Ceramics International xxx (xxxx) xxx–xxx

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Ceramics International
journal homepage: www.elsevier.com/locate/ceramint

Enhancing the physical and mechanical properties of pellet-shaped

hydroxyapatite by controlling micron- and nano-sized powder ratios
Ade Indraa,b, Ridwan Firdausa, Ismet Hari Mulyadib, Jon Affib, Gunawarmanb,∗
a
Faculty of Engineering, Department of Mechanical Engineering, Institut Teknologi Padang, Kp Olo, 25143, Padang, West Sumatera, Indonesia
b
Faculty of Engineering, Department of Mechanical Engineering, Universitas Andalas, Limau Manis, 25166, Padang, West Sumatera, Indonesia

ARTICLE INFO ABSTRACT

Keywords: Hydroxyapatite (HA) was fabricated in microns as its basic size. The particle size distribution was controlled by
Hydroxyapatite mixing micron- and nano-sized HA to obtain the optimum amount of mixture to improve its properties. HA
Uniaxial pressing powder with a size of 2.5 μm was mixed with that with a size of 200 nm, with a variety of concentrations of up to
Sintering 20 wt%. A green body was fabricated using the uniaxial pressing method at a pressure of 200 MPa. The sintering
Strengthening mechanism
process was conducted at a temperature of 1200 °C, heating rate of 3 °C/min, and holding time of 2 h in air. The
physical characteristics of the HA sintered body were determined using X-ray diffraction, scanning electron
microscopy, linear shrinkage, and density testing. The mechanical properties of the HA sintered body were tested
using compressive strength testing. The test results indicated that the mechanical properties of the HA sintered
body increased with the addition of nano-sized HA. The mechanism of the increasing strength occurred because
nano-sized HA particles filled the gaps between the micron-sized particles. In this study, the highest mechanical
properties were obtained by adding 20 wt% nano-sized HA. The compressive strength in the sample without
added nano-sized HA was 132.2 MPa and increased significantly to 208.6 MPa with the addition of nano-sized
HA of 20 wt%. No change in the phase in HA was observed within a sintering temperature of 1200 °C.

1. Introduction In addition, adding binders that increase the tangential contact

between particles when fabricating green bodies can enhance HA me-
A biomaterial is a synthetic material functioning as a substitute for chanical properties. This method is useful in producing strong sintered
parts of living systems through direct contact with living tissues [1]. bodies, as confirmed by some studies, such as those on HA/bredigite
Hydroxyapatite (HA), Ca10(PO4)6(OH)2, is a superior biomaterial that is using uniaxial pressing at 200 MPa with 5 wt% polyvinyl alcohol (PVA)
often used for implants because of its positive biocompatibility with the [26] and β-TCP/PEG using uniaxial pressing at 8 MPa pressure with
human body [2–11]. It is also a major inorganic component of bone 10 wt% PVA [27]. Recently, we conducted a study to determine the
hard-tissues and accounts for 60–65% of the mineral phase in human optimum amount of PVA to use as a binder. The addition of PVA of
bones [12]. However, its poor mechanical properties such as brittleness 2.5–10 wt% was observed to increase the tangential bonding between
and low fracture toughness inhibit its application to bone implants that particles. The hardness of a sintered body was observed to significantly
support heavy loads [13,14]. increase from 4.4 to 6.0 GPa by the addition of 5 wt% PVA [28].
These poor mechanical properties can generally be improved by Another factor in improving HA mechanical properties is the sin-
fabricating composite products such as mullite-CNT [15] and mullite- tering process at the correct temperature. A study reviewed the effect
TiB2-CNTs hybrid composites [16]. Many studies have been conducted and behavior of sintering on the properties of commercial HA. In that
on the fabrication of HA composites with metals and ceramic materials, study, the sintering temperature varied from 1000 to 1450 °C with a
such as HA/Al2O3 [17,18], HA/t-ZrO2 [19], HA/P2O5 [20], HA/silica- holding time of 2 h. The optimum sintering temperature was obtained
coated graphene (S-G) [21], and HA/Ag [22]. In some studies, HA was at 1250 °C for micron-sized HA particles and was proved by a relative
observed to improve in its mechanical properties by creating functional density of > 99% and hardness value of 6.08 GPa [29].
coatings on the surface of metal products [23–25]. Although mechan- Mechanical properties are influenced by the shape and size of par-
ical properties improve, HA composites with other materials are con- ticles, as well as their size distribution [30]. In this study, the latter was
versely considered to exhibit decreased biocompatibility. an interesting observation. However, we did not found any research

∗
Corresponding author.
E-mail address: gunawarman@eng.unand.ac.id (Gunawarman).

https://doi.org/10.1016/j.ceramint.2020.03.136
Received 31 January 2020; Received in revised form 28 February 2020; Accepted 12 March 2020
0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Ade Indra, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.03.136
A. Indra, et al. Ceramics International xxx (xxxx) xxx–xxx

Fig. 1. Illustration of sample preparation.

Table 1
Main peaks of HA powder from XRD test.
HA 2.5 μm HA 200 nm 80:20 wt% HA reference code 01-072-1243

Position [o2θ] Intensity [%] Position [o2θ] Intensity [%] Position [o2θ] Intensity [%] Position [o2θ] Intensity [%]

31.7403 100.00 31.6145 100.00 31.6608 100.00 31.741 100.0

32.8353 54.38 32.8487 43.32 32.8300 66.11 32.179 45.3
34.0639 24.62 33.9486 26.49 34.0373 29.89 32.868 56.4

2. Material and method

2.1. Materials

This study employed analytical-commercial powdered HA in two

sizes, i.e., 2.5 μm and 200 nm, which were obtained from Sigma-Aldrich
Co., USA. PVA (liquid) (UD. Jaya Kimia., Indonesia) was used as the
lubricant. All the materials were directly used without any treatment.
Pure water (product of PT. Brataco., Indonesia) was used in all the
experiments.

2.2. Method

2.2.1. Powder characterization

The crystalline phases for both HA types and a mixture of micron-
and nano- HA were characterized using X-Ray diffraction (XRD-
Fig. 2. XRD of HA powder (a) HA powder mixture of 2.5 μm:200 nm (80:20) wt PANalytical, Type PW3040/60, Netherlands). The X-ray diffractometer
%, (b) HA powder of size 2.5 μm, (c) HA powder of size 200 nm, (d) HA re- was conditioned to use cupper radiation as an anode (CuKα,
ference code 01-072-1243. λ = 1.54060 Å, 2θ = 10–100) at 40 kV and 30 mA. The scan time in
the step was 7.14 s. The HA powder was observed using scanning
that focused on controlling the particle size distribution to improve electron microscopy (SEM-Hitachi Horiba S–3400 N, Japan).
mechanical properties.
In this study, HA sized at 2.5 μm was mixed with that sized at 2.2.2. Sample preparation
200 nm to obtain a controlled particle size distribution. This was as- Two types of HA powder, i.e., sized 2.5 μm and 200 nm, were mixed
sumed to improve the mechanical properties of the products after the with varying compositions of 100:0, 95:5, 90:10, 85:15, and 80:20 wt%.
sintering process. Each composition was mixed using a rotary drum mixer (drum size:
diameter = 40 mm, length = 55 mm, PM-MELab, ITP), at a rotation of

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Fig. 3. SEM image of HA powder, 2000 × (a) powder of size 2.5 μm, (b) powder of size 200 nm.

Table 2
Main peaks of HA sintered body from XRD test.
100:0 wt% 90:10 wt% 80:20 wt% HA reference code 01-072-1243

o o o
Position [ 2θ] Intensity [%] Position [ 2θ] Intensity [%] Position [ 2θ] Intensity [%] Position [o2θ] Intensity [%]

31.7664 100.00 32.2112 100.00 32.0159 100.00 31.741 100.0

32.9179 88.90 33.3605 81.79 33.1725 72.09 32.179 45.3
34.1144 15.54 34.5348 19.33 34.4834 4.98 32.868 56.4

Fig. 5. Effect of adding nano-sized HA on linear shrinkage of the sintered body.

compressive strength. The linear shrinkage was indicated by the dif-

ferences in diameter and weight between the green and sintered bodies
due to shrinkage after sintering. The actual density was measured by
adopting Archimedes' principle using pure water, in which the results
Fig. 4. XRD of HA sintered body, sintered at a temperature of 1200 °C, heating were then divided by the theoretical density of HA, which was 3.156 g/
rate of 3 °C/min, holding time of 2 h, (a) 80:20 wt%, (b) 90:10 wt%, and (c) cm3 in this study [29,31–35], to obtain the relative density value. The
100:0 wt%, (d) HA reference code 01-072-1243. microstructure of the sintered body, which was previously polished and
thermally etched at 1000 °C, was examined using the Hitachi Horiba
30 rpm for 2 h. The drum mixer had 20 pieces of 4.8-mm steel balls in S–3400 N SEM. Finally, the value of the compressive strength was
it. Before being compacted, 5 wt% PVA was added to each composition tested using the universal testing machine LCT-15T, which was
as a binder and the composition was mixed for 1 h using the rotary equipped with a DAQ 16-bit data acquisition system. Both surfaces of
drum mixer. The samples were compacted using a pellet-shaped uni- the loaded sample were previously levelled. The tests were conducted
axial press at a pressure of 200 MPa. The resulting green body was at room temperature (25 °C) and by referring to the ASTM-C1424
8 mm in diameter and 5 mm thick. The sintering process (using a Standard for Monotonic Compressive Strength of Advanced Ceramics.
Nabertherm furnace, Germany) was conducted in an air environment at
a temperature of 1200 °C, heating rate of 3 °C/min, and holding time of
3. Results and discussion
2 h. Fig. 1 illustrates the preparation of the samples.
3.1. Powder characterization
2.2.3. Characterization of the sintered body
The characteristics of the sintered body, including the physical and The characterization of the crystalline phase was conducted using XRD
mechanical properties, were examined thoroughly. The physical prop- for each particle size group and mixture to ensure the purity of the com-
erties included linear shrinkage, density, relative density, and micro- mercial HA used in this study. Only those with a variation of 80:20 wt%
structure, and the mechanical properties were evaluated from the were considered. The results, as illustrated in Table 1 and Fig. 2, indicated

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Table 3
Properties of HA sintered body.
Sample Linear Shrinkage Density (g/cm3) Relative Density (%) Compressive Strength (MPa)

Diameter shrinkage (%) Weight Shrinkage (%)

100:0 wt% 18.09 ± 0.28 13.67 ± 0.41 2.928 ± 0.017 92.76 ± 0.55 132.2 ± 8.5
95:5 wt% 17.09 ± 0.22 13.78 ± 0.44 2.963 ± 0.023 93.88 ± 0.74 135.7 ± 8.5
90:10 wt% 16.02 ± 0.21 13.19 ± 0.55 2.973 ± 0.029 94.22 ± 0.92 163.7 ± 12.6
85:15 wt% 13.49 ± 0.36 12.99 ± 0.58 3.023 ± 0.017 95.80 ± 0.53 166.7 ± 6.1
80:20 wt% 13.80 ± 0.47 12.65 ± 0.46 3.041 ± 0.019 96.34 ± 0.62 208.6 ± 6.7

Number of samples: linear shrinkage = 15, Density = 15, and Compressive strength = 12.

Fig. 6. SEM image of HA sintered body (a–e) 4000 × , (f) 10000 × , sintering temperature at 1200 °C, (a) 100:0 wt%, (b) 95:5 wt%, (c) 90:10 wt%, (d) 85:15 wt%,
and (e)–(f) 80:20 wt%.

that the main peaks from the 2-theta position of the 2.5 μm HA powder 3.2. Characterization of the sintered body
were identified at the angles of 31.74, 32.83, and 34.06. The main peaks of
the 2-theta position for the 200 nm HA powder were observed at the 3.2.1. X-ray diffraction
angles of 31.61, 32.84, and 33.94. These results were consistent with those The characterization of the crystalline phases of the HA sintered
observed in previous studies [36,37]. In the 80:20 wt% mixture of 2.5 μm bodies with the compositions of 100:0, 90:10, and 80:20 wt% were
and 200 nm HA powders, the main peaks of the 2-theta position were at observed at the angles of 31.76, 32.91, and 34.11; 32.21, 33.36, and
the angles of 31.66, 32.83, and 34.03. These values were similar to a 34.53; and 32.01, 33.17, and 34.48, respectively. No change oc-
powder diffraction pattern from the International Centre Data Diffraction curred in all the samples. The crystallinity would change if there was
with a reference code of 01-072-1243 and chemical formula of Ca10(PO4) a significant difference in sintering temperature [40]. Another study
6(OH)2. Fig. 3 shows the characterization of the forms of HA powders demonstrated that the HA phase change occurs at a sintering tem-
sized 2.5 μm and 200 nm, i.e., the micron- and nano-spherical forms, re- perature of 1300 °C [41]. Additionally, the results were consistent
spectively [38,39]. Fig. 3 (a) indicates that the powder size distribution with those of commercial HA, as reported in previous studies
varied from 0.5 to 3 μm, and Fig. 3 (b) indicates that it varied according to [39,42]. The results of the characterization are shown in Table 2 and
the initial specification, i.e., below 200 nm. Fig. 4.

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Fig. 7. Effect of adding nano-sized HA on the density and relative density of the
sintered body.

Fig. 9. Illustration of the strengthening mechanism (a) micron size powder

only, (b) micron size powder with the addition of nano-sized powder.

Fig. 8. Effect of adding nano-sized HA on the compressive strength of the sin-

tered body.

3.2.2. Linear shrinkage

Fig. 5 shows the effect of adding the nano-sized HA on the linear
shrinkage of the sintered body. The diameter of the sintered body
contracted as the percentage of added nano-sized HA increased. A de-
crease from 18.09 to 13.49% occurred in the addition of nano-sized HA
of 0–15 wt% (Table 3). No significant change was observed in the ad- Fig. 10. Effect of adding nano-sized HA on compressive strength and diameter
dition of nano-sized HA of 15–20 wt%. The addition of nano-sized HA shrinkage of the sintered body.
of 15 wt% was assumed to be the optimum condition of the mixture
between the micron- and nano-sized HA. The diameter shrinkage me-
between the micron-sized powder particles. Fig. 6(d) and (e), respec-
chanism probably occurred because the pores between micron particles
tively, indicate that in the 85:15 and 80:20 wt% mixtures, nano-sized
were filled by the nano-sized particles, causing a decrease in the dia-
powders filled the pores between micron-sized ones, causing the density
meter shrinkage. Another study reported that the linear shrinkage of
of the sintered body to increase. Fig. 6 (f) shows the comparison be-
HA at a sintering temperature of 1200 °C was approximately 19% [43].
tween the micron- and nano-sized powders. Nano-sized powders were
The shrinkage of weight did not change significantly because the two
considered to experience grain growth at a sintering temperature of
HA powders used were of the same type but differed in size only.
1200 °C, as the temperature of nano-sized HA sintering is reported to
range from 800 to 1000 °C [44].
3.2.3. Characteristics of the microstructure
Fig. 6 shows the SEM results of the HA sintered body. Fig. 6 (a) 3.2.4. Density and relative density
indicates that many larges pores were observed in the sintered body of Both the density and relative density of the sintered body are shown
pure micron-sized powder. Fig. 6 (b) shows that the pores of the sin- in Fig. 7. The results indicated an increase in density. The relative
tered body with a mixture of micron- and nano-sized powders at a ratio density of the sintered body to which nano-sized HA of 0 wt% was
of 95:5 wt% were still relatively large but smaller than those in Fig. 6 added was 92.76%. It continuously increased to 96.34% (Table 3), i.e.,
(a). Fig. 6 (c) shows that the sintered body with a mixture of micron- until the addition of nano-sized HA of 20 wt%. This value was similar to
and nano-sized powders at a ratio of 90:10 wt% had fewer pores. The the results of the research stating that a nano-sized HA sintered body
addition of nano-sized powder predictably minimized the pores has a relative density of 95.94% at a sintering temperature of 1250 °C

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Table 4
Comparison of the performance of physical and mechanical properties with other studies.
HA (powder size compacting pressure Sintering temperature (oC/ Relative density (%) Linear shrinkage (%) Compressive strength References
(MPa) holding) (MPa)

HAp (nano) 20 1150 90 – 200 [14]

1250 96 – 230
HA (< 25 μm) 10 1200/3 h – – 115 [45]
HA 200 1200/2 h – 19 – [43]
HA (nano) 1250/3 h – – 120 [46]
HA (nano) 285 1250/40 min 95.94 ± 2.64 – – [11]
HA (nano) 200 950/2 h – – 170 [47]
HA (micron) 200 1200/2 h 96 31 – [29]
HA (micron) 25 1300/2 h 98 – – [8]
HA (nano) 1200/2 h – 7.2 157 ± 2 [48]
HA (nano) 200 1000/15 min – – 153.6 [49]
HA (micron/nano 80:20 wt 200 1200/2 h 96.36 ± 0.62 13.80 ± 0.47 208.6 ± 6.7 present study
%)

[11]. The increase in density and relative density was considered to be Declaration of competing interest
caused by the addition of nano-sized powder, resulting in an increase in
density and a decrease in porosity, as shown in Fig. 6. As Fig. 6 (a)–(c) The authors declare that they have no known competing financial
show, pores were observed between micron-sized particles with a di- interests or personal relationships that could have appeared to influ-
minishing number. In Fig. 6 (d)–(e), no pores were observed, because ence the work reported in this paper.
nano-sized powders filled the gaps between micron-sized powders. The
value of relative density in this study was approximate to that in other Acknowledgement
previous studies, where the HA powder size was 0.4–10 μm, compacted
at 200 MPa at a sintering temperature of 1200 °C, and obtained a re- This work was funded by the Doctoral Dissertation Research Grant,
lative density value of approximately 96% [29]. The value of the re- Ministry of Research Technology and Higher Education, Indonesia
lative density would still increase if the sintering temperature increases [Grant No. 051/SP2H/LT/DRPM/2019]. The authors would like to
to 1250 °C [44]. thank the Mechanical Engineering Laboratory of Institut Teknologi
Padang and Metallurgy Laboratory of Universitas Andalas for sup-
3.2.5. Compressive strength porting this study.
Fig. 8 shows that compressive strength increased with the addition
of nano-sized HA. The compressive strength in the pure sample was Appendix A. Supplementary data
132.2 MPa and increased with the addition of nano-sized HA of 5, 10,
15, and 20 wt%. The highest compressive strength in this study, with a Supplementary data to this article can be found online at https://
significant increase of 57.8%, was 208.6 MPa, for the addition of nano- doi.org/10.1016/j.ceramint.2020.03.136.
sized HA of 20 wt% (Table 3). A nano-sized HA body sintered at
1250 °C was previously reported to have a compressive strength of References
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