Biomedical Engineering Reference
In-Depth Information
mechanical properties of the film and substrate. The test is sensitive to loading rate, stylus
shape, environment (humidity), and scratch speed. Thus, it is difficult to relate the critical
load to film adhesion.
Three- and four-point bending methods have been used to ascertain the interfacial frac-
ture resistance between dissimilar materials and films on substrates and to provide quan-
titative measures of the interfacial fracture energy. The thicknesses of these substrates are
usually much smaller than their lateral dimensions, so that simple beam bending mechan-
ics can be applied to describe their elastic response (Saha and Nix 2002).
A fracture-mechanics-based method for four-point bending developed by Charalambides
et al. (1989) for interfaces between dissimilar materials has been applied in many areas
including biomaterials research (Angelelis et al. 1998; Kuper et al. 1997; Dauskardt et al. 1998;
Hofinger et al. 1998; Suansuwan and Swain 2003) to measure interface fracture energy. The
method requires a bend bar of the coated substrate with a notch machined in the coated
layer (Figure 2.12c). As the bending moment increases, a crack initiates from the notch and
propagates to the interface. For a sufficiently weak interface bond, the cracks deflect and
propagate along the interface. The interfacial fracture energy is given by Equation 2.7:
(
)
2 2
2
21
P l
E b h
1
−
ν
c
s
=
γ
(2.7)
2 3
16
s
where
b
is the specimen or beam width,
h
is the total thickness,
l
is the moment arm (dis-
tance between inner and outer loading lines),
P
c
is the critical load for stable crack propaga-
tion, and
v
s
and
E
s
are Poisson's ratio and Young's modulus of the substrate, respectively.
This method has the advantage that the interface fracture energy is independent of the
crack length as long as the crack tip is not too close to the precrack or the loading points.
In bulge and blister testing, the film-substrate system is pressurized with a fluid or gas
through a hole in the substrate. The height of the resulting hemispherical bulge in the film
is then measured by optical microscopy or an interferometer. The pressure and deflection
height can be used to provide in-plane information on elastic, plastic, and time-dependent
deformation.
In blister testing the pressure is increased until the film starts to debond from the sub-
strate. The interfacial energy can be determined from the critical pressure for debonding
as shown in Equation 2.8 (Bennet et al. 1974):
(
)
2
2
4
p
3
16
−
r
ν
f
=
γ
(2.8)
3
E t
f f
where
p
is the applied pressure,
r
is the radius of the hole,
E
f
is Young's modulus of the film,
v
f
is Poisson's ratio of the film, and
t
f
is the film thickness.
Two of these commonly used methods, namely, microtensile testing and nanoindenta-
tion, are discussed in more detail below given their relevance in biomaterial property
evaluations and based on studies by the main author's research group.
Microtensile Testing
The method is suitable for determining properties of both thin and thick coatings on a vari-
ety of ductile substrates and provides insights into interfacial delamination susceptibility
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