Biomedical Engineering Reference
In-Depth Information
longer treatment time. However, more detailed examination is needed in order to discuss the
reason for the presence of such an optimal treatment time. Now investigation of the bonding
mechanism in the HHP process is now in progress.
Fig. 19. Relationship between HA coating adherence and hydrothermal treatment time.
5. Micro structure of HA ceramics by HHP method
SEM photograph of the detached HA layer surface is shown in Fig.20. As exemplified in the
figure, the microstructure of the HA layers prepared in this study is composed of the
number of pores. The maximum dimension of the pores was observed to be approximately
20 m. The microstructure of the HA layers prepared with the different treatment times was
similar to the one shown in Fig.20. The density of the HA layers as determined by the
Archimedes method was approximately 1.9 g/cm
3
, regardless of the different treatment
times. The relative density was calculated to be approximately 60 %, assuming the
theoretical density of HA (3.16 g/cm
3
). HA ceramics have been also synthesized using the
conventional hydrothermal hot-pressing (HHP) method in our previous section, where a
uniaxial load was applied to the starting powers using upper and lower loading rods in
order to produce high pressure environments. The density of the HA ceramics prepared by
the conventional HHP method has been measured to be approximately 1.9 g/cm
3
, when the
treatment temperature and pressure were the same as the conditions used in this study. The
agreement in the density suggests that it may be possible to produce suitable hydrothermal
conditions employing the HHP and the DC-HHP method developed in this study. It is seen
that the density of the HA layers prepared falls in the range of those for human bone (1.6-2.1
g/cm
3
). Therefore, it may be possible to adjust and tailor the elastic modulus of HA coatings
using the DC-HHP method and to mitigate the stress shielding due to the misfit in the
elastic modulus [Hench, 1998]. The above-mentioned physical properties and the porous
microstructure of the HA ceramic coatings prepared by the DC-HHP method may be
