Biomedical Engineering Reference
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The molten/unmolten ratio of HVOF-sprayed HA powders was experimentally deter-
mined through image analysis. It was found that during the present HVOF spraying,
most HA powders were only partially melted, which is schematically shown in Figure
4.12 (Khor et al. 2004). After appropriate grinding and polishing, from the figure, the melt
fraction in particle (MFP) of HA powders was defined as the ratio of melted volume to the
whole particle's volume (Khor et al. 2004)
3
3
V
V
R
r
MFP
=
melted
total
=
×
100%
(4.2)
3
R
where R and r are the radius of HA particle and the unmelted core, respectively.
Determination of MFP was performed through deriving the area ratio obtained using the
ImagePro image analysis software to volume ratio. From the image analysis, the area ratio
of molten part ( A ) and area of unmelted part ( B ) can be determined
2
2
A
B
R
r
a
=
=
(4.3)
2
r
Combined with the above definition, the melt fraction of alternative HA particles can be
calculated according to the following formula:
3
2
1
MFP = −
1
(4.4)
1
+
a
The SEM cross-sectional morphology obviously demonstrates the miscellaneous powders'
melt state (Figure 4.13) (Khor et al. 2004). It reveals that with the increase in particle size,
the melt fraction of the particles decreases. The statistical analysis on the melt fraction
of the powders (MFP) is shown in Figure 4.14 (Khor et al. 2004). Under the same spray
conditions, fine powders (C-7, 30 ± 10 μm) contained MFP values in excess of 90%, while
the large powder (C-4; 50 ± 10 μm) have MFP values that stretches down to ~20%. It also
shows that apart from the influence of starting powder size, the increase in flow rate of
oxygen and hydrogen, which contributes to an increased flame temperature, also results
in increased melt fraction values (cf. C-2 and C-5 in Figure 4.14). Furthermore, the melt-
resolidification phenomenon induces a densified structure that compared favorably to the
r
R
FIGURE 4.12
Sketch of HA particle during HVOF spraying showing partially melted state. R , radius of HA particle; r , radius
of unmelted core. (From Khor et al., Biomaterials , 25, 1177-1186, 2004. With permission.)
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