**6. Experimental methods**

A summary of the characteristics of the various coating techniques for calcium phosphate is presented in Table 2. Each technique has its own benefits and drawbacks. However, sputter‐ ing is a promising method due to its ability to produce dense and thin coatings, as well as

**Techniques Advantages Disadvantages Coating thickness**

*Coating thickness.* The HA coating thickness varies. Molagic [42] succeeded in producing HA/ZrO2 coatings with an average thickness of 3.2 µm. Hong *et al.* [43] manufactured a 500 nm thick coating of crystalline HA using magnetron sputtering. Ding [44] sputter de‐ posited HA/Ti coatings with a film thickness of 3-7 µm onto a titanium substrate. Thian *et al.* [45] succeeded in incorporating silicon in hydroxyapatite (Si-HA) using magnetron sputtering and discovered its potential use as a bio-coating. The Si-HA film thickness was

*Bond strength.* An *in vitro* and *in vivo* experiment on coatings using the sputtering technique revealed coating detachment problems. Cooley *et al.* [46] reported that HA coatings were re‐ moved after 3 weeks of implantation. A bond layer coating was suggested to overcome this weak adhesion at the interface and subsequent delamination. Ievlev *et al.* [47] measured the adhesion strength of HA coatings with a sublayer and revealed that the adhesion strength

shapes, Sometimes expensive < 1 µm

Delamination and resorption. High temperature leads to decomposition

The use of alkali heat treatment could reduce mechanical strength. Requires much time

Needs annealing for

Tends to crack 25 µm

crystalline structure < 1 µm

50 - 100 µm

10 - 30 µm

provide good bond strength [39-41].

32 Titanium Alloys - Advances in Properties Control

Electrodeposition

Plasma Spray

Biomimetic

up to 700 nm.

Sol-gel Flexible in coating complex

Sputtering Dense, homogenous coating.

*5.5.1. Properties of sputtered hydroxyapatite coatings*

**Table 2.** Summary of various techniques for calcium phosphate coatings

Flexible in coating complex shapes. Low energy process, can be scaled down to deposition of a few atoms or scaled up to large dimensions.

Able to coat high and low melting materials. High deposition rate

Flexible in coating complex shapes and flexible in controlling chemical composition of the coating. Homogenous.

Excellent adhesion

#### **6.1. Design and preparation of titanium alloys**

Tin and niobium were chosen as alloying elements because both metals are biocompatible and non-cytotoxic. The titanium alloy composition was designed using the molecular orbital DV-Xα method [53]. The calculation of the nominal composition of the alloys was based on two parameters, known as the bond order (Bo) and d-orbital energy level (Md). The parame‐ ter Bo is the covalent bond strength between titanium and an alloying element, while the parameter Md represents the d-orbital energy level of a transition alloying metal that corre‐ lates with the electro-negativity and the atomic radius of element. The list of Md and Bo val‐ ues for each alloying elements (Ti, Nb and Sn) was obtained from a study conducted by Abdel Hady *et al.* [54].

using 15, 9, 6, and 1 µm diamond compounds progressively. All metallic discs were then ul‐

Sputtered Hydroxyapatite Nanocoatings on Novel Titanium Alloys for Biomedical Applications

http://dx.doi.org/10.5772/54263

35

Silica thin films and nanocrystalline hydroxyapatite coatings were successively deposited onto the prepared titanium alloy substrates by e-beam evaporation and sputtering techni‐ ques. A HV thin film deposition system (CMS-18 Kurt J. Lesker, USA) was used. Both the ebeam evaporation and the sputtering processes were performed at room temperature. The

A 200 nm SiO2 thin film was deposited at a working pressure of 6.6 x 10-4 Pa and a depo‐ sition rate of 10 nm/s. During the sputtering process, the working pressure was set at 0.8 Pa. The sputtering power was 90 W. The distance between the substrate and sputtering target was kept at 30 cm. During deposition, the substrate holder rotated in order to ach‐ ieve uniform coating. Heat treatment of samples was performed at 500°C for 2 h in a vac‐

The elemental composition was analyzed using an energy dispersive X-ray spectrometer (EDS, Oxford instruments INCA suite v.4.13) interfaced with a field-emission scanning elec‐ tron microscope (FE-SEM, ZEISS SUPRA 40 VP) operated at 15 kV. Surface morphology of the samples was observed using scanning electron microscopy, and phase identification was performed using the X-ray diffraction method (XRD, Bruker D8 Advance), operated with CuK<sup>α</sup> radiation in the Bragg-Brentano mode at a scanning rate of 0.5°/min over a 2θ range of

30-80°. Phase analysis was conducted using the database PDF-2 version 2005.

*<sup>ε</sup>* <sup>=</sup> (1 - *<sup>ρ</sup> ρs*

The porosity of the scaffold was characterised by gravimetry using the formula [13]:

where *ρ* and *ρ*s are the actual and theoretical densities of the porous alloy, respectively.

The X-ray diffraction pattern of sintered Ti14Nb4Sn is shown in Figure 5. Alpha peaks were observed at 39.0° and 40.5°, which are indexed as the reflection planes (101) and (103), while β peaks were observed at 38.5°, which is indexed as (110). The titanium alloy consisted of both α and β phases. Weak niobium peaks were also detected, while tin was not detected. Elemental analysis using EDS was performed concurrently with the SEM examination to

) × 100 (1)

trasonically cleaned using ethanol for 5 min.

base pressure of the system was 6.6 x 10-6 Pa.

uum furnace.

**6.3. Characterization**

**7. Results and discussion**

**7.1. Physico-chemical properties of the Ti14Nb4Sn alloy**

**6.2. E-beam evaporation and sputtering**

Titanium alloys were fabricated using the powder metallurgy technique. Titanium powders (purity 99.7%), tin powders (purity 99.0%) and niobium powders (purity 99.8%) with parti‐ cle sizes less than 45 µm were used. Each component was first weighted to give the desired composition of Ti14Nb4Sn. Ammonium hydrogen carbonate (NH4HCO3) was used as a space holder material. The particle size chosen was 300-500 µm in diameter.

The desired porosity and pore size were controlled by adjusting the initial weight ratio of NH4HCO3 to metal powders and the particle size of NH4HCO3. These components were mixed and blended in a planetary ball milling for 4 h with a weight ratio of ball to powder of 1:2 and a rotation rate of 100 rpm. A small amount of ethanol was employed during the mixing of the ammonium hydrogen carbonate with elemental metal powders to prevent segregation. After mixing the ammonium hydrogen carbonate with the metal powders, the mixture was pressed into green compacts in a 50 ton hydraulic press.

The green compacts were sintered at a pressure of 1.3 x 10-3 Pa using a vacuum furnace. Two steps of heat treatment were employed to produce porous structures. The first step was to burn out the space holder particles at 200°C for 2 h. The second step was to sinter the com‐ pacts at 1200°C for 10 h. Dense samples were prepared using powder metallurgy with the absence of space holder particles, and heat treatment was carried out at 1200°C. The dimen‐ sions of dense and porous titanium alloy samples were 9 mm in diameter and 2 mm in thickness for subsequent sample characterization. The sintering process was conducted at 1200°C for 10 h. A schematic diagram of the fabrication sequence for titanium alloys is pre‐ sented in Figure 4.

**Figure 4.** Schematic of Ti14Nb4Sn fabrication process by powder metallurgy route

Titanium alloy discs with 6 mm in diameter and 2 mm in thickness were gently wet ground‐ ed using (i) silicon carbide paper of 600 grit, (ii) followed by 1200 grit, and (iii) fine polished using 15, 9, 6, and 1 µm diamond compounds progressively. All metallic discs were then ul‐ trasonically cleaned using ethanol for 5 min.

#### **6.2. E-beam evaporation and sputtering**

Silica thin films and nanocrystalline hydroxyapatite coatings were successively deposited onto the prepared titanium alloy substrates by e-beam evaporation and sputtering techni‐ ques. A HV thin film deposition system (CMS-18 Kurt J. Lesker, USA) was used. Both the ebeam evaporation and the sputtering processes were performed at room temperature. The base pressure of the system was 6.6 x 10-6 Pa.

A 200 nm SiO2 thin film was deposited at a working pressure of 6.6 x 10-4 Pa and a depo‐ sition rate of 10 nm/s. During the sputtering process, the working pressure was set at 0.8 Pa. The sputtering power was 90 W. The distance between the substrate and sputtering target was kept at 30 cm. During deposition, the substrate holder rotated in order to ach‐ ieve uniform coating. Heat treatment of samples was performed at 500°C for 2 h in a vac‐ uum furnace.

#### **6.3. Characterization**

two parameters, known as the bond order (Bo) and d-orbital energy level (Md). The parame‐ ter Bo is the covalent bond strength between titanium and an alloying element, while the parameter Md represents the d-orbital energy level of a transition alloying metal that corre‐ lates with the electro-negativity and the atomic radius of element. The list of Md and Bo val‐ ues for each alloying elements (Ti, Nb and Sn) was obtained from a study conducted by

Titanium alloys were fabricated using the powder metallurgy technique. Titanium powders (purity 99.7%), tin powders (purity 99.0%) and niobium powders (purity 99.8%) with parti‐ cle sizes less than 45 µm were used. Each component was first weighted to give the desired composition of Ti14Nb4Sn. Ammonium hydrogen carbonate (NH4HCO3) was used as a

The desired porosity and pore size were controlled by adjusting the initial weight ratio of NH4HCO3 to metal powders and the particle size of NH4HCO3. These components were mixed and blended in a planetary ball milling for 4 h with a weight ratio of ball to powder of 1:2 and a rotation rate of 100 rpm. A small amount of ethanol was employed during the mixing of the ammonium hydrogen carbonate with elemental metal powders to prevent segregation. After mixing the ammonium hydrogen carbonate with the metal powders, the

The green compacts were sintered at a pressure of 1.3 x 10-3 Pa using a vacuum furnace. Two steps of heat treatment were employed to produce porous structures. The first step was to burn out the space holder particles at 200°C for 2 h. The second step was to sinter the com‐ pacts at 1200°C for 10 h. Dense samples were prepared using powder metallurgy with the absence of space holder particles, and heat treatment was carried out at 1200°C. The dimen‐ sions of dense and porous titanium alloy samples were 9 mm in diameter and 2 mm in thickness for subsequent sample characterization. The sintering process was conducted at 1200°C for 10 h. A schematic diagram of the fabrication sequence for titanium alloys is pre‐

Titanium alloy discs with 6 mm in diameter and 2 mm in thickness were gently wet ground‐ ed using (i) silicon carbide paper of 600 grit, (ii) followed by 1200 grit, and (iii) fine polished

space holder material. The particle size chosen was 300-500 µm in diameter.

mixture was pressed into green compacts in a 50 ton hydraulic press.

**Figure 4.** Schematic of Ti14Nb4Sn fabrication process by powder metallurgy route

Abdel Hady *et al.* [54].

34 Titanium Alloys - Advances in Properties Control

sented in Figure 4.

The elemental composition was analyzed using an energy dispersive X-ray spectrometer (EDS, Oxford instruments INCA suite v.4.13) interfaced with a field-emission scanning elec‐ tron microscope (FE-SEM, ZEISS SUPRA 40 VP) operated at 15 kV. Surface morphology of the samples was observed using scanning electron microscopy, and phase identification was performed using the X-ray diffraction method (XRD, Bruker D8 Advance), operated with CuK<sup>α</sup> radiation in the Bragg-Brentano mode at a scanning rate of 0.5°/min over a 2θ range of 30-80°. Phase analysis was conducted using the database PDF-2 version 2005.

The porosity of the scaffold was characterised by gravimetry using the formula [13]:

$$
\varepsilon = \left(1 - \frac{\rho}{\rho\_\*}\right) \times 100 \tag{1}
$$

where *ρ* and *ρ*s are the actual and theoretical densities of the porous alloy, respectively.
