Fracture behavior of lithia disilicate- and leucite-based ceramics

Dental Materials (2004) xx, xxx–xxx
Fracture behavior of lithia disilicate- and
leucite-based ceramics
Alvaro Della Bona a, *, John J. Mecholsky Jr. b , Kenneth J. Anusavice c
a School of Dentistry, The University of Passo Fundo, P.O. Box 611/613, Passo Fundo, RS 99001 970, Brazil
b Department of Material Sciences and Engineering, University of Florida, Gainesville, FL, USA
c Department of Dental Biomaterials, University of Florida, Gainesville, FL, USA
Received 8 July 2003; received in revised form 3 February 2004; accepted 19 February 2004
KEYWORDS
Structural reliability;
Fracture; Fractography;
Fracture toughness;
Weibull modulus
Introduction
The appeal of ceramics as structural dental
materials is based on their esthetics, low density,
high hardness, chemical inertness, and wear resistance.
A major goal of ceramic research and development
is to produce stronger, tougher ceramics
that are structurally reliable in dental applications.
Limited information is available on the fracture
behavior and toughness of hot-pressed dental
*Corresponding author. Tel.: þ55-54-3115142; fax: þ55-54-
3168403.
E-mail address: dbona@upf.br
ARTICLE IN PRESS
www.intl.elsevierhealth.com/journals/dema
Summary Objective: This study was designed to characterize the fracture behavior of
ceramics and test the hypothesis that variation in strength is associated with a
variation in fracture toughness.
Methods: The following four groups of 20 bar specimens (25 £ 4 £ 1.2 mm) were
fabricated (ISO standard 6872): E1, a hot-pressed leucite-based core ceramic
(IPS Empress); E2, a hot-pressed lithia-based core ceramic (IPS Empress 2); ES, a
hot-pressed lithia-based core ceramic (Experimental); and GV, a glass veneer (IPS
Empress2 body). Specimens were subjected to four-point flexure loading in 37 8C
distilled water. Fractographic analysis was performed to determine the fracture origin
ðcÞ for calculation of fracture toughness ðK ICÞ: Weibull analysis of flexure strength ðsÞ
data was also performed.
Results: Differences in mean s and K IC were statistically significant for E1 and GV
ðp , 0:05Þ: These differences are associated with processing effects and composition.
Significance: The higher mean s and K IC values of E2 and ES core ceramics suggest
potentially improved structural performance compared with E1 although the Weibull
moduli of E1 and E2 are the same.
Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
ceramics. Brittle fracture is a complex process. 1,2
Quantitative fractographic analysis facilitates the
identification of fracture mechanisms through the
use of fracture mechanics. 3
Fracture toughness is often used to characterize
the fracture resistance of brittle materials 4–6 and it
is usually controlled by the fracture in Mode I
(opening mode, tensile load). Irwin 16 defined
failure at the point when the Mode I stress intensity
(KI) reaches or exceeds a critical value ðK I $ K ICÞ:
The critical stress intensity factor ðK ICÞ is in many
cases a material constant and is one measure of the
toughness of the material, i.e. the resistance
to crack propagation. Therefore, the fracture
0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2004.02.004

2
Figure 1 Diagram of the typical fracture surface features occurring in brittle materials. The regions are not drawn to
scale. Adapted from Mecholsky (1993). 27
toughness or critical stress intensity factor ðK ICÞ can
often be determined using the Griffith–Irwin
equation:
K IC ¼ Ys fc 1=2
ð1Þ
where Y is a geometrical factor that accounts for
the location and geometry of the critical flaw and
type of loading, 7 s f is the stress at fracture, and c is
the radius of an equivalent semicircular flaw for a
semi-elliptical crack of semiminor axis ‘a’ and
semimajor axis ‘b’ (Fig. 1). 8,9
Little information is available on the structural
reliability of hot-pressed lithia disilicate-based and
leucite-based ceramics, and the sensitivity of processing
variables is unknown. This study was designed to
characterize the fracture behavior and fracture
toughness of a glass veneer (GV), a leucite-based
ceramic, and two lithia disilicate-based ceramics
using fractographic principles. The objective of this
study was to test the hypothesis that variation in
strength is associated with the variation in fracture
toughness for the same surface preparation.
Materials and methods
The ceramic materials (E1, E2, ES, and GV) and
firing procedures used for bar specimens
Table 1 Firing temperatures ðTÞ and times ðtÞ for the four ceramics.
Dental ceramics Starting T
(8C)
(25 £ 4 £ 1.2 mm) are summarized in Table 1. The
GV specimens were fabricated using a metal mold
and fired according to the manufacturer’s instructions
(Table 1). The hot-pressed leucite-based core
ceramic (E1) and the two hot-pressed lithia disilicate-based
core ceramic specimens (E2 and ES)
were prepared using the lost-wax technique. 10
After removal of the hot-pressed ceramic from
the investment, the interaction layer was removed
by grit blasting with 80 mm glass beads at a pressure
of 2 bar (30 psi). The bar specimens were cleaned in
1% hydrofluoric acid (HF) for 30 min, grit blasted
with 100 mm Al2O3 at a pressure of 2 bar, and
polished through 1200 grit SiC metallographic paper
to a thickness of 1.2 mm. All specimens were
finished with 1 mm polishing alumina (Mark V
Laboratory, East Granby, CT, USA) to the final
dimensions (25 £ 4 £ 1.2 mm). They were ultrasonically
cleaned in distilled water and steam cleaned
using distilled water. Specimens were examined for
flaws using light microscopy at 30 £ (microscope
model SCW30L, Fisher Scientific, Thailand). Specimens
with any obvious large flaws would be
eliminated, if detected. However, no major flaws
were detected. The specimens were stored for 48 h
in distilled water at 37 8C and then subjected to
four-point flexure loading (applied along rollers
at 1/3 and 2/3 of the length of the bars) at
Heating rate
(8C/min)
Firing T
(8C)
Holding t
(min)
Vacuum T on–off
(8C)
E1—IPS Empress core a (leucite-based ceramic) 700 60 1180 20 500–1180
E2—IPS Empress2 core a (lithia disilicate-based ceramic) 700 60 920 20 500–920
ES—Experimental core a (lithia disilicate-based ceramic) 700 60 910 15 500–910
GV—IPS Empress2 body a (amorphous glass) 403 60 800 2 450–799
a Ivoclar AG, Schaan, Liechtenstein.
ARTICLE IN PRESS
A. Della Bona et al.

a cross-head speed of 0.5 mm/min in a universal
testing machine (Model 1125, Instron Corp., Canton,
MA, USA) while immersed in distilled water.
Note that any local residual stresses induced during
preparation, i.e., around any flaws, were relieved
by the storage under water because of slow crack
growth in a conducive environment. An Isotemp
Immersion Circulator (Model 730, Fisher Scientific,
Pittsburgh, PA, USA), which was connected to the
testing chamber, was used to maintain the water at
37 8C. 11 Failure loads were recorded and the
flexural strength values were calculated using the
following equation:
s ¼ PL=wt 2
ð2Þ
where P is the applied load at failure, L is the
length of the outer (total) span, w is the width of
the specimen, and t is the thickness of the
specimen. 11,12
Fracture surfaces were examined using optical
microscopy (OM) and scanning electron microscopy
(SEM) to determine the mode of failure based on the
fracture origin and fractographic principles.
3,13 – 15
In preparation for SEM (JSM 6400, Jeol Ltd, Tokyo,
Japan) examination, the fracture surfaces were
sputter-coated with gold–palladium for 3 min in a
Hummer II Sputter Coater (21020, Technics Inc.,
Alexandria, VA, USA) at a current of 10 mA and a
vacuum of 130 mTorr.
The critical flaws were located and their size
ðcÞ was determined using fractographic principles.
The flaw size calculation ½c ¼ðabÞ 1=2 Š is the same as
that used for an equivalent semicircular surface
crack and corner crack. In the case of a corner
crack, ‘c’ corresponds to the distance from the
crack corner to the limit of the critical flaw-mirror
region (Fig. 1).
The mean crack size and strength values were
used to calculate the fracture toughness ðK ICÞ via
Eq. (1). The geometrical factor ðYÞ was calculated
based on Randall’s 7 interpretation of Irwin’s 16
work:
Y ¼½0:515Q 1=2 Š 21
ð3Þ
Q ranged in value from 1.00 for long, shallow
cracks ðY ¼ 1:94Þ to 2.46 for semicircular cracks
ðY ¼ 1:24Þ: For a corner crack, the factor Y can be
approximated by
Y ¼ 1:12 2 2=p 1=2
ð4Þ
where 1.12 2 represents the surface flaw corrections
and 2=p 1=2 represents the correction for a pennyshaped
embedded crack, i.e. Y ¼ 1:4. 17
Extra samples of each ceramic were fabricated
and ground to a powder, mixed with an adhesive
ARTICLE IN PRESS
Fracture Behavior of Ceramics 3
(collodian–amyl acetate in a ratio of 1:7), and
placed on a glass slide sample area. To identify the
ceramic crystal phases, X-ray diffraction (XRD) was
performed using a 3720 Phillips room temperature
diffractometer (serial #1470, Phillips, Netherlands)
and PW1877 automated powder diffraction software.
A copper tube anode in a continuous scan
from 3.01 to 89.978 (,3–908) with step size of 0.028
every 0.4 s was used at a voltage of 40 kV, and
current of 20 nA. A slow scan from 10 to 408 was also
used. The IPS Empress2 GV was analyzed for any
crystal phase using a DRIFT (Diffuse Reflectance
Infrared Fourier Transform, Spectra Tech Inc.,
Shelton, CT, USA) analyzer with a Gemini Stage
(Nicolet Magna 760, Nicolet Inc., Madison, WI, USA)
and OMNIC 4.1b software (Nicolet Inc., Madison,
WI, USA). The following four powder samples were
analyzed: (1) 0.5 g of KBr powder (Spectra Tech
Inc., Shelton, CT, USA) for the background control;
(2) 0.5 g of KBr powder mixed with 0.025 g of
unfired incisal GV powder; (3) 0.5 g of KBr powder
mixed with 0.025 g of unfired dentin GV powder;
and (4) 0.5 g of KBr powder mixed with 0.025 g of
fired dentin GV powder. Analysis was performed in
64 scans for four wave number resolutions. The data
were corrected for H2O and CO2 interferences and
results were recorded.
Ceramic disks specimens of each material used in
this part of the study (Table 1) were fabricated
and polished through 1 mm alumina abrasive. The
specimen thickness was measured using a caliper
(Digimatic caliper, Mitutoyo Co., Kawasaki, Japan)
and the weight was obtained using an analytical
balance (Mettler H31, Mettler Instrument Corp.,
Hightstown, NJ, USA). The density ðrÞ was obtained
using the Helium Pycnometer (Accupyc 1330,
Micromeritics Instrument Co., Norcross, GA, USA)
after calculating the volume.
The elastic modulus ðEÞ and Poisson’s ratio ðnÞ
were determined by means of ultrasonic waves and
a computer program (ECALC and Sigview-F software,
Nuson Inc., Boalsburg, PA, USA) based on a
set of equations that uses the time of flight,
density, and thickness. Piezoelectric transducers
(Ultran Laboratories Inc., Boalsburg, PA, USA) and
an ultrasonic pulse apparatus (Ultima 5100, Nuson
Inc., Boalsburg, PA, USA) were used to determine
the time of flight through the ceramic specimens.
Longitudinal and shear (transverse) time of flight
values were obtained and used to calculate n and E
of the materials. 11 A hardness tester (Model MO
Tukon Microhardness Tester, Wilson Instruments
Inc., Binghampton, NY, USA) with a Vickers diamond
was used to measure the hardness of these
ceramic specimens. Each specimen was indented at
four different locations under a load ðPÞ of

4
9.8 N. The dimensions of the indentation diagonals
were measured using an optical microscope with a
filar eyepiece. The hardness ðHÞ values were
calculated by
H ¼½2P sinðu=2ÞŠ=a 2
ð5Þ
where P is the indentation load, u is the angle
between opposite diamond faces (1368), and a is the
mean diagonal length of the indentation.
Flexural strength and K IC data were analyzed
statistically using one-way ANOVA and Tukey’s HSD
test. Weibull regression analysis was performed on
the strength data as previously described 12 and as
presented in a previous report. 11
Results
The mean values of density ðrÞ; elastic modulus ðEÞ;
Poisson’s ratio ðnÞ; and Vickers hardness ðHÞ of the
ceramics used in this study are summarized in
Table 2.
XRD analyses showed that high-expanding mineral,
leucite (K2O·Al2O3·4SiO2), is the crystal phase
in the IPS Empress core ceramic (E1), and the
lithium disilicate (Li2O·2SiO2) is the crystal phase in
the IPS Empress 2 (E2) and Experimental (ES) core
ceramics. These crystal phases are shown in Fig. 2.
The length of the elongated Li2Si2O5 crystals ranged
from 0.5 to 4 mm for E2 and from 0.5 to 2 mm for ES.
XRD analysis of the IPS Empress2 GV produced a
typical amorphous glass plot. The absence of any
crystal phase was confirmed by FTIR analysis.
Representative photomicrographs of the ceramics
microstructure are presented in Fig. 2. As
expected, there is a positive correlation between
the elastic modulus ðEÞ; hardness ðHÞ; flexure
strength ðsÞ; and fracture toughness ðK ICÞ for the
four ceramics studied (E1, E2, ES, and GV).
One-way ANOVA and Tukey’s test subsets
revealed significant statistical differences for the
mean s and K IC values between E1 and GV ceramics
ðp , 0:05Þ: The two lithia disilicate-based ceramics
(E2 and ES) showed greater mean s and K IC values
Table 2 Mean values of density ðrÞ; elastic modulus ðEÞ;
Poisson’s ratio ðnÞ; and Vickers hardness ðHÞ of Empress
ceramics.
Material properties E1 E2 ES GV
Density (r, g/cm 3 ) 2.47 2.51 2.56 2.53
Elastic modulus (E, GPa) 86 96 96 65
Poisson’s ratio ðnÞ 0.27 0.26 0.24 0.23
Vickers hardness (H, GPa) 5.9 6.3 6.3 5.4
ARTICLE IN PRESS
compared with the leucite-based ceramic (E1) and
the GV (Table 3) and the differences in mean values
were statistically significant ðp # 0:05Þ:
All fractures initiated along the tensile surfaces
between the inner load points. Corner flaws (CF),
either cracks or voids, were identified as the
fracture origin in 20–30% of the specimens (Fig. 3,
Table 3).
Discussion
A. Della Bona et al.
Structural failure associated with an applied load
occurs when the induced stress is greater than the
strength of the material. The failure location is
dependent on the size of the initiating crack and the
fracture toughness. 9 Usually, fracture of the ceramic
originates from the most severe flaw. The
large number of pre-existing ceramic cracks of
differing sizes, coupled with a low fracture toughness,
limits the strength of ceramics and causes a
large variability in strength and the time dependence
of strength. The size and spatial distribution
of flaws justify the necessity of a statistical
approach to failure analysis 18 to determine the
reliability of ceramics. 11,19
Investigations of clinically failed all-ceramic
restorations have shown that the fracture origin is
typically located at the inner tensile surface.
20 – 22
However, for strength tests to yield relevant
service information, the test and service environments
must be similar and the strength-controlling
flaw population must approximate that responsible
for failure in clinical service. 19 The distribution of
flexural strengths under wet environmental conditions
is a potential indication of ceramic performance.
The greater mean s and K IC values of E2
and ES core ceramics are indicative of potentially
improved structural performance. However, resistance
to stress corrosion processes and a small range
of strength values are also factors in structural
reliability. Thus, a combination of good fracture
toughness, excellent resistance to stress corrosion,
and resistance to loading are required for structural
reliability. If one of these elements is missing, then
there may be a false sense of security in designing
ceramic prostheses made from these materials. For
example, in the case of the Group GV, the Weibull
modulus, m; is twice as large as that for each of the
other groups and the flaw size is smaller than the
flaw sizes of the other groups. This appears to be a
positive result. However, because of the low value
of fracture toughness, the strength is much less
than that of the other groups and this, most likely,
would not be a reliable material.

Under certain service and/or environmental
conditions, stable crack extension or slow crack
growth can occur at stress intensities that are less
than the critical value, K C: In this study all fracture
surfaces were produced from specimens immersed
in 37 8C distilled water. The loading rate used
limited, but did not eliminate, slow crack growth,
when tested in water. 11 Thus, the strengths
reported are less, but more conservative than
those expected for tests conducted in air. Yet
the calculated fracture toughness does not depend
on the crack growth since the flaw size was
measured. The strength values are correlated to
the flaw size to calculate the value of toughness.
ARTICLE IN PRESS
Fracture Behavior of Ceramics 5
Figure 2 SEM micrographs of lightly etched ceramics. (A) IPS Empress 2 GV revealing light and dark glass phases
containing very similar compositions. (B) XRD analysis showed the presence of leucite crystals ( p ) in a glassy phase of
IPS Empress core ceramic (E1). XRD analysis showed the presence of lithium disilicate crystals ( p ) in a glassy phase of
(C) IPS Empress 2 core ceramic (E2) and (D) Experimental lithia disilicate-based core ceramic (ES).
Previous reports on the fracture toughness using the
single edge notched beam (SENB) method performed
at room conditions showed KIC values
comparable to those presented in this study.
23 – 25
Fracture surface analysis is well established as a
means of failure analysis in the field of glasses and
ceramics. 3 It has been recognized as a powerful
analytical tool in dentistry. 20,22 Fracture in glass
and ceramics occurs when pre-existing cracks
propagate under excessive tensile stresses.
These cracks can be produced by mechanical
means, e.g. grinding or polishing, by processing,
or by intrinsic defects, e.g. imperfections in the
structure. Fractographic analysis identified the type
Table 3 Mean values and standard deviation (SD) of flexural strength ðs fÞ; critical crack size ðcÞ; fracture toughness ðK ICÞ;
characteristic strength ðs 0Þ; Weibull modulus ðmÞ; and fracture origins for the four ceramic groups.
Groups s f (SD) (MPa) s 0 (MPa) m c (SD) (mm) K IC (SD) (MPa m 1/2 ) Fracture origin a (%)
E1 85 (15) B b
91.3 5 110 (62) 1.3 (0.3) B b
SF (70); CF (30)
E2 215 (40) A 231 5 140 (56) 3.4 (0.6) A SF (70); CF (30)
ES 239 (36) A 252 7 110 (25) 3.1 (0.4) A SF (80); CF (20)
GV 63.8 (5.8) C 65.7 14 95 (35) 0.8 (0.1) C SF (80); CF (20)
a SF, surface flaw (void or crack); CF, corner flaw (void or crack).
b The values in each column with same letters do not differ significantly ðp , 0:05Þ:

6
Figure 3 SEM micrographs of ceramic fracture surfaces showing representative critical flaws outlined by white arrows.
(A) Fracture surface of E1; line from flaw corner, c ¼ 84 mm (500 £ ). (B) Fracture surface of E2; measured line
represents the semiminor axis, a ¼ 44 mm (500 £ ). (C) Fracture surface of ES; measured line represents the semiminor
axis, a ¼ 35 mm (600 £ ). (D) Fracture surface of GV; note the tailed fracture markings (top right) pointing toward the
crack origin; measured line represents the semiminor axis, a ¼ 55 mm (500 £ ).
of origins and confirmed the presence of characteristic
markings of the fracture process (Fig. 3).
Ceramic specimens tested in bending are very
sensitive to edge or surface machining damage. 19
Fractographic analysis has shown that all failures
started from either a surface (Fig. 3B–D) or a corner
flaw (Fig. 3A) located along the tensile surface of
the specimens. These results are consistent with a
previous report, which suggested that surface
failures started only at the tensile side of specimens
tested in four-point bending. 26 However, the
number of fractures originating from corner flaws
(20–30%) in this study suggests that careful manual
chamfer or rounding of specimen edges may
improve the reproducibility of test results produced
through ISO standard 6872 for dental ceramics.
Although machine rounding of edges can produce
even more defects, careful manual rounding of the
edges is preferred, by some investigators, to
machining 908 corners.
The Weibull modulus ðmÞ is a measure of the
distribution of critical flaws. As the Weibull moduli
are similar for three of the four ceramics, and the
crack sizes ðcÞ are comparable (Table 3),
ARTICLE IN PRESS
A. Della Bona et al.
the differences in mean strength cannot be
explained by a difference in crack sizes. In fact,
E2 has the largest mean crack sizes, and yet, has
one of the largest mean strength values. Since the
fracture toughness is a constant (Eq. (1)), a large
crack size would result in a low strength value for
the same toughness. Thus, the differences in mean
strength can only be explained by differences in
fracture toughness that are related to processing
and composition variables. The GV contains
relatively few, if any, crystals and would be
expected to have a toughness value the same as,
or slightly greater than those of silicate glasses as
shown in Table 3. The E1 core ceramic has crystals
dispersed in a glassy matrix. However, the volume
fraction of crystals is very small (Fig. 2) compared
with those of E2 and ES ceramics and is expected to
have a toughness value that lies between that of the
GV and those of the E2 and ES ceramics. The fine
dispersion of crystals in the E2 and ES ceramics
leads to the increased toughness values observed
relative to either the GV or the E1 ceramic core
material. The differences in the crystal sizes
between E2 and ES ceramics are not great enough

within the sampling error to determine if these
differences can affect the toughness values.
However, the amount and sizes of crystals are
enough to increase the magnitude of toughness to a
greater level compared with GV and E1. Thus, the
strength values are proportional to the toughness
values as stated previously.
Acknowledgements
This paper is based on a dissertation submitted to
the graduate faculty, University of Florida, in
partial fulfillment of the requirements for the PhD
degree.
This study was partially supported by CNPq-Brazil,
grant 300659/03-2, and NIH-NIDCR grant DE06672.
The authors thank Dr C. Shen for performing the
statistical analysis and Mr Robert B. Lee for his
assistance in processing the ceramics. We also
thank Ivoclar AG (Liechtenstein) for providing the
ceramic materials used in this study.
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