Materials Science and Engineering C 29 (2009) 1674–1680
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Materials Science and Engineering C
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
The inﬂuence of sintering temperature on the properties of compacted
M.K. Herliansyah a,b,c,⁎, M. Hamdi a, A. Ide-Ektessabi c, M.W. Wildan b, J.A. Toque d
Department of Engineering Design and Manufacture, University of Malaya, Kuala Lumpur, Malaysia
Department of Mechanical Engineering, Gadjah Mada University, Yogyakarta, Indonesia
International Innovation Center, Kyoto University, Yoshida Honmachi, Kyoto, 606-8501, Japan
Department of Mechanical Engineering, University of the Philippines, Diliman, Quezon City, Philippines
a r t i c l e i n f o a b s t r a c t
Article history: On the sintering characteristic of hydroxyapatite (HA), the resulting microstructure and properties are inﬂuenced
Received 13 July 2007 not only by the characteristic and impurities of materials but also are found to be dependent on the thermal history
Received in revised form 6 August 2008 during the fabrication process. This research is concerned with the effect of sintering temperature on the relative
Accepted 12 January 2009
density, hardness, and phase purity after sintering process. Bovine HA (BHA) powder obtained from heated local
Available online 20 January 2009
cortical bovine bone at 900 °C for 2 h was uniaxially pressed at 156 MPa into green bodies using a 20 mm cylindrical
dies. The compacted green body was pressurelessly sintered in air atmosphere at temperatures ranging from 1000 to
Bovine hydroxyapatite 1400 °C, at a furnace ramp rate of 5 °C/min and dwell time of 2 h. The BHA starting powder was characterized using
Sintering temperature XRD and FTIR. SEM was also used for observing the microstructures of the starting material. The sintered BHA
Compacted specimens were analyzed using Archimedes method for measuring density; XRD for phase stability; and Vickers
Cortical bone method for hardness measurement. The analysis results show that the starting BHA powder and the sintered BHA
specimens contain HA. The intensity of the three main peaks of HA decreases with increasing sintering temperature
which may be due to decomposition of HA at high temperature. The density and hardness of BHA increases with
increasing sintering temperature based on the results obtained.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction Additionally, many investigations on HA have centred on a wide range of
powder processing techniques, composition and experimental conditions
In the last two decades, hydroxyapatite [HA, Ca10(PO4)6(OH)2] with the aim of determining the most viable synthesis method and
ceramics have attracted attention since it may be possible to use them conditions to produce well-deﬁned particle morphology [1–4].
as an alternative to autogenous free bone grafting, because of its excellent The sintering temperature and atmosphere are seen as important
osteoconductive and bioactive properties. This is due to its chemical factors that could alter the strength and toughness of HA . For
composition, biological, and crystallographic similarity with the mineral instance, sintering at elevated temperatures has the tendency to
portion of hard tissues . Despite its excellent biocompatibility, the eliminate the functional group OH in the HA matrix and this would
brittle nature and low fracture toughness of HA have limited its result in the decomposition of HA phase to form α-tricalcium phosphate
biomedical applications to non-load bearing applications . As a result, (α-TCP), β-tricalcium phosphate (β-TCP) and tetracalcium phosphate
a great deal of studies have been carried out to improve the mechanical (TTCP) . Kutty and Ramesh reported that decomposition of HA
properties of sintered HA [1,3,4]. Because of the attractive properties of HA suppressed densiﬁcation and was accompanied by a decreased in
powders, various techniques have been and are being developed to mechanical properties . Most of the researches in bovine Hydro-
produce hydroxyapatite. There are two main ways of producing HA; the xyapatite [19–21] were focused on synthesizing stoichiometric HA and
ﬁrst one is inorganic synthesis such as  wet-chemical method in producing nanocrystalline HA from bovine bone due to bovine bone has
aqueous solutions , sol–gel method [7,8], hydrothermal method , a high potential as a raw material of natural HA because it is
thermal deposition , continuous precipitation  and solid state morphologically and structurally similar to human bone , easy to
reaction method . The other technique involves extracting HA from obtain, low cost, available in unlimited supply and simple to process.
natural sources such as corals , egg shells [13,14], cuttleﬁsh shells However, effect of sintering temperature to mechanical properties of
[15,16], natural gypsum , natural calcite  and bovine bone [19–21]. Natural HA (HA from natural source) especially HA from bovine bone
(BHA) has not been fully understood and researches in this area are still
⁎ Corresponding author. Department of Mechanical Engineering, Gadjah Mada University,
wide open. In this paper, the effect of sintering temperature on the
Yogyakarta, Indonesia. relative density, hardness, and phase purity of compacted bovine
E-mail address: firstname.lastname@example.org (M.K. Herliansyah). Hydroxyapatite (BHA) powder was studied and reported. The results
0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
M.K. Herliansyah et al. / Materials Science and Engineering C 29 (2009) 1674–1680 1675
of the investigation are also expected to shed light on the preparation of
dense Hydroxyapatite bone graft for medical application.
2. Experimental procedures
2.1. BHA powder preparation
Cortical bovine bones were collected from local slaughter houses. The
procured bone samples were cleaned from macroscopic adhering
impurities and substances, which include the ligaments and tissues
stuck on the bone. The clean bone samples were defatted using boiling
followed with sun drying to remove organic substances and collagen. This
was done to avoid soot formation in the material during the heating
process. The dried cortical bone samples were cut into rectangular prism
shape (10 mm ×10 mm×5 mm) using hacksaw, followed by heating in air
atmosphere at 900 °C; with a temperature rate of 5 °C/min. The
temperature was maintained for 2 h to remove the organic matrix. The
Fig. 1. The microstructure of raw bovine bone was highly dense due to the presence of bovine bone pieces were cooled to room temperature by slow furnace
organic substances impregnated with inorganic minerals associated with the bovine bone. cooling then crashed using mortar pestle and sieved to obtain BHA
powder. Only BHA powder with particle size less than 400 μm was used in
2.2. BHA powder characterization
The phases present in the BHA powder was analyzed using XRD (X-ray
Diffraction, model XRD 6000 Shimadzu) with a monochromated CuKα
radiation. A scan speed of 7°/min and a step scan of 0.02° were used.
Fourier transform infrared (FTIR) spectrum of the powder was obtained
using a Nicolet Avatar 360 FTIR spectrometer. Prior to FTIR analyses, the
powder was mixed with KBr and pressed into pellets. Morphology of the
bovine bone (before and after heating process) and BHA powder were
examined using scanning electron microscope (SEM: Hitachi S4200B).
The samples were mounted on steel stub pins and subsequently coated
with palladium (Pd) using a sputter coater (E1030 Ion Sputter, Hitachi) to
provide a conducting layer and to prevent charging from occurring in the
microscope. The BHA powder particle size distribution was analyzed by
screening method using sieve shaker machine (Retsch AS200). Screening
analysis was done using stack of screens of increasing size with the
Fig. 2. The microstructure of raw bovine bone after heating at 900 °C for 2 h shows smallest opening size at the bottom. (Start from 20 μm, 45 μm,150 μm and
numerous pores and few small intergranular or intragranular pores. 300 μm on the top of the screen stack). The BHA powder (300 gram) was
Fig. 3. XRD pattern of BHA obtained from heated bovine bone at 900 °C shows that this material is highly crystalline and correspond to the characteristic peak of HA (JCPDS No. 9-432).
1676 M.K. Herliansyah et al. / Materials Science and Engineering C 29 (2009) 1674–1680
Peaks position of BHA powder and HA JCPDS 9-432.
BHA Plane JCPDS 9-432
2-Theta d h k l 2-Theta d
21.760 4.0810 2 0 0 21.819 4.0701
22.840 3.8904 1 1 1 22.902 3.8800
25.320 3.5147 2 0 1 25.354 3.5101
25.840 3.4451 0 0 2 25.879 3.4400
28.120 3.1708 1 0 2 28.126 3.1701
28.900 3.0869 2 1 0 28.966 3.0801
31.760 2.8152 2 1 1 31.773 2.8141
32.160 2.7811 1 1 2 32.196 2.7780
32.900 2.7202 3 0 0 32.902 2.7200
34.040 2.6317 2 0 2 34.048 2.6311
35.440 2.5308 3 0 1 35.48 2.5281
39.280 2.2918 2 1 2 39.204 2.2961
39.780 2.2642 3 1 0 39.818 2.2621
41.980 2.1504 3 1 1 42.029 2.1481
43.860 2.0625 1 1 3 43.804 2.0650
Fig. 5. BHA particle size distribution shows that the average size of the powder used in
45.280 2.0011 2 0 3 45.305 2.0000
this research was between 45 μm and 150 μm.
46.680 1.9443 2 2 2 46.711 1.9431
48.080 1.8909 3 1 2 48.103 1.8900
48.580 1.8726 3 2 0 48.623 1.8710
49.460 1.8413 2 1 3 49.468 1.8410
the theoretical density of hydroxyapatite as 3.156 g cm− 3. The crystalline
phases present in the samples were identiﬁed by X-ray diffraction (XRD) in
reference to standard JCPDS 9-432 cards available in the system software.
loaded onto the top of screen and the screen stack is vibrated for a period Hardness of the sintered compacts was measured using a Vickers indenter
60 min. After vibration, the amount of powder in each size interval is at a load of 153.2 N (15.62205 kgf) applied for 10 s. Prior to indentation, the
weighed and the interval percent calculated for each size fraction. The surface of the sample was polished to mirror surface ﬁnish. Average
particle size data were plotted in a particle size distribution histogram. hardness value was taken from three indents for each sample.
2.3. Sample preparation
3. Results and discussion
The BHA powder was uniaxially pressed at 156 MPa into green bodies
The BHA powder obtained using the method mentioned in the
using a 20 mm cylindrical dies. The compacted green body was
preceding section has white color and small particle size. The
pressurelessly sintered in air atmosphere at various temperatures of
microstructure of the raw bovine bone with its various pores size is
1000, 1100, 1200, 1300, and 1400 °C, at a furnace ramp rate of 5 °C/min
shown in Fig. 1. The bovine bone contains large pores and small pores
and dwell time for 2 h.
but generally the microstructure of raw bovine bone was highly dense
due to the presence of organic substances impregnated with inorganic
2.4. Characterization of sintered body minerals associated with the bovine bone. After the bovine bone was
heated at 900 °C for 2 h, the microstructure changes due to the
The linear shrinkage of the sintered samples was determined by removal of the water (around 9 wt.%) and organic contain such as
comparing the difference in the diameter of the green body and sintered collagen (around 20 wt.%), proteins, polysaccharides, and lipids that
body. The density of the sintered samples was determined by Archimedes' are also present in small quantities. Only the hard inorganic part with
method using distilled water. The relative density was obtained by taking high porosity is left as shown in Fig. 2. This ﬁgure also shows the bone
Fig. 4. FTIR spectra of BHA obtained from heated bovine bone at 900 °C shows characteristic of IR spectroscopy of HA.
M.K. Herliansyah et al. / Materials Science and Engineering C 29 (2009) 1674–1680 1677
Chaki reported that the dehydroxylation phenomenon could also be
observed by simply comparing the XRD position of the sintered
material with that of the standard JCPDS data for stoichiometric HA
. Table 1 presents the position of XRD peaks of BHA powder which
corresponds to the plane (211), (300) and (202). The three main peaks
are very similar in position. It indicates that the heated bovine bone is
of hydroxyl carbonate apatite, which is beneﬁcial for biomedical
purposes due to its similarity with the bone apatite.
The XRD analysis of BHA powder conforms to the FTIR result as
shown in Fig. 4. The IR spectrum of BHA shows only the characteristic
absorption peaks of HA. A large number of bands in the spectra
(3570.91, 3440.24, 2076.38, 1993.10, 1459.73, 1413.12, 1101.13, 633.68,
and 568.91 cm− 1) match the bands in the HA reference spectrum and
are in agreement with literature data on HA [27–29].
The absorption peaks at 3570.91 and 633.68 cm− 1 may be due to the
presence of hydroxyl (OH−) group of BHA. The FTIR spectra also indicate
the presence of phosphate (PO3−) and carbonate (CO2−) ions in the heated
Fig. 6. Scanning electron micrograph of BHA powder exhibiting the irregular particles shape. bovine bones. Results show that IR spectrum of the BHA sample shows a
major peak at ~630 cm− 1 and ~3570 cm− 1 due to the presence of the
hydroxyl group. The intensity bands at about 1410 cm−1 and 1450 cm− 1 in
like characteristics of the sintered matrices. From the surface the spectrum of BHA powder are attributed to components of the ν3 mode
morphology, the pores seemed to be interconnected. This structure of a trace amount of CO2− and ν2 CO2− band at about 875 cm−1.
has high porosity, brittle and easy to be crushed using mortar and The particle sizes distribution of BHA powder after crushing using
pestle to produce BHA powder. mortar and pestle is shown in Fig. 5. The graph shows a broad range of
particle sizes distribution. The average particle size was between
45 μm and 150 μm. There is also presence of irregular particles shape
3.1. Powder characterization as shown in Fig. 6. The irregular morphology of BHA powder can give a
higher green strength (the interparticle bonds that form due to
The XRD pattern of the BHA powder is shown in Fig. 3 within the compaction) because of mechanical interlocking during green body
2θ range from 20° to 50°. The well resolved XRD peaks of BHA could be samples preparation or compaction process .
easily indexed on the basis of hexagonal crystal system of space group
P63/m  with respect to JCPDS ﬁle no. 9-432. The Bragg peaks at
~ 21, 22, 25, 28, 31, 32, 34, 35, 39, 41, 43, 45, 46, 48 and 49° (2θ) 3.2. Phase stability
observed for BHA corresponded to the characteristic peaks of
stoichiometric HA (JCPDS 9-432). XRD analysis also indicated the Fig. 7 shows the XRD plots of BHA specimens sintered at 1000, 1100,
absences of secondary phases, such as TCP or calcium oxide (CaO). 1300 and 1400 °C. Those XRD results show that the intensity of major
Thus, it can be said that BHA powder production method resulted in peaks of sintered BHA at 1000, 1100, 1300 and 1400 °C decreases with
high crystallinity and single phase HA and is also believed to be pure increasing sintering temperature which may indicate decomposition of
HA as indicated by the peak of the diffraction patterns (JCPDS 9-432). BHA. Ooi et al. and Toque et al. reported that the BHA may decompose to
The XRD pattern of the apatite has been indexed on the basis of α-TCP (alpha TCP), β-TCP (beta TCP), and T-TCP (tetra TCP) [19–20]. The
hexagonal structure and the lattice parameters obtained by the comparison of the three major peaks of BHA (i.e. the (h k l) of (211), (3 0 0)
standard least-squares reﬁnement of the diffraction lines. Wang and and (1 1 2)) sintered at 1000, 1100, 1300 and 1400 °C is shown in Fig. 8.
Fig. 7. X-ray diffraction pattern of BHA sintered for 2 h at (a) 1000 °C; (b)1100 °C; (c) 1300 °C; (d) 1400 °C (∇ = TTCP).
1678 M.K. Herliansyah et al. / Materials Science and Engineering C 29 (2009) 1674–1680
Fig. 8. The intensity of the three main peaks of BHA as a function of sintering temperature.
3.3. Relative density 1300 °C. This point marks the temperature where maximum
densiﬁcation starts to take place. It can be noted from Fig. 9 that the
The effect of sintering temperatures on the relative density and linear maximum densiﬁcation did not occur until the sintering temperature
shrinkage of BHA is shown in Fig. 9. It was found that the linear shrinkage was increased to 1200 °C.
increases from 1.59% at 1000 °C to 10.66% at 1400 °C with maximum
shrinkage of 10.81% occurring at 1300 °C. The relative density plot which 3.4. Vickers hardness
showed an increase in measured density from 76.52% (sintered at
1000 °C) to 87.80% (sintered at 1400 °C) also exhibited a similar trend. The The variation of the average Vickers hardness of samples sintered at
maximum density of N87% was measured for samples sintered between various temperatures is shown in Fig. 10. It can be noted that the lowest
1300 and 1400 °C (see Fig. 9). The trend of these measured densities hardness value of 0.311 GPa was measured for sample sintered at 1000 °C,
seems a little bit differ from the one presented by Ruys et al.  and also whereas the maximum hardness value of 2.403 GPa was obtained for
Muralithran and Ramesh . They reported that the density of sintered sample sintered at 1300 °C. According to Fig. 10, the hardness increases
HA decreased signiﬁcantly on the specimen sintered above 1350 °C slowly from 1000 to 1100 °C and then increased rapidly, by a factor of more
indicating decomposition of HA to TCP (tricalcium phosphate). than 200%, when the temperature was increased from 1100 to 1300 °C.
The particle size used by Ruys et al.  and Muralithran and However, further increase in temperature N1300 °C resulted in a decrease
Ramesh  is much smaller than that of used in this paper. Both of in the hardness property. The measured hardness value for sample
that research used commercial HA with average particle size of 4.2 μm sintered at 1400 °C was 1.346 GPa.
for Ruys et al.  and ~ 10.6 μm for Muralithran and Ramesh . The relatively low hardness obtained for BHA samples sintered at
Their results yielded high relative density at 1200–1350 °C. Although temperatures b1200 °C was not due to decomposition of HA but
the density of bovine HA in this research still increases up to 1400 °C attributed to the low bulk density of the material. The relationship
but the density value is lower than that of reported by both of them for between relative density and hardness is shown in Fig.10. Below 1300 °C
the corresponding temperature. Sintering of HA in air is complicated the hardness trend correlates with the change in relative density where
due to two possible processes in which dehydroxylation and the hardness increases with increasing relative density (see Fig. 10).
decomposition of HA at elevated temperature may occur . Another factor which could have, in part, contributed to the decline in
In general, both the curves in Fig. 9 resemble very closely to each hardness after sintering at elevated temperatures N1300 °C is the
other, exhibiting a sigmoidal shape with an inﬂexion point around decomposition of BHA which would have hindered the formation of
Fig. 9. The effect of sintering temperature on the relative density and linear shrinkage of BHA.
M.K. Herliansyah et al. / Materials Science and Engineering C 29 (2009) 1674–1680 1679
Fig. 10. The effect of sintering temperature on the relative density and Vickers hardness of BHA.
strong inter-particle bonding in the ceramic matrix. This result strongly follows: bulk density of N87% dense and has linear shrinkage of about
correlated with the result of XRD analysis (see Fig. 7.) that shown very 10.81%. Maximum hardness value of 2.403 GPa was also measured for
low peak intensity for BHA sintered at 1400 °C. Jarcho et al.  have samples sintered at 1300 °C.
reported that the presence of decomposition products at grain 6. As would be expected, sintering at lower temperatures (b1200 °C)
boundaries of HA sintered in air at 1250 °C for 1 h. Wang and Chaki resulted in lower density.
 have also agreed that the degradation of mechanical properties in 7. It has been shown in the present work that decomposition of BHA
HA was due to weakening of the grain boundary as a result of segregation was detrimental to sintering, densiﬁcation and hardness property.
of decomposition product at grain boundary regions. Therefore, based on
these observations and the results obtained in the present work, it can be Acknowledgment
inferred that both the presence of secondary phases and larger grain
sizes are detrimental to the densiﬁcation and hardness of BHA. The authors would like to thank JICA AUN/SEED Net for providing
In general, the hardness of sintered BHA in this research is lower the funds necessary to accomplish this project under its collaborative
than hardness of sintered HA presented by Muralithran and Ramesh research program for the year of 2006–2007.
 and Tan et al.  which may be attributed to the difference in the
characteristics of the starting powder. In addition, to improve the References
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