Biomaterials 23 (2002) 4263–4275
Microstructural dependence on relevant physical–mechanical
properties on SiO2–Na2O–CaO–P2O5 biological glasses
V. Rajendrana,*, A. Nishara Beguma, M.A. Azoozb, F.H. El Batalb
a
Department of Physics, Mepco Schlenk Engineering College, Mepco Engineering College (PO), Virudhunagar (DT), Tamilnadu 626 005, India
b
Glass Research Department, National Glass Research Centre, Dokki, Cairo, Egypt
Received 14 June 2001; accepted 16 May 2002
Abstract
Bioactive glasses of the system SiO2–Na2O–CaO–P2O5 have been prepared by the normal melting and annealing technique. The
elastic moduli, attenuation, Vickers hardness, fracture toughness and fracture surface energy have been obtained using the known
method at room temperature. The temperature dependence of elastic moduli and attenuation measurements have been extended
over a wide range of temperature from 150 to 500 K. The SiO2 content dependence of velocities, attenuation, elastic moduli, and
other parameters show an interesting observation at 45 wt% of SiO2 by exhibiting an anomalous behaviour. A linear relation is
developed for Tg ; which explores the influence of Na2O on SiO2–Na2O–CaO–P2O5 bioactive glasses. The measured hardness,
fracture toughness and fracture surface energy show a linear relation with Young’s modulus. It is also interesting to note that the
observed results are functions of polymerisation and the number of non-bridging oxygens (NBO) prevailing in the network with
change in SiO2 content. The temperature dependence of velocities, attenuation and elastic moduli show the existence of softening in
the glass network structure as temperature increases. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Ultrasonic velocity measurements; Bioactive glasses; Elastic properties; Fracture toughness and fracture surface energy; Structural
softening
1. Introduction
In recent years biomaterials, namely traditional
ceramics, metals, bioactive glass and bioglass ceramics
or a combination of these materials have gained more
interest in the field of medicine in view of their variety of
potential applications such as replacement of damaged/
diseased body parts [1–3]. The most important biomaterial in this series, which finds immense application in
the field of medicine, is bioactive glasses. Some of the
bioactive glasses and bioglass ceramics with a specific
composition will form bond with natural bone and
hence, it is known as bioactive glass ceramics [4].
For the effective bonding of the bioactive glass to
bone, a selection of proper composition is more
essential. The suitable composition of the bioactive
glasses has been selected through an optimisation
procedure [5]. Thus, several compositions of the
*Corresponding author. Tel.: +91-4562-351720; fax: +91-4562351520.
E-mail address: vee rajendraan@yahoo.com (V. Rajendran).
bioactive glasses have been developed with an aim to
improve the bioactivity and mechanical properties. The
optimisation of bioactive glasses also require the proper
understanding of their physical, chemical, biological and
mechanical properties. Hench [6] reported first, the
formation of an apatite layer on bioactive glasses in
Na2O–CaO–SiO2–P2O5 system, in vitro as well as vivo.
Recently, attempts have been made by many researchers [7–14] on the successful implementations of
bioactive glasses for a variety of biomedical applications
employing different preparation methods, changing
their composition, subjecting them to different thermal
treatment conditions, etc. Some of the bioactive glasses
such as Al2O3–K2O–Na2O–CaO–P2O5 glass ceramics
[15] with different heat treatment conditions lead to an
important application in dental restoration due to the
change in microstructure, mechanical and chemical
properties. The structural strength and reactions occurring inside the rigid porous bioactive glass bodies
immersed in a simulated body fluid (SBF) have been
studied by Heimo et al. [16]. Thus, the biocompatible
and bioactive glass–ceramics have been treated by many
0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 4 2 - 9 6 1 2 ( 0 2 ) 0 0 1 8 9 - 8
4264
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
researchers for replacing bone structure in human
medicine and restorative dentistry [17–20].
In order to develop new bioactive glasses, which are
closely related to bone stiffness, the determination of
elastic properties is more essential as it involves the
mechanical interaction between implant and surrounding tissues. However, the elastic properties of bioactive
glasses have not been studied systematically in most of
the bioactive glasses. This is possibly due to the fact that
most of the available techniques such as static bending,
resonance, etc., are destructive and semi-destructive
testing methods [21] and also the materials will be
subjected to harm during the above testing. Nevertheless, a review of literature indicates that only very few
attempts [22–24] have been made on the measurement of
elastic moduli, hardness, thermal expansion and fracture
toughness on binary and multi-compositional bioactive
glasses, to explore the microstructural, physico-chemical
and mechanical properties, which widen the range of
options for clinical application of this material. The
ultrasonic technique is a non-destructive testing (NDT)
and it has been found to be one of the best techniques
for complete characterisation of bioactive glasses
[25,26]. The various advantages of ultrasonic technique
over mechanical and other methods include, the
determination of the material properties without harming, comparative analysis to physical testing as a
function of materials loading and also, it provides the
information about grain size, orientation and materials
anisotropy. This is possibly due to interaction of
ultrasonic waves with macro, micro and sub-microscopic particles during wave propagation into the
bioactive glasses and also the availability of multi-mode
and wide range of frequency selection.
Therefore, in the present investigation, it is aimed: (i)
to prepare bioactive glasses of the SiO2–Na2O–CaO–
P2O5 system for different SiO2 contents, (ii) to study the
change in structure of bioactive glasses with change in
SiO2 content in light of the measured ultrasonic
velocities, attenuation and elastic properties, (iii) to
measure the fracture toughness and hence, to correlate
the observed results with the ultrasonic NDT studies,
(iv) to study the effect of the change in frequency of
ultrasonic waves on bioactive glasses and finally, (v) to
evaluate the structure and stability of the bioactive
glasses as a function of temperature from 150 to 500 K.
2. Experimental procedure
2.1. Preparation of bioactive glasses
The bioactive glasses of the system
25.5Na2O–25.5CaO–6P2O5 (named as B43),
24.5Na2O–24.5CaO–6P2O5 (named as B45),
22Na2O–22CaO–6P2O5 (named as B50) and
43SiO2–
45SiO2–
50SiO2–
55SiO2–
19.5Na2O–19.5CaO–6P2O5 (named as B55) have been
prepared using the normal melting and annealing
technique. The chemicals used in the present investigation were AR grade with purity 99.99% of SiO2, 99.99%
of Na2O, 99.99% of CaO and 99.99% of P2O5 used
without further purification. The materials used include
fine-grained quartz for silica. Lime and soda were
introduced in the form of their respective anhydrous
carbonates. P2O5 was added in the form of ammonium
hydrogen phosphate. The mixture was melted in a
platinum 2% Rh crucible for 2 h in an electric furnace.
The melting was carried out at 1400751C. The melt was
stirred before casting two or three times in order to
achieve a homogenous melt. Then, the homogenised
melt was cast into a preheated stainless steel mould of
1 1 4 cm3 dimension. The same procedure was
employed to prepare the remaining bioactive glasses.
The bioactive glasses have been removed from the block
and then cooled to room temperature at a rate of
201C h1 after annealing at 4301C in a muffle furnace
well below Tg values. The required size of the bioactive
glasses for ultrasonic velocities and attenuation measurements was cut from the above-prepared bioactive
glasses. The two opposite faces of the bioactive glasses
were highly polished. Further, all bioactive glasses were
cleaned with acetone to remove the foreign particles.
2.2. Density measurements
Density of all bioactive glasses was measured using
Archimedes principle employing CCl4 as a buoyant. The
density was obtained employing the relation
Wa
r¼
r ;
ð1Þ
W a Wb b
where Wa is the weight in air, Wb the weight in buoyant
and rb the density of buoyant. All the weight measurements have been made using a digital balance (M/s.
Sartorius, Model: BP221S, USA). The accuracy in the
measurement of weight is 70.1 mg. The experiment was
repeated five times to get an accurate value in density.
The overall accuracy in the density measurement is
70.5 kg m3 and hence, the percentage error in the
measurement of density is 70.05%.
2.3. Ultrasonic velocity measurements
The ultrasonic velocities (both longitudinal and shear)
and attenuation measurements were carried out using a
high-power ultrasonic pulser receiver system (M/s.
Fallon Ultrasonics Inc. Ltd., Model–FUI1050, Canada)
employing the cross-correlation technique [27]. A
specially designed and fabricated separate experimental
set-up in the author’s laboratory [28] was used for the
measurement of velocities and attenuation in the
low (liquid nitrogen to room temperature) and high
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
(300–500 K) temperature studies. The block diagram of
the experimental set-up used for the ultrasonic velocities
and attenuation measurements is shown in Fig. 1. The
frequency of ultrasonic (longitudinal and shear) waves
used for the measurement of velocities and attenuation
from 150 to 500 K is 5 MHz. On the other hand, a wide
spectrum (5, 10, 15 and 20 MHz) of high-frequency
ultrasonic (longitudinal waves only) waves was used to
measure the effect of frequency on attenuation at room
temperature in all bioactive glasses. A high-power
ultrasonic pulser/receiver along with a digital storage
oscilloscope (M/s. Hewlett-Packard, Model: 54600B,
US) and a computer were used for recording the
ultrasonic (r.f.) signals. The different steps involved in
the precise ultrasonic transit time for low temperature
measurements are: (i) acquisition, digitisation and
storage of the r.f. signal from the transducer, (ii) crosscorrelation of two desired echoes for finding the
approximate time delay, (iii) adopting cubic spline
interpretation method to the peak portion of the crosscorrelated function to arrive at the exact time delay and
calculating the ultrasonic velocity from the sample
thickness and measured time delay. Thus, using the
above technique [27,29], one can measure the transit
time with an accuracy of 70.2 ns. The precise ultrasonic
velocities (UL and US ) in each of the bioactive glasses
were obtained by measuring the precise transit time in
micron resolution and the sample thickness in micrometer resolution using the relation [29]
U¼
2d
;
t
ð2Þ
where d is the thickness of the sample and t the precise
transit time.
For high temperature ultrasonic measurements, a
heating rate of 0.8 K min1 was employed with the help
Sample inside the
furnace / low temperature
experimental setup
PID temperature
controller
High power ultrasonic
pulser / receiver
IEEE interface
4265
of a PID controller by monitoring both heater and
bioactive glass temperature separately. The error in the
measurement of temperature is 70.1 K. Knowing the
bioactive glass thickness (d) in micron resolution, and
transit time (Dt) in nanosecond resolution, which is the
transit time difference before and after introducing the
bioactive glass between the two buffer rods [28],
ultrasonic velocity is measured employing the relation
U¼
d
:
Dt
ð3Þ
The other phenomena to be considered in the determination of accurate velocities in bioactive glass are the
variation in the thickness and plane parallelism between
the two opposite faces. These are eliminated by surface
grinding of bioactive glasses and obtaining the plane
parallelism between the two opposite surfaces of
bioactive glasses with an accuracy of 75 mm. Thus,
the overall accuracy in the velocity measurement is
75 m s1. The percentage error in velocity measurement
is 70.1%.
2.4. Attenuation measurements
Attenuation of the ultrasonic waves (longitudinal
waves only) at 5, 10, 15 and 20 MHz was measured using
contact type transducer. A uniform pressure was
maintained between the transducer and bioactive glass
during the contact measurements by taking the necessary care to avoid the near field effects. A suitable
couplant has been selected to get a steady back wall echo
train on the oscilloscope screen. The peak amplitude of
the first and successive back wall echoes from the
bioactive glasses was used to determine the attenuation
coefficient.
The attenuation coefficient of all bioactive glasses was
measured using the relation
20
Im
a¼
log
;
ð4Þ
2ðm nÞd
In
where Im and In are, respectively, the maximum
amplitude (voltage) of the mth and nth pulse echoes
(longitudinal waves only). The percentage error in the
attenuation measurement is 72%.
DSO
2.5. Vicker’s hardness and toughness measurements
Personal
computer
IEEE interfacing cable
via measurement
/ storage module
Results
Fig. 1. Block diagram of the experimental set-up used for velocity and
attenuation measurements.
Vicker’s hardness measurements have been carried
out in all bioactive glasses at room temperature. A sharp
diamond indentor, a three-sided pyramid with the same
area-to-depth ratio was used as a Vicker’s indentor. A
small indentation at a precise position on the polished
surface of the bioactive glass was made by the system
using a load of 1 kg for a period of 20 s. For each
bioactive glass, at least 10 indentations were made with
similar conditions and the average value of the diagonal
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
4266
length of the indentation was used for the hardness
measurement.
The energy absorbed by the material, i.e., toughness
during plastic deformation is the work required to
fracture the material. The toughness of the bioactive
glasses was measured using indentation fracture method
[22]. The fracture toughness KIC was determined using
relation [30]
0:5 Y
F
KIC ¼ 0:016
;
ð5Þ
H
C 1:5
where Y ; H and F are Young’s modulus, hardness and
applied load, respectively. C is the radius of the welldeveloped median crack.
defined by the relation
1=3
1
2
1
þ
:
Um ¼
3 US3 UL3
ð12Þ
The relative percentage change in velocity was determined by the relation
DU
U Umin
;
ð13Þ
¼
U
Umin
where Umin is the minimum sound velocity in the
experimental temperature range from 150 to 300 K and
U the sound velocity (longitudinal/shear) at temperature
T K.
2.6. XRD and thermogravimetry studies
4. Results
The X-ray diffraction (XRD) pattern (M/s. JEOL,
Model: JDX 8027) of B43, B45, B50 and B55 bioactive
glasses was made to study the glassy nature. The glass
transition temperature (Tg ) (Polymer laboratories,
Model: STA 1500, UK) of all bioactive glasses were
measured from differential thermal analysis (DTA)
curve with a heating rate of 10 K min1.
The X-ray diffraction pattern of B43, B45, B50 and
B55 bioactive glasses confirms the amorphous nature
before heat treatment. All the bioactive glasses showing
the amorphous nature are shown in Fig. 2. The
velocities and attenuation data are given in Table 1
along with glass transition temperature (Tg ) and
nominal composition of bioactive glasses. For a
constant temperature, say at room temperature (at
5 MHz), an increase in ultrasonic velocities (both
longitudinal and shear) and a decrease in attenuation
with increase of SiO2 (B43–B45) content were observed.
A further increase in SiO2 content (B45–B50 & B55),
leads to a decrease in velocities and hence, an increase in
attenuation. The SiO2 dependent density shows a linear
decrease as shown in Fig. 3. An initial increase in bulk
3. Theory
The elastic moduli of all bioactive glasses were
determined [26] from the experimental values of density
(r), longitudinal velocity (UL ) and shear velocity (US )
using the relations
Longitudinal modulus L ¼ UL2 r;
ð6Þ
Shear modulus G ¼ US2 r;
ð7Þ
Bulk modulus K ¼ L Poisson’s ratio s ¼
4
G;
3
ðL 2GÞ
;
2ðL GÞ
Young’s modulus Y ¼ ½1 þ s 2G:
ð8Þ
ð9Þ
ð10Þ
The average velocity of ultrasonic waves in bioactive
glasses was used to determine the Debye temperature
(yD ) as
h 3PN 1=3
yD ¼
Um ;
ð11Þ
KB 4V p
where h is the Planck constant, KB the Boltzmann
constant, N the Avogadro number, V the molar volume
calculated from the effective molecular weight and
density (i.e., M=r), P the number of atoms in the
molecular formula and Um the mean sound velocity
Fig. 2. XRD pattern with CuKa radiation of B43, B45, B50 and B55
bioactive glasses showing the amorphous nature before heat treatment.
B43: 43% SiO2–25.5% Na2O–25.5% CaO–6% P2O5; B45: 45% SiO2–
24.5% Na2O–24.5% CaO–6% P2O5; B50: 50% SiO2–22% Na2O–22%
CaO–6% P2O5; B55: 55% SiO2–19.5% Na2O–19.5% CaO–6% P2O5.
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
(K) and Young’s (Y ) modulus up to 45 wt%, which is
followed by a gradual decrease in K & Y after showing a
minimum at 45 wt% of SiO2 content is observed as
shown in Fig. 4. Debye temperature (yD ), which depends
on SiO2 content, shows a similar variation as that of
density with change in SiO2 content (Fig. 5). On the
other hand, the variation in Poisson’s ratio (Fig. 5) with
change in SiO2 content is almost negligible.
The hardness (H), fracture toughness (KIC ) and
fracture surface energy gf of the bioactive glasses
(Fig. 6) show a similar variation as that of velocities
(Table 1) with change in SiO2 content including the
observed maximum at 45 wt% of SiO2 content. However, a gradual increase in hardness, fracture toughness
and fracture surface energy with increase in Young’s
modulus from 76.74 to 84.02 GPa (on the other hand,
SiO2 content decreases from 55 to 45 wt%) which is
followed by a decrease in H; KIC and gf with further
increase in Young’s modulus from 84.02 to 83.57 GPa
(while SiO2 content decreases from 45–43 wt%) has been
noticed as represented in Fig. 7. The frequency dependence of attenuation coefficient shows more or less a
linear variation as illustrated in Fig. 8.
The temperature dependence of both longitudinal and
shear velocities for all bioactive glasses are shown in
Fig. 9. The observed results indicate that both longitudinal and shear velocities decrease gradually with
increase in temperature in all bioactive glasses (elastic
moduli were not shown graphically after 300 K, since
the shear velocity was not measured after 300 K;
Density (x10-3 kgm-3)
2.9
2.8
2.6
42
49
4267
56
Content of SiO2 (wt.%)
Fig. 3. Variation in density with change in SiO2 content on SiO2–
Na2O–CaO–P2O5 bioactive glasses at 303 K. The lines are guides to the
eyes.
Table 1
Experimental velocities (UL and US ), attenuation (a) at 5 MHz of SiO2–Na2O–CaO–P2O5 glasses at 303 K along with glass transition temperature
(Tg )
Sample
B43
B45
B50
B55
UL
US
a
Tg a
SiO2 wt%
Na2O wt%
CaO wt%
P2O5 wt%
m s1
m s1
dB cm1
1C
43
45
50
55
25.5
24.5
22.0
19.5
25.5
24.5
22.0
19.5
6
6
6
6
6025
6156
6098
6041
3416
3434
3424
3347
1.49
1.14
1.26
2.08
509
530
484
452
86
64
80
59
E
K
Y
K
74
42
Bulk modulus (GPa)
The error in the measurement of Tg is o71%.
Young's modulus (GPa)
a
Nominal composition
49
54
56
Content of SiO2 (wt.%)
Fig. 4. Variation in Young’s (Y ) and bulk (K) modulus with change in SiO2 content on SiO2–Na2O–CaO–P2O5 bioactive glasses at 303 K. The lines
are guides to the eyes.
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
0.31
367
0.28
θD
Series1
Serie
σ 2
42
385
121
449
(a)
Fracture toughness (MPa m1/2)
Fig. 5. Variation in Debye temperature (yD ) and Poisson’s ratio (s)
with change in SiO2 content on SiO2–Na2O–CaO–P2O5 bioactive
glasses at 303 K. The lines are guides to the eyes.
(b)
K IC
γf
1.09
(b)
7.95
7.62
1.15
KCIC
γf
Fracture surface energy (Jm-2)
1.21
7.29
42
75
7.29
85
80
Young's modulus (GPa)
Fig. 7. Relation between (a) hardness (H) and (b) fracture toughness
(KIC ) and fracture surface energy (gf ) with change in Young’s modulus
(Y ) on SiO2–Na2O–CaO–P2O5 bioactive glasses at 303 K. The lines are
guides to the eyes.
385
1.09
85
7.95
7.62
1.15
417
49
1.8
Attenuation (dBcm-1)
Hardness (MPa)
417
0.24
56
49
Content of SiO2 (wt.%)
Fracture toughness (MPam1/2)
(a)
Fracture surface energy (Jm-2)
352
449
Hardness (MPa)
382
Poisson's ratio
Debye temperature (K)
4268
B43
B45
B50
B55
1
56
Content of SiO2 (wt.%)
Fig. 6. Relation between (a) hardness (H) and (b) fracture toughness
(KIC ) and fracture surface energy (gf ), with change in SiO2 content on
SiO2–Na2O–CaO–P2O5 bioactive glasses at 303 K. The lines are guides
to the eyes.
however, the measured longitudinal velocity shows a
continuous decrease up to 500 K, which is not included
in Fig. 9). The bulk and Young’s modulus (Fig. 10)
follow the same trend of variation as that of velocities
(Fig. 9) with increase in temperature. The temperature
dependence of longitudinal and shear modulus shows a
similar trend of variation as that of bulk and Young’s
0.2
4
13
22
Frequency (MHz)
Fig. 8. Frequency dependence of attenuation on SiO2–Na2O–CaO–
P2O5 bioactive glasses for different SiO2 contents at 303 K. The lines
are guides to the eyes. For key please refer to Fig. 2.
modulus, and hence is not represented graphically. The
percentage variation in ultrasonic velocities as a function of temperature for different SiO2 contents is
represented in Fig. 11.
The temperature dependence of attenuation coefficient shows anomalies in the temperature range
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
6
(a)
B43
B45
B50
B55
6225
5950
Relative percentage change (%)
Longitudinal velocity (ms-1)
6500
3530
B43
B45
B50
B55
3430
310
225
Relative percentage change (%)
Shear velocity (ms-1)
(b)
3330
140
Fig. 9. Dependence of (a) longitudinal (UL ) and b) shear (US ) velocity
on SiO2–Na2O–CaO–P2O5 glasses with change in temperature for
different SiO2 contents. The lines are guides to the eyes. For key please
refer to Fig. 2.
(b)
B43
B45
B50
B55
2
310
225
Temperature (K)
4
56
90
Young's modulus (GPa)
0
3
(a)
64
B43
B45
B50
B55
3
Fig. 11. Relative percentage change of (a) longitudinal velocity
(DUL =UL ) and (b) shear velocity (DUS =US ) with change in temperature
on SiO2–Na2O–CaO–P2O5 glasses for different SiO2 contents. The
lines are guides to the eyes. For key please refer to Fig. 2.
Attenuation (dBcm-1)
Bulk modulus (GPa)
72
B43
B45
B50
B55
(a)
B43
B45
B50
B55
0
140
Temperature (K)
(b)
B43
B45
B50
B55
3
1
140
340
Temperature (K)
540
Fig. 12. Temperature dependence of attenuation on SiO2–Na2O–
CaO–P2O5 glasses for different SiO2 contents. The lines are guides to
the eyes. For key please refer to Fig. 2.
82
74
140
4269
225
310
Temperature (K)
Fig. 10. Dependence of (a) bulk (K) and (b) Young’s (Y ) modulus on
SiO2–Na2O–CaO–P2O5 glasses with change in temperature. The lines
are guides to the eyes. For key please refer to Fig. 2.
260–310 K, in all bioactive glasses as noticed in Fig. 12.
A gradual decrease in a which is followed by an increase
after showing a dip with increase in temperature was
noticed in all bioactive glasses. It is interesting to note
that as the content of SiO2 increases, the observed dip in
attenuation coefficient curve is shifted towards the low
temperature and also the dip area broadens as shown in
Fig. 12.
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
4270
5. Discussion
5.1. XRD, density and Tg studies
Typical XRD pattern of all bioactive glasses (as
prepared) showing the amorphous nature is shown in
Fig. 2. The absence of any diffraction peak in XRD
pattern in all bioactive glasses confirms the amorphous
nature. The glass transition temperature (Tg ) plays a
vital role in understanding the physical properties of
bioactive glasses. In bioactive glasses, it is particularly
sensitive in optimisation of the composition of the
bioactive glasses. The glass transition temperature as a
single linear function of composition of Na2O (in wt%)
has been developed theoretically using regression
analysis as
Tg ð1CÞ ¼ 232:223 þ 11:4451 ðNa2 O in wt%Þ;
R2 ¼ 84:45%;
ð14Þ
o ¼ 11:431C;
where R2 is the regression coefficient and o the residual
standard deviation. Eq. (14) explains the importance of
Na2O content in bioactive glasses. The decrease in Tg
values according to Eq. (14) is about 11.431C for every
wt% unit of Na2O that is replaced by any other
component in the present bioactive glasses. In all
bioactive glasses, it can be seen that there is a fairly
good agreement between the estimated Tg values using
linear equation and the experimentally determined
values. It is inferred from the above studies that the
above model is more useful in optimisation of the
bioactive glass composition. A similar linear dependence
of the Na2O content in SiO2–Na2O–CaO–P2O5–Al2O3–
B2O3 bioactive glass [31] system has been discussed with
the influence of Na2O. From Eq. (14), it is clear that the
value of Tg depends on the Na2O composition as
reported in SiO2–Na2O–CaO–P2O5–Al2O3–B2O3 glasses
[31] in or near the bioactive region.
Density is an effective tool in exploring the change in
structure, co-ordination and cross-link density of glasses
[32]. The addition of SiO2 in Na2O–CaO–P2O5 bioactive
glasses shows a monotonic decrease in density (Fig. 3)
from 2.8348 to 2.6792 103 kg m–3 without showing
any anomalies. However, it is inferred from the above
results that by the addition of SiO2, even though the
ratio of Ca/P changes from 4.3 to 3.3 wt%, the
percentage change in density and volume initially
increases (respectively, 1.37–2.33% and 1.51–2.46%
for density and volume) up to 45 wt% of SiO2, which
is followed by a decrease in the same after showing a
maximum at 45 wt% of SiO2 (2.33–1.89% and
2.46–2.03%, respectively, for density and volume). The
decrease in density with addition of SiO2 content leads
to an increase in volume due to the loose packing of
atoms resulting in structural softening. In vitreous SiO2,
all of the oxygens were shared between two SiO4
tetrahedra, forming a fully polymerised network. In
alkali silicate glasses, non-bridging oxygens are successively formed by the addition of alkali oxide (Na2O) and
hence, the splitting of the network takes place [33]. The
formation of non-bridging oxygens causes a weakening
of the glass structure. The incorporation of the divalent
oxide (CaO) into the glass structure can be described in
a very similar way as the introduction of an alkali oxide.
It seems that the presence of high percentage of both
Na2O and CaO (respectively, 24.5 and 24.5 wt%) in
bioactive glass than their ratios (respectively 16% and
6%) in traditional soda-lime–silica glasses used for the
preparation of sheet and table-ware glasses, leads to
somewhat different chemical and physical behaviour.
Alkaline sodium hydroxide solution was found [34] to
corrode traditional soda-lime–silica glass far more than
the corrosion by acidic solutions, while the reverse is
obtained in bioactive glass. It has been shown [35] that
the base bioactive glass and the related compositions are
easily nucleated in contrast to traditional soda-lime–
silica glass with the main phase of Na2O–2CaO–3SiO2.
Mastelaro et al. [36] assumed that there must be a
relationship between the nucleation of Na2O–2CaO–
3SiO2 bioactive glass and a short-range order in its
structure. Short-range order or polymerisation in alkali
and alkaline earth silicate glasses is commonly denoted
by Qn ; where n is the number of non-bridging oxygens
per tetrahedral cation [37]. The average value of n is
determined by the glass composition. A recent work by
Schneider et al. [38] using MAS & NMR studies shows
that the structural units in the range of the two easily
nucleated glasses of the compositions Na2O–2CaO–
3SiO2 and 2Na2O–CaO–3SiO2 are mainly made up of
Q2 groups (two bridging and two non-bridging oxygen).
They also added that further work is needed to check
whether there is or not a general trend for glasses near
the metasilicate composition. Thus, softening of the
network structure in bioactive glasses is expected to be
different at high SiO2 (>45 wt%) content through
density measurements rather than at low SiO2
(p45 wt%) content. This is because at high SiO2
content, the internal structure of the bioactive glass is
assumed to consist mostly of Q3 units. Similarly, the
structure of the silicate glasses has been discussed based
on the density measurements [39].
5.2. Ultrasonic studies
In the present bioactive glasses, the SiO2 content lies
between 43 and 55 wt% (o60 wt%), while the content of
Na2O and CaO also ranges between 25.5 and 19.5 wt%,
in addition to Ca/P ratios varying between 4.3 and
3.3 wt%. Thus, the composition of the bioactive glasses
selected in the present study has abundant alkali and
alkaline earth oxides content, and is highly reactive
when it is exposed to an aqueous medium [6]. It is
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
inferred from the earlier study that the high Ca/P ratio
aids the ability to form bond to bone [6]. On the other
hand, bioactive glasses with low Ca/P ratio will not
bond to bone [40].
The following are the changes observed, in view of the
presence of SiO2 content in the range from 43 to 55 wt%
in silicate glasses:
(i) In bioactive glass B43, the total network modifiers
Na2O+CaO content is higher than the network
formers SiO2 and P2O5 (invert glass). In this
bioactive glass, the internal structure seems to
consist of discrete or separate units. The silicate
groups are believed to be mainly of Q1 units
(without bridging oxygens).
(ii) Increasing the SiO2 content in bioactive glass B45,
the total Na2O+CaO content is higher than the
SiO2 content alone. In this bioactive glass, the
internal structure is believed to contain Q1 units
(with the bridging oxygen) plus Q1 units.
(iii) By further increasing the SiO2 content in bioactive
glasses B50 and B55, the modifier Na2O and CaO
content progressively decreases, and the SiO2
increases on the same ratio. In these two bioactive
glasses namely B50 and B55, the internal structure
is assumed to contain Q2 units+Q1 and Q1 units.
The ratio of the first two units is possibly dominant
in the network.
The observed initial increase in velocities (both UL
and US ) and decrease in attenuation (a) up to 45 wt% of
SiO2 result from the rearrangement of structural units in
the glass network. Even though, a monotonic decrease
in density was observed (Fig. 3), the observed increase in
percentage change in density and the observed maximum percentage variation at 45 wt% of SiO2 content
support the previous observation. A further addition of
SiO2 content causes the formation of different structural
units. Therefore, the formation of such new units results
in the change in rigidity and hence, velocities as
observed in Table 1. Thus, the decrease in velocities
contributes a decrease in modulus (Fig. 4) with increase
in SiO2 content. For high alkali and alkaline earth oxide
content (NaO+CaO), the concentration of non-bridging oxygens and the isolation of SiO4 tetrahedra are
expected to be very high. It is inferred from our recent
investigation [41] that NBOs and bridging oxygen
between SiO4 tetrahedra on the same bioactive glasses
are known to exist. The earlier study on silicate and
borate [42] glasses reveals that the strength of the glass
network increases up to a least wt% of alkali and then,
the appearance of NBO in the case of saturation results
in the breaking of the network.
The bulk modulus [43] is more sensitive in exploring
the change in cross-link density and bond stretching
force constant. The initial increase in velocities (Table 1)
4271
and moduli (Fig. 4) at low SiO2 (p45 wt%) content is
presumed to the conversion of three connected (PO3/2)
tetrahedra into four connected (PO4/2) tetrahedra in the
network and hence, the formation of NBO is expected to
be very less. The existence of either single or polymer
chain network of SiO4 and phosphate (PO4) groups is
reported in our recent IR studies [41]. Damodran et al.
confirmed the increase in velocities and moduli due to
the transformation of (POO3/2) units into (PO4/2) units
in PbO–MoO3–P2O5 glasses [44] with addition of PbO,
which support the observation made in the present
study. The alkali ions are incorporated into the
interstitial sites in the glass network through NBOs
and hence, the structure compactness of the present
glasses is weaker than with vitreous silica [33] as it has
lower binding energy than Si–O bonds. Thus, the
increase in rigidity or change of structural group
arrangement (increase in velocities) up to 45 wt% of
SiO2 and then a decrease in rigidity (decrease in
velocities) of the network up to 55 wt% of SiO2 content
lead to an initial increase in moduli, which is followed by
a decrease in moduli as shown in Fig. 4. It is evident
from the modulus data that the increase in the strength
of the bridging bonds up to 45 wt% of SiO2 content
beyond which the formation of Q1 and Q2 units
accompanied with structural rearrangement and polymerisation are known to exist.
Generally, the change in cross-link density is measured from the magnitude of the Poisson’s ratio [26] i.e.,
a high cross-link density will have a low Poisson’s ratio
and vice versa. In the present glass, even though a
maximum in Poisson’s ratio (Fig. 5) at 45 wt% of SiO2
has been noticed, the overall change in Poisson’s ratio is
negligible i.e., it changes from 0.263 to 0.278 when the
SiO2 content increase from 43 to 55 wt%. Thus, the
change in Poisson’s ratio suggests that the change in
cross-link density of these specific bioactive glasses is
less pronounced. The observed results support the
observation made on moduli, velocities and attenuation
studies.
Debye temperature (yD ) is another useful parameter
to understand the change in structure and the semiconducting properties of the glasses. The observed yD
values (Fig. 5) are in good agreement with those
reported on similar oxide glasses. The decrease in yD is
very negligible for the initial change in SiO2 (43–
45 wt%), which is followed by a steep decrease in yD
with further addition of SiO2 content shows the possible
change in network structure. The change in slope of the
compositional dependence of yD after 45 wt% of the
SiO2 content indicates that the rearrangements of the
glass network will contribute more to yD : The increase
in softening of the glasses with increase in NBO has
been revealed by Rajendran et al. and Mallawany
et al., respectively, on V2O5–Bi2O3–TeO2 [45] and
V2O5–TeO2–Ag2O [46] glasses.
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
4272
5.3. Hardness, fracture toughness and fracture surface
energy
processes such as plastic deformation, crack, branching
and vibrational energies.
Young’s modulus (Y ), hardness (H) and fracture
toughness (KIC ) play a key role in elucidating the
physico-chemical and mechanical properties of bioactive
glasses. In general, as discussed earlier, the change in
chemical bond and bond strength in the glass structure
are normally incorporated in Young’s modulus, while
the hardness explores the information about the elastic
strength and surface behaviour of glasses. On the other
hand, Y is one of the important parameters in
determining the fracture behaviour involved in glasses.
Similar to Y (Fig. 4), hardness (Fig. 6) of bioactive
glasses increases up to 45 wt% of SiO2 and then, the
same was found to decrease with further increase in SiO2
content. The initial increase in H and Y is ascribed to
the increase in bond strength in unit volume as studied
in densification of magnesium and meta–phosphate
glasses [47]. Similarly, the decrease in H and Y is
correlated with the structural unit arrangement and the
number of bridging and non-bridging oxygens as
studied in Na2O–K2O–SiO2 bioactive glasses [48] by
Rizkalla et al. Fracture surface energy gf was obtained
using the relation [49]
5.4. Effect of frequency
2
gf ¼ KIC
½1 2s
;
2Y
ð15Þ
where KIC and Y are, respectively, the fracture toughness and Young’s modulus.
According to Eq. (15), the fracture surface energy (gf )
mostly depends on the average bond strength in the
fracture path and hence, the introduction of Si–O bond
into the glass network lowers the fracture toughness.
The value of both KIC and gf initially increase with
increase in Y and then show a monotonic decrease with
further increase in Y : The relationship obtained between
KIC and Y ; and gf and Y (Fig. 7b) in the present
bioactive glasses shows a similar trend as observed in
CaO–P2O5 and MgO–P2O5 glasses [47], while it shows
an opposite trend of behaviour when compared with the
general relationship between KIC and Y ; and gf and Y in
meta-phosphate glasses [50].
It is interesting to note that the abundance of nonbridging than bridging oxygens plays a dominant role in
controlling Young’s modulus, hardness, fracture toughness and fracture surface energy. A decrease in Y ; H and
KIC due to the existence of NBOs and weakening of
Si–O bonds has been studied by Rizkalla et al. [48] on
Na2O–K2O–SiO2 glasses. Similarly, in lead silicate
glasses [51], the introduction of weaker Pb–O bonds at
high PbO content lowers the fracture toughness and
fracture surface energy in the glass structure. Attempts
have been made [22,30,51] to explore the crack
propagation at the crack tip employing irreversible
In general, the attenuation in a solid material can be
related as
a ¼ a1 aa þ a2 ab þ ac þ ad ;
ð16Þ
where aa is the true absorption, ab the scatter absorption, ac the coupling absorption and ad the diffraction
absorption. The other losses which contribute to
attenuation are non-parallelism of the bioactive glass
surfaces and surface conditions. However, in the present
study, the total contribution to the attenuation is only
true, coupling and diffraction absorption, since the
parallelism of the bioactive glass surface and surface
condition are taken care. Therefore,
a ¼ a1 aa þ ac þ ad ;
ð17Þ
machine oil and non-aqueous stopcock grease with very
less thickness have been used as couplant, respectively,
for high temperature and low temperature studies
between the bioactive glass and transducer. By adding
the couplant corrections (both separately for machine
oil and non-aqueous stopcock grease) to the measured
attenuation and neglecting the absorption and diffraction, the ac and ad terms are neglected (Fig. 12). It is also
inferred from the variation of a with change in
frequency (Fig. 8), the contributions due to scattering
absorption is less and hence, prevails only true or
thermo-elastic absorption as studied in BaTiO3 doped
lead bismuth semiconducting oxide glasses [29]. The
attenuation can be written as
a ¼ af ;
ð18Þ
where a is a constant depending on bioactive glass and
its composition. The absence of any rapid changes in a
with increase in frequency indicates (Fig. 8) that the
dispersion effect is less pronounced in the present
bioactive glasses. A similar trend of attenuation with
increase in frequency has been studied on alkali borate
glasses [52].
5.5. Effect of temperature
Elastic moduli are more sensitive to structural
changes as a function of temperature. A gradual
decrease in velocities (Fig. 9) and moduli (Fig. 10) with
increase in temperature without showing any anomalies
were noticed. The absence of anomalies and gradual
decrease in velocities and moduli indicates the nonexistence of structural phase changes as observed in
many other glasses such as V2O5–Bi2O3–TeO2 glasses
[45]. As temperature increases, the softening of the
network structure takes place due to the easier ionic
V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
motion as a result of structural relaxation. The
continuous decrease in relative percentage change in
velocities (Fig. 11) from high value (4.83% (B43), 3.72%
(B45), 3.76% (B50), 2.92% (B55) and 1.46% (B43),
1.11% (B45), 1.14% (B50), 2.15% (B55), respectively,
for longitudinal and shear velocities) to zero supports
the softening of the network as the temperature
increases. The recent studies on the relative percentage
change in velocities of vanadate based [45,53] glasses
support the softening of the glass network as observed in
the present study.
The observed anomalies in attenuation coefficient as a
function of temperature from 150 to 500 K are similar to
that observed for V2O5–PbO glasses [53] and are
different from the behaviour of glasses such as V2O5–
GeO2 [54] and CuO–P2O5 glasses [55], wherein a peak in
attenuation was noticed. The observed anomalies in
attenuation in V2O5–GeO2 [54] and CuO–P2O5 [55]
glasses have been explained based on the existence of the
different valence states of transition metal ions (TMI)
namely vanadium and copper in the respective glasses,
with change in composition. The existence of such
valence states of TMI has been studied through the
electrical properties in the above glasses. The earlier
study [56] on the stability of the glass network suggest
that the stable glasses will have close packed structures
while unstable glasses will have loose packed ones. Thus,
the structure of the present bioactive glasses becomes
loose packed when the SiO2 content exceeds 45 wt%.
The anomalies and the magnitude of the dip observed in
the attenuation coefficient broaden when SiO2 content
increases from 43 to 55 wt%, which also support the
above observations. Out of the different compositions
studied in the present work, the 45SiO2–24.5Na2O–
24.5CaO–6P2O5 (45S5s) glass is the first bioactive glass
prepared by Hench [6]. In most of the bioactive glasses
studied, the content of SiO2 was between 45 and
55 wt%. At low SiO2 content, it dissolves easily into
the surrounding body fluids [57]. The observed distinct
behaviour in the measured velocities (Fig. 9b), Young’s
modulus (Fig. 10b) and attenuation (Figs. 8 and 12) of
B55 bioactive glass is presumed due to the incorporation
of high content of SiO2 (55 wt%) which is comparatively
12% higher than the 45S5 glass. The addition of more
SiO2 content (>50 wt%), which leads to the change of
the state of polymerisation or the presence of different Q
units as discussed at the beginning of our discussion is
responsible for the observed distinct behaviour in UL ;
US ; K; Y and a:
6. Conclusions
The bioactive glasses in the (942x)SiO2–xNa2O–
xCaO–6P2O5 system have been prepared for different
SiO2 contents by keeping the Ca/P ratio between 4.3 and
4273
3.3 wt%. The following are the conclusions made in the
present investigation:
(a) The measured acoustical parameters such as velocities, attenuation, moduli, Debye temperature and
Poisson’s ratio suggest that the strength (compactness) of the glass network continues up to 45 wt%
of SiO2 content.
(b) With further addition of SiO2 (>45 wt%), a
decrease in the above parameters after showing a
maximum at 45 wt% reveals the breaking of Si–O–
Si bonds and the formation of non-bridging oxygen
(NBO) results in the softening of the glass network.
(c) A linear relation between KIC and Y ; and gf and Y
of the bioactive glasses have been revealed. The Y ;
H; KIC ; and gf have been observed as a function of
chemical and physical properties of bioactive
glasses.
(d) The developed linear Eq. (14) explores the importance and the influence of Na2O content on Tg
values in bioactive glasses.
(e) The frequency-dependent attenuation indicates the
prevailing nature of true absorption and less
dispersion effect.
(f) A continuous decrease in elastic moduli and
velocities as a function of temperature in all
bioactive glasses reveals the existence of structural
softening in the glass network.
(g) The relative percentage change in velocities as a
function of temperature, and attenuation coefficient
reveals that the bioactive glasses with low SiO2
(p45 wt%) content are different in structural
arrangement than those with high SiO2
(>45 wt%) content.
(h) The observed distinct behaviour in velocities,
Young’s modulus, and attenuation coefficient in
B55 bioactive glass are presumed to the change in
the state of polymerisation or the presence of
different Q units.
The present investigation confirms that the ultrasonic
non-destructive characterisation of bioactive glasses are
more informative in exploring the structural changes,
stability and mechanical properties, which are essential
for optimisation of the bioactive glasses for different
biomedical applications.
Acknowledgements
The authors (VR and AN) are grateful to Prof. G.
Shanmugam, Principal and Thiru Yennarkay R. Ravindran, Correspondent, Mepco Schlenk Engineering
College for their constant encouragement to this
collaborative work. The authors (VR and AN) are
thankful to IGCAR, Kalpakkam for providing the
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V. Rajendran et al. / Biomaterials 23 (2002) 4263–4275
financial support (IGC/SHINEG/SED/TPS/200-813).
The authors are thankful to Mr. N. Palanivelu, SRF
for his assistance in the experimental studies. The
authors are thankful to the referee for his explicit
comments on the above paper.
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