**Figure 1.**

*Schematic of preparation of precursor solution.*

the white compound. The characteristic peak assignable to carboxyl group can be observed at 1620 cm<sup>−</sup><sup>1</sup> . The peaks at 1200, 2980, and 3360 cm<sup>−</sup><sup>1</sup> can be assigned to H5O2 + ion. **Figure 3** shows TG-DTA curves of the obtained white compound. The endothermic peak with the mass loss at 235°C can be assigned to the elimination of the coordinated water molecules. It was clarified by these results that the number of coordinated water is 2. The exothermic peak with the mass loss from 324 to 464°C corresponds to the combustion of EDTA. As a result, the chemical component of the isolated white powder was confirmed as (H)(H5O2)[Ca(edta)] and consistent to the elemental analysis.

The isolated Ca complex of EDTA is not soluble to ethanol, producing a suspended solution when the compound is added into the solvent. However, by the addition of dibutylamine to the suspended solution, a clear solution can be facilely obtained. In addition, the dibutylammonium salt of the Ca complex with EDTA produces an amorphous film on the Ti substrate by spin-coating before firing, under the presence of phosphate ion.

**Figure 2.** *FT-IR spectra of EDTA and white compound.*

**Figure 3.** *TG-DTA curves of white compound.*

When an aqueous solution of 85 mass% phosphoric acid was directly added to the dibutylammonium salt of the Ca complex with EDTA in ethanol instead of the dibutylammonium diphosphate, the gelation of the mixed solution was observed after several days at ambient temperature. This may be caused by the hydrolysis involving Ca2+ and PO4 <sup>3</sup><sup>−</sup> species due to the copresence of the large amount of water molecules derived from the aqueous solution of phosphoric acid. It is important to note that the present molecular precursor solution does not occur in such a hydrolysis for several months, due to the negligible amount of water in the solution.

#### **2.2 Fabrication of the apatite films**

Machined commercially pure wrought Ti disks were used as a substrate material. The molecular precursor solution was dropped onto the Ti surface to cover the entire area of the disk. The precursor films formed on the disks were dried at 60°C for 20 min and then fired at 300, 400, 500, 600, and 700°C for 2 h using a furnace in air.

**Figure 4** shows the XRD patterns of the coating films on the substrate at different firing temperatures. No peak could be observed when the precursor films were fired at 300 and 400°C, in the exception of the peaks assignable to Ti. Thus, the heat treatment below 400°C did not produce calcium phosphate crystals on the Ti substrate. Thermal analysis of the molecular precursor gel demonstrated that the decomposition of organic materials occurred at around 360°C and that complete decomposition of organic materials can be achieved at a firing temperature higher than 400°C. By the heat treatment at 500°C, the thickness of the coated film decreased more than that at 300°C. Also, in the XRD patterns of the heat-treated films at 500°C, the peak intensities assignable to Ti substrate increased in comparison with those obtained at 300 and 400°C. These changes in film thicknesses and detection of the Ti substrate are due to the combustion and removal of organic material in the precursor film. The organic materials in the precursor film on the Ti disk disappeared completely at the temperatures in the range 600–700°C.

The CA film formed at a firing temperature at 500°C was almost amorphous, but the films formed at firing temperatures of 600 and 700°C showed a crystalline structure. The greater intensities of the rutile and anatase peaks resulted from heat treatment of the Ti substrate. It is thus suggested that a firing temperature at 600°C is suitable for the production of a thin CA film on the Ti substrate.

The surface of the coating film is quite smooth with no crack nor pinhole.

*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 4.** *XRD patterns of CA film firing at several temperatures.*

### **2.3 Characterizations of the fabricated film**

**Table 1** lists the changes of film thickness and Ca/P ratio, which was measured by electron probe microanalysis (EPMA), of the coating films fired at 600°C on titanium disk by immersing them in phosphate-buffered saline (PBS, pH = 7.4). It was thus shown that this crystalline apatite film on the Ti substrate is robust to the immersion in PBS. Only a slight crack could be observed after immersion for 7 and 28 days, respectively. No distinct degradation of the deposited coatings was detected. However, the gradual decreases of the Ca/P ratio of the coating film were observed after immersion in PBS. It is suggested that ion exchange reaction between the coating film and PBS occurred gradually via the dissolution/precipitation process.

The tensile bond strength of the coated film onto the Ti substrate was measured to evaluate the degree of adherence. **Table 2** lists the tensile bond strength of CA film on the Ti substrate. The tensile bond strengths of several coating films onto Ti have been reported as follows: 31.9 MPa by the plasma spraying method


**Table 1.**

*Change in thickness of CA film after immersion in PBS.*


#### **Table 2.**

*Tensile bond strength of CA film on Ti substrate.*

[30], 8.02–45.82 MPa by ion beam sputtering deposition [31], 59.0 MPa by ion beam dynamic mixing [6], and 32.50 MPa by plasma spraying coating to titanium plasma-sprayed titanium [32], respectively. The bond strength obtained in the present study, 40 MPa, was compatible with these reported values, although it was impossible to obtain the real interfacial bond strength by this method owing to the cohesive failure of the epoxy glue used to fix the samples (**Table 2**).
