Biomedical Engineering Reference
In-Depth Information
represent similar microstructures to the as-sprayed HACs, and 150°C hydrothermally
treated HACs display the highest coating density of all the conditions. However, note
that an obvious cracking feature (as indicated by arrows) and an interfacial TiO
2
layer can
be recognized under both of the high-temperature heating processes and the formation
of TiO
2
layer (characterized by XRD analysis, a rutile phase) significantly causes serious
interfacial fracturing and coating debonding at elevated heating temperatures [120,209]. In
addition, the deterioration of bonding strength for heat-treated HACs in vacuum and in
the air should also depend on the above-mentioned detrimental crystallization-induced
contraction, CTE mismatch between the coating and substrate [64,120,210] in Section 6.2.3,
and the variation of residual stresses [100,210] during high-temperature heat treatments,
especially in the temperature range of 600°C to 800°C. The relationship between the heat-
ing temperature and the bonding strength is found to be related to the crystallization
behavior, and the affecting factors will be clarified in the following paragraphs.
According to the criterion of ASTM C633, the variation of bonding strength in situ is
suggested to be governed by the cohesive strength of coatings and the adhesive strength
of a coating to a metal substrate. The affecting factors of the adhesive strength of a coating
and substrate interface include the surface roughness of substrate and the residual stress.
As for the cohesive strength of coating, the factors include the crystallinity (IOC) and the
densification of a coating that appears on Young's modulus of a coating. Since Young's
modulus is a measure of the interatomic binding forces, the evolution of Young's modulus
for the HACs depends on the extent of crystallization. In addition, the microstructural fea-
, the microstructural fea-
tures such as defects, lamellar structure, and the contact between splats boundaries should
also be considered as other affecting factors on Young's modulus of a coating [211,212].
Through Young's modulus measurements, it can help to clarify the effect of crystallization
on the bonding strength and the failure mechanism. Several methods can measure Young's
modulus of a material, including the bending test, single-edge notch test, compact tension
test, and so forth, and a standard three-point bending test that follows Equation 6.24 is a
suitable method applied for measuring Young's modulus of plasma-sprayed HACs [213].
In addition, the microstructural fea-
3
PL
wt
E
=
(6.24)
3
4
δ
E
(GPa) is the Young's modulus,
P
(N) is the load,
L
(mm) is the span between sup-
ports,
w
(mm) is the specimen width,
t
(mm) is the specimen thickness, and
δ
(mm) is
the specimen deflection at midspan. For Young's modulus measurements, the HA coat-
For Young's modulus measurements, the HA coat-
ing test pieces, which are better if they are larger than 1 mm thick, are carefully cut from
the substrates by a low-speed diamond saw along the coating/substrate interface. Then
substrate-removed HA coating test pieces are heat-treated and then they can be used for
Young's modulus measurements. The loading direction should be perpendicular to the
spraying deposition surface. As shown in Figure 6.24a, note that Young's modulus of the
heat-treated HACs is significantly increased with increasing heating temperatures from
400°C to 800°C, and Figure 6.24b shows a linear relationship between Young's modulus
and the crystallinity (IOC) of these crystallized HACs. Although the porosity is also an
influencing factor on Young's modulus of a coating, SEM/BEI images displayed in Figure
6.24b indicate that there is no obvious microstructural difference between the HAC test
pieces with different crystallinity. Except for the influence of the porosity and defects
(such as pores and microcracks) distribution, therefore, it is reasonable to suggest that the
crystallinity is a main controlling factor in increasing Young's modulus of crystallized
For Young's modulus measurements, the HA coat-
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