Biomedical Engineering Reference
In-Depth Information
In recent years, a new hydrothermal method has been proposed that helps to prepare
thin HA ceramic coatings on Ti substrates with a curved surface at low temperature. It
has been demonstrated that a pure HA ceramic layer with a thickness of 50 μm could be
coated to a Ti cylindrical rod at about 135°C under the confining pressure of 40 MPa [138].
However, the coating layer showed a porous microstructure with the relative density of
only about 60%. Hydrothermal hot-pressing (HHP) method is also a possible route for
producing a ceramic body at relatively low temperatures (generally under 300°C) [161].
The bonding of HA and Ti alloys was achieved by the HHP method through the sur-
face modification of Ti alloys with NaOH solution at relatively low temperatures [161-163].
Because the released water can be utilized as a reaction solvent during the HHP treatment,
the compression of specimens under hydrothermal conditions can help to accelerate the
densification of HA and the joining of HA to metal was achieved simultaneously under
hydrothermal condition following the chemical reaction 6.16 [139,164].
6CaHPO
4
⋅2H
2
O + 4Ca(OH)
2
→ Ca
10
(PO
4
)
6
(OH)
2
+ 18H
2
O
(6.16)
Hydrothermal Crystallization with Self-Healing Effect of Plasma-Sprayed HACs
Some researchers have noted that morphology-controlled HA crystals and thin HA coat-
ings, which have high crystallinity and an enhanced interfacial adhesive strength, on a
curved surface of Ti substrate can be successfully synthesized through the hydrother-
mal method at low temperatures [63,138,165,166]. Although the hydrothermal method
has attracted great interest recently as a replacement for the high-temperature plasma-
spraying process, the hydrothermal synthesis, as displayed in reaction 6.16, is more com-
is more com-
plex because the purity of the raw material (dicalcium phosphate dihydrate, CaHPO
4
⋅2H
2
O,
DCPD), the concentration, and the pH value of solutions must all be carefully controlled to
obtain well-crystallized HA. Besides, the efficiency of forming a uniform surface coating
is much lower than the plasma-spraying technique. Therefore, combining the advan-
, as displayed in reaction 6.16, is more com-
as displayed in reaction 6.16, is more com-
.16, is more com-
16, is more com-
Therefore, combining the advan-
tages of plasma spraying and hydrothermal treatment can lead to improvements in the
crystallinity, microstructural homogeneity, adhesive strength, and biological responses
of HA-coated Ti-substrate implants [121,122]. The benefi ts of low-temperature hydrother-
technique. Therefore, combining the advan-
. Therefore, combining the advan-
Therefore, combining the advan-
The beneits of low-temperature hydrother-
mal crystallization of nanocrystallite HA have been clarified through x-ray photoelectron
spectroscopy (XPS), transmission electron microscopy (TEM) examinations, evaluation of
the crystallization rate, and the activation energy [167-171].
Figure 6.13 illustrates the schematic apparatus of the plasma-sprayed HA-coated
Ti-6Al-4V specimens within the hermetical autoclave during the hydrothermal treat-
ment. The deionized water is used as the source of steam atmosphere during the hydro-
121,122]. The benefi ts of low-temperature hydrother-
,122]. The benefi ts of low-temperature hydrother-
122]. The benefi ts of low-temperature hydrother-
]. The benefi ts of low-temperature hydrother-
The benefi ts of low-temperature hydrother-
is used as the source of steam atmosphere during the hydro-
thermal treatment. The specimens were isolated without immersion in the water. Figure
6.14 shows the XRD patterns of the plasma-sprayed and various hydrothermally treated
HACs (HT-HACs). The sharpening of the three strongest HA main peaks and the fl atten-
is used as the source of steam atmosphere during the hydro-
. The sharpening of the three strongest HA main peaks and the latten-
ing of the diffraction background (2θ at about 28° to 34°) mean that the plasma-sprayed
HACs significantly crystallized and the content of ACP signifi cantly decreased by per-
(HT-HACs). The sharpening of the three strongest HA main peaks and the fl atten-
-HACs). The sharpening of the three strongest HA main peaks and the fl atten-
HACs). The sharpening of the three strongest HA main peaks and the fl atten-
decreased by per-
forming the autoclaving hydrothermal treatment in an ambient saturated steam-pressure
system. The quantitative results of the crystallinity (IOC) for these HT-HACs are shown
in Figure 6.15. The IOC value increased with increasing the hydrothermal heating time
and temperature. It is worth noting that just low heating temperatures of 150°C to 200°C
are required to see a significant rise in crystallinity for the HT-HACs (the IOC is about
ACP signifi cantly decreased by per-
signifi cantly decreased by per-
significantly decreased by per-
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