Fluoridation and Mechanical Properties of Hydroxyapatite with Addition of TiF3, Al2O3 and Fe2O3 by Sintering

Muhammad Asif, Zhengyi Fu*, Shahzad Ahmad Khan, Weimin Wang, Hao Wang, Yucheng Wang, Sahibzada Shakir Rehman, Ruifang Ma

State Key Lab of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology, Wuhan 430070, China

*Corresponding author: Tel.: +86 027 87865484; Fax: +86 027 87215421; E-mail address: zyfu@whut.edu.cn

Key words: Pressureless sintering, Fluoridated hydroxyapatite, Mechanical properties.

Abstract: Fluoridated composite Hydroxyapatite biomaterial was prepared by sintering the

hydroxyapatite Ca10 (PO4)6. (OH)2[HA]powder with addition of titanium fluoride 30 wt. % -TiF3

and mixture of iron oxide and aluminum oxide [5 wt. %-Al2O3/Fe2O3 ; (5/1)]. The pellets were

prepared by conventional sol-gel technique. The as-prepared samples were sintered at various

temperatures and the effect of different temperature on grain size and mechanical properties has

been investigated. The samples were sintered at various temperatures of 1100－1400 °C. After

sintering, the sample’s mechanical properties such as compressive strength, bending strength,

Vickers hardness were improved to be 49±0.26－103±0.26 MPa, 19.6±0.20－36.2±0.20 MPa, and

7.97±0.16－12.50±0.16 GPa with their grains size 4.76±0.10－17.63±0.10 µm and bulk densities

1.8944-2.2426 g/cm3.

Introduction

Hydroxyapatite (HA)[Ca10(PO4)6.(OH)2] is a hexagonal structured ceramic which composed of

calcium and phosphate groups. Synthetic apatite HA is similar to mineral components found in

natural bone tissues. Due to this chemical similarity between synthetic HA and natural bone

components, synthetic HA is used as bone substitute in orthopedics [1]. Hydroxyapatite is bioactive

and biocompatible biomaterial. Due to it’s biocompatibility synthetic HA is used in orthopedics and

dental applications. Synthetic apatite forms the strong chemical bonds with natural bone [2].

Synthetic HA is restricted to orthopedics applications. However, due to the high dissolution rate of

biological acidic environment, synthetic HA shows poor corrosion resistance and poor chemical

stability [3]. Due to high rate of bioresorption, pure HA exhibits long term stability problem which

causes loosening and failure of HA implants [4]. For improving the biocompatibility of synthetic HA,

fluoride can be substituted with hydroxyl group of synthetic HA. Fluoridated HA has attracted recent

researchers as promising replacement for HA. Fluoridated hydroxyapatite (F−HA) demonstrates

significant acidic resistance of body fluids [5]. Titanium oxide and Titanium alloys have been used in

hydroxyapatite since last several years. Alloys of titanium and titanium oxide develop the good level

of bioactivity and favor osteogenesis and tissues attachment to biomaterials interface [6]. Metallic

materials are conventionally used for replacing the structural components of human body tissues.

Because metallic based biomaterials have superior mechanical properties. Mostly commonly used

among the metallic biomaterials are Co-Cr alloys, pure titanium, titanium alloys, Ti-6Al-4V alloys

and stainless steel [7]. Several types of composites of hydroxyapatite including metals based HA have

been studied and practiced. The aim of research work is fluoridation and improving the mechanical

properties by sintering the hydroxyapatite with addition of titanium fluoride and aluminum oxide/

iron oxide.

Experimental

Raw materials and procedure: Starting raw materials were 65 wt. % hydroxyapatite powder,

5wt. % Al2O3/Fe2O3 (5/1) powder and 30 wt. % TiF3. These commercially prepared compounds

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were purchased from the company Alfa Aesar. Initially aqueous solutions of raw materials were

prepared by dissolving in distilled water. Then the solutions were mixed and stirred vigorously at

80 °C. For obtaining homogeneous solution, the sample was stirred for three hours. The obtained

suspension was aged for 12 hours and then filtrated, washed with distilled water and ethyl alcohol

alternatively several times. The obtained white powder was subsequently dried in oven at 110 °C

for 12 hours to obtain sample powder. For obtaining compact pressed sample pellets of 200 mm

diameter, the powder was uniaxially pressed at 10 MPa using stainless steel mold. The pellets were

sintered in an automated gas furnace by heating to desired temperature, 1100 °C, 1200 °C, 1300 °C

and 1400 °C. From room temperature to 900 °C, the heating rate was of 3 °C/min was fixed. From

900 °C to onward temperature the heating rate of 5 °C/min was fixed and holding time was 60 min

(1h) for each sample. For sintering the samples, inert media nitrogen gas was used to avoid the

samples from free oxygen and let the elements to react with each other in absence of atmospheric

oxygen.

Characterizations: The bulk densities of sintered samples were determined accurately by

Archimedes principle. Vickers hardness was tested on polished surfaces under the load of 1kg. For

determining bending strength the samples were designed into bar shape of 2 mm × 5 mm × 12 mm

dimensions. For determining compressive strength the samples were shaped into 2 mm × 4 mm × 7

mm bar shape. Bending strength and compressive strength were measured in Testing Centre of

Wuhan University of Technology. The phase compositions of sintered samples were examined by

using X-rays diffraction (XRD), (PANalytical Xpert, Pro.type, Nother land, with Cu Kα radiation).

Diffracted beam of graphite monochromator was used to produce Cu Kα1 radiation. The samples

were scanned at scanning speed ten degree per minute 10°/min (2θ) in range 10°-80°. Microstructures

i.e. grains size and shapes were examined by Field Emission Scanning Electron Spectrometry

(FESEM) (Quanta FEG 250, FEI, Hillsboro, USA, equipped with energy dispersive spectroscopy

EDS). EDS analysis was carried out by using current voltage 20 kV. Elemental analysis was carried

out by using Energy Dispersive X-ray Spectrometry (EDS) analysis of the sintered pellets.

Results and discussions

10 20 30 40 50 60 70 80

Inte

nsit

y(a

.u.)

2Theta(deg.)

1100°°°

°C

CC

C

1200°°°

°C

CC

C

1300°°°

°C

CC

C

1400°°°

°C

CC

C

Fig.1, XRD patterns of samples A, B, C and D, sintered at 1100 °C, 1200 °C, 1300 °C and 1400 °C

respectively.

◆--71-0880--Ca5(PO4)3F ▲--77-0120--Ca5 F(PO4)3 ▼--89-0056--CaTiO3

◇--75-1205--FeTiO3 ○--87-0087--Al(PO4) ★--81-0659--Fe3 (PO4)2

The phase composition of sintered samples A, B, C and D, sintered at various temperatures

was identified by X− rays diffraction (XRD). For sample A and B, strong peaks of XRD have

confirmed the crystalline phase of fluoridated hydroxyapatite Ca5(PO4)3F with card no. 71－0880.

For samples C and D, which were sintered at 1300 °C and 1400 °C, the crystalline phase of

compound Ca5 F(PO4)3 have been identified with their card no. 77－0120. The fluoride has been

found attached between calcium and phosphate rather than Ca5(PO4)3F. For samples C and D, these

strong peaks were slightly changed due to change of Fluoroapatite structure. The sharp peaks have

240 Eco-Materials Processing and Design XV

confirmed the major phase of Fluoroapatite. Minor phases of Al(PO4), Fe3(PO4)2 , CaTiO3 and

FeTiO3 have been identified with their card no. 87－0087, 81－0659, 89－0056, 75－1205

respectively.

Fig.2, FESEM images of samples A,B, C and D, sintered at 1100 °C, 1200 °C, 1300 °C and

1400 °C.

The FESEM micrographs of compact sintered samples A, B, C and D sintered at 1100 °C,

1200 °C, 1300 °C and 1400 °C are shown in Fig.2. The mean grain size of the sample sintered at

1100 °C and 1200 °C were found 4.76±0.10 µm and 4.3±0.10 µm respectively. With increasing the

sintering temperature to 1300 °C and 1400 °C, the average grain size increased to 9.3±0.10 µm and

17.6306±0.10 µm respectively.

Fig.3, EDS of samples A, B, C and D, sintered at 1100 °C, 1200 °C, 1300 °C and 1400 °C.

For EDS analysis, randomly area was chosen on the surfaces of sintered pellets. Different

peaks of different intensities were obtained as shown in Fig.3. Each obtained peak has been labeled

with their related elements. EDS analysis has confirmed the presence of elements Ca, P, O, Al, Fe,

Ti and F respectively.

Fig.4, Relationship Fig.5, Relationship Fig.6, Relationship Fig.7, Relationship

between average grain between Vickers hard- between bending strength between compressive

Size. ness and temperature. and temperature. and temperature.

Materials Science Forum Vol. 804 241

Fig.4. displays the relationship between grain size and temperature. It has revealed that the

grains size increases with the increasing temperature. As Fig.2, FESEM depicts, with increasing the

temperature, small grains have merged and developed into rod shape, which have been arranged in

parallel way. This merging behavior of grains has caused increasing the density with increasing the

temperature. After sintering the samples A, B, C and D were found to be 1.89 g/cm3, 1.586 g/cm

3,

2.046 g/cm3 and 2.243 g/cm

3 respectively. Fig.5 shows the relationship of Vickers hardness and

temperature. The increase of temperature from 1100 °C to 1400 °C has caused increasing the

Vickers hardness, which have been improved form 7.97±0.16 GPa to 12.50±0.16 GPa. Another

factor of increasing the Vickers hardness with increasing temperature is parallel arrangements of

grains also. This parallel arrangement exerts more resistance against external pressure. Fig.6

explains the relationship of temperature and bending strength. With increasing the temperature, the

bending strength has been found to be increased. After sintering the bending strength has been

improved from 19.64±0.20 MPa to 36.15±0.20 MPa. Fig.7 provides the illustration of improving

the compressive strength with increasing the temperature. After sintering the sample A at 1100 °C,

the compressive strength has been found to be 49.1±0.26 MPa. Compressive strength has been

increased with increasing the temperature. The graphical line representing the compressive strength

has reached to climax with increasing the temperature as has been shown in Fig.7. After sintering

the sample D at 1400 °C, then highest value of compressive strength has been attained to be

103.1±0.26 MPa.

Conclusions

Hydroxyapatite was sintered at various temperatures with addition of 30 wt. % titanium fluoride

and 5 wt. % mixture of aluminum oxide/Iron oxide (Al2O3/Fe2O3-5/1). The results have indicated

that the addition of TiF3, Al2O3/Fe2O3 to HA, has densified and improved hardness and strength of

sintered hydroxyapatite. Sintering temperatures also have densified and enhanced the mechanical

properties of the samples. It has revealed that titanium fluoride has not only caused the fluoridation

of hydroxyapatite, Moreover titanium with aluminum oxide and iron oxide has enhanced the

mechanical properties of the sintered sample. After sintering the sample at maximum temperature

1400 °C, the highest values of Vickers hardness, compressive strength and bending strength were

found to be 12.50±0.16 GPa, 103±0.26 MPa and 36±0.20 MPa respectively, which are greatly

higher than the strength and hardness of sintered pure hydroxyapatite. It is suggested that obtained

biomaterials can be used in orthopedics as bone implants.

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242 Eco-Materials Processing and Design XV

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