Biomedical Engineering Reference
In-Depth Information
Substrate
Cohesive
20 μm
FIGURE 1.7
Typical fracture surface of fluoridated HA coatings after pull-out tensile test: a mixed failure mode is com-
monly observed, which consists of adhesion failure occurring at the coating-substrate interface, cohesion fail-
ure occurring within the coating, and gluing failure occurring at the epoxy-coating interface. (From Zhang et
al.,
Thin Solid Films
, 516, 16, 5162-5167, 2008. With permission.)
adhesion strengths between the coating and substrate (Zhang et al. 2008). In other words,
those pull-out based testing methods are highly influenced by the coating characteristics
and infiltration of epoxy, resulting in limited information about the adhesion properties
at the coating-substrate interface. Therefore, other evaluation techniques/methods (e.g.,
scratch test) are necessary to get a sound evaluation of adhesion properties.
Evaluation of Interfacial Shear Strength
Generally, for bioceramic-coated, load-bearing implants, the adhesion behaviors of coating-
substrate interface can be roughly classified into tensile and shear adhesion. Therefore,
besides the tensile strength, the interfacial shear strength also serves as a crucial factor
for those implants used as tooth root and hip joint replacement. Although certain quan-
titative evaluations have been done with the commonly used pull-out shear test, in view-
ing the intrinsic drawbacks of the “pull-out test,” those obtained shear strength should
be the “cohesive shear strength” rather than the “interfacial shear strength” (Li, Khor,
and Cheang 2002). However, the shear lag strain approach described by Agrawal and Raj
(Agrawal and Raj 1989, 1990), which utilizes the regular crack patterns obtained through
the designed tests, appears useful and relatively straightforward for the determination of
interfacial shear strength. Basically, this method relies on the development of transverse
crack patterns in a brittle coating when the relatively ductile supporting substrate is plas-
tically deformed under an applied uniaxial load. This crack behavior can be adequately
described by a shear-lag analysis that directly relates crack density to the load transfer
capabilities of the interface, and this shear lag theory predicts the establishment of a steady
state of constant crack density observed at relatively high strain levels. For a coating of
thickness
t
, the interfacial shear strength
τ
max
can be determined by a simplified expression
(Agrawal and Raj 1989):
t
1 5
π
=
f
τ
σ
max
.
λ
