Biomedical Engineering Reference
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O H A
(i) Higher than the melting
temperature
Stoichiometric
Hydroxyl deficient
Hydroxyl and phosphate
deficient
D O H A
(ii) Very high temperature
A amorphous phase
H hydroxyapatite
O oxyapatite
E O H D
(iii) Extremely high temperature
D C
3
P and C
4
P
E C
4
P and CaO
FIGURE 4.32
Proposed model for phase formation of plasma-sprayed HA particles. (From Gross et al., in
Thermal Spray:
Meeting the Challenges of the 21st Century, Proceedings of the 15th International Thermal Spray Conference,
Nice,
France, 1998a, pp. 1133-1138. With permission.)
leading to a change in melt composition, and (2) the high cooling rate due to the thermal
process. Hydroxyl group removal promotes the amorphous phase and oxyapatite. Further
heating produces a less viscous melt facilitating decomposition of HA to TCP and TTCP.
Phosphate removal during flight produces a more calcium-rich melt preferring TTCP and
CaO formation.
The extremely high temperatures easily result in an increased amount of ACP in as-
sprayed coatings. It was believed that the cooling rate had significant influence on the
resultant phases that transformed from HA (Ruan et al. 1996). Furthermore, partial water
vapor pressure acts as an important factor in the phase determination after spraying pro-
cess. The HA stability as a function of temperature and
P
H2O
is shown in Figure 4.33. It
reveals that the decomposition of HA is easier under lower water vapor pressure and higher
temperature. (Accordingly, 75% of the water may be lost while retaining the HA crystal
T
º
C
1600
1500
1400
1300
HA + CaO
3
HA
+
TTCP
2
TTCP
+
Liq.
α
'TCP
+
TTCP
α
TCP
+
TTCP
1
T
2
0
T
1
5.25 5.50
5.75 6.00 6.25 6.50
10
4
/
T
(K)
FIGURE 4.33
HA stability as a function of temperature and
P
H
2
O
. (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.)
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