Scientific Advisory Board - Erich Schmid Institute

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE
Complex Materials
Several aspects of our wide variety of materials have been discussed in previous sections,
especially dual-phase and complex phase steels, damage tolerant materials (see section
deformation, fatigue, fracture) and nanocrystalline bulk materials (see next section Synthesis
of Bulk nano-materials and composites by Severe plastic deformation), which could be as
well listed here. other material systems we study intensively are thin film structures, which
are scientifically interesting due to the numerous internal interfaces that significantly influence
the film and film/substrate properties. in this section we want to highlight our recent research
achievements concerning adhesion and fracture of brittle and ductile films on compliant
substrates, spanning from metal/polymer to ceramic/metal systems.
Advances in Fragmentation Testing of Thin Films
the reliability of flexible electronics depends upon the ability of metal thin films, both brittle
and ductile, to bend and stretch with the supporting polymer substrate while maintaining
mechanical and electrical integrity. experiments to measure these behaviors are mostly based
on fragmentation testing. A key element to fragmentation testing is the detection of the fracture
strain or stress. this detection can be easily performed with an optical microscope or scanning
electron microscope while straining (in situ). Brittle films or coatings will form channel cracks
perpendicular to the straining direction and channel cracks are easy to detect, thus failure is
also easily determined. However, ductile films, such as Au, cu, or Ag (all materials being used
as conduction carrying elements in flexible electronics) plastically deform before channel cracks
are observed. the localized plastic deformation, or necks (necking), are the first sign of yield in
the film and are hard to observe with om or Sem. the implementation of fragmentation testing
with atomic force microscopy (Afm) is a solution that allows for the detection of initial film yield
and measurement of the deformation density and channel crack density as a function of strain.
necking and channel cracks are observed using height images and the difference in surface
profiles (fig. 13). the technique works well when there is a specific criterion to define a channel
crack versus localized deformation (necking). Brittle films match Sem experiments very well
while ductile films do not. the greatly improved height resolution of the Afm compared to the
Sem is the reason for this discrepancy. these results lead one to believe that fragmentation
Fig . 13 a) AFM height image at 2% strain with the surface profile and necks indicated by
the white arrows. b) Surface profiles at 2% and 15% strain. c) AFM height image at 15%
strain of the same area as (a) with the surface profile and channel cracks indicated with black
arrows and the position of necks with white arrows and circled areas indicate where a neck has
become a channel crack. Note the ~x2 difference in height scales in a) and c).
page 28 Scientific RepoRt 2012