EXPERIMENTAL INVESTIGATION OF THE INSTABILITY OF
BUCKLING PATTERNS: FROM STRAIGHT-SIDED TO
WORMLIKE STRUCTURES
F. Cleymand, C. Coupeau and J. Grilhe ´
Laboratoire de Me ´tallurgie Physique, UMR 6630, SP2MI, Bld P. & M. Curie 86962,
Futuroscope Cedex, France
(Received January 30, 2001)
(Accepted February 8, 2001)
Keywords: Atomic force microscopy (AFM): Thin films, Wrinkling
Introduction
The understanding of the mechanical properties of coated materials is a key factor in a number of
technological applications. In particular, the delamination of compressed thin films is one of most
limiting structural performance and presents various interesting problems in physics and mechanics.
The delamination of compressed thin films has been investigated by many experimental and
theoretical studies [1–5]. The delamination patterns result from high residual compressive stresses in the
thin film mainly due to the deposition process and involve the propagation from an initial buckle of
cracks at the film/substrate interface [6 –9]. Various shapes of debonding patterns, such as straight-sided
blisters [10] or worm-like structures [4], have been observed and characterised in different film/
substrate specimens. The worm-like buckling patterns are elongated tunnels with antisymmetric
undulation at the edges and have been recently simulated by Crosby and Bradley [11]. Recently, the
linear stability of straight delamination patterns has been performed by a numerical resolution of the
Fo ¨ppl and van Karman equations by a perturbation method [12]. One of the results is the determination
of a critical value of the Poisson’s ratio allowing the selection of stable configuration between the
antisymmetric and symmetric (or varicose) patterns. Nevertheless, this study is based essentially on
earlier experimental investigations of pre-existing worm-like blisters, explained by a simple mechanism
of instability affecting the propagation of straight-sided ones. Up to the present, the worm-like
formation has not been considered as an ageing effect.
In this paper, the evolution from straight-sided to worm-like patterns has been studied experimen-
tally by atomic force microscopy (AFM).
Experimental Results
Experiments were conduced on well-adhering 304L Stainless Steel (SS) films 58 nm thick on
polycarbonate substrates. The choice of this substrate depends on its elastic behaviour over a wide range
of strain, so that no activation of Frank-Read sources takes place. The residual stresses
i
of the
as-deposited thin films before deformation were estimated by the curvature method to be of the order
of 1-2 GPa. The specimen has been deformed by uniaxial compression to generate straight-sided blisters
perpendicular to the compression axis (Fig. 1a) [13,14].
Scripta mater. 44 (2001) 2623–2627
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