1

Influence of Magnesium Doping in Hydroxyapatite Ceramics

C. Y. Tan1, K. L. Aw

2, S. Ramesh

2 and M. Hamdi

1

1 Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia 2 Ceramics Technology Laboratory, University Tenaga Nasional, Selangor, Malaysia

Abstract—The sinterability of magnesium oxide (MgO)

doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was

investigated. Green samples were prepared and sintered in air

at temperatures ranging from 1000°C to 1400°C. Sintered

bodies were characterized to determine the phase stability,

bulk density, hardness, fracture toughness and Young’s

modulus. XRD analysis revealed that the HA phase stability

was not disrupted throughout the sintering regime employed.

In general, samples containing 1 wt% MgO and when sintered

at lower temperatures possessed better mechanical properties

than the undoped HA. The study revealed that all HA samples

achieved > 98% relative density when sintered between

1050ºC–1300ºC. The addition of 1 wt% MgO when sintered at

1150°C was found to be most beneficial throughout the studied

range as the samples exhibited the highest Young’s modulus of

136.3 GPa, Vickers’s hardness of 7.66 GPa and fracture

toughness of 1.48 MPam1/2 as compared to 113.57 GPa, 7.10

GPa and 1.08 MPam1/2 for the undoped HA.

Keywords— hydroxyapatite, magnesium oxide, bioceramics,

sinterability, mechanical properties

I. INTRODUCTION

As reviewed by Suchanek et al. [1], hydroxyapatite

ceramic, HA [(Ca10(PO4)6(OH)2] has been identified as one

of the most suitable biomaterial for medical implants due to

its close chemical resemblance with the mineral compo-

nents and crystal structure to apatite in human skeletal sys-

tem of natural bone and teeth. Moreover, its identical cal-

cium to phosphorus ratio to natural bone promotes strong

bonding between bony tissues and the implant surface [2].

In addition to that, evidence of rapid bone formation in the

HA implant and subsequent healing around the damaged

sites in the body were also observed, thus demonstrating its

potency as a viable biomaterial for clinical usage [3].

The applicability of HA as a biomaterial in clinical

orthopaedic and dental applications, nevertheless, is limited

to only non-stressed loaded regions owning to the brittle

nature and the low fracture toughness (0.8-1.2 MPam1/2

) of

the bioceramic [1]. Thus, the development of bioactive HA

that has enhanced mechanical properties coupled with ex-

cellent biocompatibility are desirable. As a result, various

studies ranging from powder processing techniques, compo-

sition and experimental conditions are carried out with the

aim of determining the most effective synthesis method and

conditions to produce improved mechanical properties of

sintered HA accompanied with superior biocompatibility

with hard tissues [4]. Suchanek et al. [5] found that HA

composites, particularly with HA/HA whiskers, could im-

prove the reliability of HA ceramics. However, the authors

had also revealed the difficulty in densifying the HA-based

composites by pressureless sintering. This problem associ-

ated with powder consolidation could be overcome by using

other techniques such as hot pressing [6] and hot isostatic

pressing [7] at the expense of high set-up as well as running

cost.

An alternative economical technique to obtain highly

dense HA body is by incorporating appropriate low-

temperature sintering additives [8]. Various attempts have

been carried out in hope to enhance the mechanical proper-

ties of HA and biocompatibility through the incorporation

of sintering additives such as zirconia, ceria and iron [9-11].

Among these additives, the inclusion of magnesium oxide

on HA ceramics has not been studied extensively. There-

fore, the primary objective of the present work was to study

the phase stability and sinterability of synthesized HA ce-

ramics by wet precipitation method when doped with up to

1 wt% magnesium oxide (MgO).

II. METHODS & MATERIALS

The HA powder used in the present work was prepared according to a novel wet chemical method comprising pre-cipitation from aqueous medium involving calcium hydrox-ide and orthophosphoric acid [12]. In the current work, up to 1 wt% MgO was mixed into HA powder by wet milling. After the mixing, the wet slurry was dried, crushed and sieved to obtain fine powder. The green samples were uni-axial compacted at about 1.3 MPa to 2.5 MPa into rectangu-lar bar (4 x 13 x 32 mm) and circular discs (20 mm diame-ter) samples. The green compacts were subsequently cold isostatically pressed at a pressure of 200 MPa (Riken Seiki, Japan). This was followed by consolidation of the particles by pressureless sintering performed in air using a rapid heating furnace (ModuTemp, Australia), over the tempera-ture range of 1000ºC to 1400ºC, with ramp rate of 2

oC/min.

(heating and cooling) and soaking time of 2 hours for each firing. All sintered samples were then polished to a 1 µm finish prior to testing. The phases present in the powders and sintered samples were determined using X-Ray diffrac-

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tion (XRD) (Geiger-Flex, Rigaku Japan). The bulk densities of the compacts were determined by the water immersion technique (Mettler Toledo, Switzerland). The Young’s modulus (E) by sonic resonance was determined for rectan-gular samples using a commercial testing instrument (Grin-doSonic: MK5 “Industrial”, Belgium). The modulus of elasticity or Young’s modulus was calculated using the experimentally determined resonant frequency [13]. The microhardness (Hv) and fracture toughness (KIc) of the sam-ples were determined using the Vickers indentation method (Matsuzawa, Japan). The indentation load (< 200 g) was applied and held in place for 10 seconds. Five indentations were made for each sample and the average value was taken. The KIc value was calculated using the equation de-rived by Niihara [14].

III. RESULTS & DISCUSSION

The sinterability of the HA powder were compared in terms of HA phase stability, relative density, Vickers hard-ness, fracture toughness, grain size measurement and Young’s modulus.

All the samples, regardless of sintering conditions, did not show any cracking or distortion after sintering. The XRD traces indicate that the sintering of MgO-doped and undoped HA samples did not result in the formation of secondary phases throughout the sintering regime employed as typically shown in Figure 1.

a

b

c

d

e

f

increasing dopant

Fig. 1 XRD patterns of HA samples sintered at 1400ºC showing no signs

of HA decomposition. (a) undoped HA and HA containing (b) 0.05 wt%,

(c) 0.1 wt%, (d) 0.3 wt%, (e) 0.5 wt% and (f) 1 wt% MgO, respectively. The XRD results indicated that the phase stability of

HA was not disrupted by the sintering schedule and tem-perature, pressing conditions prior to sintering as well as the dopant addition.

Sintering at high temperatures, above 1300ºC, has been reported in the literature to be detrimental as HA phase instability was observed. However, in the present work, decomposition of HA phase was not observed even when

sintered at 1400ºC and thus contradicted the findings of Royer et al. [15] and Wang et al. [16]. In general, sintering of HA can lead to the partial thermal decomposition of HA into tricalcium phosphate (TCP) and/or tetracalcium phos-phate (TTCP) [17]. The thermal decomposition is normally accompanied in two steps i.e. dehydroxylation and decom-position. Dehydroxylation to oxyhydroxyapatite proceeds at temperatures about 850ºC to 900ºC by the fully reversible reaction in accordance to equation 1 [17]:

Ca10(PO4)6(OH)2 → Ca10(PO4)6(OH)2-2xOx + xH2Ogas (1)

The decomposition to TCP and TTCP occurs at tem-

peratures greater than 900ºC according to the reaction given in equation 2 [17]:

Ca10(PO4)6(OH)2 → 2Ca3(PO4)2 + Ca4P2O9 + H2Ogas (2)

According to these equations, both the dehydroxylation

and decomposition reactions include water vapour as a product. The rates at which these reactions proceed depend on the partial pressure of H2O in the furnace atmosphere [17]. Therefore, the secondary phase formation during sin-tering could be suppressed by controlling the moisture con-tent in the sintering atmosphere. The high moisture content present in the sintering atmosphere has the tendency to slow down the decomposition rate by preventing the dehydration of the OH group from the HA matrix. The difference in result in the present work with Royer et al. and Wang et al. could in part be attributed to the difference in relatively humidity in the sintering atmosphere and also the nature of the starting synthesized powder.

The densification curves as a function of sintering tem-peratures is shown in Figure 2.

3.000

3.040

3.080

3.120

3.160

1000 1050 1100 1150 1200 1250 1300 1350 1400

Sintering Temperature (ºC)

Bu

lk D

ensi

ty (

Mg

m-3

)

0

0.05 wt% Mg

0.1 wt% Mg

0.3 wt% Mg

0.5 wt% Mg

1 wt% Mg

Fig. 2 Bulk density variation as a function of sintering temperatures for

HA.

In general, the bulk density variation of all the compo-

sition studied exhibited a similar trend with increasing sin-

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tering temperature. A general observation that can be made from Figure 2 is that the dopant incorporated has marginal effect on the measured bulk density of HA samples. All the samples attained above 98% of theoretical density when sintered above 1050ºC.

The relationship between the Young’s modulus of the sintered body, sintering temperature and MgO additions are shown in Figure 3. In general, the inclusion of MgO in HA lattice, particularly for the higher dopant concentration, was found to be beneficial in enhancing the stiffness of the sin-tered HA body. As shown in Figure 3, the highest value of 136.3 GPa is recorded for HA samples containing 1wt% MgO and when sintered at 1150ºC.

90

95

100

105

110

115

120

125

130

135

140

1000 1050 1100 1150 1200 1250 1300 1350 1400

Sintering Temperature (ºC)

Yo

un

g's

Mo

du

lus

(GP

a)

a

0 0.05 wt% Mg

0.1 wt% Mg 0.3 wt% Mg

0.5 wt% Mg 1 wt% Mg

Fig. 3 The effect of MgO and sintering temperatures on the Young’s

modulus of HA The variation of the average Vickers hardness and frac-

ture toughness of samples sintered at various temperatures is shown in Figure 4 and Figure 5, respectively.

2

3

4

5

6

7

8

1000 1050 1100 1150 1200 1250 1300 1350 1400

Sintering Temperature (ºC)

Vic

ker

's H

ard

nes

s (G

Pa

) a

0

0.05 wt% Mg

0.1 wt% Mg

0.3 wt% Mg

0.5 wt% Mg

1 wt% Mg

Fig. 4 Effect of sintering temperature and MgO addition on the Vickers

hardness of HA.

The beneficial effect of MgO especially for the 1 wt% addition in enhancing the hardness and toughness of HA has been revealed. A general observation that can be made from Figure 4 is that the measured hardness of all the samples revealed a similar trend, i.e. the hardness increased rapidly to a maximum value and then decreased slowly with in-creasing sintering temperatures. For example, the hardness of the undoped HA ceramic peaked at 1050ºC (7.23GPa) and further increase in temperature > 1050

oC resulted in a

decrease in the hardness. The results also revealed that the addition of 1 wt% MgO was beneficial as samples exhibited higher hardness value in the sintering regime employed as compared to undoped HA. Throughout the studied range, the highest hardness value of 7.66GPa is obtained for HA doped with 1 wt% MgO and when sintered at 1150ºC.

0 .7 5

0 .8 5

0 .9 5

1.0 5

1.15

1.2 5

1.3 5

1.4 5

1.5 5

1.6 5

1.7 5

1.8 5

10 0 0 10 5 0 110 0 115 0 12 0 0 12 5 0 13 0 0 13 5 0 14 0 0

S inte ring Te m pe ra ture ( ºC )

0

0 .0 5 wt% M g

0 .1 wt% M g

0 .3 wt% M g

0 .5 wt% M g

1 wt% M g

Fig. 5 The effect of MgO addition on the fracture toughness of HA.

Generally, the fracture toughness of all compositions

exhibited very similar trend as the sintering temperature increased. Further analysis indicates that this trend is in good agreement with the variation in Vickers hardness as described earlier. The results also show that the addition of MgO was effective in enhancing the fracture toughness (KIc) of the synthesized HA, particularly when sintered at 1150ºC. The 1 wt% MgO-doped HA samples exhibited the highest fracture toughness of 1.48 ± 0.17 MPam

1/2 as com-

pared to 1.28 ± 0.05 MPam1/2

measured for the undoped HA. It should be highlighted here that the KIc value ob-tained for the MgO-doped HA is very encouraging, as most researchers had reported that the experimental KIc values for HA varied from 0.9 MPam

1/2 to about 1.2 MPam

1/2 [18, 19].

IV. CONCLUSIONS

The present research demonstrated that the additions of

small amount of magnesium oxide can be beneficial in

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enhancing the mechanical properties without affecting the

HA phase stability even when sintered at 1400ºC. MgO

additions, however was found to have a marginal effect on

the bulk density of sintered HA regardless of the sintering

temperature employed. The addition of 1 wt% MgO and

when sintered at 1150°C was found to be most beneficial as

the HA samples exhibited the highest Young’s modulus of

136.3 GPa, hardness of 7.66 GPa and fracture toughness of

1.48 MPam1/2

ACKNOWLEDGMENT

The authors gratefully acknowledge the support provided

by Universiti Malaya, UNITEN and SIRIM Berhad.

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Author: Tan Chou Yong

Institute: Universiti Malaya

Street:

City: Kuala Lumpur

Country: Malaysia

Email: chouyong@perdana.um.edu.my