Keywords

1 Introduction

Ti and Ti-based alloys are used as artificial 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 (Ca10(PO4)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 films with low crystallinity coated on Ti implants dissolve rapidly when Ti is implanted into a human body. The crystallinity and microstructure of coated films is an important factor for establishing a good interface between the bone and implants [4, 5].

Recently, calcium titanate (CaTiO3) has gained considerable attention as a biomaterial. CaTiO3 coatings with controlled thickness and crystallinity are effective for bone formation because CaTiO3 is chemically stable at low pH and can form HAp in SBF [6–8]. CaTiO3 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 CaTiO3 coating. Chemical vapor deposition (CVD) is a versatile technique to prepare various ceramic films and is widely used in the industry. Sato et al. reported the synthesis of CaTiO3 films by CVD using metal organic precursors followed by apatite formation on the film surface upon immersion in SBF [16]. Auxiliary energies such as plasmas and lasers could be employed to accelerate chemical reactions and prepare highly crystalline films with controlled morphology and crystal phases at high deposition rates [17–21]. In this study, we demonstrate the synthesis of Ca–Ti–O films by laser CVD, and the effects of deposition parameters on crystal phases, morphology, and deposition rate are investigated. Laser CVD can produce Ca n+1Ti n O3n+1 films which exhibited a significant formability of calcium phosphate precipitates on the coating surface in the SBF immersion.

2 Laser Chemical Vapor Deposition

CVD is a gas-phase deposition process, comprising several chemical reactions between source gases (precursors). Dense films 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 artificial bones. Hence, bio-ceramic coatings of well-crystallized Ca–P–O system compounds, such as HAp, α- and β-Ca3P2O8, Ca4P2O9, and α- and β-Ca2P2O7, 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, fine crystals, columnar crystals, dendritic crystals, whiskers, plate-like crystals, and epitaxial single-crystal films. In this study, aluminum nitride (AlN) was first 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 film characteristics. Based on the insight into the correlation between the CVD parameters and the film characteristics using the AlN substrates, bioactive Ca–Ti–O films were coated on metallic Ti substrates under optimum laser CVD conditions.

Fig. 4.1
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Schematic of laser CVD apparatus

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3 Bio-ceramic Coating of Ca–Ti–O by Laser CVD [25, 26]

The phase diagram of a CaO–TiO2 pseudo-binary system is shown in Fig. 4.2 [27, 28]. At a Ca/Ti ratio of 1.0, the CaTiO3 phase exists, which is the most common calcium titanate compound. No other phases are stable in the Ti-rich region between TiO2 and CaTiO3, whereas Ca n+1Ti n O3n+1 phases exist in the Ca-rich region between CaTiO3 and CaO. The crystal structures of CaTiO3 and Ca n+1Ti n O3n+1 are illustrated in Fig. 4.3. Further CaTiO3 has a perovskite structure with a space group of Pnma, comprising the corner-sharing TiO6 octahedra surrounded by Ca ions with a 12-fold coordination [29]. The Ca n+1Ti n O3n+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+1Ti n O3n+1. Two phases, Ca2Ti3O7 (n = 2) and Ca3Ti4O10 (n = 3), have been reported to exist in the TiO2–CaO system. Although CaTiO3 films 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+1Ti n O3n+1 films as a biomaterial [31]. Laser CVD can be used to synthesize CaTiO3 and Ca n+1Ti n O3n+1 by controlling deposition parameters, such as the Ca/Ti supply ratio of the precursors and deposition temperature depending on the laser power. Figure 4.4 depicts the effects of deposition temperature and Ca/Ti supply ratio on the phase formation of Ca–Ti–O films by laser CVD. At a Ca/Ti supply ratio of approximately 1.0, single-phase CaTiO3 films were formed at deposition temperatures below 1100 K. At deposition temperatures above 1100 K, CaTiO3 films contained TiO2, Ca–Al–O compounds (e.g., CaAl2O4 and CaAl4O7) and Al2O3, 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+1Ti n O3n+1 films were deposited at relatively low deposition temperatures (<1000 K), whereas films prepared at deposition temperatures higher than 1000 K comprised CaO and CaTiO3. In the Ti-rich compositional region between CaTiO3 and TiO2, several Ca–Ti–O compounds were reported. Bertaut and Blum [32] and Bright et al. [33] reported the synthesis of CaTi2O4 by electrolysis of TiO2 and CaTiO3 in a CaCl2 melt. CaTi2O4 single crystals were synthesized by a flux method from CaTiO3 in CaCl2 and Ti metal [34]. The existence of CaTi4O9 and CaTi2O5 was reported in a wet chemical method and sol–gel method [35–38]. Ancora et al. published patents on the production of CaTi2O5 and CaTi5O11 [39], where the CaTi2O5 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 CaTiO3 and TiO2 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 films by laser CVD in this study were similar to those of CaTi2O5 and CaTi5O11 reported by Ancora; however, the phase identification was difficult because the films may comprise a mixture of phases and have preferred orientations. The Ti-rich Ca–Ti–O films were transformed into TiO2 and CaTiO3 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.

Fig. 4.2
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Phase diagram of CaO–TiO2 system [27, 28]

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Fig. 4.3
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Crystal structures of CaTiO3, Ca3Ti2O7, and Ca4Ti3O10

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Fig. 4.4
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Phase formation relation between deposition temperature and Ca/Ti precursor supply ratio for Ca–Ti–O films formed by laser CVD

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Figure 4.5 illustrates the effect of the deposition temperature on the crystal orientations of CaTiO3 films prepared at a total pressure of 800 Pa, where the texture coefficient (TC) is the degree of crystal orientation. A TC value of 10 corresponds to perfect orientation, whereas a TC value of 1 corresponds to random orientation [40]. At temperatures below 800 K, CaTiO3 films 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 CaTiO3 films having no preferred orientation. The preferred orientation during the growth of CaTiO3 films can be controlled not only by the deposition temperature but also by the total pressure in the chamber. The (121)-oriented CaTiO3 films 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). Figure 4.6 shows the typical surface and cross-sectional morphologies of CaTiO3 films with the preferred orientations ((011), (101), and (121)) and without orientation. (011)-oriented CaTiO3 films have a cone-like morphology with pyramidal facets, as shown in Fig. 4.6a. Square facets, which are several micrometers in size, were formed in (101)-oriented CaTiO3 films, as shown in Fig. 4.6c. (121)-oriented CaTiO3 films had a granular morphology with fine grains smaller than several micrometers in size (Fig. 4.6e). These CaTiO3 films with strongly preferred orientations were grown in the columnar regime (Fig. 4.6b, d, f). CaTiO3 films 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.

Fig. 4.5
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Effect of deposition temperature on TC(022) and TC(101) of CaTiO3 films

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Fig. 4.6
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SEM images of typical CaTiO3 films deposited by laser CVD on AlN substrates. (a, b) (011)-oriented CaTiO3 film at 795 K and 800 Pa, (c, d) (101)-oriented CaTiO3 film at 935 K and 800 Pa, (e, f) (121)-oriented CaTiO3 film at 855 K and 400 Pa, and (g, h) CaTiO3 film with random orientation at 1080 K and 800 Pa

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Figure 4.7 depicts the detailed microstructures with crystal structure models of the corresponding textures in the (011)- and (101)-oriented CaTiO3 films. The cone-like morphology of the (011)-oriented CaTiO3 film 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 CaTiO3 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 CaTiO3 film is shown in Fig. 4.8a. Figure 4.8b shows the terrace on the top surface of the square facet corresponding to the (101) plane, along which the corner-sharing TiO6 octahedra are aligned. Here, the lateral planes of the square facet were {101} and {010}. In 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 films and metallic substrates [41–43].

Fig. 4.7
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Microstructure and oriented texture of (011)-oriented CaTiO3 film. (a) Surface SEM image at high magnification and (b) crystallographic texture of the pyramidal facet

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Fig. 4.8
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Microstructure and oriented texture of (101)-oriented CaTiO3 film. (a) Surface SEM image at high magnification, (b) crystallographic texture of the square facet, (c) cross-sectional TEM image of the square facet, and (d) relation between crystallographic texture and the formation of nanopores

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Figure 4.9 shows the surface and cross-sectional morphologies of the Ca n+1Ti n O3n+1 film 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 films [44]. The Ca/Ti composition of this film was 1.54 by EPMA, which was nearly the same as that of Ca3Ti2O7. However, it was difficult to identify the detailed phases in the Ca n+1Ti n O3n+1 films, because the XRD powder pattern of Ca3Ti2O7 was similar to that of Ca4Ti3O10 owing to the same type of long-range perovskite-related structure.

Fig. 4.9
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SEM images of a CanTi n+1O3n+1 film at 777 K and 800 Pa. (a) Surface morphology, (b) enlarged image of cauliflower-like grains, and (c) cross-sectional image

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Fig. 4.10
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Effect of deposition temperature on deposition rates of CaTiO3 films formed by laser CVD. For comparison, plots of the deposition rates of CaTiO3 films formed by conventional thermal CVD are also included

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Figure 4.10 shows the temperature dependence of the deposition rate for CaTiO3 films by laser CVD and conventional thermal CVD in an Arrhenius format. The deposition rates of the CaTiO3 films by laser CVD reached 230 μm h−1 in the temperature range of 800–1000 K. For the case of conventional thermal CVD, CaTiO3 films 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 CaTiO3 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 CaTiO3 and Ca n+1Ti n O3n+1 films coated on the AlN substrates before and after immersion in SBF (Hanks’ solution) for 3 days. Although no significant change in the randomly faceted grains of the CaTiO3 films 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 CaTiO3 film 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 CaTiO3 into Hanks’ solution, indicating the biosolubility of the CaTiO3 coating. The surface cone-like morphology with pyramidal facets of (011)-oriented CaTiO3 films became 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 film’s surface after immersion in Hanks’ solution. The biosolubility and calcium phosphate formation of CaTiO3 films are affected by the morphology and preferred orientation. Figure 4.12 depicts the change in the surface morphology of Ca n+1Ti n O3n+1 films caused by immersion in the Hanks’ solution. The cauliflower-like grains of the as-deposited Ca n+1Ti n O3n+1 film became smooth, and calcium phosphate precipitate was formed after immersion for 1 day (Fig. 4.12b). The entire surface of the film was covered by calcium phosphate precipitate after 3 days, as shown in Fig. 4.12c. Compared with the conventional perovskite CaTiO3 films (Fig. 4.11), the Ruddlesden–Popper-type Ca n+1Ti n O3n+1 films exhibited significant changes in the surface morphology and high calcium phosphate formation ability after the short-term immersion in SBF.

Fig. 4.11
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Effect of immersion in Hanks’ solution for 3 days on the surface morphologies of CaTiO3 films coated on AlN substrates. (a, b) CaTiO3 film with random orientation, (c, d) CaTiO3 film with (101)-orientation, and (e, f) CaTiO3 film with (011)-orientation. Images (a, c, and e) show as-deposited films, whereas (b, d, and f) show films after the immersion

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Fig. 4.12
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The effect of immersion in Hanks’ solution on the surface morphologies of CanTi n+1O3n+1 films coated on AlN substrates; (a) as-deposited, (b) after immersion for 1 day, and (c) after immersion for 3 days

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Ca n+1Ti n O3n+1 film was coated on a CP-Ti substrate. Figure 4.13 shows the surface morphologies of the Ca n+1Ti n O3n+1 film prepared at a deposition temperature of 620 K on a CP-Ti substrate. The as-deposited Ca n+1Ti n O3n+1 film had a cauliflower-like 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 cauliflower-like grains became smooth, and the grain boundaries were obscured. The entire film 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+1Ti n O3n+1 film on Ti substrates, and this coating is promising for enhancing the osteoinductivity of Ti-based implants.

Fig. 4.13
figure 13

The effect of immersion in Hanks’ solution on the surface morphologies of CanTi n+1O3n+1 films coated on Ti substrates; (a) as-deposited, (b) after immersion for 1 day, and (c) after immersion for 3 days

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4 Summary

Well-crystallized Ca–Ti–O films with various crystal phases and microstructures were produced at high deposition rates by laser CVD. Highly (011)-, (101)-, and (121)-oriented CaTiO3 films were obtained, forming cauliflower-like, granular, and faceted morphologies. These various preferred orientations and morphologies affected the solubility, regeneration of calcium phosphate, and bio-inertness of CaTiO3 films. For the Ca-rich compositions, Ca n+1Ti n O3n+1 films with a Ruddlesden–Popper-type crystal structure were formed and exhibited promising bioactivity for calcium phosphate regeneration.