the biomaterial coatings. 2. Thick oxide coating process It is stringently required in various engineering fields to obtain thick coatings ranging from several to several hundred micrometers in thickness with strong adherence to
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Fig. 2. A systematic diagram of plasma spray (a) and its typical microstructure (b).
substrates. The typical application of thick coating is thermal barrier coatings (TBCs) of YSZ for gas turbine blades commonly made of Ni-base super-alloys [12]. Technological issues on TBCs are similar to those of biomaterial coatings, adherence of ceramic coatings on metal substrates. Therefore, the recent study on TBCs would be applicable to develop biomaterial coatings. Fig. 2 depicts a schematic diagram of plasma spray which has been widely employed for hydroxyapatite (HAp) thick coatings [13]. The source powders are instantly evaporated in a plasma torch and its vapor will deposit on the substrate. The powders would be partially unmelted or solidified too early in a gas phase, which would often cause unfavorable bsplat texture,Q as shown in Fig. 2(b). The coatings by plasma spray would contain many defects of cracks and pores leading to partial delamination and short lifetime of coatings. On the other hand, electron beam physical vapor deposition (EB-PVD), shown in Fig. 3, has been intensively studied recently [12]. A YSZ ingot rod is evaporated by a high-power electron beam, and a typical columnar microstructure can be obtained by EBPVD, as demonstrated in Fig. 3(b). Since the thermal expansion of oxide coatings is generally smaller than that of metal substrates, significant thermal stress would arise at the coating/metal interface. However, the columnar textures intrinsically possess many gaps between each column, and these gaps would contribute to relax the thermal stress at the interface. Furthermore, the columnar texture commonly contains a large amount of pores in YSZ grains typically prepared by EB-PVD, as depicted in Fig. 4 [14]. These pores in
Fig. 3. A systematic diagram of EB-PVD and its typical microstructure (b).
T. Goto / International Congress Series 1284 (2005) 248–256
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Fig. 4. Nano-pores in YSZ prepared by EB-PVD.
nano-size (nano-pores) are also effective in reducing the thermal expansion mismatch between oxide coatings and metal substrates. Therefore, it is generally understood that the columnar texture with a large amount of nano-pores would be essential to achieve thick oxide coating on metal substrate to attain excellent adherence. 3. Thick oxide coatings by CVD Since the CVD process occurs through chemical reactions of molecules or clusters at the substrate surface, CVD coatings would have excellent adherence or conformal coverage to the substrate. Therefore, CVD has wide applications particularly in semiconductor devise applications. However, the deposition rates of conventional CVD could be around a few Am/h, being too small to apply thick coatings such as TBCs. In order to develop high-performance TBC coatings, many efforts have been conducted to increase the deposition rate of YSZ coating by conventional CVD. Fig. 5 depicts the deposition temperature dependence of deposition rates for YSZ coatings by CVD [7,8,15– 17]. The highest deposition rate of YSZ coatings (108 Am/h) has been achieved by using Zr(dpm)4 (dpm: dipivaloylmethanate) precursors in CVD by our group [8]. Fig. 6(a) demonstrates the cross-section of CVD-YSZ coating on Ni-base alloy. The columnar texture was clearly observed with significant (200) preferred orientation. Detailed nano-
Fig. 5. Deposition temperature dependence of deposition rates of YSZ coatings by CVD.
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Fig. 6. (a) A cross-sectional TEM image of a CVD-YSZ coating on a Ni-base alloy, (b) nano-pores in a YSZ coating prepared by CVD.
structure by transmission electron microscopy (TEM) observation is shown in Fig. 6(b). The YSZ coating prepared at a relatively high deposition rate (52 Am/h) contained a large amount of nano-pores. Although the shape of nano-pores in YSZ coating by EB-PVD usually has been spheres or ellipsoids, YSZ coatings by CVD have contained angular shaped pores. The inside of the angular shaped pores could be surrounded by specific crystal planes. Such angular shaped pores might be similar to bnegative crystalQ suggesting a crystallographically stable structure [18]. We have confirmed that nano-pores were fairly stable after heat treatments at high temperatures (~ 1400 8C) for a long time (100 h). 4. Thick oxide coatings by LCVD LCVD has been intensively studied since the 1970s and mainly applied in semiconductor devises as thin film coating at low substrate temperatures [10]. Fig. 7 demonstrates the typical LCVD for the application in repairing an electric circuit by focusing a laser beam (Fig. 7(a)) and constructing a three-dimensional structure in micrometer scale by scanning a laser beam (Fig. 7(b)) [19]. LCVD can be generally categorized into two types; pyrolytic LCVD where laser is mainly used as a heat source, and photolytic LCVD where laser is used as a high-energy photon source. The pyrolytic LCVD can often fabricate dots or wires at extremely high speeds, but only in the size of the laser beam, usually less than 1 mm. However, no thick
Fig. 7. Repairing of an electric circuit (a) and a three-dimensional deposit (b) by conventional LCVD.
T. Goto / International Congress Series 1284 (2005) 248–256
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Fig. 8. A YSZ coating on a Ni-base super-alloy substrate by LCVD.
oxide coatings have been fabricated on wide-area substrates more than several millimeters in diameter by LCVD. We have found that a fairly high power laser emission significantly enhanced the deposition rate of oxides in CVD [11]. Fig. 8 demonstrates an YSZ coating in a thickness about 100 Am on Ni-base super-alloy substrate (25 mm in diameter) by LCVD. The whole surface was uniformly coated by YSZ. The deposition rate was tremendously increased to more than several 100 Am/h at more than a critical laser power of 70 W. In this study, the highest deposition rate (660 Am/h) of YSZ coatings was achieved on a wide-area substrate. We have prepared other oxide films such as Y2O3, Al2O3 and TiO2 at high deposition rates at most 100 to 3000 Am/h. These values were about 100 to 1000 times higher than those obtained by conventional CVD. Since Nd:YAG laser has relatively low photon energy (1.17 eV) and unable to break chemical bondings in source materials, the present LCVD could be a kind of pyrolytic laser CVD. However, the laser in the present LCVD cannot be a simple heat source, because the enhancement of deposition rate was so significant. Fig. 9(a) demonstrates the change of substrate temperature during the LCVD process, in which the laser radiation was started at time (t) of zero. The substrate temperature increased by laser radiation up to 150 to 200 8C after 60 s, and then source gases were introduced into the chamber. An abrupt temperature increase occurred accompanying a strong light emission after 100 s. According to spectroscopic analyses, this light emission had a continuous dispersion similar to the plank distribution corresponding to a black-body temperature of about 4000 8C [20]. The Langmuir probe measurements detected charged species in the bright light emission zone indicating the formation of plasma. The
Fig. 9. Change of substrate temperature during (a) LCVD process, (b) effect of deposition temperature on the deposition rate of YSZ coating in LCVD.
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deposition efficiency of the present LCVD reached 85%, whereas that of conventional CVD has been usually less than a few percent. The high-temperature thermal plasma together with some activation effect on the substrate surface by laser radiation may be associated with the enhancement of deposition rates in the LCVD. Fig. 9(b) demonstrates the effect of deposition temperature on the deposition rate of YSZ coatings in LCVD. With increasing the laser power, the plasma intensity increased resulting to the increase in deposition temperature. The deposition rate saturated around 980 8C, partially due to powder formation in gas phase at higher temperatures. In conventional CVD, the deposition rate increases with increasing deposition temperature, and then decreases at a higher temperature commonly around 650 to 800 8C. In the present LCVD, the deposition rate still increased even at 900 8C. The laser and accompanying plasma could heat or activate very thin layer of substrate surface, and the powder formation in a gas phase must be suppressed. The charged ions formed in the plasma zone could be also extremely reactive resulting in the deposition rate and high efficiency in the present LCVD. Fig. 10 depicts a cross-sectional SEM image of YSZ coating prepared at laser power ( P L) of 200 W. The YSZ coating was obtained at a deposition rate of 450 Am/h with a well-developed columnar structure having a significant (200) orientation. The (200) orientation and columnar alignment became more significant with increasing deposition temperature and P L. The top surface of each column has a faceted structure, and feather-like porous texture was observed near the top surface. Fig. 11 demonstrates the TEM micrographs of columnar grains near the surface (Fig. 11(a)), middle of column (Fig. 11(b)) and near the substrate (Fig. 11(c)). Slightly wide gaps between columns with zigzag shapes were observed in Fig. 11(a). In the middle part of columns, relatively large pores about 10 nm in diameter and smaller nano-pores about a few nanometers were identified inside the columns. The gaps and pores between columns could be effective to relax the thermal stress between YSZ coating and metal substrate. A large amount of nano-pores was also observed at the coating/substrate interface. These nano-pores could contribute to the relaxation of thermal stress resulting in good adherence to the substrate. Excellent thermal shock resistance of the YSZ-coated Ni-base super-alloy was confirmed by thermal shock tests changing the temperature cyclically between 500 and 1400 8C, and the YSZ coating has survived even after 1200 cycles. Although such extreme thermal shock resistance would not be required in biomaterial coatings, the strong
Fig. 10. A cross-sectional SEM image of YSZ coating by LCVD.
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Fig. 11. TEM micrographs of columnar grains near the surface (a), middle of column (b) and near the substrate (c).
adherence can be achieved in thick biomaterial coating such as HAp on Ti-base alloy implants by introducing the columnar texture and nano-pores. 5. Summary Biomaterial coatings, mainly those such as HAp on Ti-base alloys, have been intensively conducted by various processing, where the thickness of the coatings have been usually less than a few micrometers due to weak adherence between ceramic coatings and metal substrates. We have developed thick oxide coatings on metal substrates with strong adherence by laser CVD. Due to the different nature of the thermo-mechanical properties between ceramics and metals (mainly thermal expansion coefficient), the relaxation of thermal stress at the interface is a key issue for thick coatings. The columnar texture containing a large amount of nano-pores could be promising in overcoming the thermal expansion mismatch between ceramics and metals. Such particular texture can be obtained by LCVD. The laser radiation was also extremely effective in enhancing the deposition rate in CVD. Several thick coatings of YSZ, Y2O3, Al2O3 and TiO2 were obtained at most 100 to 1000 times higher deposition rates than those by conventional MOCVD. Thick oxide coating on metal substrates by LCVD could be promising for biorelated applications. Acknowledgement This research is partially supported by the Nano-coating Project sponsored by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] D.M. Liu, Q. Yang, T. Troczynski, Sol–gel hydroxyapatite coatings on stainless steel substrates, Biomaterials 23 (2002) 691 – 698. [2] M.T. Carayon, J.L. Lacout, Study on the Ca/P atomic ratio of the amorphous phase in plasma-sprayed hydroxyapatite coatings, J. Solid State Chem. 172 (2003) 339 – 350.
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[3] M. Manso, et al., Electrodeposition of hydroxyapatite coatings in basic conditions, Biomaterials 21 (2000) 1755 – 1761. [4] W. Weng, J.L. Baptista, Alkoxide route for preparing hydroxyapatite and its coatings, Biomaterials 19 (1998) 125 – 131. [5] W.A. Bryant, The fundamentals of chemical vapour deposition, J. Mater. Sci. 12 (1977) 1285 – 1306. [6] C.F. Powell, Chemical vapor deposition, in: C.F. Powell, J. HOxley, J.M. Blocher Jr. (Eds.), Vapor Deposition, John Wiley, New York, 1966, pp. 249 – 276. [7] G. Wahl, et al., Chemical vapor deposition of TBC: an alternative process for gas turbine components, Trans. ASME 123 (2001) 520 – 524. [8] R. Tu, T. Kimura, T. Goto, Rapid synthesis of yttria partially-stabilized zirconia films by metal-organic chemical vapor deposition, Mater. Trans. 43 (2002) 2354 – 2356. [9] B. Preauchat, S. Darwin, Properties of PECVD-deposited thermal barrier coatings, Surf. Coat. Technol. 142–144 (2001) 835 – 842. [10] C. Duty, D. Jean, W.J. Lackey, Laser chemical vapor deposition: materials, modeling, and process control, Int. Mater. Rev. 46 (2001) 271 – 287. [11] T. Kimura, T. Goto, Rapid synthesis of yttria-stabilized zirconia films by laser chemical vapor deposition, Mater. Trans. 44 (2003) 421 – 424. [12] D.R. Clarke, C.G. Levi, Materials design for the next generation thermal barrier coatings, Annu. Rev. Mater. Res. 33 (2003) 383 – 417. [13] P.A. Kammer, Plasma spray, in: D.A. Glocker, S.I. Shah (Eds.), Flame Spray. Handbook of Thin Film Process Technology, IOP Pub, 1996, pp. A4.1:1 – A4.1:16. [14] T.J. Lu, et al., Distributed porosity as a control parameter for oxide thermal barriers made by physical vapor deposition, J. Am. Ceram. Soc. 84 (2001) 2937 – 2946. [15] Y. Akiyama, T. Sato, Imaishi, Reaction analysis of ZrO2 and Y2O3 thin film growth by low-pressure metalorganic chemical vapor deposition using h-diketonate complexes, J. Cryst. Growth 147 (1995) 130 – 147. [16] N. Bourhila, et al., Deposition and characterization of ZrO2 and yttria stabilized ZrO2 films using injectionLPCVD, in: M.D. Allendorf, C. Bernard (Eds.), Proc. 14th Intern Conference Electrochem Soc, vol. 97–25, 1997, pp. 417 – 424. [17] M. Pulver, W. Nemetz, G. Wahl, CVD of ZrO2, Al2O3 and Y2O3 from metalorganic compounds in different reactors, Surf. Coat. Technol. 125 (2000) 400 – 406. [18] J.D. Powers, A.M. Glaeser, Oriental effects on the high-temperature morphological evolution of pore channels in sapphire, J. Am. Ceram. Soc. 83 (2000) 2297 – 2304. [19] D. Bauerle, Direct Writing: Laser Processing and Chemistry, 3rd ed., Springer, Berlin, 2000, pp. 337 – 360. [20] H. Miyazaki, T. Kimura, T. Goto, Acceleration of deposition rates in a chemical vapor deposition process by laser irradiation, Jpn. J. Appl. Phys. 42 (2003) L316 – L318.
www.ics-elsevier.com
Surface coating technology for biomaterials—morphology and nano-structure control Takashi Goto * Institute for Materials Research, Tohoku University, 2-1-1 Katahira Aoba-ku Sendai 980-8577, Japan
Abstract. Thin film coatings have been widely employed, particularly in the semiconductor industry; however, thick oxide coatings also have promising applications including biomaterial coatings. High-speed deposition processes, commonly plasma spray or electron beam physical vapor deposition, have been utilized for thick coatings, whereas another deposition route has been pursued to obtain high-performance thick coatings. This paper introduces a new high-speed deposition process, laser chemical vapor deposition (LCVD), for thick coatings and its versatility of morphology and nano-structure control are advantageous to adherence of ceramic coatings to metal substrates. LCVD has achieved extremely high deposition rates ranging from 100 to 3000 Am/h for various oxide coatings such as ZrO2, Y2O3, Al2O3 and TiO2. Excellent adherence can be developed by columnar texture containing a large amount of nano-pores in the grain of oxide coatings. D 2005 Elsevier B.V. All rights reserved. Keywords: Laser chemical vapor deposition; Morphology; Nano-structure; Oxide coating; Columnar texture; Nano-pore
1. Introduction Since thin-film coating has been an essential key technology for semiconductor devises, a variety of coating processes have been developed. On the other hand, thick-film coating process has not matured well in spite of wide-range engineering applications including biomaterial coatings. The thickness of biomaterial coatings could be changed from submicron to several hundred micro-meters depending on the circumstance of usage for coatings, and therefore, an appropriate coating process should be chosen. Fig. 1 * Tel.: +81 22 215 2105; fax: +81 22 215 2107. E-mail address: [email protected]. 0531-5131/ D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2005.06.007
T. Goto / International Congress Series 1284 (2005) 248–256
249
Fig. 1. A systematic diagram coating processes for biomaterials.
summarizes the coating processes employed for biomaterials. The process temperature for the coating should be less than about 600 8C due to the degradation of mechanical strength of metal substrates, usually Ti-base alloys. Many kinds of processes such as sol–gel [1], thermal spray [2], electrophoretic deposition [3] and dip-coating [4] have been applied for biomaterial coatings, where lower process temperature and stronger adherence are significantly important. Among these coating processes, chemical vapor deposition (CVD) can be a promising process due to the controllability of morphology and nano-structure of deposits ranging from dense, columnar and porous coatings [5]. As CVD has been regarded as a thin-film deposition technique, thick coatings have been scarcely fabricated by conventional CVD [6]. On the other hand, due to the stringent requirement of high-performance thick oxide coatings, particularly thermal barrier coatings (TBCs) – commonly yttria-stabilized zirconia (YSZ) coatings on Ni-base super-alloys – thick YSZ coatings have been obtained at high deposition rates around 50 [7] to 100 Am/h [8]. For further enhancement of deposition rate, auxiliary energy of plasma has been supplied in CVD process, and high deposition rate of 200 Am/h for YSZ coatings has been achieved by using plasma CVD [9]. Another auxiliary energy of laser may significantly enhance the deposition rates in CVD [10]; however, no work using laser CVD (LDVD) has been reported for thick oxide coatings on wide-area substrates. We have recently developed a new LCVD process to fabricate several oxide coatings at extremely high deposition rates up to 3000 Am/h with wellcontrolled morphology and nano-structure with strong adherence to metal substrates [11]. Since the deposition rate generally decreases with decreasing deposition temperature, the process temperature (i.e., deposition temperature) can be lowered by using a high-speed deposition process. In biomaterial coatings, the deposition temperature should be less than 600 to 700 8C as the substrate metal of Ti-base alloy will be degraded at high temperatures. This paper briefly reviews the recent study of thick oxide YSZ coatings on metal substrates by using CVD and LCVD as a case study for the biomaterial coatings. 2. Thick oxide coating process It is stringently required in various engineering fields to obtain thick coatings ranging from several to several hundred micrometers in thickness with strong adherence to
250
T. Goto / International Congress Series 1284 (2005) 248–256
Fig. 2. A systematic diagram of plasma spray (a) and its typical microstructure (b).
substrates. The typical application of thick coating is thermal barrier coatings (TBCs) of YSZ for gas turbine blades commonly made of Ni-base super-alloys [12]. Technological issues on TBCs are similar to those of biomaterial coatings, adherence of ceramic coatings on metal substrates. Therefore, the recent study on TBCs would be applicable to develop biomaterial coatings. Fig. 2 depicts a schematic diagram of plasma spray which has been widely employed for hydroxyapatite (HAp) thick coatings [13]. The source powders are instantly evaporated in a plasma torch and its vapor will deposit on the substrate. The powders would be partially unmelted or solidified too early in a gas phase, which would often cause unfavorable bsplat texture,Q as shown in Fig. 2(b). The coatings by plasma spray would contain many defects of cracks and pores leading to partial delamination and short lifetime of coatings. On the other hand, electron beam physical vapor deposition (EB-PVD), shown in Fig. 3, has been intensively studied recently [12]. A YSZ ingot rod is evaporated by a high-power electron beam, and a typical columnar microstructure can be obtained by EBPVD, as demonstrated in Fig. 3(b). Since the thermal expansion of oxide coatings is generally smaller than that of metal substrates, significant thermal stress would arise at the coating/metal interface. However, the columnar textures intrinsically possess many gaps between each column, and these gaps would contribute to relax the thermal stress at the interface. Furthermore, the columnar texture commonly contains a large amount of pores in YSZ grains typically prepared by EB-PVD, as depicted in Fig. 4 [14]. These pores in
Fig. 3. A systematic diagram of EB-PVD and its typical microstructure (b).
T. Goto / International Congress Series 1284 (2005) 248–256
251
Fig. 4. Nano-pores in YSZ prepared by EB-PVD.
nano-size (nano-pores) are also effective in reducing the thermal expansion mismatch between oxide coatings and metal substrates. Therefore, it is generally understood that the columnar texture with a large amount of nano-pores would be essential to achieve thick oxide coating on metal substrate to attain excellent adherence. 3. Thick oxide coatings by CVD Since the CVD process occurs through chemical reactions of molecules or clusters at the substrate surface, CVD coatings would have excellent adherence or conformal coverage to the substrate. Therefore, CVD has wide applications particularly in semiconductor devise applications. However, the deposition rates of conventional CVD could be around a few Am/h, being too small to apply thick coatings such as TBCs. In order to develop high-performance TBC coatings, many efforts have been conducted to increase the deposition rate of YSZ coating by conventional CVD. Fig. 5 depicts the deposition temperature dependence of deposition rates for YSZ coatings by CVD [7,8,15– 17]. The highest deposition rate of YSZ coatings (108 Am/h) has been achieved by using Zr(dpm)4 (dpm: dipivaloylmethanate) precursors in CVD by our group [8]. Fig. 6(a) demonstrates the cross-section of CVD-YSZ coating on Ni-base alloy. The columnar texture was clearly observed with significant (200) preferred orientation. Detailed nano-
Fig. 5. Deposition temperature dependence of deposition rates of YSZ coatings by CVD.
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Fig. 6. (a) A cross-sectional TEM image of a CVD-YSZ coating on a Ni-base alloy, (b) nano-pores in a YSZ coating prepared by CVD.
structure by transmission electron microscopy (TEM) observation is shown in Fig. 6(b). The YSZ coating prepared at a relatively high deposition rate (52 Am/h) contained a large amount of nano-pores. Although the shape of nano-pores in YSZ coating by EB-PVD usually has been spheres or ellipsoids, YSZ coatings by CVD have contained angular shaped pores. The inside of the angular shaped pores could be surrounded by specific crystal planes. Such angular shaped pores might be similar to bnegative crystalQ suggesting a crystallographically stable structure [18]. We have confirmed that nano-pores were fairly stable after heat treatments at high temperatures (~ 1400 8C) for a long time (100 h). 4. Thick oxide coatings by LCVD LCVD has been intensively studied since the 1970s and mainly applied in semiconductor devises as thin film coating at low substrate temperatures [10]. Fig. 7 demonstrates the typical LCVD for the application in repairing an electric circuit by focusing a laser beam (Fig. 7(a)) and constructing a three-dimensional structure in micrometer scale by scanning a laser beam (Fig. 7(b)) [19]. LCVD can be generally categorized into two types; pyrolytic LCVD where laser is mainly used as a heat source, and photolytic LCVD where laser is used as a high-energy photon source. The pyrolytic LCVD can often fabricate dots or wires at extremely high speeds, but only in the size of the laser beam, usually less than 1 mm. However, no thick
Fig. 7. Repairing of an electric circuit (a) and a three-dimensional deposit (b) by conventional LCVD.
T. Goto / International Congress Series 1284 (2005) 248–256
253
Fig. 8. A YSZ coating on a Ni-base super-alloy substrate by LCVD.
oxide coatings have been fabricated on wide-area substrates more than several millimeters in diameter by LCVD. We have found that a fairly high power laser emission significantly enhanced the deposition rate of oxides in CVD [11]. Fig. 8 demonstrates an YSZ coating in a thickness about 100 Am on Ni-base super-alloy substrate (25 mm in diameter) by LCVD. The whole surface was uniformly coated by YSZ. The deposition rate was tremendously increased to more than several 100 Am/h at more than a critical laser power of 70 W. In this study, the highest deposition rate (660 Am/h) of YSZ coatings was achieved on a wide-area substrate. We have prepared other oxide films such as Y2O3, Al2O3 and TiO2 at high deposition rates at most 100 to 3000 Am/h. These values were about 100 to 1000 times higher than those obtained by conventional CVD. Since Nd:YAG laser has relatively low photon energy (1.17 eV) and unable to break chemical bondings in source materials, the present LCVD could be a kind of pyrolytic laser CVD. However, the laser in the present LCVD cannot be a simple heat source, because the enhancement of deposition rate was so significant. Fig. 9(a) demonstrates the change of substrate temperature during the LCVD process, in which the laser radiation was started at time (t) of zero. The substrate temperature increased by laser radiation up to 150 to 200 8C after 60 s, and then source gases were introduced into the chamber. An abrupt temperature increase occurred accompanying a strong light emission after 100 s. According to spectroscopic analyses, this light emission had a continuous dispersion similar to the plank distribution corresponding to a black-body temperature of about 4000 8C [20]. The Langmuir probe measurements detected charged species in the bright light emission zone indicating the formation of plasma. The
Fig. 9. Change of substrate temperature during (a) LCVD process, (b) effect of deposition temperature on the deposition rate of YSZ coating in LCVD.
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deposition efficiency of the present LCVD reached 85%, whereas that of conventional CVD has been usually less than a few percent. The high-temperature thermal plasma together with some activation effect on the substrate surface by laser radiation may be associated with the enhancement of deposition rates in the LCVD. Fig. 9(b) demonstrates the effect of deposition temperature on the deposition rate of YSZ coatings in LCVD. With increasing the laser power, the plasma intensity increased resulting to the increase in deposition temperature. The deposition rate saturated around 980 8C, partially due to powder formation in gas phase at higher temperatures. In conventional CVD, the deposition rate increases with increasing deposition temperature, and then decreases at a higher temperature commonly around 650 to 800 8C. In the present LCVD, the deposition rate still increased even at 900 8C. The laser and accompanying plasma could heat or activate very thin layer of substrate surface, and the powder formation in a gas phase must be suppressed. The charged ions formed in the plasma zone could be also extremely reactive resulting in the deposition rate and high efficiency in the present LCVD. Fig. 10 depicts a cross-sectional SEM image of YSZ coating prepared at laser power ( P L) of 200 W. The YSZ coating was obtained at a deposition rate of 450 Am/h with a well-developed columnar structure having a significant (200) orientation. The (200) orientation and columnar alignment became more significant with increasing deposition temperature and P L. The top surface of each column has a faceted structure, and feather-like porous texture was observed near the top surface. Fig. 11 demonstrates the TEM micrographs of columnar grains near the surface (Fig. 11(a)), middle of column (Fig. 11(b)) and near the substrate (Fig. 11(c)). Slightly wide gaps between columns with zigzag shapes were observed in Fig. 11(a). In the middle part of columns, relatively large pores about 10 nm in diameter and smaller nano-pores about a few nanometers were identified inside the columns. The gaps and pores between columns could be effective to relax the thermal stress between YSZ coating and metal substrate. A large amount of nano-pores was also observed at the coating/substrate interface. These nano-pores could contribute to the relaxation of thermal stress resulting in good adherence to the substrate. Excellent thermal shock resistance of the YSZ-coated Ni-base super-alloy was confirmed by thermal shock tests changing the temperature cyclically between 500 and 1400 8C, and the YSZ coating has survived even after 1200 cycles. Although such extreme thermal shock resistance would not be required in biomaterial coatings, the strong
Fig. 10. A cross-sectional SEM image of YSZ coating by LCVD.
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Fig. 11. TEM micrographs of columnar grains near the surface (a), middle of column (b) and near the substrate (c).
adherence can be achieved in thick biomaterial coating such as HAp on Ti-base alloy implants by introducing the columnar texture and nano-pores. 5. Summary Biomaterial coatings, mainly those such as HAp on Ti-base alloys, have been intensively conducted by various processing, where the thickness of the coatings have been usually less than a few micrometers due to weak adherence between ceramic coatings and metal substrates. We have developed thick oxide coatings on metal substrates with strong adherence by laser CVD. Due to the different nature of the thermo-mechanical properties between ceramics and metals (mainly thermal expansion coefficient), the relaxation of thermal stress at the interface is a key issue for thick coatings. The columnar texture containing a large amount of nano-pores could be promising in overcoming the thermal expansion mismatch between ceramics and metals. Such particular texture can be obtained by LCVD. The laser radiation was also extremely effective in enhancing the deposition rate in CVD. Several thick coatings of YSZ, Y2O3, Al2O3 and TiO2 were obtained at most 100 to 1000 times higher deposition rates than those by conventional MOCVD. Thick oxide coating on metal substrates by LCVD could be promising for biorelated applications. Acknowledgement This research is partially supported by the Nano-coating Project sponsored by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] D.M. Liu, Q. Yang, T. Troczynski, Sol–gel hydroxyapatite coatings on stainless steel substrates, Biomaterials 23 (2002) 691 – 698. [2] M.T. Carayon, J.L. Lacout, Study on the Ca/P atomic ratio of the amorphous phase in plasma-sprayed hydroxyapatite coatings, J. Solid State Chem. 172 (2003) 339 – 350.
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