Biomedical Engineering Reference
In-Depth Information
σ
x
o
Coating
Interface
crack
Substrate
(a)
σ
x
Coating
σ
n
σ
x
o
Stress
concentration
Substrate
(b)
FIGURE 6.26
Effect of delamination on coating fracture driving force: (a) attached configuration and (b) delaminated con-
figuration.
σ
x
° and
σ
x
denote in-plane compressive stress in attached and delaminated coating, respectively.
σ
n
is induced through-thickness tensile stress.
The in-plane compressive residual stress, which resulted from the compressive residual
strains as shown in Figure 6.25, would induce through-thickness tensile stress (the
σ
n
in
Figure 6.26b) acting in the direction normal to the interface of the HA coating and the Ti
substrate. The induced normal tensile stress will neutralize the bonding force of a coating
on the substrate and weaken the interfacial adhesive bonding strength of the HA coating
to substrate, especially for those HACs heat-treated under temperatures higher than
600°C. Thus, from the above demonstrations, it is difficult to simultaneously acquire high
crystallinity without the microstructural deterioration and the degradation of adhesive
bonding strength for plasma-sprayed HACs through the high-temperature vacuum and
atmospheric heat treatments.
With regard to the hydrothermal treatment performed at relatively lower temperatures,
Figure 6.27a shows a linear relationship between Young's modulus and the IOC values of
As-sprayed HACs
Crystallized HACs in Ref. [120]
HT HACs in Ref. [122]
33
30
Hydrothermal crystallization
of HA
27
24
(a)
1 µm
(b)
0
20
40
60
80
100
Index of crystallinity, IOC (%)
FIGURE 6.27
(a) A linear correlation between Young's modulus and crystallinity (IOC) of HACs. (b) Self-healing effect of
hydrothermal crystallization (as indicated by arrows) on HT-HACs observed at coating fracture surface after
three-point bending test.
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