Biomedical Engineering Reference
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4.10a-1). Very few have the enlarged size of 0.4 to 1.2 μm in height and 80 to 120 nm in
width. While other coatings do not have such features (Figure 4.10b and c). Research
showed evidence of different melt state of the sprayed powder particles during the HVOF
spraying (Khor et al. 2004), which will be discussed in a later part. It is further confirmed
that during the HVOF spraying, bigger powder particles experienced partial melt state,
while the smallest particles were entirely molten. Plasma-sprayed HA particles usually
get entirely molten. There is no doubt that the melting of the particles accounts for the
reorganization of the rod-shaped nanosized grains in the starting feedstock. The forma-
tion of the nanosized grains with hexagonal prismatic morphology indicates the influence
of both the melt state and temperature of the HA particles upon their impingement. The
partial melt state during HVOF spraying and overheating state during plasma spraying of
the HA particles could result in a faster cooling rate of the molten part upon their impinge-
ment on the room-temperature substrate. The high cooling rate would influence the forma-
tion of the hexagonal grains. Even though Chraska et al. (2001) reported in their findings
that the top surface of the first solidified splat causes epitaxial growth of columnar grains
in subsequent splats for plasma-sprayed YSZ splats, the study on HA revealed spherical
nanosized grains within the second folded splat and there is no experimental evidence
showing the presence of columnar grains in other parts in the coatings other than in the
first layer splats.
It is noted that the nanostructures in all the coatings showed markedly different fea-
tures from those in the starting feedstock (Figure 4.5). After the high-temperature spray
processing, the rod-shaped nanograins have turned to be nanosized spheres (with a size
of 30-120 nm). This suggests that during the melting/resolidification process, individual
nanorods were mostly fractured into several individual parts, and with the aid of surface
tension, the nanosized spheres were accomplished. Therefore, it can be expected that finer
nanostructures in starting feedstock would induce finer nanostructures in the coatings. It
nevertheless indicates that the existence of nanosized grains in the starting feedstock is
essential for formation of nanostructures in the coatings.
TEM and FESEM observation of the HA splats further reveals the presence of the nano-
structures (Figure 4.11) (Li et al. SCT 2006). It is clear that at the areas near to splats' fringes,
the structure is actually nanostructures composed of ~30-nm grains (Figure 4.11a and b).
The HA particles may have different thermal history during HVOF and plasma spraying.
This may influence both the microstructure and phases in the resultant splats/coatings.
The unmelted part of the HVOF-sprayed HA splat was also characterized using TEM. The
sample was prepared through further ion milling the individual HA splats. As shown in
Figure 4.11c, the unmelted part of the splat shows enlarged rod-shaped grains with the
dimension of <400 nm in diameter and <550 nm in length. The HVOF coating made from
partially melted powder, hence, must have a mixed nano- and microstructure. Enhanced
cooling of substrate during coating deposition could effectively increase the content of the
nanosized grains in the HA coatings. The nanostructures exhibited within the molten part
of the HA splats are consistent with the nanosized grains revealed in the coatings. This,
on the other hand, further suggests that during the accumulation of individual HA splats
to form bulk coating, there is no obvious grain growth for the nanosized grains. Studies
showed the structural features are actually associated closely with melt state of the HA
particles experienced during the coating deposition.
The distinguishing structural features could be related to melt state of the sprayed HA
powder particles (Khor et al. 2004). The term “melt-state” refers arbitrarily to the extent of
melting that took place in the particles prior to impact, and subsequent solidification on
the substrate. It would be possible that the phase composition and properties of resultant
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