Fracture behavior of lithia disilicate- and leucite-based ceramics

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