Chemical Vapor Deposition of Ca–P–O Film Coating
Ca–P–O system bio-ceramic films were coated by chemical vapor deposition (CVD). CVD is a versatile technique for controlling crystal phase and microstructure, which significantly affect bio-compatibility. By introducing auxiliary energy, laser and plasma, in CVD, much wider range of Ca–P–O coatings can be synthesized. Hydroxyapatite regeneration of the Ca–P–O coatings prepared by CVD techniques were evaluated in a simulated body fluid (SBF).
KeywordsApatite regeneration Calcium phosphate Crystal structure Laser and plasma CVD
Metallic bio-materials, typically Ti and Ti alloys, can be used as artificial bones or dental implants because they are non-allergenic, have good corrosion resistance in the human body and possess comparable mechanical properties with bone. However, these metallic bio-materials do not have sufficient tissue compatibility; therefore, they require a few months for bone-regeneration. Since human bone is similar in makeup calcium hydroxyapatite (Ca10(PO4)6(OH)2) ceramics, materials of the Ca–P–O system are commonly used as bio-ceramic coatings on metallic bio-materials to accelerate the bone regeneration. Several coating techniques, such as plasma spray, sol–gel, alkaline treatment and magnetron sputtering, have been proposed . Although chemical vapor deposition (CVD) has been widely used to prepare various forms of materials, i.e., films, powders and bulks as electric devices and anti-abrasive coatings , CVD has rarely been used to synthesize bio-ceramic coatings. However, CVD has advantages in controlling crystal phase and microstructure, providing well-adhered coatings even on complex-shaped metal substrates. CVD is a promising technique for the preparation of bio-ceramic coatings because it can optimize their microstructure to enhance bio-compatibility. The authors of this review have prepared Ca–Ti–O , Ca–Si–O  and Ca–P–O bio-ceramic coatings  by CVD. This review briefly describes the CVD preparation of Ca–P–O bio-ceramic coatings and their bone (hydroxyapatite) regeneration behavior in a simulated body fluid (SBF).
9.2 Chemical Vapor Deposition (CVD)
PECVD (Fig. 9.1a) uses plasma as an auxiliary energy source. An electromagnetic field with radio frequency (RF: 13.5 MHz) or micro-wave (2.45 GHz) can be applied to a deposition zone to form the plasma. The gas can be discharged and dissociated to activate ions, radicals and electrons. These activated species are significantly reactive, even at low temperatures, forming non-equilibrium or quasi-equilibrium films . The authors first utilized PECVD for preparing bio-ceramic coatings as shown later.
Lasers can be an auxiliary energy source of light and heat in CVD, and thus LCVD (Fig. 9.1c) can be categorized into two types: photolytic LCVD and pyrolytic LCVD . Since a source gas may absorb a specific laser wavelength, photolytic LCVD can prepare films without substrate heating. The laser passes through a gas phase, directly decomposing source gases. Photolytic LCVD using a high energy laser, typically an ultra-violet or Excimer laser, has the advantage of low temperature deposition without thermal degradation of the substrate. However, photolytic LCVD cannot create a wide-area coating at a high deposition rate. In pyrolytic LCVD, infra-red lasers, such as CO2 and Nd:YAG lasers, are generally used. Pyrolytic LCVD heats locally at a small area of the substrate by focusing the laser beam; thus, source gases can easily access the heated area. The deposition rate of pyrolytic LCVD can be significantly high, reaching several 100 m/h . However, the deposition area is usually less than several mm2. Therefore, pyrolytic LCVD are generally understood to not make large-area coatings on substrates with complicated shape. The authors first developed LCVD to prepare oxide and non-oxide films at high deposition speeds (more than several 100 μm/h) on wide-area substrates (around several cm2) by using a high power laser (several 100 W of CO2, Nd:YAG and diode lasers), as shown later [11, 12].
9.3 CVD of Ca–P–O Films and Their Bio-Characteristics
Figure 9.5b depicts a crystal structure of HAp, which is illustrated by tetrahedra of PO4 and polyhedra of Ca ions coordinated with seven and nine oxygen atoms. HAp and OAp have ‘apatite layers’, composed of two Ca-PO4 columns and one Ca-Ca column parallel to the ac plane, and hydroxyl ions in HAp and oxygen ions in OAp are located at the center of Ca hexagons parallel to the ab plane. HAp or OAp have been frequently studied because HAp is bio-active and similar to human bones. Although many techniques including solid-state sintering, sol–gel and magnetron sputtering  have been employed to prepare OAp or HAp, Darr et al. prepared fluorine-containing carbonated hydroxyapatite by thermal CVD using Ca(tmhd)2 (where tmhd=2,2,6,6,-tetramethylheptane-3,5-dione) and P2O5 . The crystal structure of the film was not investigated, but the Ca/P content of the film was about 1.3 which is different from that of HAp (Ca/P = 1.7). The bio-compatibility of this film was not reported. Since OH is easily evaporated in a vacuum, preparing OH-containing HAp by dry processes (vacuum processes), such as CVD and sputtering, is difficult. OAp film prepared by magnetron sputtering did not contain OH , whereas OAp film prepared by thermal CVD contained a small amount of OH. In this review, the OAp film prepared by CVD containing a small amount of OH is described as OAp film. The authors first prepared a crystalline OAp film in as-deposited form.
CVD is a promising process for bio-ceramic coatings because it can provide well-defined crystal phases and microstructures through the control of process parameters. Auxiliary energy sources, such as laser and plasma, are particularly useful to fabricate materials that cannot be synthesized by conventional thermal CVD. The Ca–P–O system has many useful bio-ceramics, i.e., bio-inert, bio-active and bio-resorbable materials. CVD and in particular LCVD and PECVD are promising methods for the preparation of metastable or unstable bio-ceramic materials. Since CVD has many process parameters, CVD can prepare optimized microstructures, crystal phases and preferential orientations for HAp regeneration.
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