Bio-ceramic Coating of Ca–Ti–O System Compound by Laser Chemical Vapor Deposition

Bio-ceramic Ca–Ti–O system compound ﬁ lms were prepared by laser chemical vapor deposition (laser CVD). Laser CVD is a high-speed technique for coating ﬁ lms with versatile controllability of microstructures and crystal phases. Highly oriented CaTiO 3 ﬁ lms with speciﬁ c textures and Ca n +1 Ti n O 3 n +1 ﬁ lms with the Ruddlesden–Popper-type structure were prepared at high deposition rates. The formation of calcium phosphate in simulated body ﬂ uid (SBF) was promoted by Ca n +1 Ti n O 3 n +1 ﬁ lms.


Introduction
Ti and Ti-based alloys are used as artifi cial bones and dental implants because of their acceptable mechanical properties, low weight, and adequate corrosion resistance in the human body. However, they suffer certain disadvantages, such as poor osteoinductive properties and a duration of several months for the reconstruction of the bone/implant interface with adequate adhesion. The osseointegration of an orthopedic implant involves a cascade of cellular and extracellular biological events that occur at the bone/implant interface [ 1 ]. The processes can be enhanced by the surface treatments and bio-ceramic coating on implants [ 2 , 3 ]. The plasma-sprayed hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HAp) coating on Ti is practically used for dental implants [ 3 , 4 ]. However, the low interface bonding strength and coating toughness can cause a fracture in the interface between HAp and Ti implants. HAp fi lms with low crystallinity coated on Ti implants dissolve rapidly when Ti is implanted into a H. Katsui (*) • T. Goto Institute for Materials Research , Tohoku University , 2-1-1 Katahira, Aoba-ku , Sendai , Miyagi 980-8577 , Japan e-mail: katsui@imr.tohoku.ac.jp human body. The crystallinity and microstructure of coated fi lms is an important factor for establishing a good interface between the bone and implants [ 4 , 5 ].
Recently, calcium titanate (CaTiO 3 ) has gained considerable attention as a biomaterial. CaTiO 3 coatings with controlled thickness and crystallinity are effective for bone formation because CaTiO 3 is chemically stable at low pH and can form HAp in SBF [ 6 -8 ]. CaTiO 3 has also been proposed as an intermediate layer to improve the adhesion between HAp and Ti-based implants [ 9 -13 ]. To date, a variety of techniques, such as sol-gel [ 13 ], hydrothermal reactions [ 14 ], ion implantation [ 15 ], sputtering [ 8 ], and anode oxidation techniques [ 7 ], were employed for CaTiO 3 coating. Chemical vapor deposition (CVD) is a versatile technique to prepare various ceramic fi lms and is widely used in the industry. Sato et al. reported the synthesis of CaTiO 3 fi lms by CVD using metal organic precursors followed by apatite formation on the fi lm surface upon immersion in SBF [ 16 ]. Auxiliary energies such as plasmas and lasers could be employed to accelerate chemical reactions and prepare highly crystalline fi lms with controlled morphology and crystal phases at high deposition rates [ 17 -21 ]. In this study, we demonstrate the synthesis of Ca-Ti-O fi lms by laser CVD, and the effects of deposition parameters on crystal phases, morphology, and deposition rate are investigated. Laser CVD can produce Ca n +1 Ti n O 3 n +1 fi lms which exhibited a signifi cant formability of calcium phosphate precipitates on the coating surface in the SBF immersion.

Laser Chemical Vapor Deposition
CVD is a gas-phase deposition process, comprising several chemical reactions between source gases (precursors). Dense fi lms can be coated by CVD even on rough surfaces with high adherence and good conformal coverage. This is advantageous for bio-ceramic coatings on complex-shaped dental implants and artifi cial bones. Hence, bio-ceramic coatings of well-crystallized Ca-P-O system compounds, such as HAp, αand β-Ca 3 P 2 O 8 , Ca 4 P 2 O 9 , and αand β-Ca 2 P 2 O 7 , have been performed using CVD [ 22 -24 ]. Generally, the deposition rate of CVD is lower than that of plasma spray and electron beam physical vapor deposition. In conventional thermal CVD, the chemical reaction at the interface between the gas and substrate surface is driven by thermal energy. Laser irradiation can accelerate the chemical reactions and enable low-temperature deposition to avoid degradation and corrosion of the substrate materials. Figure 4.1 shows a schematic of the laser CVD apparatus for the coating of Ca-Ti-O compounds. The source materials (precursors) of Ca and Ti were evaporated, and the source vapors were introduced into a CVD reaction chamber. Oxygen was separately introduced into the chamber. A substrate was placed on a hot stage for preheating. The substrate surface was irradiated by an Nd:YAG laser (wavelength 1064 nm) through a quartz window. By controlling deposition parameters, such as laser power, deposition temperature, total pressure, and precursor supply conditions, various forms of deposits can be obtained, e.g., amorphous, fi ne crystals, columnar crystals, dendritic crystals, whiskers, plate-like crystals, and epitaxial single-crystal fi lms. In this study, aluminum nitride (AlN) was fi rst used as the substrate, because it is thermochemically stable at high temperature, and its good workability enables us to investigate the effect of a wide range of CVD parameters on the Ca-Ti-O fi lm characteristics. Based on the insight into the correlation between the CVD parameters and the fi lm characteristics using the AlN substrates, bioactive Ca-Ti-O fi lms were coated on metallic Ti substrates under optimum laser CVD conditions.

Bio-ceramic Coating of Ca-Ti-O by Laser CVD [ 25 , 26 ]
The phase diagram of a CaO-TiO 2 pseudo-binary system is shown in Fig. 4.2 [ 27 , 28 ]. At a Ca/Ti ratio of 1.0, the CaTiO 3 phase exists, which is the most common calcium titanate compound. No other phases are stable in the Ti-rich region between TiO 2 and CaTiO 3 , whereas Ca n +1 Ti n O 3 n +1 phases exist in the Ca-rich region between CaTiO 3 and CaO. The crystal structures of CaTiO 3 and Ca n +1 Ti n O 3 n +1 are illustrated in Fig. 4.3 . Further CaTiO 3 has a perovskite structure with a space group of Pnma , comprising the corner-sharing TiO 6 octahedra surrounded by Ca ions with a 12-fold coordination [ 29 ]. The Ca n +1 Ti n O 3 n +1 phases have perovskite-related structures, the so-called Ruddlesden-Popper structure, formed by alternate stacking of perovskite blocks and CaO layers, as shown in Fig. 4.3 [ 30 ]. The stacking sequence in a unit cell corresponds to the n value in Ca n +1 Ti n O 3 n +1 . Two phases, Ca 2 Ti 3 O 7 ( n = 2) and Ca 3 Ti 4 O 10 ( n = 3), have been reported to exist in the TiO 2 -CaO system. Although CaTiO 3 fi lms fabricated by various methods and their bioactivities were investigated using in vivo and in vitro experiments [ 3 , 6 , 8 , 10 , 11 , 13 ], there are few reports of the synthesis of Ca n +1 Ti n O 3 n +1 fi lms as a biomaterial [ 31 ]. Laser CVD can be used to synthesize CaTiO 3 and Ca n +1 Ti n O 3 n +1 by controlling deposition parameters, such as the Ca/Ti supply ratio of the precursors and deposition temperature depending on the laser power. and Al 2 O 3 , resulting in a reaction between the source gases and the AlN substrate at high temperatures. At Ca/Ti supply ratios <0.8, Ti-rich Ca-Ti-O compounds were formed; however, no phases were thermodynamically stable according to the phase diagrams [ 27 , 28 ]. Under Ca-rich conditions (Ca/Ti supply ratio >1.0), Ca n +1 Ti n O 3 n +1  [ 27 , 28 ] fi lms were deposited at relatively low deposition temperatures (<1000 K), whereas fi lms prepared at deposition temperatures higher than 1000 K comprised CaO and CaTiO 3 . In the Ti-rich compositional region between CaTiO 3 and TiO 2 , several Ca-Ti-O compounds were reported. Bertaut and Blum [ 32 ] and Bright et al. [ 33 ] reported the synthesis of CaTi 2 O 4 by electrolysis of TiO 2 and CaTiO 3 in a CaCl 2 melt. CaTi 2 O 4 single crystals were synthesized by a fl ux method from CaTiO 3 in CaCl 2 and Ti metal [ 34 ]. The existence of CaTi 4 O 9 and CaTi 2 O 5 was reported in a wet chemical method and sol-gel method [ 35 -38 ]. Ancora et al. published patents on the production of CaTi 2 O 5 and CaTi 5 O 11 [ 39 ], where the CaTi 2 O 5 crystal structure differed from that produced by Limar and Kisel [ 35 , 36 ]. Since these Ti-rich phases were considered to be metastable and decomposed into CaTiO 3 and TiO 2 at high temperatures and the synthesis process was limited, the detailed crystal structures and compositions remain unknown. In this study, the X-ray diffraction (XRD) patterns of Ti-rich Ca-Ti-O fi lms by laser CVD in this study were similar to those of CaTi 2 O 5 and CaTi 5 O 11 reported by Ancora; however, the phase identifi cation was diffi cult because the fi lms may comprise a mixture of phases and have preferred orientations. The Ti-rich Ca-Ti-O fi lms were transformed into TiO 2 and CaTiO 3 by heat treatment (post annealing) at 1273 K. Further investigation of the detailed chemical compositions and microstructure was required for the Ti-rich Ca-Ti-O compounds.   [ 40 ]. At temperatures below 800 K, CaTiO 3 fi lms with (011) orientation were formed. With an increase in the deposition temperature, the preferred orientation changed from (011) to (101) at approximately 800 K. Further increases in the deposition temperature resulted in the formation of CaTiO 3 fi lms having no preferred orientation. The preferred orientation during the growth of CaTiO 3 fi lms can be controlled not only by the deposition temperature but also by the total pressure in the chamber. The (121)-oriented CaTiO 3 fi lms were deposited in the total pressure range of 400-600 Pa at a deposition temperature of 825-855 K, whereas the preferred orientation was (101) at a total pressure of 800 Pa in the same deposition temperature range (Fig. 4.5 ).  Fig. 4.6a . Square facets, which are several micrometers in size, were formed in (101)-oriented CaTiO 3 fi lms, as shown in Fig. 4.6c . (121)-oriented CaTiO 3 fi lms had a granular morphology with fi ne grains smaller than several micrometers in size (Fig. 4.6e ). These CaTiO 3 fi lms with strongly preferred orientations were grown in the columnar regime ( Fig. 4.6b, d, f ). CaTiO 3 fi lms without preferred orientation prepared at a high deposition temperature composed randomly arranged faceted grains (several micrometers in size) with a dense and smooth cross section, as shown in Fig. 4.6c, f .  cone-like morphology of the (011)-oriented CaTiO 3 fi lm comprised pyramidal facets, which are several tens nanometers in size. Considering the preferred (011) orientation and the shapes of the grains, the pyramidal texture could be associated with the CaTiO 3 crystal structure, and the faceted planes would be {010} and {110} as shown in Fig. 4.7b . The microstructure of the square facets in the (101)-oriented CaTiO 3 fi lm is shown in Fig. 4.8a . Figure 4.8b shows the terrace on the top surface    Fig. 4.8c , the cross-sectional transmission electron microscopy (TEM) image of the square facet revealed that nanopores formed along the (110) and (011) planes, which are the close-packed planes of Ca-O atoms. These nanopores may relax the stress between the bio-ceramic fi lms and metallic substrates [ 41 -43 ]. Figure 4.9 shows the surface and cross-sectional morphologies of the Ca n +1 Ti n O 3 n +1 fi lm prepared at a Ca/Al supply ratio of 1.6 and a deposition temperature of 777 K. The surface exhibited a cone-like morphology with a grain size of approximately 5-10 μm (Fig. 4.10a ). Each cone-like grain comprised granules that were several tens nanometers in size. The cross section was cone-like, which is a typical morphology for CVD-deposited fi lms [ 44 ]. The Ca/Ti composition of this fi lm was 1.54 by EPMA, which was nearly the same as that of Ca 3 Ti 2 O 7 . However, it was diffi cult to identify the detailed phases in the Ca n +1 Ti n O 3 n +1 fi lms, because the XRD powder pattern of Ca 3 Ti 2 O 7 was similar to that of Ca 4 Ti 3 O 10 owing to the same type of long-range perovskite-related structure. Figure 4.10 shows the temperature dependence of the deposition rate for CaTiO 3 fi lms by laser CVD and conventional thermal CVD in an Arrhenius format. The deposition rates of the CaTiO 3 fi lms by laser CVD reached 230 μm h −1 in the temperature range of 800-1000 K. For the case of conventional thermal CVD, CaTiO 3 fi lms without preferred crystal orientation were grown at the deposition rates in the range of 10-30 μm h −1 and at deposition temperatures above 900 K. Laser CVD enables the preparation of CaTiO 3 with several types of oriented textures at lower deposition temperatures and considerably higher growth rates compared with those obtainable by thermal CVD. Figure 4.11 depicts the surface morphologies of the CaTiO 3 and Ca n +1 Ti n O 3 n +1 fi lms coated on the AlN substrates before and after immersion in SBF (Hanks' solution) for 3 days. Although no signifi cant change in the randomly faceted grains of the CaTiO 3 fi lms without preferred orientation occurred during immersion, the grain boundaries and the faceted edges became slightly obscured (Fig. 4.11a, b ). On the other hand, for the as-deposited CaTiO 3 fi lm comprising square-faceted grains with strong (101) orientation, the edges and corners of the facets became round and smooth after immersion in Hanks' solution ( Fig. 4.11c, d ). These changes in grain boundaries and facet edges could be caused by the dissolution of CaTiO 3 into Hanks' solution, indicating the biosolubility of the CaTiO 3 coating. The surface cone-like morphology with pyramidal facets of (011)-oriented CaTiO 3 fi lms became  Images ( a , c , and e) show as-deposited fi lms, whereas ( b , d , and f) show fi lms after the immersion smooth, as shown in Fig. 4.11f . Figure 4.11f shows that a small amount of calcium phosphate precipitate (several hundred nanometers in size) with a bright contrast appeared on the fi lm's surface after immersion in Hanks' solution. The biosolubility and calcium phosphate formation of CaTiO 3 fi lms are affected by the morphology and preferred orientation. Figure 4.12 depicts the change in the surface morphology of Ca n +1 Ti n O 3 n +1 fi lms caused by immersion in the Hanks' solution. The caulifl owerlike grains of the as-deposited Ca n +1 Ti n O 3 n +1 fi lm became smooth, and calcium phosphate precipitate was formed after immersion for 1 day (Fig. 4.12b ). The entire surface of the fi lm was covered by calcium phosphate precipitate after 3 days, as shown in Fig. 4.12c . Compared with the conventional perovskite CaTiO 3 fi lms (Fig.  4.11 ), the Ruddlesden-Popper-type Ca n +1 Ti n O 3 n +1 fi lms exhibited signifi cant changes in the surface morphology and high calcium phosphate formation ability after the short-term immersion in SBF. Ca n +1 Ti n O 3 n +1 fi lm was coated on a CP-Ti substrate. Figure 4.13 shows the surface morphologies of the Ca n +1 Ti n O 3 n +1 fi lm prepared at a deposition temperature of 620 K on a CP-Ti substrate. The as-deposited Ca n +1 Ti n O 3 n +1 fi lm had a caulifl owerlike morphology similar to that on an AlN substrate (Fig. 4.12a ). After the SBF immersion for a day (Fig. 4.13b ), the surface of the caulifl ower-like grains became smooth, and the grain boundaries were obscured. The entire fi lm surface was covered with calcium phosphate precipitate after immersion for 3 days, as shown in Fig. 4.13c . Therefore, laser CVD enables the bio-ceramic coating of Ca n +1 Ti n O 3 n +1 fi lm on Ti substrates, and this coating is promising for enhancing the osteoinductivity of Ti-based implants.

Summary
Well-crystallized Ca-Ti-O fi lms with various crystal phases and microstructures were produced at high deposition rates by laser CVD. Highly (011)-, (101)-, and (121)-oriented CaTiO 3 fi lms were obtained, forming caulifl ower-like, granular, and faceted morphologies. These various preferred orientations and morphologies affected the solubility, regeneration of calcium phosphate, and bio-inertness of CaTiO 3 fi lms. For the Ca-rich compositions, Ca n +1 Ti n O 3 n +1 fi lms with a Ruddlesden-Popper-type crystal structure were formed and exhibited promising bioactivity for calcium phosphate regeneration.