ICMCTF2013 Session F5-1: Coatings for Compliant Substrates

This paper reports on the mechanical behaviour of nanostructured W/Cu thin films deposited on a polyimide substrate under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (Saint-Aubin, France). The elastic-plastic-failure behaviour of the composite metallic film - polymeric substrate can be investigated under equi-biaxial and non-equi-biaxial loading conditions. The in-situ tensile tests were carried out combining synchrotron X-ray diffraction (XRD) and digital-image correlation (DIC) techniques. The combination of these two techniques can accurately measure deformations at two different scales, namely the in- grain scale and the macroscopic scale. The results show that the two strain measurements, i.e. lattice strain in the crystalline part of the W component of the film measured by XRD and macroscopic strain in the substrate measured by DIC, match to within 1x10^-4 in the linear elastic domain. This result clearly demonstrates that the applied strain in the elastic domain is transmitted unchanged through the film-substrate interface, and thus the W/Cu thin film elaborated by magnetron sputtering exhibit a good adhesion to the polymeric substrate without adhesion layer. The second part of the paper deals with higher strains response under equi-biaxial tensile tests. The elastic limit of the nanostructured W/Cu thin films was determined at the bifurcation point between the XRD lattice strain and the DIC macroscopic strain. Deformation mechanisms such as film fragmentation are proposed.

Nanocrystalline metallic coatings of sub-micron thickness are widely used in modern microelectronic applications. Their deformation properties differ considerably from those of thicker and more coarse-grained counterparts, due to obstacles to dislocation motion presented by the grain boundaries, and the proximity of the free surface and the interface.

In X-ray diffraction experiments to determine both the residual and ‘live’ stresses in nanocrystalline coatings, one difficult challange that comes up invariably is the determination of the strain-free lattice spacing d_o. The present study addresses this challenge as described below.

Previously, detailed experimental analysis by Digital Image Correlation and Finite Element modelling have been used to demonstrate that Focused Ion Beam (FIB) ring-core drilling can produce full stress relief of circular and rectangular “islands” [1]. We used this approach to generate a built-in strain-free reference by patterning a 50×50μm² region of the coating by FIB milling to produce an array of small stress-relieved “islands” ~0.4×0.4μm² each.

Transmission X-ray diffraction setup was used for data collection at DIFFABS beamline (Synchrotron Soleil, France). A 400nm-thick nanocrystalline gold coating on PMMA substrate [2] and a 240nm-thick multi-layered W-Cu nano-composite thin films on Kapton substrate [3] were studied. The samples were loaded incrementally using a compact uniaxial loading device, and micro-beam diffraction data were collected on and away from the reference array. It was shown experimentally that the “island” array remained strain free throughout the experiment, providing an on-board d_o lattice spacing reference. The changing lattice spacing d in the coating was also monitored away from the array, to deduce the elastic strain evolution during deformation. The results and their implications are presented and discussed.

[1] Korsunsky, A.M., (2009), Materials Letters, Vol. 63, p.1961-1963.

[2] Girault, B. et al. (2011), J. of Applied Physics, (109) 014305.

[3] Eve, S. et al. (2011), Int. J. Theoretical and Applied Multiscale Mechanics, Vol. 2, No. 1, p.38–45.

Keywords: gold thin film, W-Cu, polymer substrate, stress relaxation, Synchrotron X-ray Diffraction, Focused Ion Beam

The electrical, optical, and structural integrity of flexible transparent electrodes is paramount in the design and fabrication of optoelectronic devices, such as organic light emitting diodes, liquid crystal displays, touch panels, solar cells, and solid state lighting applications. The electrodes may corrode due to acrylic acid containing pressure sensitive adhesives. In addition, structural failure may occur due to external applied loading. The combined action of mechanical loading and corrosion can aggravate the failure of the electrodes.

In this study we investigate the effects of acrylic acid concentration, film thickness, number of bending cycles, and applied strain on the electrical and structural integrity of carbon nanotube and indium tin oxide films on polyethylene terephthalate substrates.

In situ electrical resistance measurements are conducted during corrosion, bending, corrosion-bending, fatigue, and fatigue-corrosion experiments in order to determine the crack onset strain. Crack density calculation is performed on images acquired using optical microscopy. In addition, scanning electron microscopy is conducted in order to determine failure mechanisms.

Thin hydrogenated amorphous carbon (a-C:H) coatings suffer from insufficient load bearing capacity for high load applications when they are deposited on relatively soft substrates like Al-alloys. In this work, the contact damage behaviour of an a-C:H coating system deposited on Al-alloy sheets was investigated by indentations with different indenter geometries and scratch tests. The investigated coating system consists of a silicon rich adhesion layer, an adjacent ramp layer with a graded chemical composition and a 2 µm thick a-C:H top coat deposited by PECVD. Using an accumulative roll bonding (ARB) process the flow stress as well as the grain size of the Al-alloy has been modified. Additionally shot peening was applied to increase the load strength and residual stresses in the Al-alloy sheet. Doing so, the mechanical properties of the substrate were varied systematically without changing its chemistry. The aim of this treatment was to enhance the load bearing capacity of the coating-substrate system. During indentation with a spherical tip, several discontinuities (pop-ins) occurred in each load displacement curve. Using Focused Ion Beam (FIB) cross-sections, these events were attributed to the formation of cracks in the coating allowing for an analysis of the fracture toughness of the a-C:H layer. The pop-in behaviour also depends on the mechanical properties of the substrate and reflects the different load bearing capacities. Finally, the concepts for increasing the load bearing capacity are discussed with the help of FE models.

Tensile tests are an important tool to improve the understanding of the mechanical behavior of coatings and their interaction with compliant substrates. When submitted to an increasing uniaxial tensile load, the coating presents nucleation and propagation of transversal cracks, up to a critical load in which the space between two consecutive cracks remains predominantly constant. Beyond this load, the stress state in the film, which is affected by the difference in lateral contraction of the film and the substrate, may lead to an adhesive failure and possible spallation of the coating. This work aims improving the understanding of the mechanical behavior of the coating after the saturation of inter-crack spacing and before spallation. A set of 3D finite element analyses was conducted to evaluate the influence of the stress state at the coating/substrate interface on the adhesive failure. Results showed non-uniform stress distributions on film-strips between two consecutive cracks, even when cracks were distributed uniformly. This non-uniformity led to distinct patterns of stress distribution that are function of the mechanical properties of the system components.