Biomedical Engineering Reference
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coatings increases. However, they showed detrimental effect on adhesive strength (Figure
4.24b). Tensile failure analysis has suggested an entire adhesive failure mode. This in turn
indicates that the coating/substrate interface plays a key role in determining the adhe-
sion. The difference of the mechanical properties can be correlated to the nanostructural
features (Li et al. SCT 2006). The perpendicular-to-substrate grains might have changed
the residual stresses at the coating/substrate interface, or they weakened the mechani-
cal interlocking of the coating with the rough substrate. For a high adhesive strength,
formation of the grains with hexagonal prismatic morphology near to the coating/sub-
strate interface must be avoided. The mechanism why the nanostructures benefit high
Young's modulus is not clear yet. The presence of nanosized pores among the nanosized
grains might be responsible for the increased value, as it has been discussed that irregular
pores and large pores can result in decreased modulus (Sevostianov et al. 2000; Shen et al.
1995). Furthermore, high packing density of constituent atom exhibit high values of elastic
modulus (Soga 1982). Even though the plasma-sprayed HA coating comprises compatible
content of nanostructures, both the low Young's modulus and adhesive strength might be
related to the phases present in the coating and the low velocity of in-flight particles (this
brought about high porosity). It must be noted that within the nanostructures in the HA
splats, there are a lot of phases apart from HA (e.g., ACP, α-TCP, β-TCP) being revealed.
These may not be desirable since these phases come from HA decomposition and have less
bioactivity than HA (Cleries et al. 2000; Ducheyne et al. 1993). Furthermore, the presence
of the phases in the plasma-sprayed HA coating may also account for the low modulus of
the coating.
It should be noted that the overall properties exhibited by bulk HA coating could be
mainly attributed to those of separate HA splats. In order to reveal the approximate con-
tribution of different phases, sprayed HA powders were collected and subsequently nano-
indentation tests were conducted on their polished cross sections (Khor et al. 2004). For
comparison, the nanoindentation test was also conducted on the polished cross sections
of starting HA powders. Concerning the small indentation area, less than 9 × 10
4
nm
2
, the
Young's modulus and microhardness data are reliable in reflecting the relations between
microstructure and properties. It was found that different parts of the sprayed HA particle
exhibits remarkably different Young's modulus (Khor et al. 2004). The resolidified zone
exhibits an average Young's modulus value of 41.25 GPa with a vast range from 23.1 to 65.3
GPa, which indicates complex phase components within that zone. And the corresponding
microhardness shows an average value of 3.37 GPa with a data range from 1.72 to 5.93 GPa.
Since the indentation area is extremely small, it is possible that those values reflect the
properties of different phases. In other words, the alternation of Young's modulus is attrib-
uted to different phases. The unmelted part of the HA powders shows a Young's modulus
value of 83.9 (±9.4 GPa) and a microhardness value of 5.22 (±0.87 GPa). Since the starting
powders were composed of crystalline HA, it can be claimed from the indentation results
that crystalline HA exhibits the highest
E
-values. The existence of other phases, such as
α-TCP, ACP, could be responsible for the decrease in Young's modulus. The results corre-
spond well to those obtained from bend tests on HA coatings. The nanoindentation tests
further confirmed the significant influence of different phases on overall properties of
resultant coatings. It therefore states that optimization of phase components in HA coat-
ing is a useful way toward satisfactory mechanical properties. Meanwhile, it is noted that
compared to the nanoindentation result on bulk HA coating, which exhibited a Young's
modulus of 118 (±5.21 GPa), the sprayed HA particles showed low Young's modulus. The
residual stress formed during coating formation could be responsible for the increased
E
values exhibited by HA coatings.
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