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Figure 9. Calibration diagram showing the linear dependence of the Ni K
α
intensity (XRFS analy-
sis) and thickness of the electrolessly deposited Ni films on the initial conductance 1/R
o
of these
films. For films obtained from the same plating bath, a simple electrical measurement provides an
indication of the film thickness.
relative elongation
L/L
o
where L
o
is the initial distance between the two
metallic contacts. These curves are characteristic of Ni / PI systems in which the
polymer surface has been plasma-treated in NH
3
and subsequently “activated” ei-
ther (a) by immersion in a simple acidic PdCl
2
solution or (b) through the Pdac
route. In both cases, it is possible to distinguish different zones associated with
the progress of the fragmentation process of the Ni thin film. Indeed, there is a
first fragmentation step during which many random cracks appear (upward-
sloping parts of curves (a) and (b)) and form parallel metal strips. As the applied
strain increases, the initial step of the fragmentation process progressively gives
rise to a second step which consists of the division of the widest segments at their
midpoint (downward parts of curves (a) and (b)). Note that the boundary between
the two corresponding zones cannot be precisely determined since the random
cracks formed probably coexist with the cracks resulting from the division of the
widest segments. When curves (a) and (b) tend to a minimum (
ε
(%) =
∆
~ 12-14%) crack
density saturation is reached. This means that, at this stage, the primary process of
segment division can be considered as nearly completed. Eventually, a secondary
fragmentation process resulting from the contraction of the metal coating perpen-
dicular to the straining direction and controlled by Poisson's ratio of the polymer
substrate begins to appear. This results in the formation of cracks parallel to the
straining direction, and, therefore, in the formation of secondary fragments. In the
experiments relative to curves (a) and (b) of Figure 10, this secondary cracking
mode leads only to a very slight increase of the
ε
R/R values. This means that the
conduction mode is practically not changed and that most of the secondary frag-
δ
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