Biomedical Engineering Reference
In-Depth Information
by the application of controlled external stresses (Filiaggi et al. 1996; Wang et al. 1998; Roest
et al. 2004; Latella et al. 2007a, 2007b).
In this adhesion test, the coating is deposited on a tensile coupon, which can then be
pulled in a universal testing machine or a specialized device that can be placed under
the objective lens of an optical microscope or in a scanning electron microscope. Figure
2.13a shows a schematic of a tensile sample with a thin coating layer that can be uniaxially
loaded at defined rates. The cracking, damage evolution, and failure in the coating can
be viewed in situ or ex situ after application of specific strains. Brittle coatings on ductile
substrates when uniaxially stressed produce parallel cracks in the coating layer perpen-
dicular to the tensile axis and normal to the interface as demonstrated in Figure 2.13b for
a sol-gel silica film on stainless steel strained to about 15%. These cracks generally extend
through the thickness of the coating and along the width of the sample and increase in
number with additional elongation, leading to a decrease in the crack spacing. For some
systems, cracks may also be accompanied by localized delamination of the coating from
the substrate. Eventually, delamination of the coating signals the end of the lifetime of the
coated system.
For softer and more compliant films, cracking can be irregular and film debonding
reduced substantially. The drawback with these semibrittle films is that quantitative anal-
ysis becomes more difficult.
Tensile testing is advantageous in that the stress field is uniform along the gauge length
of the sample and relatively small specimens can be used. Similarly, using optical or scan-
ning electron microscopy to view the damage in situ during loading offers useful insights
into material failure mechanisms (Ignat et al. 1999; Latella et al. 2004). The only prereq-
uisite for this type of test is that for analysis of the coating behavior, the residual stress,
and Young's modulus of the coating are required by other means, such as from substrate
curvature measurements and nanoindentation, respectively.
Microtensile tests provide insights into delamination and fracture processes and for
ranking the practical adhesion performance of different coatings. The film debond-
ing behavior is essential to the reliability of film-substrate systems, particularly where
mechanical loading and chemical interactions are encountered. For films that crack and
debond in a controlled fashion, one can use linear elastic fracture mechanics relations to
(a)
(b)
6 um
FIGURE 2.13
(a) Shows a schematic of the tensile test specimen, (b) parallel cracks and damage in a sol-gel silica coating on
stainless steel strained to about 15% (tensile axis is in the horizontal direction). Small debonded film region
between cracks can be seen in the center of the image.
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