Biomedical Engineering Reference
In-Depth Information
Fracture toughness
K
IC
of human enamel, measured by indentation, was in the range
[41]
0.4
1.6 MPa m
1/2
, depending on the tested place and indenter orientation. Also other factors play
a role. For example, old enamel is more brittle than young. Dentin is much tougher than enamel, and
the attempts to nucleate cracks in it by nanoindentation were unsuccessful so far.
A disadvantage of this method is that elastic modulus and hardness must be known in addition
to the crack nucleation and measurement. This needs two groups of tests. Recently, Zhang et al.
[38]
recommended another approach. They have shown that a direct proportionality exists between the
ratio of hardness and elastic modulus (
H
/
E
) and the ratio of the elastic and total work (
W
el
/
W
tot
) in
a load-unload indentation cycle. These works correspond to the areas under the force-displacement
curve (CABC in
Figure 17.1
for
W
el
and OABO for
W
tot
), and can be obtained easily from the
P
-
h
data. This enables the determination of fracture toughness in one load-unload test, using the follow-
ing modification
[38]
of Eq. (17.22):
1 2
/
3 2
/
K
λ
(
W W
/
)
P c
/
(17.24)
IC
tot
el
with the constant
λ
0.0695 for cube-corner indenter. However, this value was obtained by the cali-
bration on three silicate glasses and Si only, and its validity for enamel and other hard tissues has not
been verified yet. Moreover, a recent paper
[42]
has brought a review of various indentation meth-
ods for the determination of fracture toughness, and its conclusions were rather skeptic regarding
their accuracy. For similar reason, some authors recommend the term
indentation toughness
instead
of
fracture toughness
[43]
. Nevertheless, indentation methods do characterize the material resistance
against crack propagation, and can be, at least, useful for comparison or ranking of various materials
or treatments.
17.7
SCRATCH TESTS FOR THE EVALUATION OF FRICTION
AND WEAR RESISTANCE
One cause of gradual teeth deterioration is the wear due to the friction between the tooth and another
object, such as the opposite tooth, a hard toothbrush, piece of food or a foreign particle. The main
mechanisms of teeth wear are attrition, abrasion, and erosion. The knowledge of factors that influence
the wear rate is important for the development of teeth-care tools and treatments, for the development
of restorative materials, including their surface modifications or coatings, but also in the development
of foods and beverages.
Wear can be studied in two ways. In standard wear tests, a pin of a suitable shape slides along the
test sample in rotating or reciprocating way. The normal and tangential (friction) force,
P
n
and
P
t
,
are measured and used for the determination of the coefficient of friction,
μ
P
t
/
P
n
. Also the rate of
material removal is measured, and the appearance of the surface is observed under a microscope. This
“macro” approach is suitable for the study of the influence of various factors on the friction, material
loss, and the time to reaching a certain damage.
The details of wear processes and their mechanisms on micro- or nanolevel are better studied via
scratch tests. These can be performed by nanoindenters (scratch-testers), which enable not only the
movement of the indenter into the specimen, but also their mutual sliding. In the test, a suitable stylus
(or indenter) is moved along the surface, usually under increasing load, and the normal and tangen-
tial force are measured. A videomicroscope enables observation of the scratch trace on the surface.
