Biomedical Engineering Reference
In-Depth Information
Therefore for thermal-sprayed HA/ZrO
2
coatings, prevention, or alleviation of the chemi-
cal reaction is a challenge.
Co-/PostsprayTreatmentofHACoatings
Influence of HA Feedstock
Thermal spray is basically a high-temperature processing approach. As discussed ear-
lier, HA can easily decomposite to other calcium phosphates at the temperatures beyond
~1000°C. Reciprocal transformation among these phases can prevail under certain condi-
tions. The equilibrium phase diagram for these transformations depending on tempera-
ture and content of P
2
O
5
is shown in Figure 4.31, which was usually utilized as a useful
tool in predicting the phases that could form upon solidification of sprayed HA powder.
The values of
T
1
(HA starts to fully transform to TCP and TTCP) and
T
2
(HA starts to trans-
form to TTCP) are dependent on the vapor pressure.
Generally, the temperature of plasma jet is approximately up to 20,000°C (Bertagnolli et
al. 1995), while the melting point of HA is around 1470°C and its transformation to other
phases usually takes place from around 1000°C. Therefore, at elevated temperatures dur-
ing plasma deposition, phase transformation from crystalline HA to other phases such as
TCP or CaO (Aoki 1994; Weng et al. 1994; Vu et al. 1997) seems inevitable. The following
decomposition reactions represent the usual chemical decomposition of crystalline HA
powders as they are heated and accelerated in the plasma jet or during splat formation:
Ca
10
(PO
4
)
6
(OH)
2
→ 2Ca
3
(PO
4
)
2
+ Ca
4
P
2
O
9
+ H
2
O↑
(4.13)
(Liu et al. 1994; Gross et al. 1998a; Ogiso et al. 1998b)
TCP +
TTCP
TCP
+
Liq.
T
1
CaO + TTCP
TTCP
HA
T
2
TCP
+
HA
TCP
+
DCP
CaO + HA
CaO
TTCP HA TCP P
2
O
3
% P
2
O
3
FIGURE 4.31
Schematic representation of CaO-P
2
O
5
-H
2
O system at fixed
P
H2O
. (From Harris, D.H., in
Thermal Spray Research
and Applications, Proceedings of the Third NTSC
, Long Beach, CA, May 1990, pp. 419-423. With permission.)
Search WWH ::
Custom Search