Microtribology of Human Teeth - Dental Biotribology - page 119

Fig. 5.4 Microstructure observations of ( a ) the polished enamel surfaces and ( b ) nanoindentations
on the occlusal section
to 1 μm in turn. Cutting, grinding, and polishing were conducted under water-
cooling conditions to avoid local overheat that can result in dehydration and changes
in both the microstructure and chemistry of human teeth. After polishing, speci-
mens were stored in distilled water at 4 °C. The average roughness R a of the speci-
mens was controlled at 0.10 μm using a high-resolution surface profilometer
(TALYSURF6, Taylor Hobson Limited, Leicester, UK). All specimens were cleaned
with alcohol before testing.
Four specimens, which were randomly selected from five specimens of the
occlusal section, were used to measure the nanoindentation hardness and elastic
modulus of the enamel. In order to orient the indentations, the surface of each speci-
men was etched for 1 min in an aqueous 0.001 M citric acid solution before nanoin-
dentation. It was found that such slight etching did not significantly change the
surface profile, composition, and mechanical properties of the enamel [ 17 ]. By
using a nanoindenter (CSM Instruments, Peseux, Switzerland), 200 indentations
under a peak load of 2 mN were made on each specimen surface. The space between
the indentations was 3 μm. At each test, the loading speed was adjusted to keep 30 s
of loading time, 2 s of delay at peak load, and 30 s of unloading time. The whole
process of indentations was completed at 25 °C and at a relative humidity of 60 %.
Because surface roughness could influence the measurement accuracy of the
nanoindentation to some extent, an extremely low load should not be applied during
the nanoindentation process. In addition, the moderate space between indentations
must be taken into account in order to avoid their interaction. Hence, the number of
indentations within a single enamel rod was limited, as shown in Fig. 5.4 [ 1 ].
Considering that the nanoindentation test zone was really small on the surface of
each specimen in this study, it was hypothesized that both the shape and mechanical
properties of enamel rods were similar to each other within the test zone. Based on
the above hypothesis, a coordinate system was set up for each rod so that every
indentation was oriented by its coordinate position. As a result, all the indentations
can be superimposed in the same coordinate system, as shown in Fig. 5.5 .