Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 165

10 μm (Li et al. 2004a). Furthermore, three categories of the pores exist in the splat: open
pores, through-thickness pores, and sealed pores (Figure 4.22) (Li et al. 2004a).
It is noted that for the starting HA particles, a porous structure was demonstrated
(Figure 4.5b). During plasma spraying of the HA powders, the particles were assumed
to be fully melted, which can be deduced from the complete flattening of the splats. The
formation of sealed pores was likely to be the result of a rate balance between gas bubble
escaping and particle solidification. Under the same spray conditions, formation of either
open pores or through-thickness pores could be attributed to the locations of the original
gas bubbles. Partial-melt state of the HA particles during HVOF spraying could be respon-
sible in part for the difference in pore morphology within a splat from that deposited by
plasma spray. An earlier study on assessing the significance of the melt state of HVOF-
sprayed HA revealed that typically, the melt fraction in the particles ranged from 20% to
over 90% depending on the starting particle size. Particles around 20 μm would likely to
be over 80% melted. Therefore, apart from porosity of starting powders, the melt state of
the particles during spraying is an important factor influencing the final pore structure of
bulk coating.
The subsequent droplet (or partially melted particle) could have remarkable influence on
the open pores as exhibited by the former splat due to the considerably high velocity of the
droplet. Typical morphology changes of the folded splats induced by the in vitro incuba-
tion for 1.5 h are shown in Figure 4.23 (Li et al. 2004a). Surprisingly, a trace of liquid filling

(a) (b)

rough-
thickness
pore

Sealed pore

20 kV ×1700 10 µm 17 44 7Pa 20 kV ×1190 10 µm 17 44 7Pa

(c)

Open pore

Sealed pore

20 kV ×1400 10 µm 17 44 7Pa

FIGURE 4.22
Typical plasma sprayed HA splats showing three types of pores: open pore, through-thickness pore, and sealed
pore. (From Li et al., Biomaterials , 25, 3463-3471, 2004a. With permission.)