Fracture behavior of lithia disilicate- and leucite-based ceramics

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.