Biomedical Engineering Reference
In-Depth Information
curve of sol-gel derived HA and fluoridated HA coatings. The curves indicate a good
adhesion between the coating and substrate, and reveal that the coating-substrate interfa-
cial failure mode changes from brittle to ductile with fluorine ion were incorporated into
the HA lattice structure (Zhang et al. 2006). That is to say, the scratch test can provide a lot
of remarkable information about the coating-substrate interface, even though some of the
data are just qualitative. Therefore, by considering the features of the pull-out tensile test,
shear lag strain method, and scratch test, the combination of these three testing methods
could be more helpful in getting a comprehensive understanding of the coating-substrate
adhesion properties.
Toughness of Sol-Gel Derived HA Coating
Fracture toughness serves as a decisive factor in evaluating the functionality of coated
implants and determines the level to which the material can be stressed in the presence
of cracks, or equivalently, the magnitude of cracking that can be tolerated at a specific
stress level. Regarding the interfacial fracture toughness of HA-coated Ti6Al4V implants,
Filliaggi, Coombs, and Pilliar (1991) used a short bar chevron notch test and obtained the
K
IC
values of 0.6-1.41 MPa m
1/2
. By using a single-edge notch-bend test, Tsui, Doyle, and
Clyne (1998) reported some similar values of
K
IC
of about 0.28-1.1 MPa m
1/2
. In addition,
an indentation based method was also employed by Li, Khor, and Cheang (2002), and the
corresponding value was reported as 0.48 MPa m
1/2
for
K
IC
.
However, for HA coatings prepared with the sol-gel method, indentation-based
energy analysis method is preferred due to the limitation of the thickness of HA coating.
Theoretically, the energy approach examines the energy difference before and after the
cracking, which is responsible for the fracture of the coating. The energy difference would
then be the energy release in the through-thickness cracking in the coating. The energy
release can be obtained from a “step” that would be observed in the load-displacement
curve for the indentation. Therefore, based on the energy difference before and after the
crack generation, the fracture toughness of the coating can be determined as (Li, Diao, and
Bhushan 1997):
∆
U
t
E
K
=
f
(
)
IC
2
2
C
1
−
ν
π
R
f
where
ν
f
is the Poisson's ratio of the coating, 2
π
C
R
is the crack length in the coating plane,
t
and
E
f
are the coating thickness and elastic modulus, respectively, and
Δ
U
is the strain
energy difference before and after cracking. Figure 1.9 displays a typical load-displacement
curve of indentation together with the corresponding SEM micrograph, which can be used
readily for toughness evaluation of the HA coating (Zhang et al. 2008).
Residual Stress Measurement
Residual stress is inherently induced in any coating deposited by a method with a high
temperature process due to the differences in the thermal properties between the coating
and the substrate. Residual stress in the coating might vary with coating thickness, deposi-
tion parameters, etc. Therefore, both tensile and compressive residual stresses have been
