Biomedical Engineering Reference
In-Depth Information
starting powders, which exhibit a porous structure. From this point of view, different melt
fractions of HA powders correspond to different coating structures. In order to reveal the
influence of different melt state of the powders on phase composition of resultant coat-
ings, the coatings, C-4, C-6, and C-7 were analyzed. Figure 4.15 shows the XRD patterns of
the three types of coatings. It is found that decrease in powder size results in an increase
in ACP phase and extent of thermal transformation of HA. The following formula was
widely suggested to describe the decomposition of HA (Ogiso et al. 1998b; Wang et al. 1998;
McPherson et al. 1995):
Ca
10
(PO
4
)
6
(OH)
2
→ 3Ca
3
(PO
4
)
2
+ CaO + H
2
O
(4.5)
The decomposition of HA during coating deposition can result in some CaO simulta-
neously, but because of the low content (<1 wt.%), it could not be detected by XRD analysis.
The formation of the ACP was apparently associated with partial dehydroxylation of HA
(Radin et al. 1992) during melting and subsequent rapid solidification of the powders (Cao
et al. 1996). The melted portion of the powder is in direct response to the formation of
ACP. Moreover, crystalline HA phase is generally attributed to the retention of original
HA phase in the sprayed powders. For near crystalline coatings (crystallinity >90%, deter-
mined arbitrarily from XRD technique; Girardin et al. 2000), content of α-TCP was also
determined using the Rietveld method, which is shown in Figure 4.16 (Khor et al. 2004).
Content of TCP directly reflects the extent of the transformation that occurred following
solidification of the droplets on the substrate. Liao et al. (1999) reported that transforma-
tion to TTCP occurred at about 1360°C through some transient phases. It has been found
that no transformation to α-TCP from a stoichiometric HA (with a Ca/P ratio of 1.67) could
be detected up to 1200°C with prolonged heating (Zhou et al. 1993). The appearance of
β-TCP in the HA coatings is possibly attributed to the phase transformation from α-TCP
at about 1100°C (Reser 1983). Studies already showed that once HA powders had been
HA
α-TCP
β-TCP
TTCP
CaO
(4)
(3)
(2)
(1)
20
30
40
50
60
2
θ
(deg)
FIGURE 4.15
XRD patterns of the HA coatings (a) and the starting HA powder (b). (1) HVOF HA coating (50 ± 10 μm powder),
(2) HVOF HA coating (40 ± 10 μm powder), (3) HVOF HA coating (30 ± 10 μm powder), and (4) plasma HA coat-
ing (50 ± 10 μm powder). (From Li, H., Khor, K.A.,
Surf. Coat. Technol.
, 201, 2147-2154, 2006. With permission.)
Search WWH ::
Custom Search