Biomoteria/s
17 (1996)2059-2064
0 1996 Elsevier Science Limited
in Great Britain. All rights reserved
Printed
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Effect of calcination on sintering of
hvdroxvaDatite
u
J
J
1
Horng Yih Juang and Min Hsiung Hon
Department
of Materials
Four different
hydroxyapatite
sintering
temperatures
powder
behaviours.
and distribution,
gated
by dilatometry
to dominate
(about
Keywords:
and Engineering,
(700-1000°C)
before
press
The results
which
found
strength
Science
changes
and density
the properties,
55 MPa) with finer
Hydroxyapatite,
were
forming
show
from
grain
calcination,
to study
treatment
Fluidity
0
of powder
sintering,
1996 Elsevier
bending
Tainan, Taiwan
treatment
the effect
increases
The sintering
at 900°C and sintering
size.
the average
behaviours
and driving
force
1250°C results
Science
strength,
of as-received
of calcination
on the
particle
were
for sintering
in a higher
size
investiwere
bending
Limited
particle
size distribution
1996
Shimadzu) using Cu Kcc radiation at 30mA, 30 kV.
Scans were performed for 28 values of 20-50°C at a
rate of 2°C min-‘.
The calcination treatment was applied by heating HA
powder in an oven with an Sic heater at a heating rate
of 5°C min-’
up to 700, 800, 900 and 1000°C
respectively to hold for 4 h, then cooled in the oven.
The calcined HA powder was then sieved through 60
mesh and pressed inside a steel die under a uniaxial
stress of 150MPa. For lubrication, the interior surface
of the die was coated with a suspension of stearic acid
in ethylene alcohol. The pressed green specimens
were then sintered in air for 1 h in the temperature
range of 1200-1400°C.
For particle size distribution
measurement,
the
samples were analysed by a sedigraph (Model 5100,
Micrometrics Instrument Corp.). Before measurement,
the powders were excited ultrasonically for 10 min.
The shrinkage of green compact with temperature
was measured
with a vertical
dilatometer
(high
temperature dilatometer, Setaram Corp.] at a heating
rate of 5°C min-‘.
The bulk densities of the sintered specimens were
measured by the Archimedes method. The morpholoand sintered
specimens
gies of powder
were
investigated
with
scanning
electron
microscopy
(Model no. Joel 5200, Joel Corp.) with an accelerating
voltage of 25 kV.
The bending strength was determined by 4-point
bending methods on a universal testing machine (AGS5OOD, Shimadzu, Japan) which applied force with
cross head speed of 0.5 mmmin~’ to the specimens
until failure and recorded the strength. The sintered
test bars with dimensions 40 x 4 x 3 mm3 were ground
(800 grit) on both sides before testing. Four specimens
were tested for a single point.
PROCESS
Sample preparation
Extra pure reagent HA (Hayashi, Japan) was used as the
starting material. X-ray diffraction was performed on
powder
compact
with a diffractometer
(XD-Dl,
to
for calcination
to monomodal.
measurement.
Commercially available hydroxyapatite (HA) is similar
in structure to the mineral phase of bone and therefore
holds great promise as a biomaterial because it has the
ability of bonding to bone. There have been various
publications
concerning the sintering of dense HA
owing to its potential
applications
in skeletal
reconstruction’-3.
On the contrary, as HA can be
prepared in very fine powder form and hence its
surface is considered to interact actively with the
it was also chosen as a
surrounding atmosphere,
potential candidate for a gas sensor materia14. Closer
inspection of these works indicates that calcination
treatment of HA powder at 800-900°C1’5-g
was often
applied before forming and sintering, but no further
discussion about why calcination
was utilized and
how calcination
affects the sintering behaviours or
microstructures of HA was given. Calcination is a heat
treatment process used to modify the raw powder to
acquire various properties for subsequent processing.
The effect of calcination
on powder properties,
compaction and sintering behaviour for other ceramics
was studied10-‘3.
In this study, four different temperatures were chosen
to calcine HA powder before forming to investigate how
calcination affects the powder properties and sintering
behavior of HA.
Correspondence
chosen
that calcination
Calcining
Kung University,
Cheng
and sintering
trimodal
Received 31 July 1995; accepted 30 January
EXPERIMENTAL
National
Dr H.Y. Juang.
2059
Biomaterials
1996.
Vol.
17 No. 21
2060
Effect of calcination
RESULTS AND DISCUSSION
Characteristics
and
MM.
Han
,
z shows
Sintering behaviour of powders
The linear shrinkage of HA green compact heated from
room temperature
to 1450‘C measured by dilatometry
at constant
heating
rate is shown
in Figure 3. It
indicates
that sintering
begins at higher temperature
for the calcined powders, whereas the final dimension
change does not vary apparently
for uncalcined
and
calcined 700 to 900 C HA powder compacts, which is
around 27%. The shrinkage
of 1000 C calcined
HA
compact is decreased to 17.5%.
Figure 4 is differentiated
from Figure 3. Because of
the constant heating rate, the meaning of shrinkage vs
temperature
as shown in Figure 4 is equivalent
to
shrinkage
vs time and can be considered
as the
sintering kinetics of HA powder compact. Calcination
at 700 C does not make a significant
difference
to the
sintering
behaviour
compared
with uncalcined
HA
powder compact. Its shrinkage rate is faster than those
of the other calcination
temperatures,
which indicates
that the sintering
reaches
the final stage at lower
temperature.
For samples calcined
at 800°C or over,
the shrinkage
rate decreases,
which
indicates
that
higher temperature
is necessary to reach the final stage
of sintering.
For the HA samples
uncalcined
and calcined
at
700 C the sintering begins at a lower temperature
and
Biomaterials
of HA: H.Y. Juang
of powders
the particle size distributions
of HA
powder
as-received
and
calcinated
at different
temperatures
for 4 h. The mean particle sizes of HA
powder for as-received
and calcined at 700, 800, 900
and 1000°C are 0.97, 1.09, 2.47, 3.01 and 4.19Aim
respectively.
Figure I indicates
that as-received
HA
powder has a wide particle size distribution which can
be classified into three populations
of sizes (trimodal).
The calcination
treatment mainly transforms the ‘finer’
population
to the
‘coarser’
population.
Although
increasing
the calcination
temperature
results
in a
larger mean particle size, the particle size distribution
becomes narrower. When the calcination
temperature
was up to 900°C the distribution
was found to contain
one population (monomodal).
Figure
Z(a)-(e)
shows
the effect
of calcination
treatment on the morphologies
of HA at temperatures
from 700 to 1000°C.
Many very small crystals
in
agglomerates
can be observed
which are formed by
Van der Waals forces between fine crystals. The results
obtained
from the sedimentation
method
can be
thought of as ‘agglomerate
size’ rather than ‘particle
size’.
The coalescence
of fine crystals
or agglomerates
formed in calcination
is due to initial stage sintering.
For higher temperature
more apparent agglomeration
can be observed, which is confirmed by the increase of
particle size distribution
measured by the sedimentation method.
The phases of HA powder as-received and calcined at
700 to 1OOO’C were investigated by XRD and no phase
transformations
were
detected.
Calcination
only
increased the crystallinity
of as-received HA powder.
Figure
90
on sintering
1996, Vol. 17 No. 21
1
100
1
10
0
ParticleSze(um)
Figure 1 Particle
size
received
and calcined
(u.c., uncalcined
HA).
distributions
at different
of HA powder
temperatures
for
as4h
the temperature
when shrinkage
stops is also lower.
However, higher temperature
is usually necessary
for
pore diminishing
in the final sintering stage, and the
grain growth also occurs simultaneously.
Therefore,
the chance
of grain
growing
accompanied
with
densification
is increasing
for HA as-received
and
calcined
700°C.
For 800 and 900°C
calcined
HA
powder
compact
higher
sintering
temperature
is
needed but the wider temperature needed for shrinkage
to proceed makes grain growth less possible.
A sudden decrease in total shrinkage of the compact
made of HA calcined
at 1000°C indicates
that this
calcination
temperature
is too high for powders and
makes the powder
too coarse,
which
inhibits
the
sintering behaviour of powder compact.
Figure 5 shows that the green and sintered bulk
densities depend on mean agglomerate size. The green
densities of the powder compact are 37.2, 38.2, 38.7,
43.6 and 46.5%
of theoretical
density for powders
uncalcined
and
calcined
from
700
to
1OOO’C
respectively,
which indicates that packing efficiency is
increased
with increasing
agglomerate
sizes owing to
an increase
of calcination
temperature.
Although
higher calcination
temperature
results in higher green
density, the final sintering does not follow the trend.
As stated previously,
the calcination
treatment
has
some delaying effect on the sintering. The as-received
and calcined
HA powder
compacts
reached
96%
theoretical
density at 125O‘C and temperature
increase
does not raise the density
significantly.
For HA
powder
calcined
at 800 and 9OO’C, similar
level
sintered bulk densities were obtained by sintering at
1300 C and the powder calcined at 1000°C was 1350-C.
Mechanical properties and morphology
Figure 6 shows the 4-point bending strength of sintered
HA specimens
for different calcination
temperatures.
The highest
strength
values were obtained
for asreceived
and 700, 800, 900°C calcined
HA powders
sintered at 1250-C with 40.5, 42.4, 49.6 and 54.8MPa
respectively,
whereas for the samples made of powder
calcined
at 1000 C the highest
bending
strength
(42.6 MPa) was obtained by sintering at 1400°C.
Effect
of calcination
on sintering
of HA: H.Y. hang
and M.H. Hon
2061
40-
04
02
1
-.5 oo5
-0.2-
-240 -26.0I
0
I
300
I
I
600
I
I
I
900
I
1200
:::j
I
1500
0
Temperature (“C)
Figure 3
Linear shrinkage
of green compact
powder
heated
from
room temperature
to
uncalcined
HA).
,
,
300
,
,
,
,
,
900
600
,
1200
,
,
1500
Temperature CC)
for calcined
1450°C (US.,
Figure 4
Linear shrinkage
rate
made of calcined
powder heated
1450°C (u.c., uncalcined
HA).
curves for green compact
from room temperature
to
and Chakig and Ruys et 01.~~ indicated
that the
decomposition
makes the mechanical
strength poorer
than HA, but Royer7 showed that the best mechanical
strength can be achieved for HA containing
TCP. The
Figure
7 shows the diffraction
patterns
of HA
specimens
sintered at 1400°C. It indicates that there is
no significant
decomposition
of HA into /?- or atricalcium
phosphate
(TCP) during sintering.
Wang
-
Biomaterials
1996,
Vol. 17 No. 21
2062
Effect
of calcination
24
1.0
00
4.0
3.0
2.0
Mean Agglomerate
5.0
Figure 5 The dependence
of green
and sintered
bulk
densities
on mean agglomerate
size of powders
by calcining at different
temperatures
(u.c., uncalcined
HA; TD,
Theoretical
Density).
0.00
’
J_
I
I
1250
1150
Sintering
Figure 6
specimens
tures.
I
YC
/
1
1350
Temperature
1450
Four-point
bending
strength
of sintered
HA
made of powders
calcined
at different
tempera-
1996,
Vol.
17 No. 21
32
36
and
M.H.
Han
40
Figure 7 X-ray diffraction
patterns
of sintered
specimens
at 1400°C
made
of HA powders.
(a) As-received
and
calcined
at: (b) 700°C; (c) 800°C; (d) 900°C; (e) 1000°C (H,
hydroxyapatite
peak).
According to the Hall-Petch
equation, crf = o, + kd-‘!‘,
where err is fracture strength, d is grain size, k and o0
are constants,
and
the
strength
decreases
with
increasing
grain size. The sample made of powder
calcined
at 900°C and sintered
at 1250°C
has the
smallest
grain size and thus the highest
strength.
Heating
over 1250°C
results
in grain growth
and
strength decrease. Although a finer grain size can be
attained before 125O”C,
many porosities
exist in the
sample,
which
govern
the strength.
For a porous
ceramic
material,
the relation between
strength and
porosity essentially
agrees with of = (T, exp (-bp),
in
which
p designates
the residual
porosities.
Below
125O”C,
the effect of decreasing
grain size cannot
compensate
the decrease of strength by the effect of
increase of porosity.
For the HA powder calcined at lOOO”C, the densification is inhibited because of coarse agglomerate; thus, a
higher temperature
(1350°C at least) is necessary
for
sintering to obtain a similar level of strength as the others.
(“c)
effect of TCP on the mechanical
strength
remains
unclear.
In this study, because
no significant
phase
transformation
occurs during sintering, morphology
is
considered
as the most significant factor governing the
strength.
Figures
8(a)-(e)
are SEM
morphologies
showing
the grain size and porosity
for different
temperature
calcined
HA sintered
at 1250°C.
As
indicated in Figure 5, the relative densities of sintered
HA for uncalcined
and calcined
at 700 to gOo”C
powder are all over 98% and no significant
increment
can be made on the densities. With the same density
level (98%), grain size is thought to be the dominant
factor for mechanical
properties
of sintered samples.
Biomaterials
28
of HA: H.Y. Juang
2%
Size (urn )
,
I
on sintering
CONCLUSIONS
Calcination
changes the particle size distribution
of
as-received
HA powder from multi-modal
to unimodal because
of coalescence
of finer particles
during calcination,
but no phase transformation
occurs.
Calcination
treatment at 700°C does not affect the
sintering behaviours
of HA significantly:
however,
calcination
at 800°C or over delays the initiation of
sintering
and the shrinkage
proceeds
in a wider
temperature
range, which is thought to decrease
the probability of grain growth during sintering.
Finer HA powder in uncalcined
HA provides more
surface as driving force of sintering; however, Van
der Waal force then becomes
more significant,
which retards the packing of HA powder. Calcina-
Effect
of calcination
on sintering
Figure 8 Morphologies
e, lOOO”C, then sintered
4.
of HA: H.Y. Juang and M.H. Hon
of sintered
at 1250°C.
HA samples
made
of powders.
tion increases the efficiency of powder packing,
though driving force is sacrificed in the mean time.
In this study, calcination treatment during TOO to
900°C is beneficial for sintered specimens’ properties because of a compromise between the two
effects described above.
The sample made of powder calcined at 900°C and
sintered at 1250°C results in the best mechanical
properties (about 55 MPa) because of the finer grain
size.
Calcination
not only improves the properties
of
sintered HA, it is also beneficial for forming process
like injection moulding, slip casting etc., for various
kinds of practical
applications
because of better
packing and fluidity capability.
2063
a, Uncalcined
Williams
2
5
6
7
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