journal of the mechanical behavior of biomedical materials 40 (2014) 95 –101

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Research Paper

Effect of alumina contents on phase stability and mechanical properties of magnesium fluorapatite/ alumina composites M.S. Hejazia,b,n, M. Ahmadiana,b, M. Meratiana, M.H. Fathia,c a

Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran c Dental Materials Research Center, Isfahan University of Medical Sciences, Isfahan, Iran b

art i cle i nfo

ab st rac t

Article history:

The aim of the present work was twofold: to prepare biphasic magnesium fluorapatite

Received 28 December 2013

(MFA) composites with different amounts of alumina using a two-step sintering process,

Received in revised form

and to evaluate the effects of various amounts of alumina on the mechanical properties,

2 August 2014

phase stability, and densification of the composite samples. Initially, MFA powders were

Accepted 10 August 2014

prepared with different amounts of alumina by mechanical activation and the MFA

Available online 21 August 2014

composite samples were subsequently prepared using the two-step sintering (TSS)

Keywords:

method. In order to determine the appropriate temperature of the first step sintering,

Magnesium fluorapatite

conventional sintering of MFA/50% alumina was carried out at temperatures in the range

Composite

of 1000–1300 1C. X-ray diffraction and scanning electron microscopy (SEM) techniques were

Two-step sintering

used to characterize the prepared MFA/alumina composites. The results showed fracture

Mechanical properties

toughness and hardness in the MFA/50% alumina composite samples to increase as a result of alumina addition to their maximum values of 5.8271.05 MPa m1/2 and 22.0973.5 GPa, respectively. & 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Hydroxyapatite (HA) and fluorapatite (FA) are members of the apatite group (Hidouri et al., 2003). Hydroxyapatite (Ca10(PO4)6 (OH)2) has an excellent biocompatibility and bioactivity. It has become one of the most promising materials for such human tissue implants as bones and teeth (He et al., 2005). It is well known that fluorine ions associated with optimization of n

Corresponding author. E-mail address: [email protected] (M.S. Hejazi).

http://dx.doi.org/10.1016/j.jmbbm.2014.08.004 1751-6161/& 2014 Elsevier Ltd. All rights reserved.

solubility and formation of apatite. It is also, magnesium ions affected the thermal behavior of solid solution. However, changes have been observed to occur in the physical and chemical properties of the above-mentioned materials as a result of substituting these cations and anions (Hidouri et al., 2003; Cacciotti et al., 2009). The fluorine ion (F  ) has been investigated as an essential element for bone and dental formation in human body (Fathi and Zahrani, 2008). Magnesium is an abundant,

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journal of the mechanical behavior of biomedical materials 40 (2014) 95 –101

essential cation in the human body as it has significant effects on human metabolism (Gu and Zheng, 2010). Alumina may act as a bioceramic reinforcement that shows a combination of excellent properties such as wear resistance, high strength, and biocompatibility. Moreover, it has been used in load-bearing parts, orthopedic applications, and dental implants (Guidara et al., 2011; Ayed and Bouaziz, 2008). Many studies have focused on producing biphasic calcium phosphate composites and investigated the effects of various amounts of Al2O3 at different temperatures. It has been reported that mechanical properties of biphasic calcium phosphate as HA, FA and FA-TCP including sinterability, biocompatibility and direct bone formation with adjacent hard tissue increased with alumina addition (Guidara et al., 2011; Ayed and Bouaziz, 2008; Chiba et al., 2003). Moreover, MFA materials showed higher strength, toughness, young modules and hardness in comparison to FA (Hidouri et al., 2009). Therefore, in this case, to obtain to higher mechanical properties, MFA/alumina composites as a new biomaterial were considered. The two-step sintering (TSS) developed by Wang et al. (2006) has been employed for suppressing grain growth to obtain nanosized grains in various materials such as alumina–zirconia composites (Wang et al., 2009), alumina (Hesabi et al., 2009), hydroxyapatite (Mazaheri et al., 2009), and yttria-substituted zirconia (Lourenc et al., 2011). The present work focuses on preparing biphasic MFA composites with different amounts of alumina using the two-step sintering method to suppress grain growth. The mechanical properties and density of the composites prepared are finally characterized by scanning electron microscope (SEM) and X-ray diffraction.

2.

Materials and method

Mg-substituted fluorapatite and MFA/alumina composites were synthesized by a conventional solid-state reaction. A mixture of phosphorous pentoxide (P2O5, Merck), calciumhydroxide (Ca(OH)2, Merck), magnesium hydroxide (Mg(OH)2, Merck), and calcium fluoride (CaF2, Merck) powders were mixed and milled in a high-energy planetary ball mill using zirconia media at room temperature in the air for 12 h. Commercially pure α-Al2O3 (Aldrich, USA) and the already made MFA nanopowder were mixed and milled for 3 h. Green pellets of the nanopowders were prepared by uniaxial pressing under 300 MPa before being sintered using the TSS process. To determine the temperature of the first sintering step, conventional sintering of MFA/50% alumina was carried out at temperatures in the range of 1000–1300 1C at a heating rate of 10 1C min  1 for 1 min. TSS1 was employed for the composites and TSS2 for MFA, respectively, (Table 1). The Archimedes water immersion method was employed to measure density. The particle size of the TSS processed composite samples was estimated using SEM micrographs and the average values of about 100 readings were reported. Table 1 – Conditions for applied TSS processes for one minute in t1 and heating rate of 10 1C min  1. Applied TSS

T1 (1C)

T2 (1C)

t2 (h)

TSS1 TSS2

1200 1050

1100 950

12 20

Mechanical properties including hardness and fracture toughness were measured using the indentation method on polished surfaces applying a load of 10 kg over an average of five separate locations. Fracture toughness (KIC) was calculated according to the Evans and Charles model repeated below (Lourenc et al., 2011): KIC ¼ 0:0824P=C3=2 where, P stands for applied load (N) and C for crack length. The vickers hardness (Hv) was determined using the following equation: Hv ¼ 1:854P=d2 where, d stands for the diagonal length of the indentation. A Philips diffractometer (40 kV) with radiation (λCuKα ¼ 0.15406 nm) was used to evaluate the structural changes and phase transformations in the samples. The microstructure of the sintered samples was studied using a scanning electron microscope (Phillips XL 30). Differential thermal analysis (DTA–TGA, STA) was performed at a heating rate of 10 1C min  1 in the air for the samples in powder shape.

3.

Results and discussion

3.1.

Studying the microstructure of the prepared samples

The microstructure of MFA composites with 0, 10, 25, and 50 wt% alumina sintered via two-step-sintering (TSS) is shown in Fig. 1(a)–(d). Fig. 1(a) shows fine particles and different distributions of particle size associated with the formation of agglomerates in pure MFA. In this condition, the samples exhibit intergranular porosities which increase with increasing alumina percentage (Fig. 1(a–d)). The increasing porosity is associated with lower densification and higher decomposition while higher porosity is favorable to bone ingrowth requirements. The particles bonded during the twostep sintering process to form a continuous structure with intergranular porosities (Fig. 1(b, c)). Sinterability decreased with alumina content in the MFA/alumina composites so that minor amounts of the powders did not lead to the sintering of MFA/50% alumina but accumulated on the surface (Fig. 1(d)). Finally, particle size in the composites increased with increasing alumina as illustrated in Fig. 5.

3.2.

Sintering behavior

In order to determine the temperature of the first sintering step, conventional sintering was carried out at temperatures varying from 1000 to 1300 1C. Following Wang et al. (2006), T1 was obtained once the density of the samples reached 75% of their theoretical values. Table 2 presents the resulting density variations with temperature for the MFA/50% alumina composites. Fig. 2(a)–(c) shows the characteristic peaks for MFA (JCPDS # 15-0876), α-Al2O3 (JCPDS # 10-0173), and β-TCP (JCPDS # 9-0169). In order to prove to match the XRD patterns with JCPDS cards, the values of diffraction angles and d-spacing of (0 2 10) plane relative to β-TCP is presented in Table 3. It can be seen that values are very close to those reported for β-TCP (JCPDS # 9-0169). Calcium aluminate phases nucleated at

journal of the mechanical behavior of biomedical materials 40 (2014) 95–101

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Fig. 1 – The microstructures of sintered MFA composites with (a) 0, (b) 10, (c) 25 and, (d) 50% alumina via TSS. Table 2 – Density variations for MFA/50% alumina composites for temperatures ranging 1000–1300 1C. Sintering temperature (1C) Density (% theatrical density)

1000 69.0

1100 71.5

1200 75.0

1300 79.5

Fig. 2 – XRD patterns of composite samples sintered for different temperatures (a) 1000, (b) 1100, (c) 1200 and (d) 1300 1C.

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journal of the mechanical behavior of biomedical materials 40 (2014) 95 –101

1200 1C and the exothermic peak in the DTA curve was observed at T41100 1C (Fig. 3(a)). In fact, calcium aluminates were produced at high temperatures by MFA decomposition into CaO and Ca3(PO4)2 (seen as an endothermic peak at about 980 1C in the DTA curves for MFA/50% alumina and pure MFA (Fig. 3(a) and (b)). Ca3(PO4)2 then reacted with alumina to form calcium aluminates. Furthermore, the differential thermal analysis curve for MFA (Fig. 3(b)) exhibits an exothermic peak at 850 1C. This could be due to powder crystallization and Mg2FPO4 formation (JCPDS # 24-0704). Besides, the exothermic peak at about 980 1C observed in the XRD patterns of MFA conventionally sintered at 900 1C is attributed to the decomposition of MFA into β-TCP compared to MFA subjected to TSS (Fig. 4(a) and (b)). The endothermic peak at 1090 1C is Table 3 – Diffraction angles and d-spacing values of β-TCP from XRD patterns of composite samples sintered at different temperatures and comparison with selected JCPDS. Sintering temperature (1C)

1000 1100 1200 1300 JCPDS# 9-0169

(0 2 10) Plane d-Spacing (Å)



2.8700 2.8716 2.8758 2.8739 2.8800

31.138 31.120 31.073 31.095 31.026

associated with the formation of a liquid phase reported for fluorapatite without Mg (Hidouri et al., 2003; Ayed and Bouaziz, 2008). Consequently, intensity decreased in MFA peaks, indicating the occurrence of decomposition. Based on the CaO–Al2O3 system, CaAl2O4 formation must have taken place as described below (Rivas Mercury et al., 2005): 7α-Al2O3þ12CaO-Ca12Al14O33The first transitory calcium aluminate formation (Ca12Al14O33) took place at about 1030 1C corresponding to the exothermic peak observed in the DTA curve (Fig. 3(a)). Then, the second transitory calcium aluminate CaAl4O7 formed in association with an exothermic peak at about 1100 1C (Fig. 3(a)) according to the following equation: (2) Al2O3þCaAl14O33-12CaAl4O7 Subsequently, the reaction between the first and second transitory calcium aluminates occurred to produce CaAl2O4 as expressed below: 5CaAl4O7þCa12Al14O33-17CaAl2O4

(3)

There is no evidence in the XRD patterns indicating the formation of Ca12Al14O33 or CaAl4O7. It is also seen that MFA partially decomposed into β-TCP at about 980 1C in the MFA/ 50% alumina composite. Increasing temperature led to enhanced, MFA decomposition and transport rate while all MFA peaks disappeared at 1300 1C (Fig. 2(d)). Moreover, only βTCP and calcium aluminate (CaAl2O4) were detected in the XRD patterns as seen in Fig. 2(d). It may be concluded that a

Fig. 3 – DTA curves for (a) MFA/50% alumina and (b) MFA only.

journal of the mechanical behavior of biomedical materials 40 (2014) 95–101

temperature of 1200 1C is suitable for the first sintering step because relative density at this temperature reaches 75% of its theoretical value while MFA is not entirely decomposed. In other words, once T1 reaches 1300 1C, MFA is entirely decomposed into β-TCP. In the case of MFA composites containing 10 and 25% alumina, the sintered densities reached 82% and 79% TD, respectively, for the same T1 value. Based on the above considerations, the two-step sintering (TSS1) with T1 ¼1200 1C, t1 ¼1 min, T2 ¼ 1100 1C, and t2 ¼12 h was

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employed in this work for the composite samples. For comparison, magnesium-fluorapatite was sintered at T1 ¼1050 1C, t1 ¼ 1 min, T2 ¼950 1C, and t2 ¼12 h (TSS2).

3.3.

Mechanical properties

Fig. 5 shows the relative density and particle size of MFA/ alumina composites sintered by the TSS process. Clearly, relative density decreased and particle size increased with

Fig. 4 – XRD patterns of MFA in (a) CS at 900 1C and (b) in used TSS.

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journal of the mechanical behavior of biomedical materials 40 (2014) 95 –101

increasing alumina content. The reduced density is attributed to the higher decomposition once alumina is increased. Alumina sintering has been reportedly carried out at temperatures in the range of 1400–1750 1C (Guidara et al., 2011). In the present work, however, the first sintering step was accomplished at 1200 1C. Thus, sinterability decreased as a result of adding alumina to MFA/alumina composites. The values for the hardness and fracture toughness of MFA/Al2O3 composites are presented in Table 4. MFA hardness values equal to those reported in the literature for the secondary phase Mg2FPO4 have a significant effect on the mechanical properties of the composite (Hidouri et al., 2009). It can be seen that fracture toughness and hardness of MFA/Al2O3 composite increased with increasing alumina content as crack resistance increased due to the reinforcement of the

Fig. 5 – Relative density and particle size variations versus MFA/alumina composites sintered using TSS process.

Fig. 6 – XRD patterns of MFA composites with (a) 10, (b) 25 and (c) 50% alumina.

MFA matrix as a result of introducing Al2O3. It should also be noted that alumina is harder than MFA. In the present work, the composite samples showed higher hardness values compared to those reported in the literature; this difference may be due to the conventional sintering process used in previous studies (Evis and Doremus, 2007; Golestani-fard et al., 2011). However, as the TSS process was used in this study, hardness and fracture toughness increased due to grain growth suppression while nonosized components were also observed in the composite samples. The fracture toughness and hardness of MFA/A12O3 composites reached their maximum values of 5.8271.05 MPa m1/2 and 22.0973.5 GPa in the MFA/50% alumina composite, respectively. Clearly, the increased alumina content in the composite improved the hardness and fracture toughness of MFA/alumina composites, indicating that alumina played a key role in improving the mechanical properties of the composites making them suitable for load-bearing parts in biomedical applications such as bone and teeth. In all the composite samples, MFA decomposed into β-tricalcium phosphate (β-TCP) during TSS. It has been shown that this decomposition and the consequent biphasic ceramic formation are favorable to bone regeneration and repair (Viswanath et al., 2005). The XRD patterns for the sintered MFA/alumina composites via TSS are shown in Fig. 6. All the diffraction peaks correspond to the mixture of MFA, alumina, β-TCP, and CaAl2O4. The intensity of the alumina, β-TCP, and calcium aluminate peaks increased as a result of increasing alumina, which indicates the degree of MFA decomposition into β-TCP.

4.

Conclusions

Alumina was added to MFA/alumina composites to obtain higher mechanical properties. Biphasic MFA/alumina composites with various amounts of alumina treated at different temperatures produced new phases of β-TCP and calcium aluminates. MFA composite with 50 wt% alumina treated at 1300 1C for 1 min entirely decomposed into β-TCP and calcium aluminates while the density reached 79.5% of its theoretical value. Higher densities at higher temperatures indicated MFA consumption in the composite samples. The achievement of 75% of the theoretical density at 1200 1C led to the selection of the first sintering step temperature (T1) for the two-step sintering of the composites. The TSS process also yielded a fine particle size in the composites. However, particle size increased from 175 to 590 nm as a result of introducing alumina into the composite, which thereby improved their mechanical properties. The fracture toughness and hardness of MFA/A12O3 composites reached

Table 4 – Mechanical properties of MFA/Al2O3 composites sintered via TSS. Composite samples

Sintering method

Hardness (GPa)

Toughness (MPa m1/2)

MFA MFA/10% Al2O3 MFA/25% Al2O3 MFA/50% Al2O3

TSS1 TSS2 TSS2 TSS2

6.3871.29 12.3970.86 16.6276.4 22.0973.5

2.270.82 1.9770.98 3.8771.38 5.8271.05

journal of the mechanical behavior of biomedical materials 40 (2014) 95–101

their maximum values of 5.8271.05 MPa m1/2 and 22.097 3.5 GPa in the MFA/50% alumina composite. Lower densification occurred by alumina addition. The decomposition of MFA into β-TCP increased with increasing alumina content in all the samples. Finally, the MFA/50% alumina composite was found favorable not only to bone regeneration and repair but also for load-bearing parts in biomedical applications due to the highest hardness and fracture toughness resulting from the greater porosity thus achieved. In sum, addition of alumina to MFA/alumina composite samples led to enhanced mechanical properties, particle size, and MFA decomposition into β-TCP as well as reduced densification of the composites.

Acknowledgments The authors are grateful to Isfahan University of Technology for supporting the present research.

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alumina composites.

The aim of the present work was twofold: to prepare biphasic magnesium fluorapatite (MFA) composites with different amounts of alumina using a two-ste...
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