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(a)
(b)
20 kV ×1000 10µm 17 44 7Pa
20 kV ×1000 10 µm 17 44 7Pa
(c)
20 kV ×2500 10 µm 17 44 7Pa
FIGURE 4.23
Microstructure changes of folded HA splats after 1.5-h incubation in vitro showing a healing effect of sub-
sequent splat on the open pores of former splat, (a) before the incubation, (b) after the incubation, the subse-
quent splat dissolved into the SBF entirely, and (c) under higher magnification of image (b). (From Li et al.,
Biomaterials,
25, 3463-3471, 2004a. With permission.)
from the subsequent droplet is revealed (Li et al. 2004a). The folded splat shows significant
resealing effect on the micropores of the former splat formed. It has been pointed out that
the phases brought about by the HA transformation have marked dissolvability in the
SBF (Ducheyne et al. 1993). It is clear that the injected part is composed mainly of dissolv-
able (bioresorbable) phases, such as TTCP, TCP, and ACP, and in a previous report, it has
been claimed that phase transformation of HA took place mainly within the melted part
of the particle (Li et al. 2000). Liquid with high impingement speed can be spilled into the
open (through-thickness) pores easily, and hence, decreases the porosity of resultant coat-
ing. It is therefore acknowledged that particle velocity is crucial as it directly governs the
impact, with which the particle impinges on the former splat to affect pore healing (seal-
ing). This could at least be the partial reason why HVOF coatings are typically denser than
plasma-sprayed coatings, given that powder feedstock and powder feed-rate are similar.
The filling of the liquid materials into the micropores can also be suitably explained by
the large impingement pressure brought about by the subsequent splat, which has been
described in an earlier report (Li et al. 2003). It should be noted that only liquid could
achieve such improvement. With regard to the healing effect, it can be concluded that only
the sealed pores and the unbonded areas among splats contribute predominately to the
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