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A New Thick Film Coating Technology-Laser Chemical Vapor Deposition
Chapter 10.4
A New Thick Film Coating Technology-Laser Chemical Vapor Deposition Takashi Goto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980-8577, Japan
Chapter Outline 1. 2. 3. 4.
Introduction CVD for High-Speed Deposition YSZ Thermal Barrier Coating a-Al2O3 Coating for Cutting Tools
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1. INTRODUCTION Nowadays, highly functional materials are used under severe conditions such as high-temperature, high-stress, and corrosive environments. Metallic materials have often been used as structural materials because of their superior properties such as high strength and ductility. Ceramic materials, in contrast, have inferior mechanical properties than metallic materials do. However, they have various functionalities such as high thermal/chemical stability and higher hardness than metallic materials have. Therefore, ceramic-coated metallic materials can function as good high-performance multifunctional materials. In general, coating technology can be divided into dry (vacuum) and wet (solution) processes [1]. The dry process is further categorized into two subtypes: physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD, a source material (target) is transported to a substrate forming film by a physical process, such as ablation and sputtering, without an accompanying chemical reaction. The composition may be readily conserved from the target to the film, where the thickness of the coating can range from nanometers to several millimeters. Because PVD would encompass the sequential processes of collision, adsorption, and deposition of particles and/or clusters, it is fundamentally
Handbook of Advanced Ceramics. http://dx.doi.org/10.1016/B978-0-12-385469-8.00045-9 Copyright Ó 2013 Elsevier Inc. All rights reserved.
5. HAp Coating for Dental Implants 6. Summary References
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a “line-of-sight” process. Thus, the backside or inside of holes can hardly be coated by PVD. CVD, in contrast, accompanies the chemical reactions of source gases and takes place where the gases can arrive. CVD is then not a line-of-sight process and has good adherence and conformal coverage. However, it is a thermally activated process; thus, it requires a high temperature. As a result, it is difficult to use CVD for coating on nonrefractory substrates such as low melting point metals and polymers, but it can be used to prepare thermally stable refractory ceramic films even on substrates with complex shapes [2]. CVD has many deposition parameters, such as temperature, total pressure, partial pressure of each source gas, and the geometry of the CVD chamber (how to heat the substrate, etc.). By controlling these parameters, various microstructures, that is, amorphous fine-grained polycrystals and epitaxial single crystals, can be prepared [3]. CVD has wide-ranging applications, typically in semiconductor thin film devices and for coating on cutting tools. The deposition rate of CVD is generally a few to several tens of micrometer per hour. Thus, it is mainly applied to the thin film coatings. However, thick film coatings also have many useful industrial applications, and CVD can be applicable for such coatings. This chapter discusses the
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high-speed CVD used in the preparation of thick film coatings.
2. CVD FOR HIGH-SPEED DEPOSITION The main deposition process in CVD is the diffusion of gases in a gas phase and their chemical reactions on a heated substrate. A schematic diagram of the CVD deposition process is depicted in Figure 1 [4]. The source gases diffuse from the main gas stream to the substrate through the gas boundary layer and are absorbed/reacted on the substrate. Then, the byproduct gases desorb from the substrate and diffuse out to the main gas stream through the gas boundary layer. These sequential steps are all thermally activated processes; therefore, CVD is often termed thermal CVD. Figure 2 depicts the general relationship between the logarithmic deposition rate and the reciprocal deposition temperature [5]. In a low-deposition-temperature region, the rate-controlling step could be a chemical reaction on the substrate, where the deposition rate exponentially increases with increasing deposition temperature. The activation energy in this temperature region is generally several tens to hundreds of kilojoules per mol. In a higher-depositiontemperature region, the deposition rate still increases, while the activation energy becomes rather small, generally less than tens of kilojoules per mol, because the rate-controlling step could be mass transport (diffusion) of gases (source gas or byproduct gas) through the gas boundary layer. Therefore, to increase the deposition rate of the CVD so that a thick film is obtained, as depicted in Figure 2 (broken line), the region in which the chemical reaction occurs should be expanded to a high-temperature region while keeping source gases easily accessible to the substrate.
For the highest deposition rate, CVD can be conducted at a high deposition temperature under the mass transport controlled region. The source gases should be appropriately thermally stable in the high-temperature region without becoming depressed via a homogeneous reaction in a gas phase. Halide gas is commonly used because of its moderate stability at a high temperature. This reaction is called halide CVD. The chemical/thermal stability of substrate materials is also crucial to high-temperature deposition. Graphite substrate is often used for preparing nonoxide films in a nonoxidizing atmosphere. Nonoxide thick films, typically Si3N4 and SiC, are prepared using halide precursors (such as SiCl4 and CH3SiCl3) and graphite substrates at high deposition rates of 1e2 mm/h at 1700e1800 K [6,7]. Metalorganic (MO) compound precursors can also be used in CVD, termed Metalorganic Chemical Vapor Deposition (MOCVD), but MO precursors are easily decomposed and product films (mainly oxides) are reactive with the substrate material at a high temperature. Thus, the deposition rate of MOCVD is generally lower than that of halide CVD. The precursor gases are heated before arriving at the substrate, and the deposition reaction occurs in various places inside the CVD chamberdpartially in a gas phase to form powder and partially on a CVD chamber wall to form premature deposits. The deposition rate of CVD is therefore significantly affected by the manner in which the precursor gases and the substrate are heated, that is, whether the substrate is indirectly heated by the chamber wall or is directly heated without heating the CVD chamber wall. CVD is also categorized by the type of chamber wall, that is, the cold wall type CVD and the hot wall type CVD. Figure 3 depicts the schematics of cold and hot wall CVD
FIGURE 1 A schematic diagram of the deposition process of CVD.
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FIGURE 2 Relationship between deposition rate and temperature.
chambers. In the cold wall type CVD, the substrate is directly heated by either radiofrequency or microwave induction heating and electric current joule heating. In these heating methods, the gas phase reaction (homogeneous reaction) can be minimal; only radiation from the substrate may cause the homogeneous reaction. The deposition rate and deposition efficiency are generally high, whereas the uniform temperature zone in the CVD chamber is rather narrow. In the hot wall type CVD, a wide area in the CVD chamber can be uniformly heated. Thus, a large number of substrate pieces or wafers can be simultaneously coated; however, the deposition rate is usually low, around several micrometers per hour. Lasers are versatile energy sources and have been applied to CVD, termed laser CVD. Laser CVD is a typical cold wall type CVD in which a laser directly irradiates the substrate (or only gas) but not the chamber wall [8]. Because a laser has high-energy photons, it can directly debond molecules photochemically, thereby causing deposition reactions. This process is called photolytic laser CVD. Because the deposition occurs without heating the substrate, CVD can be conducted at almost the room temperature. By using photolytic laser CVD, semiconductor device films have been prepared on n- or p-type Si single crystal substrates without affecting the substrate’s dopant distribution [8]. In contrast, a laser such as a CO2 laser or an Nd:YAG laser (infrared wavelength laser) has high thermal energy as that of a heat source. By focusing a laser beam on a specific area of a substrate, thermally activated chemical reactions and grain growth can occur in a localized area without a homogeneous reaction in a gas phase (lasers usually do not interact with gases), as
FIGURE 3 Schematics of cold (a) and hot wall CVD chambers (b).
depicted in Figure 4. In common thermal CVD, the deposition rate would be limited by diffusion in a gas phase in a high-temperature region, as shown in Figure 2. In pyrolytic laser CVD, the deposition zone (i.e. the laser beam size) is usually small; thus, the source gas can easily access the deposition zone. As a result, the deposition rate is not limited by diffusion and can be enormously high, more than several hundreds of meters per hour. Figure 5 demonstrates
FIGURE 4 Thermally activated chemical reactions that result from focusing a laser beam on the substrate surface.
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FIGURE 5 Deposition rate of pyrolytic laser CVD as a function of laser power.
the deposition rate of pyrolytic laser CVD as a function of laser power [9]. However, it is believed that pyrolytic laser CVD can be used to prepare only thin rods, whiskers, and nanotubes (i.e. one-dimensional deposits) at a high deposition rate. Therefore, wide-area films or plates cannot be prepared at a high deposition rate by pyrolytic laser CVD. We have developed a new laser CVD to prepare thick films at deposition rates 100e1000 times greater than that of conventional CVD on wide-area and complex-shaped substrates [10]. The laser is broadly irradiated to a substrate and only the substrate surface is heated, that is, the entire substrate body is not heated. This methodology enables thick ceramic film coatings to be prepared on metal substrates at a high deposition rate. In this chapter, we discuss refractory thick film coatings on metal substrates using laser CVD for applying a thermal barrier yttriastabilized zirconia (YSZ) coating on an Ni-based superalloy, an antiabrasive a-Al2O3 coating on cutting tools, and a hydroxyapatite (HAp) coating on a Ti implant.
3. YSZ THERMAL BARRIER COATING YSZ is widely adopted for thermal barrier coatings (TBCs) on gas turbine blades made of an Ni-based superalloy because of its high thermal stability, low thermal conductivity, and a relatively large thermal expansion, which is close to that of the metal substrate. An Ni-based superalloy can withstand a temperature of around 1400 K, while the gas turbine can endure a temperature of around 1800 K. The temperature difference (several hundred degrees Celsius) between the working temperature of the gas turbine and the refractory temperature of the metal substrate is partially attributable to TBC. Atmospheric plasma spray or electron beam physical vapor deposition (EBPVD) has been applied to TBC [11]; however, the working temperature of TBC should be raised, and the
Handbook of Advanced Ceramics
lifetime should be extended by improving the quality of the YSZ coating. Thus, a new and more advanced coating process should be developed. CVD is advantageous for preparing a YSZ coating with good conformal coverage and adherence and is also a promising technique for TBC [12]. However, the precursor and oxygen source gases are generally reactive in gas phase to form powder (homogeneous reaction) at a high gas concentration, and the deposition rate of oxide films is generally too low (usually several tens of micrometers per hour) to prepare a TBC coating with a thickness of around several hundreds of micrometers. Laser CVD can be used for preparing YSZ thick film coatings at high deposition rates without causing the homogeneous reaction in the gas phase because the laser would not generally interact with the source gases. Figure 6 depicts the cross-section of the YSZ coating prepared by laser CVD [13]. A typical columnar texture, similar to that of EBPVD, can be prepared. Although the thermal expansion coefficient of YSZ is close to that of an Ni-based superalloy, a small difference still exists between the two, which might have caused thermal stress and delamination at the interface. The columnar texture is vertically aligned. Furthermore, the crack formation resulting from severe thermal stress would extend vertically as well. This tendency would prevent catastrophic delamination of the entire coating. Figure 6 (b) shows a magnified view of the columnar grains. Many voids exist at the columnar boundary. Similar voids are also observed in the YSZ coatings by EBPVD, and they are known as being effective in the relaxation of thermal stress at the interface. The CVD of YSZ films has been widely investigated because YSZ is used for oxide ion conductor films and as a buffer layer between a metal tape and a YeBaeCueO superconductor film. Figure 7 demonstrates the deposition rates of YSZ films as a function of deposition temperature by conventional CVD [14e18]. Attempts have been made for preparing a YSZ coating using CVD so that the coating can be applied as TBC. A deposition rate of 50e100 mm/h has been attained by employing a cold wall type CVD. However, the conventional CVD still does not have a suitable deposition rate for practical application. In contrast, the deposition rate of YSZ film by laser CVD is several to hundred times greater than that of conventional CVD. The chemical reaction around the substrate can be highly activated by laser irradiation. Therefore, the rate-controlling step of the deposition process is not a chemical reaction but rather a diffusion reaction (mass transfer) in a gas phase with low activation energy. Figure 8 demonstrates the nanostructure of the columnar grains of the YSZ coating by laser CVD [5]. A large number of nanopores of about a few nanometers in diameter can be observed in the grains. Figure 9 depicts the relationship between the deposition rate and the thermal
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FIGURE 6 Cross-section of the YSZ coating prepared by laser CVD (b) shows the magnified view of the columnar grains of (a).
FIGURE 9 Relationship between deposition rate and thermal conductivity of YSZ coating by laser CVD. FIGURE 7 Deposition rates of YSZ films as a function of deposition temperature by conventional CVD.
conductivity of the YSZ coating by laser CVD [12]. The number of nanopores (i.e. density) is almost proportional to the deposition rate; in contrast, thermal conductivity is inversely proportional to the deposition rate. The thermal conductivity of the YSZ coating was one-third to onefourth that of YSZ bulk ceramics. Thus, this coating is a promising candidate for TBC.
4. a-Al2O3 COATING FOR CUTTING TOOLS
FIGURE 8 Nanostructure of the columnar grains of the YSZ coating by laser CVD.
WCeCo cemented carbide has been widely used as a cutting tool because of its high hardness and ductility. It is commonly coated with a-Al2O3 thick film to protect the substrate from thermal and mechanical degradation. CVD is most commonly used for the a-Al2O3 coating [19]. Al2O3 has many poly types, mainly a, g, k, q, d; a-Al2O3 is the
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FIGURE 10 Deposition rates of Al2O3 coating as a function of deposition temperature by conventional and laser CVD.
most stable and hardest among them. The volume change by phase transformation at a high temperature during service will cause delamination of the Al2O3 coating on WCeCo. Therefore, the top coat on WCeCo should comprise the most stable a-Al2O3 coating [20]. Because a-Al2O3 is a high-temperature form of Al2O3, the deposition temperature for preparing its coating should be generally high, that is, >1200e1300 K. Such a high deposition temperature will often cause degradation of the WCeCo substrate; thus, the deposition temperature should be lowered. W is also a rare resource, and the substitution of WCeCo into TiNeNi cermet has been investigated. The deposition temperature should be further lowered to
prepare the a-Al2O3 coating on TiNeNi cermet because the outward diffusion of Ni and many interface reactions become significant at high temperatures. Therefore, a lowtemperature and high-speed a-Al2O3 coating process should be developed. We have applied laser CVD to prepare the a-Al2O3 coating for enabling low-temperature and high-speed deposition [21e23]. Figure 10 depicts the deposition rates of the Al2O3 coating as a function of deposition temperature by both conventional CVD and laser CVD [24e28]. The a-Al2O3 coating has been commercially prepared by halide CVD using mainly AlCl3 as the Al source and CO2 gas as an oxidant. This coating has commonly been prepared by halide CVD at >1300 K. A strong temperature dependence of deposition rate whose activation energy is 90e170 kJ/mol is characteristic of halide CVD, as depicted in Figure 10. The MOCVD has also been applied to prepare the Al2O3 coating, and amorphous and the g-Al2O3 coating has often been prepared. The deposition rates by MOCVD are higher than those by halide CVD at low temperatures <1000 K. The a-Al2O3 coating can be prepared by laser CVD even at a low-deposition temperatures of around 1000 K at a significantly high deposition rate of several hundreds of micrometers per hour. This value is almost several hundred times greater than that of conventional CVD. Figure 11 presents the relationship among deposition temperature, Al source rate, and microstructure [21]. The microstructures are depicted in Figure 12(a)e(c). The orientation can change from (300) to (006), primarily because of an increase in the Al source rate. In practical applications, (006)-oriented a-Al2O3 is preferred because of its high hardness and smooth surface. Figure 13
FIGURE 11 Relationship among deposition temperature, Al source rate, and microstructure a-Al2O3 prepared by laser CVD.
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FIGURE 12 Microstructure of a-Al2O3 prepared by laser CVD at Tdep ¼ 1100 K, PL ¼ 88 W (a), Tdep ¼ 1190 K, PL ¼ 172 W (b), Tdep ¼ 1,60 K, PL ¼ 172 W (c), marked in Figure 11.
demonstrates the cross-section of a-Al2O3 prepared by laser CVD. Nanopores are contained in the a-Al2O3 film as well as the YSZ coating, which is as effective as a thermally protective coating on cutting tools.
5. HAp COATING FOR DENTAL IMPLANTS
FIGURE 13 CVD.
Cross-section of (104)-oriented a-Al2O3 prepared by laser
Ti-based alloys have been widely used for dental implants and artificial bone because of their high strength, relatively low Young’s modulus (which is close to that of the human bone), and their tendency to cause no allergy in living tissues. However, the regeneration of a bone on the Ti-based alloy takes a long time, usually several months in the human body. To accelerate this regeneration of bone (osteoconductivity), the Ti-based alloy surface should be coated with a bioceramic film [1]. A typical bioceramic coating consists of bioabsorbable tricalcium phosphate (TCP) and a bioactive HAp film. Plasma spray, solegel, and sputtering are common to prepare bioceramic coatings [29]; however, highly oriented and well-adhered coatings have not been prepared by these techniques. Laser CVD can be used for preparing various calcium phosphate compounds with a well-controlled morphology and orientation. Figure 14 demonstrates the effects of deposition temperature and Ca to P molar ratio in source
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FIGURE 14
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Effects of deposition temperature and Ca to P molar ratio on the crystal phase of calcium phosphate at PL ¼ 30 W (a) PL ¼ 150 W (b).
FIGURE 15
(002)-Oriented HAp (a) and (400)-oriented b-TCP films (b) prepared by laser CVD.
FIGURE 16 Regenerated HAp on HAp-coated Ti in Hanks’ solution after 259.2 (a) and 604.8 ks (b).
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FIGURE 17 HAp regeneration rate on various bioceramic coatings in SBF.
gases on the crystal phase of calcium phosphate at laser powers of 30 and 150 W [30]. The crystal phase and microstructure are dependent on laser power, deposition temperature, and Ca to P molar ratio. At a laser power of 30 W, HAp, b-TCP, and tetra calcium phosphate (TTCP, Ca4P2O9) are formed depending on conditions. CaO codeposits at a low-deposition temperature. In a single phase, HAp can be prepared at an intermediate Ca to P molar ratio. At a laser power of 150 W, a-TCP and HAp are obtained in a single phase. Calcium pyroclore (Ca2P2O7) and TTCP are also prepared together with a-TCP and HAp. Laser CVD is advantageous in preparing highly oriented coatings, and it is known that the orientation of the coating significantly affects the regeneration of HAp in a simulated body fluid (SBF) such as Hanks’ solution. Figure 15 presents the (002)-oriented HAp and the (400)-oriented b-TCP films prepared by laser CVD. Cauliflower-like and pyramid-like microstructures are commonly observed at a low and high deposition temperatures, respectively. Figure 16(a) and (b) demonstrate the regeneration of HAp on HAp-coated Ti in Hanks’ solution after 259.2 and 604.8 ks, respectively. The HAp regeneration rate on HApcoated Ti is higher than that on TCP-coated Ti. HAp demonstrates a hexagonal structure and has anisotropic
crystal growth. The regeneration rate also significantly depends on the orientation of the HAp coating. Figure 17 summarizes the HAp regeneration rate on various bioceramic coatings in an SBF. The (002)-oriented HAp coating prepared by laser CVD has the highest regeneration rate among the other HAp coatings [31].
6. SUMMARY Thin film coating processes by PVD and CVD have been indispensable technologies in many engineering fields. In contrast, thick film processing has not been well developed despite various useful applications, such as thermal barrier, anticorrosion, and antiabrasive coatings. Laser CVD has been traditionally employed to prepare small-scale deposits such as thin film or dots, whiskers, and nanotubes. Further, this technique cannot be applied to prepare thick and widearea coating films. However, by using a high-power laser and maintaining appropriate conditions, the thick film coating even on a complex-shaped substrate can be prepared with a significant orientation. Laser CVD is also applicable in preparing oxide and nonoxide thick coatings [32,33]. In addition, it is a promising technique for a wide range of engineering applications.
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REFERENCES [1] Goto T, Narushina T, Ueda K. Bio-ceramic coating on titanium by physical and chemical vapor deposition. In: Zhang S, editor. Biological and biomedical coatings handbook-processing and characterization. CRC Press; 2011. p. 295e328. [2] Bryant WA. Reviewdthe fundamentals of chemical vapour deposition. J Mater Sci 1977;12:1285e306. [3] Pattanaik AK, Sarin VK, Park J-H. Basic principles of CVD thermodynamics and kinetics. In: Chemical vapor deposition, surface engineering series, vol. 2. ASM Intern.; 2001. p. 23e43. [4] Goto T, “Chemical methods” In: Inorganic compound, 5th series experimental chemistry, ed. by Chem Soc Jpn, 2005. p. 4e7. [5] Goto T. Heat resisting coating by laser CVD. J Surf Finish Soc Jpn 2009;60:709e15. [6] Hirai T, Goto T, Kaji T. Preparation of silicon carbide by chemical vapor deposition. J Ceram Soc Jpn 1983;91:502e9. [7] Niihara K, Hirai T. Chemically vapour-deposited silicon nitride, part 1. J Mater Sci 1976;11:593e603. [8] Duty C, Jean D, Lackey WJ. Laser chemical vapour deposition: materials, modelling, and process control. Inter Mater Rev 2001;46:271e87. [9] Goto T, Kimura T. High-speed oxide coating by laser chemical vapor deposition and their nano-structure. Thin Solid Films 2006;515:46e52. [10] Garcia JRV, Goto T. Thermal barrier coatings produced by chemical vapor deposition. Sci Technol Adv Mater 2003;4:397e402. [11] Clarke DR, Levi CG. Materials design for the next generation thermal barrier coatings. Annu Rev Mater Res 2003;33:383e417. [12] Goto T. Thermal barrier coating by chemical vapor deposition. In: Singh M, Ohji T, Asthana R, Mathur S, editors. Ceramic integration and joining technology: from macro to nanoscale. Wiley; 2011. p. 393e413. [13] Goto T. Thermal barrier coatings deposited by laser CVD. Surf Coat Tech 2005;198:367e71. [14] Tu R, Kimura T, Goto T. Rapid synthesis of yttria-partially-stabilized zirconia films by metal-organic chemical vapor deposition. Mater Trans 2002;43:2354e6. [15] Akiyama Y, Sato T, Imaishi N. Reaction analysis for ZrO2 and Y2O3 thin film growth by low-pressure metalorganic chemical vapor deposition using b-diketonate complexes. J Crys Growth 1995;147:130e46. [16] Wahl G, Nemetz W, Giannozzi M, Rushworth S, Baxter D, Archer N, et al. Chemical vapor deposition of TBC: an alternative process for gas turbine components. Trans ASME 2001;123:520e4. [17] Bourhila N, Felten F, Senateur JP, Schuster F, Madar R, Abrutis T, et al. “Deposition and characterization of ZrO2 and yttria-stabilized ZrO2 films using injection-LPCVD”. In: Allendorf MD, Bernard C,
[18]
[19]
[20] [21]
[22]
[23]
[24]
[25] [26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
editors. Proc. 14th Intern. Conf. and EUROCVD-11, Electrochem Soc Proc, vols. 97e25, 1997. p. 417e24. Pulver M, Nemetz W, Wahl G. CVD of ZrO2, Al2O3 and Y2O3 from metalorganic compounds in different reactors. Surf Coatings Tech 2000;125:400e6. Ruppi S. Deposition, microstructure and properties of texturecontrolled CVD a-Al2O3 coatings. Refrac Mat Hard Mater 2005;23:306e16. Vourinen S, Karlsson L. Thin Solid Films 1992;214:132. Kadokura H, Ito A, Kimura T, Goto T, Tu R. Moderate temperature and high-speed synthesis of a-Al2O3 films by laser chemical vapor deposition using Nd:YAG laser. Surf Coat Tech 2010;204: 2302e6. You Y, Itoh A, Tu R, Goto T. Low-temperature deposition of a-Al2O3 films by laser chemical vapor deposition using a diode laser. Appl Surf Sci 2010;256:3906e11. You Y, Itoh A, Tu R, Goto T. Orientation control of a-Al2O3 films prepared by laser chemical vapor deposition using a diode laser. J Ceram Soc Jpn 2010;118:366e9. Park C, Kim J, Chun JS. The effects of reaction parameters on the deposition characteristics in Al2O3 CVD. J Vac Sci Technol A 1983;1:1820e4. Ruppi S, Larsson A. Chemical vapour deposition of k-Al2O3. Thin Solid Films 2001;388:50e61. Maruyama T, Arai S. Aluminum oxide thin films prepared by chemical vapor deposition from aluminum acetylacetonate. Appl Phys Lett 1992;60:322e3. Devi A, Shivashankar S, Samuelson A. MOCVD of aluminium oxide films using aluminium b-diketonates as precursors. J Phys IV 2002;12:139e46. Pflitsch C, Viefhaus D, Bergmann U, Atakan B. Organometallic vapour deposition of crystalline aluminium oxide films on stainless steel substrates. Thin Solid Films 2007;515:3653e60. Ueda K, Narushima T, Goto T, Katsube T, Nakagawa H, Kawamura H, et al. Evaluation of calcium phosphate coating films on titanium fabricated using RF magnetron sputtering. Mater Trans 2007;48:307. Sato M, Tu R, Goto T, Ueda K, Narushima T. Precipitation behavior in a Hanks’ solution on CaePeO films prepared by laser CVD. Mater Trans 2009;50:2455e9. Sato M, Tu R, Goto T, Ueda K, Narushima T. Apatite formation behavior on bio-ceramic films prepared by MOCVD. J Ceram Soc Jpn 2008;117:U461e5. Gong Y, Tu R, Goto T. Laser chemical vapor deposition of titanium nitride films with tetrakis (diethylamido) titanium and ammonia system. Surf Coat Tech 2010;204:2111e7. Fujie K, Ito A, Tu R, Goto T. Laser chemical vapor deposition of SiC films with CO2 laser. J Alloys Comp 2010;502:238e42.
A New Thick Film Coating Technology-Laser Chemical Vapor Deposition Takashi Goto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980-8577, Japan
Chapter Outline 1. 2. 3. 4.
Introduction CVD for High-Speed Deposition YSZ Thermal Barrier Coating a-Al2O3 Coating for Cutting Tools
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1. INTRODUCTION Nowadays, highly functional materials are used under severe conditions such as high-temperature, high-stress, and corrosive environments. Metallic materials have often been used as structural materials because of their superior properties such as high strength and ductility. Ceramic materials, in contrast, have inferior mechanical properties than metallic materials do. However, they have various functionalities such as high thermal/chemical stability and higher hardness than metallic materials have. Therefore, ceramic-coated metallic materials can function as good high-performance multifunctional materials. In general, coating technology can be divided into dry (vacuum) and wet (solution) processes [1]. The dry process is further categorized into two subtypes: physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD, a source material (target) is transported to a substrate forming film by a physical process, such as ablation and sputtering, without an accompanying chemical reaction. The composition may be readily conserved from the target to the film, where the thickness of the coating can range from nanometers to several millimeters. Because PVD would encompass the sequential processes of collision, adsorption, and deposition of particles and/or clusters, it is fundamentally
Handbook of Advanced Ceramics. http://dx.doi.org/10.1016/B978-0-12-385469-8.00045-9 Copyright Ó 2013 Elsevier Inc. All rights reserved.
5. HAp Coating for Dental Implants 6. Summary References
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a “line-of-sight” process. Thus, the backside or inside of holes can hardly be coated by PVD. CVD, in contrast, accompanies the chemical reactions of source gases and takes place where the gases can arrive. CVD is then not a line-of-sight process and has good adherence and conformal coverage. However, it is a thermally activated process; thus, it requires a high temperature. As a result, it is difficult to use CVD for coating on nonrefractory substrates such as low melting point metals and polymers, but it can be used to prepare thermally stable refractory ceramic films even on substrates with complex shapes [2]. CVD has many deposition parameters, such as temperature, total pressure, partial pressure of each source gas, and the geometry of the CVD chamber (how to heat the substrate, etc.). By controlling these parameters, various microstructures, that is, amorphous fine-grained polycrystals and epitaxial single crystals, can be prepared [3]. CVD has wide-ranging applications, typically in semiconductor thin film devices and for coating on cutting tools. The deposition rate of CVD is generally a few to several tens of micrometer per hour. Thus, it is mainly applied to the thin film coatings. However, thick film coatings also have many useful industrial applications, and CVD can be applicable for such coatings. This chapter discusses the
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high-speed CVD used in the preparation of thick film coatings.
2. CVD FOR HIGH-SPEED DEPOSITION The main deposition process in CVD is the diffusion of gases in a gas phase and their chemical reactions on a heated substrate. A schematic diagram of the CVD deposition process is depicted in Figure 1 [4]. The source gases diffuse from the main gas stream to the substrate through the gas boundary layer and are absorbed/reacted on the substrate. Then, the byproduct gases desorb from the substrate and diffuse out to the main gas stream through the gas boundary layer. These sequential steps are all thermally activated processes; therefore, CVD is often termed thermal CVD. Figure 2 depicts the general relationship between the logarithmic deposition rate and the reciprocal deposition temperature [5]. In a low-deposition-temperature region, the rate-controlling step could be a chemical reaction on the substrate, where the deposition rate exponentially increases with increasing deposition temperature. The activation energy in this temperature region is generally several tens to hundreds of kilojoules per mol. In a higher-depositiontemperature region, the deposition rate still increases, while the activation energy becomes rather small, generally less than tens of kilojoules per mol, because the rate-controlling step could be mass transport (diffusion) of gases (source gas or byproduct gas) through the gas boundary layer. Therefore, to increase the deposition rate of the CVD so that a thick film is obtained, as depicted in Figure 2 (broken line), the region in which the chemical reaction occurs should be expanded to a high-temperature region while keeping source gases easily accessible to the substrate.
For the highest deposition rate, CVD can be conducted at a high deposition temperature under the mass transport controlled region. The source gases should be appropriately thermally stable in the high-temperature region without becoming depressed via a homogeneous reaction in a gas phase. Halide gas is commonly used because of its moderate stability at a high temperature. This reaction is called halide CVD. The chemical/thermal stability of substrate materials is also crucial to high-temperature deposition. Graphite substrate is often used for preparing nonoxide films in a nonoxidizing atmosphere. Nonoxide thick films, typically Si3N4 and SiC, are prepared using halide precursors (such as SiCl4 and CH3SiCl3) and graphite substrates at high deposition rates of 1e2 mm/h at 1700e1800 K [6,7]. Metalorganic (MO) compound precursors can also be used in CVD, termed Metalorganic Chemical Vapor Deposition (MOCVD), but MO precursors are easily decomposed and product films (mainly oxides) are reactive with the substrate material at a high temperature. Thus, the deposition rate of MOCVD is generally lower than that of halide CVD. The precursor gases are heated before arriving at the substrate, and the deposition reaction occurs in various places inside the CVD chamberdpartially in a gas phase to form powder and partially on a CVD chamber wall to form premature deposits. The deposition rate of CVD is therefore significantly affected by the manner in which the precursor gases and the substrate are heated, that is, whether the substrate is indirectly heated by the chamber wall or is directly heated without heating the CVD chamber wall. CVD is also categorized by the type of chamber wall, that is, the cold wall type CVD and the hot wall type CVD. Figure 3 depicts the schematics of cold and hot wall CVD
FIGURE 1 A schematic diagram of the deposition process of CVD.
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FIGURE 2 Relationship between deposition rate and temperature.
chambers. In the cold wall type CVD, the substrate is directly heated by either radiofrequency or microwave induction heating and electric current joule heating. In these heating methods, the gas phase reaction (homogeneous reaction) can be minimal; only radiation from the substrate may cause the homogeneous reaction. The deposition rate and deposition efficiency are generally high, whereas the uniform temperature zone in the CVD chamber is rather narrow. In the hot wall type CVD, a wide area in the CVD chamber can be uniformly heated. Thus, a large number of substrate pieces or wafers can be simultaneously coated; however, the deposition rate is usually low, around several micrometers per hour. Lasers are versatile energy sources and have been applied to CVD, termed laser CVD. Laser CVD is a typical cold wall type CVD in which a laser directly irradiates the substrate (or only gas) but not the chamber wall [8]. Because a laser has high-energy photons, it can directly debond molecules photochemically, thereby causing deposition reactions. This process is called photolytic laser CVD. Because the deposition occurs without heating the substrate, CVD can be conducted at almost the room temperature. By using photolytic laser CVD, semiconductor device films have been prepared on n- or p-type Si single crystal substrates without affecting the substrate’s dopant distribution [8]. In contrast, a laser such as a CO2 laser or an Nd:YAG laser (infrared wavelength laser) has high thermal energy as that of a heat source. By focusing a laser beam on a specific area of a substrate, thermally activated chemical reactions and grain growth can occur in a localized area without a homogeneous reaction in a gas phase (lasers usually do not interact with gases), as
FIGURE 3 Schematics of cold (a) and hot wall CVD chambers (b).
depicted in Figure 4. In common thermal CVD, the deposition rate would be limited by diffusion in a gas phase in a high-temperature region, as shown in Figure 2. In pyrolytic laser CVD, the deposition zone (i.e. the laser beam size) is usually small; thus, the source gas can easily access the deposition zone. As a result, the deposition rate is not limited by diffusion and can be enormously high, more than several hundreds of meters per hour. Figure 5 demonstrates
FIGURE 4 Thermally activated chemical reactions that result from focusing a laser beam on the substrate surface.
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FIGURE 5 Deposition rate of pyrolytic laser CVD as a function of laser power.
the deposition rate of pyrolytic laser CVD as a function of laser power [9]. However, it is believed that pyrolytic laser CVD can be used to prepare only thin rods, whiskers, and nanotubes (i.e. one-dimensional deposits) at a high deposition rate. Therefore, wide-area films or plates cannot be prepared at a high deposition rate by pyrolytic laser CVD. We have developed a new laser CVD to prepare thick films at deposition rates 100e1000 times greater than that of conventional CVD on wide-area and complex-shaped substrates [10]. The laser is broadly irradiated to a substrate and only the substrate surface is heated, that is, the entire substrate body is not heated. This methodology enables thick ceramic film coatings to be prepared on metal substrates at a high deposition rate. In this chapter, we discuss refractory thick film coatings on metal substrates using laser CVD for applying a thermal barrier yttriastabilized zirconia (YSZ) coating on an Ni-based superalloy, an antiabrasive a-Al2O3 coating on cutting tools, and a hydroxyapatite (HAp) coating on a Ti implant.
3. YSZ THERMAL BARRIER COATING YSZ is widely adopted for thermal barrier coatings (TBCs) on gas turbine blades made of an Ni-based superalloy because of its high thermal stability, low thermal conductivity, and a relatively large thermal expansion, which is close to that of the metal substrate. An Ni-based superalloy can withstand a temperature of around 1400 K, while the gas turbine can endure a temperature of around 1800 K. The temperature difference (several hundred degrees Celsius) between the working temperature of the gas turbine and the refractory temperature of the metal substrate is partially attributable to TBC. Atmospheric plasma spray or electron beam physical vapor deposition (EBPVD) has been applied to TBC [11]; however, the working temperature of TBC should be raised, and the
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lifetime should be extended by improving the quality of the YSZ coating. Thus, a new and more advanced coating process should be developed. CVD is advantageous for preparing a YSZ coating with good conformal coverage and adherence and is also a promising technique for TBC [12]. However, the precursor and oxygen source gases are generally reactive in gas phase to form powder (homogeneous reaction) at a high gas concentration, and the deposition rate of oxide films is generally too low (usually several tens of micrometers per hour) to prepare a TBC coating with a thickness of around several hundreds of micrometers. Laser CVD can be used for preparing YSZ thick film coatings at high deposition rates without causing the homogeneous reaction in the gas phase because the laser would not generally interact with the source gases. Figure 6 depicts the cross-section of the YSZ coating prepared by laser CVD [13]. A typical columnar texture, similar to that of EBPVD, can be prepared. Although the thermal expansion coefficient of YSZ is close to that of an Ni-based superalloy, a small difference still exists between the two, which might have caused thermal stress and delamination at the interface. The columnar texture is vertically aligned. Furthermore, the crack formation resulting from severe thermal stress would extend vertically as well. This tendency would prevent catastrophic delamination of the entire coating. Figure 6 (b) shows a magnified view of the columnar grains. Many voids exist at the columnar boundary. Similar voids are also observed in the YSZ coatings by EBPVD, and they are known as being effective in the relaxation of thermal stress at the interface. The CVD of YSZ films has been widely investigated because YSZ is used for oxide ion conductor films and as a buffer layer between a metal tape and a YeBaeCueO superconductor film. Figure 7 demonstrates the deposition rates of YSZ films as a function of deposition temperature by conventional CVD [14e18]. Attempts have been made for preparing a YSZ coating using CVD so that the coating can be applied as TBC. A deposition rate of 50e100 mm/h has been attained by employing a cold wall type CVD. However, the conventional CVD still does not have a suitable deposition rate for practical application. In contrast, the deposition rate of YSZ film by laser CVD is several to hundred times greater than that of conventional CVD. The chemical reaction around the substrate can be highly activated by laser irradiation. Therefore, the rate-controlling step of the deposition process is not a chemical reaction but rather a diffusion reaction (mass transfer) in a gas phase with low activation energy. Figure 8 demonstrates the nanostructure of the columnar grains of the YSZ coating by laser CVD [5]. A large number of nanopores of about a few nanometers in diameter can be observed in the grains. Figure 9 depicts the relationship between the deposition rate and the thermal
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FIGURE 6 Cross-section of the YSZ coating prepared by laser CVD (b) shows the magnified view of the columnar grains of (a).
FIGURE 9 Relationship between deposition rate and thermal conductivity of YSZ coating by laser CVD. FIGURE 7 Deposition rates of YSZ films as a function of deposition temperature by conventional CVD.
conductivity of the YSZ coating by laser CVD [12]. The number of nanopores (i.e. density) is almost proportional to the deposition rate; in contrast, thermal conductivity is inversely proportional to the deposition rate. The thermal conductivity of the YSZ coating was one-third to onefourth that of YSZ bulk ceramics. Thus, this coating is a promising candidate for TBC.
4. a-Al2O3 COATING FOR CUTTING TOOLS
FIGURE 8 Nanostructure of the columnar grains of the YSZ coating by laser CVD.
WCeCo cemented carbide has been widely used as a cutting tool because of its high hardness and ductility. It is commonly coated with a-Al2O3 thick film to protect the substrate from thermal and mechanical degradation. CVD is most commonly used for the a-Al2O3 coating [19]. Al2O3 has many poly types, mainly a, g, k, q, d; a-Al2O3 is the
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FIGURE 10 Deposition rates of Al2O3 coating as a function of deposition temperature by conventional and laser CVD.
most stable and hardest among them. The volume change by phase transformation at a high temperature during service will cause delamination of the Al2O3 coating on WCeCo. Therefore, the top coat on WCeCo should comprise the most stable a-Al2O3 coating [20]. Because a-Al2O3 is a high-temperature form of Al2O3, the deposition temperature for preparing its coating should be generally high, that is, >1200e1300 K. Such a high deposition temperature will often cause degradation of the WCeCo substrate; thus, the deposition temperature should be lowered. W is also a rare resource, and the substitution of WCeCo into TiNeNi cermet has been investigated. The deposition temperature should be further lowered to
prepare the a-Al2O3 coating on TiNeNi cermet because the outward diffusion of Ni and many interface reactions become significant at high temperatures. Therefore, a lowtemperature and high-speed a-Al2O3 coating process should be developed. We have applied laser CVD to prepare the a-Al2O3 coating for enabling low-temperature and high-speed deposition [21e23]. Figure 10 depicts the deposition rates of the Al2O3 coating as a function of deposition temperature by both conventional CVD and laser CVD [24e28]. The a-Al2O3 coating has been commercially prepared by halide CVD using mainly AlCl3 as the Al source and CO2 gas as an oxidant. This coating has commonly been prepared by halide CVD at >1300 K. A strong temperature dependence of deposition rate whose activation energy is 90e170 kJ/mol is characteristic of halide CVD, as depicted in Figure 10. The MOCVD has also been applied to prepare the Al2O3 coating, and amorphous and the g-Al2O3 coating has often been prepared. The deposition rates by MOCVD are higher than those by halide CVD at low temperatures <1000 K. The a-Al2O3 coating can be prepared by laser CVD even at a low-deposition temperatures of around 1000 K at a significantly high deposition rate of several hundreds of micrometers per hour. This value is almost several hundred times greater than that of conventional CVD. Figure 11 presents the relationship among deposition temperature, Al source rate, and microstructure [21]. The microstructures are depicted in Figure 12(a)e(c). The orientation can change from (300) to (006), primarily because of an increase in the Al source rate. In practical applications, (006)-oriented a-Al2O3 is preferred because of its high hardness and smooth surface. Figure 13
FIGURE 11 Relationship among deposition temperature, Al source rate, and microstructure a-Al2O3 prepared by laser CVD.
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FIGURE 12 Microstructure of a-Al2O3 prepared by laser CVD at Tdep ¼ 1100 K, PL ¼ 88 W (a), Tdep ¼ 1190 K, PL ¼ 172 W (b), Tdep ¼ 1,60 K, PL ¼ 172 W (c), marked in Figure 11.
demonstrates the cross-section of a-Al2O3 prepared by laser CVD. Nanopores are contained in the a-Al2O3 film as well as the YSZ coating, which is as effective as a thermally protective coating on cutting tools.
5. HAp COATING FOR DENTAL IMPLANTS
FIGURE 13 CVD.
Cross-section of (104)-oriented a-Al2O3 prepared by laser
Ti-based alloys have been widely used for dental implants and artificial bone because of their high strength, relatively low Young’s modulus (which is close to that of the human bone), and their tendency to cause no allergy in living tissues. However, the regeneration of a bone on the Ti-based alloy takes a long time, usually several months in the human body. To accelerate this regeneration of bone (osteoconductivity), the Ti-based alloy surface should be coated with a bioceramic film [1]. A typical bioceramic coating consists of bioabsorbable tricalcium phosphate (TCP) and a bioactive HAp film. Plasma spray, solegel, and sputtering are common to prepare bioceramic coatings [29]; however, highly oriented and well-adhered coatings have not been prepared by these techniques. Laser CVD can be used for preparing various calcium phosphate compounds with a well-controlled morphology and orientation. Figure 14 demonstrates the effects of deposition temperature and Ca to P molar ratio in source
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FIGURE 14
Handbook of Advanced Ceramics
Effects of deposition temperature and Ca to P molar ratio on the crystal phase of calcium phosphate at PL ¼ 30 W (a) PL ¼ 150 W (b).
FIGURE 15
(002)-Oriented HAp (a) and (400)-oriented b-TCP films (b) prepared by laser CVD.
FIGURE 16 Regenerated HAp on HAp-coated Ti in Hanks’ solution after 259.2 (a) and 604.8 ks (b).
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FIGURE 17 HAp regeneration rate on various bioceramic coatings in SBF.
gases on the crystal phase of calcium phosphate at laser powers of 30 and 150 W [30]. The crystal phase and microstructure are dependent on laser power, deposition temperature, and Ca to P molar ratio. At a laser power of 30 W, HAp, b-TCP, and tetra calcium phosphate (TTCP, Ca4P2O9) are formed depending on conditions. CaO codeposits at a low-deposition temperature. In a single phase, HAp can be prepared at an intermediate Ca to P molar ratio. At a laser power of 150 W, a-TCP and HAp are obtained in a single phase. Calcium pyroclore (Ca2P2O7) and TTCP are also prepared together with a-TCP and HAp. Laser CVD is advantageous in preparing highly oriented coatings, and it is known that the orientation of the coating significantly affects the regeneration of HAp in a simulated body fluid (SBF) such as Hanks’ solution. Figure 15 presents the (002)-oriented HAp and the (400)-oriented b-TCP films prepared by laser CVD. Cauliflower-like and pyramid-like microstructures are commonly observed at a low and high deposition temperatures, respectively. Figure 16(a) and (b) demonstrate the regeneration of HAp on HAp-coated Ti in Hanks’ solution after 259.2 and 604.8 ks, respectively. The HAp regeneration rate on HApcoated Ti is higher than that on TCP-coated Ti. HAp demonstrates a hexagonal structure and has anisotropic
crystal growth. The regeneration rate also significantly depends on the orientation of the HAp coating. Figure 17 summarizes the HAp regeneration rate on various bioceramic coatings in an SBF. The (002)-oriented HAp coating prepared by laser CVD has the highest regeneration rate among the other HAp coatings [31].
6. SUMMARY Thin film coating processes by PVD and CVD have been indispensable technologies in many engineering fields. In contrast, thick film processing has not been well developed despite various useful applications, such as thermal barrier, anticorrosion, and antiabrasive coatings. Laser CVD has been traditionally employed to prepare small-scale deposits such as thin film or dots, whiskers, and nanotubes. Further, this technique cannot be applied to prepare thick and widearea coating films. However, by using a high-power laser and maintaining appropriate conditions, the thick film coating even on a complex-shaped substrate can be prepared with a significant orientation. Laser CVD is also applicable in preparing oxide and nonoxide thick coatings [32,33]. In addition, it is a promising technique for a wide range of engineering applications.
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REFERENCES [1] Goto T, Narushina T, Ueda K. Bio-ceramic coating on titanium by physical and chemical vapor deposition. In: Zhang S, editor. Biological and biomedical coatings handbook-processing and characterization. CRC Press; 2011. p. 295e328. [2] Bryant WA. Reviewdthe fundamentals of chemical vapour deposition. J Mater Sci 1977;12:1285e306. [3] Pattanaik AK, Sarin VK, Park J-H. Basic principles of CVD thermodynamics and kinetics. In: Chemical vapor deposition, surface engineering series, vol. 2. ASM Intern.; 2001. p. 23e43. [4] Goto T, “Chemical methods” In: Inorganic compound, 5th series experimental chemistry, ed. by Chem Soc Jpn, 2005. p. 4e7. [5] Goto T. Heat resisting coating by laser CVD. J Surf Finish Soc Jpn 2009;60:709e15. [6] Hirai T, Goto T, Kaji T. Preparation of silicon carbide by chemical vapor deposition. J Ceram Soc Jpn 1983;91:502e9. [7] Niihara K, Hirai T. Chemically vapour-deposited silicon nitride, part 1. J Mater Sci 1976;11:593e603. [8] Duty C, Jean D, Lackey WJ. Laser chemical vapour deposition: materials, modelling, and process control. Inter Mater Rev 2001;46:271e87. [9] Goto T, Kimura T. High-speed oxide coating by laser chemical vapor deposition and their nano-structure. Thin Solid Films 2006;515:46e52. [10] Garcia JRV, Goto T. Thermal barrier coatings produced by chemical vapor deposition. Sci Technol Adv Mater 2003;4:397e402. [11] Clarke DR, Levi CG. Materials design for the next generation thermal barrier coatings. Annu Rev Mater Res 2003;33:383e417. [12] Goto T. Thermal barrier coating by chemical vapor deposition. In: Singh M, Ohji T, Asthana R, Mathur S, editors. Ceramic integration and joining technology: from macro to nanoscale. Wiley; 2011. p. 393e413. [13] Goto T. Thermal barrier coatings deposited by laser CVD. Surf Coat Tech 2005;198:367e71. [14] Tu R, Kimura T, Goto T. Rapid synthesis of yttria-partially-stabilized zirconia films by metal-organic chemical vapor deposition. Mater Trans 2002;43:2354e6. [15] Akiyama Y, Sato T, Imaishi N. Reaction analysis for ZrO2 and Y2O3 thin film growth by low-pressure metalorganic chemical vapor deposition using b-diketonate complexes. J Crys Growth 1995;147:130e46. [16] Wahl G, Nemetz W, Giannozzi M, Rushworth S, Baxter D, Archer N, et al. Chemical vapor deposition of TBC: an alternative process for gas turbine components. Trans ASME 2001;123:520e4. [17] Bourhila N, Felten F, Senateur JP, Schuster F, Madar R, Abrutis T, et al. “Deposition and characterization of ZrO2 and yttria-stabilized ZrO2 films using injection-LPCVD”. In: Allendorf MD, Bernard C,
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