1 - Erich Schmid Institute

1
Summary
teristics. Firstly, the stresses grow nearly exponentially from −0.81GP a at the surface
to −3.39GP a at the interface. Secondly, the corresponding errors of the stresses in the
individual sublayers or the standard deviations, respectively, range from 0.11GP a at the
surface to 0.87GP a at the interface, which can be explained analyzing the deflection as
a function of film thickness. The deflection does not change significantly with the film
thickness in the region near the surface, but it shows a strong gradient near the interface.
Here, the change in the magnitude of the errors in the sublayers is caused by the uncertainty
in measuring the film or sublayer thickness, respectively because the influence of
this error is much smaller near the surface than in the vicinity of the interface owing to
the shape of the deflection versus film thickness curve.
A further development of the ILR method is the three dimensional ion beam layer removal
(3D-ILR) method presented in paper C. This technique allows the determination
of spatial residual stress profiles in near-surface structures and is presented by investigating
the residual stress field in and around the plastic zone resulting from a shallow
scratch in the Ni film investigated in paper A.
The 3D-ILR method provides two dimensional stress profiles from a number of parallel
cross sectional cuts through a spatial stress profile. In each experiment, the stress component
along the cantilever fabricated parallel to the edge is determined. The stresses
perpendicular to the cantilever relax partially, depending on the cantilever width; a
determination of these stresses would require the fabrication of another cantilever perpendicular
to the specimen edge. The experimental and calculation procedure for the
two-dimensional stress profiles is similar to the procedure of the basic ILR method.
The experimental procedure starts with FIB cut performed at the edge of the specimen
to obtain a plane view on the cross section of the scratched system. This cross section
is divided into a number of a few microns wide sections. A cantilever is fabricated from
this cross section in a way that it is located far away from the plastic zone and the ILR
method is performed to obtain the depth profile of residual stresses. Now the cantilever
is elongated by removing the substrate material in the section next to the cantilever,
which leads to a change in the deflection. Then, the film thickness is reduced gradually
in this new section and the stress distribution is calculated. This procedure of elongating
the cantilever by the length of the adjacent section, removing the thin film gradually
and measuring the corresponding deflection is repeated until the center of the scratch is
reached.
The calculated depth profiles of residual stresses as a function of the lateral position
normal to the scratch shows a general trend: far away from the scratch, the depth profile
of residual stresses exhibits a gradient growing from the surface towards the interface
as described in paper A and an average stress of 460 ± 60MP a. This stress gradient
decreases moving towards the center of the scratch, where finally the residual stresses are
homogeneously distributed over the film thickness at a mean stress of −478 ± 91MP a.
This example presents the ability of the 3D-ILR method to determine spatial stress
profiles with a depth resolution on a nanoscale and a lateral resolution in the micron
range.
Besides the residual stress profiles, fracture toughness and strength of thin films are
essential for the structural integrity and the lifetime of coated components. In paper D,
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