Microstuctural and Mechanical Properties of Zirconia-Silica- Hydroxyapatite Composite for Biomedical Applications

Key Engineering Materials Vol. 631 (2015) pp 156-159 Submitted: 30.06.2014

© (2015) Trans Tech Publications, Switzerland Revised: 30.07.2014

doi:10.4028/www.scientific.net/KEM.631.156 Accepted: 30.07.2014

Microstuctural and Mechanical Properties of Zirconia-Silica- Hydroxyapatite Composite for Biomedical Applications

Aliye Arabacı1, a *Nazlıcan Yüksel1,b and Nermin Demirkol2,c

1Istanbul University, Faculty of Engineering, Department of Metallurgical Engineering, Avcilar 34320, Istanbul, Turkey

Technical Prog.Dept., Vocational School of Degirmendere Ali Ozbay, Kocaeli University, Kocaeli, Turkey

aaliye@istanbul.edu.tr, bnzlcnyuksel92@hotmail.com, cnermin.demirkol@kocaeli.edu.tr,

* aliye@istanbul.edu.tr

Keywords: Hydroxyapatite, mechanical properties, composite, zirconia.

Abstract. Hydroxyapatite is a calcium phosphate ceramic that is used as a biomaterial. It has been studied extensively as a candidate biomaterial for prosthetic applications. Hydroxyapatite (HA) does not have the mechanical strength to enable it to succeed in long term load bearing applications. Therefore, Its mechanical properties may be improved with addition of zirconia powders. The aim of this study is to improve the mechanical properties of the hydroxyapatite by producing composite material including zirconia and silica powders. Therefore, hydroxyapatite was mixed with 5 wt% zirconia, 5 wt% silica powders and then this pressed mixture were sintered at different temperatures (1100-1300oC). The sintering behavior, microstructural characteristics and mechanical properties were investigated.

Introduction

Hydroxyapatite (HA) is a calcium phosphate ceramic that is used as a biomaterial. It is one of few materials that elicit an active response in the body. Calcium phosphate ceramics are classified primarily calcium hydroxyapatite, tricalcium phosphate and tetracalcium as materials for biomedical grafts [1-5]. Hydroxyapatite is chemically similar to the mineral component of bones and hard tissues in mammals. But HA has very low mechanical strength. Therefore, it is not suitable for load bearing applications due to its very brittle character. So, HA must be reinforced with other materials to form more load bearing composite. HA bioceramics must be reinforced with other ceramics or metals. Zirconia, titania or alumina were used as second-phase ceramic materials to improve mechanical properties [6-8].
Zirconia is one of the most important ceramic materials. Zirconia has high mechanical strength and fracture toughness [9]. There are some studies related to HA-zirconia composites in the literature. Evis [10] had studied reactions in HA-zirconia composites. Addition of zirconia caused increased decomposition of the HA composites sintered at 1100 and 1300oC, forming tri calcium phosphate (TCP). Erkmen et al. [11] had studied microstructural and mechanical properties of HA- ZrOcomposites. In this study, enamel derived HA (EHA) and commercial synthetic hydroxyapatite (CHA) were used as an matrix. The density and mechanical properties were generally increased by adding 5 wt.% PSZ (partially stabilized zirconia), especially after sintering at
1200 oC for EHA-PSZ composites. But, CHA-PSZ composites showed lower mechanical strength
values at 1200 and 1300oC sintering temperatures. Silica (SiO2)- substituted HA promotes early bonding at bone/implant interface. Also, using 2 wt.% SiOshowed good results in cell culture
studies [12-13]. Oktar [13] et. al. studied mechanical properties of bovine hydroxyapatite (BHA)
composites doped with SiOand the best mechanical properties were achieved with BHA-5 wt. % SiOcomposites sintered at 1200oC. In this composite, extended formation of glassy phase occurred at 1300oC sintering temperature.
The aim of this study is to produce zirconia-silica-hydroxyapatite composite with the addition of
5 wt.% zirconia - 5 wt.% silica and to characterize mechanical and microstructural properties of this composite.

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Materials and Methods

Key Engineering Materials Vol. 631 157

In this study, the commercial synthetic hydroxyapatite obtained from Across Company with molecular weight of 502.31 g mol-1 was used. Hydroxyapatite powder was mixed with 5 wt% commercial zirconia, 5 wt% silica powders ( Eczacıbaşı Esan Company) using ball milling for 4 h. The samples were prepared according to a British Standard for compression tests (BS 7253) [14].
After the mixtures were ball milled for 4h, the powder portions were pressed with a pellet die in order to give the pellet shape under 350 MPa pressing pressure. Then these pellets were isostatically cold-pressed (CIP) under 200 MPa. Obtained pellets were sintered between 1100 and 1300 °C for 4 h with a heating rate of 5 °C min -1. Compression strength and Vickers microhardness were measured. The compression tests were done with an universal testing machine (Shimadzu) at crosshead speed of 3 mm.min -1. The microhardness values of the samples were determined under
200g load using Vickers microhardness testing system (Zwick, ZHW10). And also the densities of the sintered pellets were determined by using the well-known Archimedes’ method (The water was used as solvent (water at 25 °C 0.997 g cm-3) in this method). Scanning electron microscopy (SEM) analyses were carried out and X-ray diffraction studies were also performed. The microstructure of the sintered samples was characterized by means of SEM using FEI Quanta FEG 450 microscope. The X-ray spectra of the sintered samples were obtained over the 2θ range of 10–90° by using Rigaku D/max-2200 PC X-ray diffractometer with Cu-Kα radiation at a scan of 2°/min.

Results and Discussion

Fig. 1 displays the XRD patterns of Zirconia-Silica-Hydroxyapatite (ZSH) composite sintered at
1100°C, 1200 °C and 1300°C for 4h. This composite includes whitlockite (W), hydroxyapatite (HA), zirconium calcium oxide (ZCO), calcium phosphate silicate (CPS) and calcium phosphate (CP) phases after sintering at two different temperatures of 1100°C and 1300°C. Additionally to these phases, ZSH composite sintered at 1100°C includes extra silica phase.

Fig.1. XRD diffraction patterns of ZSH composite sintered at 1100°C and 1300°C (W:Whitlockite, HA: Hydroxyapatite, ZCO: Zirconium Calcium Oxide, CPS: Calcium Phosphate Silicate: Calcium Phosphate (CP) and SiO2).

Scanning electron micrographs of ZSH Composite sintered at 1100°C, 1200°C and 1300°C for 4h are illustrated in Fig. 2a–c. The micrographs shown in Fig. 2a clearly show that the microstructure of composite sintered at 1100°C includes porosity. This result shows that the composite did not sinter enough and has low densification at 1100°C sintering temperature. As can be seen, the grain boundaries of Z-S-H Composite sintered at 1200 oC can not be distinguished clearly (Fig. 2b). Fully dense structure and bigger grains obtained with increasing sintering temperature as seen in Fig.2c. It indicates that the increasing sintering temperature has led to an increase in the mechanical properties (Table 1).

158 Bioceramics Volume 26


Fig.2. Scanning electron micrographs of ZSH composite sintered at 1100°C, 1200 °C and 1300°C. Table 1 shows the experimental results of density, compression strength and vickers microhardness

of the ZSH composite sintered at different sintering temperatures. Density and mechanical properties of composite increased with increasing sintering temperature. The highest density and mechanical properties were obtained with the ZSH composite sintered at 1300°C.

Table1. Compression strength, density and microhardness of Zirconia-Silica-Hydroxyapatite

Composite at different sintering temperatures.

Temperature

(°C)

Density (g/cm3)

Compression Strength

(MPa)

Vikers Microhardness

(HV)

1100

2.52

85.5

116

1200

2.58

114

119

1300

2.85

134

266

Conclusions

In this study, the microstructural and mechanical properties of zirconia-silica-hydroxyapatite composite were examined. The findings of this study are concluded as follows:
1. The dense ZSH composites were obtained using CIP.
2. The density of composite increased with increasing sintering temperature.
3. The mechanical properties of composite increased with increasing sintering temperature.
4. The highest density, compression strength and Vickers microhardness values were obtained with the samples sintered at 1300°C as 2.85 g/cm3, 134 MPa and 266 HV, respectively.

Acknowledgments

This work was financially supported by the Research Fund of Istanbul University with the project numbers45692.

Key Engineering Materials Vol. 631 159

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