**3. Aqueous spray method for apatite coating on titanium substrate**

#### **3.1 Preparation of aqueous spray solution**

We first prepared a clear solution by blowing CO2 gas into an aqueous solution of calcium hydroxide [38]. Into the solution, an aqueous solution involving phosphoric acid was then added slowly, and a clear solution could be prepared. Generally, Ca2+ ion reacts immediately with PO4 <sup>3</sup><sup>−</sup> ion in aqueous solution, and calcium phosphate compounds precipitate. However, the finally obtained solution is stable if enough concentration of CO2 is maintained in the solution. In fact, many crystals of brushite (CaHPO4.2H2O) deposited when CO2 in the solution decreased. Therefore, the periodical blowing of CO2 into the solution can prevent from precipitating such calcium phosphate compounds.

#### **3.2 Fabrication of the apatite films**

The CA film on a Ti plate was fabricated by the aqueous spray method and characterized precisely [38]. In the spraying process with a pressure of 0.2 MPa supplied by an air compressor, the mist of the aqueous solution emitted from the nozzle of an airbrush was vertically collided on a preheated Ti substrate. The thickness of the sprayed film measured by a profilometer and those of the films obtained by heat treatment of the sprayed film at 400–700°C under an Ar gas flow were in the range 1.21–1.40 μm. The well-developed network structure with many round particle was observed on the surface of the sprayed CA film, as has been shown in **Figure 8**. The characteristic network structure of the films fabricated in this present work can be also observed for those formed by the ESD method [17–20]. The size of the network structure found on the surface of the present films (10–15 μm) was twice as large as those of the films formed by ESD method (5–7 μm) [39]. The round bulged wall may be formed by spreading of the mist reached on the surface. The round particles (**Figure 7**), which were not observed for the films formed by the ESD method, may be formed by particle growth of the crystal nuclei generated in the sprayed mist during the approach to the substrate and appeared on the film surface. This difference in morphology may be due to the size of the fine drops in the mist and the boiling point of the solvent.

The apatite deposition on the preheated substrate occurs through at least the following four steps. Firstly, spraying of the aqueous solution with compressed air from the nozzle forms fog-like drops of the solution. Secondly, the fog-like drops gradually evaporate due to heat near the substrate, and a continuous increase in ion concentration in the drops occurs during the approach and before contact with the substrate. This step includes the partial removal of CO2 gas. In the next step, the concentrated sprayed drops in the mist collide with and spread on the substrate. In the fourth and final step, the spread apatite immediately deposited on the substrate by evaporation of the water molecules in the hydrated ions, and the apatite accumulates by continuous collision of the concentrated sprayed mist.

From the XRD measurement of the sprayed film, it is clear that the apatite structure is easily formed using this method. In addition, the surface morphology of the sprayed film did not change by heat treatment at 400–700°C under an Ar gas flow.

On the other hand, the amount change of the spray solution caused the drastic change of the surface morphologies. **Figure 8** shows the tilt-viewed surface morphologies for **A** whose spray amount and film thickness were 25 mL and 1.3 μm and **B** whose spray amount and film thickness were 5 mL and 0.11 μm, respectively [40]. The image of the thinner-film **B** indicated that the formation of the network structures with round particles occurred as found on the thicker-film **A**, even if the amount of the sprayed solution decreased.

On the basis of these SEM images, **Table 4** also lists the averaged border heights of 10 arbitrarily selected networks. The other two films, **A'.** and **B'.**, were prepared by heat treating **A** and **B**, respectively, at 600°C for 10 min under Ar gas flow of

**Figure 7.** *Surface morphologies of the (a) Ti substrate and (b) sprayed film (the amount of spray solution is 25 mL).*

*Fabrication of Apatite Films on Ti Substrates of Simple and Complicated Shapes by Using Stable… DOI: http://dx.doi.org/10.5772/intechopen.80409*

**Figure 8.**

*Tilte-viewed SEM images of Ti substrate (Ti), spray amount of 25 mL on Ti (***A***) and spray amount of 5 mL (***B***). The tilte angle was 85°.*

0.5 L min<sup>−</sup><sup>1</sup> . The ratios of border height to thickness of **A**, **A'.**, **B**, and **B'.** were 0.6, 0.8, 2.4, and 2.8, respectively. Thus, the degree of film shrinkage in the vertical direction by thermal densification was larger than that of border height reduction when the films **A** and **B** were heat treated in the abovementioned conditions.

### **3.3 Characterizations of the fabricated film**

The results of elemental and Fourier transform infrared (FT-IR) analyses of the powder mechanically collected from the surface of the sprayed film agreed well with those of Ca10(PO4)6(CO3)·2CO2·3H2O.

**Figure 9** shows the FT-IR spectra of the powders collected from the sprayed film before heat treatment and those heat treated at different temperatures. The bands around 1639 and 3435 cm<sup>−</sup><sup>1</sup> are likely due to water molecules, but not the hydroxy group. The peak at 2343 cm<sup>−</sup><sup>1</sup> , which was observed for both the sprayed film before heat treatment and those heat treated at 400°C and 500°C, can be assigned to the asymmetric stretching mode of the CO2 molecule [41]; the corresponding peaks for the two films heat treated above 600°C were extremely small. This unexpected behavior of the films heat treated above 600°C may be due to crystallite decay that occurs by elimination of CO2 molecules that were initially inserted in the apatite skeleton, as shown in the FT-IR spectra.

The shear stresses developed in the sprayed film before heat treatment and the sprayed films heat treated at different temperatures were measured. For the film heat treated at 700°C, the shear stress exceeded the measurable limit, indicating more than 133 MPa based on the maximum load of this instrument (0.50 kg). The shear stress of the sprayed film (21 MPa) is notably larger than the value reported (13 MPa) for an HA film formed on a Ti substrate via plasma spray deposition [42]. The spray solution contains various ions, such as hydroxide, carbonate, and phosphate ions, that may act as bridging ions and can readily form strong chemical bonds at the interface of the Ti substrate and the film. The remarkable decrease in wthe shear


**Table 4.** *Average thicknesses and average border height of network of* **A***,* **A'***,* **B***, and* **B'.**

**Figure 9.** *FT-IR spectra of the powders obtained from the sprayed film and heat-treated films.*

stresses observed for the films heat treated at 400 and 500°C can also be explained by the following assumption; at these low temperatures, a large amount of CO2 may be eliminated from the films, because the TG curve shows that CO2 moieties were removed below 600°C. As a result, the densities of these films formed at low temperatures are relatively low as compared to those obtained at higher temperatures which cause sufficient film densification.
