NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 251

depends on the creep response of the material. For small samples with
limited creep, such as enamel, it is not very reliable. In the biomaterials
field, only limited indentation creep tests have been done on biological
hard tissues. Torsion 47,48 and three-point bending tests 45 were used to
measure the relaxation elastic modulus and shear modulus of bovine
bone. Further more, because of the anisotropic property of natural
biomaterials, the loading direction must be controlled. This limitation,
along with the size of teeth, severely restricts the usage of bulk methods
for the evaluation of dental tissue.
As already mentioned, depth-sensing indentation techniques have
become widely used for mechanical property evaluation, and may be
used for the determination of creep behaviour. Indentation creep tests are
a powerful technique for studying the rheological properties and
relaxation dynamics of different materials such as metals, 51 polymers 52,53
and other materials. 54,55 In our recent investigations, 56 the creep
behaviour of enamel and dental materials were described and compared
by a one-step loading-unloading method from nanoindentation with a
Berkovich indenter. Samples were indented at the maximum force of
250 mN with a one-step loading method. Holding time at maximum
load was 900 seconds for all tests. During one-step unloading, another
holding at 5 mN was activated. This unload-holding time was 900
seconds as well. Each group of tests commenced at least two hours after
a thermal soak period in the instrument, and thermal drift was assessed
before each experiment. Thermal drift during all the tests was deemed to
be negligible (<0.02 nm/sec).
In Fig. 8-7 , the relative penetration depth, h t ', was calculated by
subtracting the initial depth at the beginning of the holding time from the
increasing depths during the holding period at maximum and minimum
loads. Creep data at maximum-load holding exhibited increasing
penetration depth with time for most of the investigated materials
( Fig. 8-7a ) . Amalgam showed the maximum creep, followed by titanium.
However, considerable scatter was observed in the data. Enamel, on the
other hand, had a relatively stable creep response. In contrast, the
indentation creep depths of hydroxyapatite and cast Cr-Co alloy were